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Long-Term Morphotectonic Evolution and Denudation Chronology of the Antioqueno Plateau, Central Cordillera, Colombia

Permanent Link: http://ufdc.ufl.edu/UFE0021950/00001

Material Information

Title: Long-Term Morphotectonic Evolution and Denudation Chronology of the Antioqueno Plateau, Central Cordillera, Colombia
Physical Description: 1 online resource (223 p.)
Language: english
Creator: Restrepo, Sergio
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: andes, anthropogenic, antioqueno, apatite, colombia, denudation, erosion, geomorphology, morphotectonic, plateau, thermochronology, zircon
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Erosion is now recognized as a fundamental component of the functioning of Earth systems. Erosion by water is one of the most efficient mechanisms of exhumation of rocks in the tropical Andes and exerts control on myriad of processes such as landscape evolution, orogenic belt development and style, tectonic process, sediment production and transfer to water bodies, weathering and soil development, chemistry of oceans and continental water bodies, climate, etc. In this investigation zircon U-Pb geochronology and Hf isotopic analysis, whole rock geochemistry, and apatite (U-Th)/He and fission track low-temperature thermochronology are used to reconstruct the long-term denudation chronology of the Antioquen tildeo Plateau in the northern portion of the Colombian Cordillera Central . This approach contributes to an understanding of the Cenozoic morphotectonic evolution of this major elevated plateau in the context of tectonic perturbations in the region. In addition, this investigation allows generating the first quantitative, landscape-specific figures of long-term rates of erosion as a backdrop that permits a preliminary comparison with modern, anthropogenically enhanced erosion rates. Morphotectonic evolution of this province seems to be strongly influenced by the Antioquen tildeo Batholith (AB). U-Pb dating and Hf isotopic analysis in zircon grains, complemented with whole-rock major and trace element analysis plus Pb and Nd isotopic data, constrain formation age, post-crystallization cooling history, tectonic setting and magma sources associated with this large granodiorite mass. Results suggest rapid assemblage from ca. 71 to 77 Ma involving discrete magmatic injections from different sources and with varying degrees of crustal contamination and/or fractional crystallization. Geochemical data point to a tectonic setting quite similar to the modern Northern Andean Volcanic Zone. Apatite low temperature thermochronology records two discrete pulses of exhumation at ca. 45 and 23 Ma separated by a period of tectonic quiescence. Both pulses are synchronous with orogenic phases throughout the Andes indicating continental-scale tectonic controls. Exhumation appears to be controlled by well documented changes in the rate of convergence between the Farallon (Nazca) and South American plates. Erosion rates at the climax of exhumation are ~0.1 mm/yr while the long-term averages are ~0.02-0.008 mm/yr. The latter are considered low rates for an active orogen and are characteristic of cratonic areas. Morphologic and structural features of the Antioquen tildeo Plateau define a decoupled morphotectonic system (non-equilibrium relict surface), that exhibits lagged response to tectonic input so that the most recent and strongest phase of orogenic activity in the Andes (Miocene-Pliocene) is not recorded by either apatite fission track or (U-Th)/He.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sergio Restrepo.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Foster, David A.
Local: Co-adviser: Martin, Ellen E.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0021950:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021950/00001

Material Information

Title: Long-Term Morphotectonic Evolution and Denudation Chronology of the Antioqueno Plateau, Central Cordillera, Colombia
Physical Description: 1 online resource (223 p.)
Language: english
Creator: Restrepo, Sergio
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: andes, anthropogenic, antioqueno, apatite, colombia, denudation, erosion, geomorphology, morphotectonic, plateau, thermochronology, zircon
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Erosion is now recognized as a fundamental component of the functioning of Earth systems. Erosion by water is one of the most efficient mechanisms of exhumation of rocks in the tropical Andes and exerts control on myriad of processes such as landscape evolution, orogenic belt development and style, tectonic process, sediment production and transfer to water bodies, weathering and soil development, chemistry of oceans and continental water bodies, climate, etc. In this investigation zircon U-Pb geochronology and Hf isotopic analysis, whole rock geochemistry, and apatite (U-Th)/He and fission track low-temperature thermochronology are used to reconstruct the long-term denudation chronology of the Antioquen tildeo Plateau in the northern portion of the Colombian Cordillera Central . This approach contributes to an understanding of the Cenozoic morphotectonic evolution of this major elevated plateau in the context of tectonic perturbations in the region. In addition, this investigation allows generating the first quantitative, landscape-specific figures of long-term rates of erosion as a backdrop that permits a preliminary comparison with modern, anthropogenically enhanced erosion rates. Morphotectonic evolution of this province seems to be strongly influenced by the Antioquen tildeo Batholith (AB). U-Pb dating and Hf isotopic analysis in zircon grains, complemented with whole-rock major and trace element analysis plus Pb and Nd isotopic data, constrain formation age, post-crystallization cooling history, tectonic setting and magma sources associated with this large granodiorite mass. Results suggest rapid assemblage from ca. 71 to 77 Ma involving discrete magmatic injections from different sources and with varying degrees of crustal contamination and/or fractional crystallization. Geochemical data point to a tectonic setting quite similar to the modern Northern Andean Volcanic Zone. Apatite low temperature thermochronology records two discrete pulses of exhumation at ca. 45 and 23 Ma separated by a period of tectonic quiescence. Both pulses are synchronous with orogenic phases throughout the Andes indicating continental-scale tectonic controls. Exhumation appears to be controlled by well documented changes in the rate of convergence between the Farallon (Nazca) and South American plates. Erosion rates at the climax of exhumation are ~0.1 mm/yr while the long-term averages are ~0.02-0.008 mm/yr. The latter are considered low rates for an active orogen and are characteristic of cratonic areas. Morphologic and structural features of the Antioquen tildeo Plateau define a decoupled morphotectonic system (non-equilibrium relict surface), that exhibits lagged response to tectonic input so that the most recent and strongest phase of orogenic activity in the Andes (Miocene-Pliocene) is not recorded by either apatite fission track or (U-Th)/He.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Sergio Restrepo.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Foster, David A.
Local: Co-adviser: Martin, Ellen E.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0021950:00001


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1 LONG TERM MORPHOTECTONIC EVOLUTION AND DENUDATION CHRONOLOGY OF THE ANTIOQUEO PLATEAU, CORDILLERA CENTRAL COLOMBIA By SERGIO ANDRS RESTREPO -MORENO A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FL ORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009

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2 2009 Sergio Andres Restrepo Moreno

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3 To my family: my preferred spiritual niche and my most precious source of love and awarenes s

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4 ACKNOWLEDGMENTS This degree is linked to what seems now an incredible amount of work. Although this research venture represents yet another step in my apparently endless academic career, it is by no means an individual conquest and many people and organizations participated in this endeavor so I could bring my doctoral project to fruition. I owe the deepest gratitude to my family for their continuing support in the various projects I have embarked, not only in academe but in every aspect of life. A nd I also owe them an apology as they generously allowed me to steal some of our precious time together. But the family has grown in this long 7 years or more so I want to specifically thank members of the old family: my parents Mariana and Miguel Angel for the gift of life and for their guidance and their infinite capacity to love and give; to my siblings Clara, Cami lo and Joche because with them I was introduced, at an early age, to the delight of laughter and the privileges of extended childhood, I am also grateful to them for their long -term solidarity and the great moments we have shared together. And, of course I want to thank the new family: another magical group of people that has resulted from sharing my life with Isa which in turn brought Luna and Inti Miguel into the scene. These three new members of my whole family have made my life even better than it already was while increasing my degree of compromise with society. To Isas family also my gratitude for their decided support to Isa, the k ids and, hence, to me The enormous task of administration and paperwork is at the basis of academic success in our Department, although the work of these invisible people usually remains unnoticed. I want thank all of those who have dedicated long h ours of work so others can engulf in book laboratory life, those who have passed through room Williamson 241 and the various generations of committed workers, in particular the generation with which I got to share more time: Ron Ozbun, Ileana McCray, Mary Ploc h, and Jody Gordon.

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5 I owe gratitude to David Foster, my supervisor. David valued my intentions of conducting an independent research project. His support came in many forms, though most appreciated were his critiques of my ideas, and his invitation to shar e his devotion for kids foot -ball, something they call soccer here in the US. Each of the other members of my commi ttee had something to teach me, Mark Brenner, Willie Harri s and Ellen Martin are thanked for their guidance through the intricacies of both academic matters and life in general. Through their company and permanent support this doctoral work truly acquired its Ph part as they were always ready to waste some time philosophizing around. Helpful academic discussions with them enhanced this wo rk. They also represent an important aspect of my n ow deep interest i n lakes, soils climate change, and the interactions between humans and the natural environment, which are all essential component s of my academic and social quandaries. I also want to th ank Dave Hodell for his opportune comments about my research, particularly during committee meeting time and Mike Binford for his interesting approach to the use of the distant eyes (remote sensing) and GIS to study the complexities of Earth. I am thankful to Mike Perfit. Although not strictly a committee member, he has provided much appreciated long term guidance and support and he played a fundamental role as a graduate advisor during my first years at UF. He was also the person in Geological Sciences that believed in my project and made everything possible so I could tap the initial financial support that allowed me to successfully pursue this track. The comradeship of Richard Barclay (geology), Phill DAmo (geology), Lara Maxwell (veterinary medicine), M onica Lipinsky (geology), Victoria Mejia (geology), Carlos Jaramillo (geology), Mary Lindenberg (geology), Pedro Santos (zoology), Greg Babitt (zoology) Anne Lindenberg (engineering) the soccer crew in zoology and botany, and a few other geology and non -geology students was fundamental in acclimatizing to the rapid changes in life that came

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6 with leaving family and land behind. Their company also deepened my thinking in very interesting realms inside and outside of academe. Thanking George Kamenov is a mus t as his support in the technical aspects of this dissertation our shared passion for foot -ball (something they call soccer here in the US), and our common nature as foreigners (and then parents!) gradually brought us close together and made us share the path of laughter quite often. Various organizations and colleagues in Colombia and The US have contributed intellectually to this project. Listing is always a bad thing to do because you constantly forget to include someone/something but here I go: I want to thank Miguel Angel Restrepo and the rest of the crew at CIER, Colombian NGO for the financial and logistic support ; Norberto Parra, Alberto Arias, and Kenneth Cabrera, great geomorphologists and hil arious friends at ICNE at Universidad Nacional de Colo mbia for the permanent academic advise ; Daniel Stockli and other researchers at the Isotope Geology Lab of the University of Kansas for a very productive time for me as their apprentice in the meticulous work of apatite helium dating. I am most grateful to the generosity of Paul OSullivan and Ray Donelick at Apatite to Zircon Inc. Their scientific work i n perfecting a new way to date apatite crystals by fission tracks using LA -ICP -MS is a break through. Not onl y are they great scientists, but also possess a very humane approach to the interaction with others in academe. They came to UF, on their budget, to teach George, David and I the fundamentals of LA -ICP -MS fission track dating sharing their knowledge, opening up a new avenue of research for one of our facilities at UF and tremendously facilitating my process of data acquisition with this tool. Big thanks also go to my colleagues and friends Carlos Suescn and Camilo Polanco for their assistance in the field, to Misty Stroud for her valuable help with w hole rock chemical analysis at UF, and to my cousin Juan David Moreno and Pa ty Monsalve for their dedicated work with some of the graphics that accompany this text. M any

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7 thanks also go t o Andre Ste phenson at A p plication Support Center at UF for his invaluable help in finishing up th is manuscript. Financial support was of paramount importance in undertaking this scientific project. The independent character of my research made it a bit more difficult but I always manage d to find very receptive individuals out there ready to contribute to my scholarly efforts. Most thanked are Anne Donnelly and Emila Hodges for the SEAGEP Fellowship and the various research grants provided Susan Jacobson for awarding me the Compton Foundation Tropical Conservation and Development Scholarship and Research Grant, Cindy Martinez and others at the American Geological Institute for a long hi story of financia l support and mentoring; administrators in the Geology Department, The Center for Latin American Studies Tinker Foundation-UF, The College of Liberal Arts and Sciences, The Office of Graduate Studies -UF, Womens Club -UF, CIER, the Office of M inorities at the Graduate School -UF, GSA, (unfortunately I have to close with and etc. right here ), etc. for the opportune financial support. Once again, I present my sincere gratitude to all those individuals and organizations that directly and indirectly contributed to this work, whether listed here or not, this dissertation is as much the product of their hard labor and dedication as it is mine.

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8 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................................... 4 LIST OF TABLES .............................................................................................................................. 11 LIST OF FIGURES ............................................................................................................................ 12 ABSTRACT ........................................................................................................................................ 14 CHAPTER 1 INTRODUCTION ....................................................................................................................... 16 Intro to an Intro ............................................................................................................................ 16 Study Site ..................................................................................................................................... 17 Objectives and Significance of this Investigation ..................................................................... 19 Scientific and Societal Importance of Erosion .......................................................................... 20 Natural Erosion .................................................................................................................... 21 Anthropogenic Erosion........................................................................................................ 24 Assessing Erosion Rates in the Colombian Andes ................................................................... 26 Modern Erosion ................................................................................................................... 26 Long -term Erosion ............................................................................................................... 27 This Investigation in the Context of the Geological Discipline ............................................... 31 Methods ....................................................................................................................................... 33 Erosional Exhumation and Low Temperature Thermochronology .................................. 33 Apatite lo w temperature thermochronology ............................................................... 33 Post Crystallization Cooling and Petrology of the Antioqueo Plateau .......................... 39 Spot U -Pb and Hf isoto pic analysis in magmatic zircons .......................................... 39 Zircon cathodoluminescence imaging (CL) ............................................................... 40 Whole rock major -trace element, and radiogenic is otopic analysis ......................... 41 Thesis Outline .............................................................................................................................. 42 2 FORMATION AGE AND MAGMA SOURCES OF THE ANTIOQUEO AND OVEJAS BATHOLITHS, CENTRAL CORDILLE RA, COLOMBIA .................................. 50 Introduction ................................................................................................................................. 50 Study Site ..................................................................................................................................... 53 Geologic Setting .................................................................................................................. 53 The Antioqueo and Ovejas Batholiths ...................................................................... 55 Previous Thermochronology and Isotopic Analysis .................................................. 56 Tectonic and Geodynamic Setting ...................................................................................... 57 Methods ....................................................................................................................................... 59 Sample Collection and Preparation .................................................................................... 60 Zircon separation, grain description and cathodoluminescence ................................ 61 Whole rock sample preparation ................................................................................... 63

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9 U/Pb and Hf Spot Analysis in Zircons ............................................................................... 64 Whole Rock Geochemical and Isotopic Analysis ............................................................. 67 Major and trace element analysis ................................................................................ 67 Nd isotopic analysis ..................................................................................................... 67 Pb isotopic analysis ...................................................................................................... 68 Res ults .......................................................................................................................................... 68 Major Elements .................................................................................................................... 68 Zircon U -Pb and Hf ............................................................................................................. 69 Pb and Nd Isotop ic Compositions ...................................................................................... 70 Rare Earth Elements ............................................................................................................ 70 Discussion and Interpretation ..................................................................................................... 71 Major Elements ............................................................................................................ 71 Zircon CL, U -Pb and Hf .............................................................................................. 72 Trace Element and Radiogenic Isotopes ..................................................................... 74 Radiogenic Pb and Nd ................................................................................................. 79 A Feasible Geodynamic Setting .................................................................................. 79 Conclusions ................................................................................................................................. 82 3 LONG TERM EROSION AND EXHUMATION OF THE ALTIPLANO ANTIOQUEO, NORTHERN ANDES (COLOMBIA) FROM APATITE (UTH)/HE THERMOCHRONOLOGY ..................................................................................................... 102 Introduction ............................................................................................................................... 102 Study Site ................................................................................................................................... 104 Geological and Physiographic Overview ......................................................................... 104 The An tioqueo Plateau .................................................................................................... 106 Tectonic Setting ................................................................................................................. 108 Apatite (U Th)/He Thermochronology .................................................................................... 110 Methods ..................................................................................................................................... 111 Results ........................................................................................................................................ 114 Discussion and Interpretation ................................................................................................... 116 Conclusions ............................................................................................................................... 123 4 FURTHER CONSTRAINTS ON LONG TERM EROSIONAL EXHUMATION OF THE ALTIPLANO ANTIOQUEO, NORTHERN ANDES, COLOMBIA, BY LAICP -MS APATITE FISSION TRACK ANALYSIS .............................................................. 138 Introduction ............................................................................................................................... 138 Study Site ................................................................................................................................... 140 Geomorphologic Considerations ...................................................................................... 141 Geological Considerations ................................................................................................ 143 Tectonic Considerations .................................................................................................... 146 Uplift a nd Denudation in the Colombian Andes ..................................................................... 147 Low Temperature Thermochronology and Erosional Exhumation ................................ 150 Apatite Fission Track Ana lysis ......................................................................................... 150 Apatite Fission Track Analysis in the Antioqueo Plateau ............................................ 152 Methods ..................................................................................................................................... 153

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10 Sample Collection and Preparation .................................................................................. 153 Age Measurements by LA ICP MS ................................................................................. 154 Track Length Measurements ............................................................................................. 156 Annealing Kinetics and Dpar Parameterization ............................................................... 156 Fission -track Modeling...................................................................................................... 157 Results ........................................................................................................................................ 158 Interpretation and Discussion ................................................................................................... 160 Conclusions ............................................................................................................................... 171 5 GENERAL CONCLUSIONS AND FURTHER RESEARCH .............................................. 186 Antioqueo and Ovejas Batholith Age and Petrogenensis ..................................................... 186 Low Temperature Ther mochronology ..................................................................................... 187 LIST OF REFERENCES ................................................................................................................. 190 BIOGRAPHICAL SKETCH ........................................................................................................... 221

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11 LIST OF TABLES Table page 2 1 Summary of major element data for the Antioqueo and Ovejas batholiths.. ................... 85 2 2 U e Antioqueo and Ovejas batholiths.. ................. 86 2 3 Whole rock trace element concentrations for the Antioqueo and Ovejas batholiths. ...... 87 2 4 Lead and Neodymium isotopic compositions for the Antioqueo and Ovejas batholiths. .............................................................................................................................. 88 3 1 Summary of (U Th)/He results for Matasanos and La Garca vertical transects. ............ 127 4 1 Instrumentation, operating conditions, and data acquisition parameters used for LA ICP -MS apatite fission track dating. ................................................................................... 173 4 2 Summary of apatite fission track data. ............................................................................... 174

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12 LIST OF FIGURES Figure page 1 1 Physiographic Units of Colombia. ........................................................................................ 46 1 2 The Antioqueo Plateau a geomorphic feature carved on the Antioqueo Batholith.. ..... 47 1 3 The concept of the partial annealing zone (PAZ) and partial retention zone (PRZ) in apatite fission track and (U Th)/He thermochronology.. ..................................................... 48 1 4 Vertical profile approach to low -temperature thermochronology in the Antioqueo Plateau.. ................................................................................................................................... 49 2 1 Geologic and structural map of the study site. ..................................................................... 88 2 2 Color photographs of samples in their hand specimen. ....................................................... 89 2 3 Photomicrographs of grey-scale CL imaging of typical zircons from the Antioqueo and Ovejas batholiths.. ........................................................................................................... 90 2 4 Classification of the Antioqueo and Ovejas batholith based on their SiO2 vs. Na2O + K2O concentrations.. ........................................................................................................... 91 2 5 Sub alkalinity degree for the Antioqueo and Ovejas batholiths. ...................................... 92 2 6 Majo r element Harker variation diagrams for the Antioqueo and Ovejas batholiths.. .... 93 2 7 U -Pb Age -Elevation relationship and range of previously reported ages for the Antioqueo and Ovejas bathol iths ........................................................................................ 94 2 8 Tera -Wasserburg diagrams. ................................................................................................... 95 2 9 U -Pb Age ...................................................................................... 96 2 10 Pb/Pb correlation diagrams for the Antioqueo and Ovejas batholiths. ............................. 97 2 11 o and Ovejas batholiths. .................................. 98 2 12 Primitive mantle normalized Rare Earth Elements patterns (REE) for the Antioqueo and the Ovejas batholiths.. ..................................................................................................... 99 2 13 Multi -element plot for the Antioqueo and the Ovejas batholiths. ................................... 100 2 14 Post -crystallization cooling path for the Antioqueo Batholith.. ...................................... 101 3 1 Shaded relief map of the Northern Andes, Colombia. T ................................................... 128 3 2 Geology of the study site.. ................................................................................................... 129

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13 3 3 Three dimensional representation and simplified topographic/geologic cross section across the Antioqueo Plateau. ........................................................................................... 130 3 4 Modern tectonic setti ng. ...................................................................................................... 131 3 5 Apatite (U Th)/He Age -elevation Relationship. ................................................................ 132 3 6 Relation between apparent (U Th)/He and fission track ages in apatite.. ........................ 133 3 7 Simplified sketch illustrating the extent of the crustal s ection removed since ca. 25 Ma ....................................................................................................................................... 134 3 8 Simplified cartoon of cross -sections depicting the paleogeographic and geologic evolution of the Northern Andes at the latitude of the study site. ..................................... 135 4 1 Regional geomorphology. .................................................................................................... 175 4 2 Geology of the study site. .................................................................................................... 176 4 3 Tectonic Setting. ................................................................................................................... 177 4 4 Age elevation relat ionship. .................................................................................................. 178 4 5 AFT Age vs. Elevation Relationship for The La Garca profile. ...................................... 179 4 6 Age vs. mean track length relationship fo r the Matasanos Porce profile. ........................ 180 4 7 Representative Time temperature paths modeled with AFTSolve. ............................... 181 4 8 Relevant HeFTy Time temperature paths.. ........................................................................ 182 4 9 AFT and AHe time -temperature paths.. .............................................................................. 183 4 10 Long -term spatially averaged erosion. ................................................................................ 184 4 11 Timing and magnitude of Cenozoic changes in convergence between the Farallo n Nazca and South American p late s .. ..................................................................................... 185

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14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy LONG TERM MORPHOTECTONIC EVOLUTION AND DENUDATION CHRONOLOGY OF THE ANTIOQUEO PLATEAU, CORDILLERA CENTRAL, COLOMBIA By Sergio Andrs Restrepo -Moreno May 2009 Chair: David A. Foster Cochair: Ellen Martin Major: Geology E rosion is now recognized as a fundamental component of the functioning of Earth systems. Erosion by water is one of the most efficient mechan isms of exhumation of rocks in the tropical Andes and exerts control on myriad of processes such as landscape evolution orogenic belt development and style, tectonic process, sediment produc tion and transfer to water bodies, weath ering and soil developmen t, chemistry of oceans and continental water bodies, climate, etc. In this investigation zircon UPb geochronology and Hf isotopic analysis, whole rock geochemistry, and apatite (U Th)/He and fission track low -temperature thermochronology are used to reconstruct the long-term denudation chronology of the Antioqueo Plateau in the northern portion of the Colombian Cordillera Central This approach contributes to an understanding of the Cenozoic morphotectonic evolution of this major elevated plateau in the context of tectonic perturbations in the region In addition, this investigation allows generating the first quantitative landscape -specific figures of longterm rates of erosion as a backdrop that permit s a preliminary comparison with modern, anthropogen ically enhanced erosion rates. Morphotectonic evolution of this province seems to be strongly influenced by the Antioqueo Batholith (AB) U -Pb dating and Hf isotopic analysis in zircon grains,

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15 complemented with whole rock major and trace element analysis plus Pb and Nd isotopic data constrain formation age post -crystallization cooling history, tectonic setting and magma sources associated with this large granodiorite mass Results suggest rapid assemblage from ca 71 to 77 Ma involving discrete magmatic injections from different source s and with varying degrees of crustal contamination and/or fractional crystallization Geochemical data point to a tectonic setting quite similar to the modern Northern Andean Volcanic Zone Apatite low temperature t hermochr onology record s two discrete pulses of exhumation at ca. 45 and 23 Ma separated by a period of tectonic quiescence. Both pulses are synchronous with orogenic phases throughout the Andes indicating continental -scale tectonic controls Exhumation appears to be controlled by well documented changes in the rate of convergence between the Farallon (Nazca) and South American plates. Erosion rates at the climax of exhumation are ~0.1 mm/yr while the longterm averages are ~0.0 2 0.008 mm/yr. The latter are consider ed low rates for an active orogen and are characteristic of cratonic areas. Morphologic and structural features of the Antioqueo Plateau define a d ecoupled morphotectonic system ( nonequilibrium relict surface), that exhibits lagged response to tectonic input so that the most recent and strongest phase of orogenic activity in the Andes (Miocene -Pliocene) is not recorded by either apatite fission track o r (U Th)/He

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16 CHAPTER 1 INTRODUCTION Intro to an Intro Overall, I devised this academic exercise as a phi losophical j ourney that would allow me to reflect, simultaneously, on scientific and societal problems, if such categories of human tribulations admit any separation at all. The initiative was to instigate a personal immersion in geological work in light o f J. Lub chenco's proposal for a new and urgent social contract for the sciences [1] I chose to work in the tropical Andes, specifically the Cordillera Central of Colombia, for various reasons including but not restricted to: 1) the i ndisputable importance of that portion of the Andean chain geologically, geographically (human and physical), biologically, and ecologically 2) the social debt I have acquired via education with the peopl es of that r egion, and 3) the spiritual affinity I have with that territory It was the concern about the interactions of humans with the landscape i.e., the technosphere [2], particularly in the context of the capacity of humans t o transform geomorphic dynamics [3 5] and the intention to understand erosional forces past and present, that pushed me into pu rsuing th is investigation The literature review that ma rked the deve lopm en t of this dissertation and my personal experience as a traveler aware of soil degradation, indicate that enhanced erosion due to human perturbation of natural ecosystems has the characterist ics of an environmental catastrophe with an associated major threat to the sustainability of the human enterprise [6 9] The trigger of this research adventure was thus related to an independently developed exploration of anthropogenic and natural erosion rates. However, and mainly due to the lack of reliable information on modem erosion rates and the difficulty in generating them over a reasonable time frame the investigation migrated to topics that are at the same level of interest

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17 Such themes include the possibility to quantitatively determine the long-term landscape evolution and the exhumation history of an i nteresting morphotectonic unit in the Colombian Cordillera Central known as the Antioqueo Plateau (Figure 1 1) in light of proposed theories for the morphotectonic, geologic and paleogeographic evolution of the region [10 17] and continental -scale tectonic perturbations [18 20] In fact, it was this latter migration of the study in new directions what turned out to be the core of the researc h project presented in this dissertation. Study Site The Colombian territory can be roughly divided into four physiographic provinces: a) five discrete mountainous areas including the Andean Region, the Sierra Nevada de Santa Marta, the Serrana de Baud, the Serrana de la Macarena, and the Serrana de Chiribiquete; b) two major plains represented by the Caribbean Plains in the north, and the vast Eastern Plains encompassing the savannas of the Orinoqua and the tropical forest of the Amazona; c) two inte randean valleys associated to the Cauca and Magdalena rivers; and d) the relatively flat areas west of the Andean Region (Pacific Coast Plains) occupied by the valleys of the rivers Atrato, San Juan and Pata (Figure 1 1 ). The mountainous domain of the And ean Region has a trident shape that encompasses the sub -parallel ranges of the Cordillera Occidental, Cordillera Central and Cordillera Orietal (Western, Central and Eastern c ordilleras ), roughly oriented in a N -S direction and separated by major intramont ane depressions. The Cauca trough separates the Cordillera Occidental and Cordillera Central while the wider Magdalena Valley defines the boundary between the Cordillera Central and Cordillera Oriental The study site falls on northern tip of the Cordiller a Central L itho l ogically the area is dominated by Paleozoic polymetamorphic basement, with small remnants of s hallow marine sedimentary sequences Lower Cretaceous in age, and late

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18 Cretaceous massive granitoids emplaced in the axis of the range such as t he Antioqueo batholith [21] Structurally speaking the region has functioned as a coherent semi rigid crustal bloc, with low internal disruption by faults [22] and bound by two major inherited structures, the Cauca -Romeral and Palestina fault system s to the west and east respectively [23] The tectonic history of this portion of the Andean orogen has been determined by subduction dynamics between the Nazca (Farallon), Caribbean and South American plates and interactions of major lithospheric blocks (Andean and Panama Choco blocks ) and an intense accretionary process from the Paleozoic to the late Cenozoic [23 26] The plateau morphology of the study site is a striki ng feature of the Andean range in Colombia [10, 12, 27, 28] This portion of the Cordillera Central known as the Antioqueo Plateau (AP), b ears resemblance with a truncated and asymmetric pyramid with a steep face coinciding with the Cauca Romeral depression (occupied by the narrow valley of the Cauca Rive r) and a more gentle decline towards the area structurally controlled by the Palestina fault system (towards the much wider Magdalena valley). This cordilleran segment is in clear contrast with the typical sierra like ranges of the Western and Eastern C ord illera s and the southern portion of the Cordillera Central In add ition, the AP is deeply incised by the Medellin Porce fluvial system (Figure 1 2) creating regional scarps with differences in elevation in excess of 1.5 km. The combination of lithologic, s tructural, and geomorphic features make the AP a n ideal milieu to conduct low temperature thermochronology studies along vertical profiles which permit s assessing the type of questions proposed in this investigation. In the following sections of this Intr oduction the objectives and significance of the project, the importance of erosion, and the status o f erosional studies in Colombia will be addressed.

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19 Additionally the advantages of the multi tool approach implemented in this research will be succinctly p resented with emphasis on the utility of low -temperature thermochronology Objectives and Significance of this Investigation To date, the morphotectonic history of the Northern Andes of Colombia remains largely unexplored. However, this portion of the Ande an chain represents an important segment of the orogen with a great potential in providing valuable information about the interactions between tectonics and surface processes particularly through the application of low temperature thermochronologic techni ques In this investigation, the lithologic, structural and morphologic characteristics of the A P are exploited to reconstruct the Cenozoic erosional and morphotectonic history of the northern segment of the Cordillera Central A better understanding of th e long term landscape development and erosional exhumation history of the AP is needed for defining the morphotectonic and uplift history of the Northern Andes, the relationships between erosion and tectonics the paleogeog raphic evolution of the region (D uque Caro, 1990; Gmez, 2005; Gregory -Wodzicki, 2000; Hooghiemstra, et al., 2006; Hoorn, et al., 1995; Van der Hammen, 1960; Van der Hammen, et al., 1973) and regional geodynamics trend s from the Late Mesozoic to the Cenozoic which have controlled large s cale phenomena from voluminous magma generation and emplacement of the Antioqueo batholith to the development of the AP as a geomorphic anomaly. The findings of this study have implications for refining morphotectonic response of the overriding plate to variations in convergence rates between the Nazca (Farallon) and the South American plates during the Cenozoic [18] and pa tterns of regional uplift Low temperature thermochronology also permits to define a base line for geologic (natural) erosion rates that will be valuable in future geomorphologic inv estigations as the back drop against which modern (anthropogenic) erosion r ates can be compared.

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20 The specific objectives of the investigation include: 1) establishing the timing and magnitude of erosional denudation events in this orogenic segment, 2) test ing whether the AP functions as a n e le vated, relict erosional surface i.e., a portion of the orogen that has not achieved equilibrium between erosion and tectonics ; 3) reconstructing long-term background erosion rates for the Paleogene and Neogene as a way to understand landscape development patterns for the AP and as the first atte m pt to quantify natural (pre anthropogenic ) erosion rates ; and 4 ) defining more precisely the geodynamic environment and the time of emplacement of the Antioqueo Batholith, the initial high temperature portion of its cooling path, and the first phase s of morphotectonic development possibly associated to this major granitic intrusion that appears to have imposed lithologic control on the morphotectonic evolution of the AP Results are analyzed and interpreted in light of proposed theories for erosional phases in the Colombian Andes [16] regional basin development [14, 29] other uplift -exhumation deformation episodes reported for the Andes an d the Caribbean [30 40] well characterized episodes of increased convergence betw een Nazca (Farallon) and South America [18 20, 41, 42] and paleogeographic evolution in Norther n South America [15, 17, 43] More precisely defining the crystallization age of the Antioqueo Batholith (through U -Pb dating) and a feasible geodynamic environment of formation (via geochemical and isot opic data) is also crucial in unraveling possible causality betwe en increased convergence, enhanced magmatism [44 47] crustal thickening, and the initial phases of uplift during the late Mesozoic -Early Cenozoic reported for the region [16] Scientific and Societal Importance of Erosion Earths surface constitute s a complex interface between the geosphere, the hydrosphere, the atmosphere, and the biosphere. At this interface, myriad of interactions betwe en the spheres take place at a great range of spatiotemporal scales and through multiple feed back mechanisms,

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21 i n a rather sophisticated and intricate fashion that Phillips (1999) called the music of the spheres. In turn, such interactions control numerous geologic, climatic and biological processes, also at various scales of space and time [48 56] In the Holocene, and particularly after the last glaciation, another sphere came into play as humans beca me increasingly capable of modifying the natural environment i.e., the technosphere [2]. This relatively new sphere has increasingly affected the natural systems of Earth leading to a conflict between the technosphere and the ecosphere leading to a global environmental crisis that is manifested in several realms such as such as global warming, loss of fertile soil, water and air pollution, pauperization of fisheries, etc. [6, 57, 58] Although neglected compared to other global environmental problems such as global warming and nit rogen pollution, accelerated erosion due to anthropogenic causes has the potential to become a major ecological, and hence social, catastrophe that certainly warrants further consideration [8, 59 61] Defining background erosion rates is an important endeavor in understanding erosion past and present not only as a natural phenomenon and in the context of the interactions between tectonics, climate and surficial process, but also as a ne cessary step in unraveling the effect of humans on natural geomorphic dynamics The techniques utilized in this investigation, particularly low temperature thermochronology, are of great importance to these goals Natural Erosion Erosion is an i ntricate a n d fundamental natural process taking place right at the interface referred to above. Erosion is affected by, and exerts direct influences on, apparently unrelated domains such as climate [49, 51, 52, 62 65] weathering and pedogenesis [66, 67] chemistry of the ocean and the atmosphere [48, 49, 68] sediment production and transport at a variety of temporal scales [69 74] basin evolution [14] geometry and kinematics of orogens [75 79] and subduction dynamics [80] Therefore, constraining variations in erosion rates across space and

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22 time in active mountain chains helps to shed light on long term landscape evolution and on the dynamic coupling between geomorphic processes, tectonics and climate [54, 8183] The study of surface processes e.g., erosion, has thus become of first -order importance to improv ing our understanding of the functioning of orogenic belts and of Earths dynami c systems in general It is usually accepted that erosional activity is sensitive to both tectonic and climatic input [51, 8487] but a definite answer to the question of which mechanism dominates over the other [52] still remains unresolved [50] A more profound comprehension of how erosional, climatic, and tecto nic forces interact to shape mountains has the potential to foster clearer insights into Earths history [54, 81Pinter, 1997 #419] This desirable situation requires not only the development of more robust physicallybased theory and models about the processes that control fluvial erosion over time [50, 88] but also improved data on erosion rates with better spatial and temporal resolution both in active and inactive tectonic settings [54] Removal of crustal material via fluvial erosion is one of the most efficient means of exhumation of deeply buried rocks in mountain chains [89 92] Geomorphic dynamics i.e., erosion and deposition, are central to the development of topography [81, 9395] and to the process es of mountain genesis [96 98] and basin evolution [14] After almost a century of visualizing erosion as the feeble sister of tectonics, many Earth scientists have begun to put forward the idea that erosion may be equally important in the overall development of mountain belts [97] no t only at the surface but deep within the orogen [77, 79, 84] Orogenic systems are now approached as the result of multiple and complex interactions and feedback mechanism among tectonic, erosional and climatic processes [81, 99] ; and recent compilations reflect that fact [100] All three processes exert control on the shape and maximum height of mountain chains, the geometry and distribution of deformation belts, the distribution of metamorphic

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23 zo nes, the thermal and mechanic characteristics o f the lithosphere, and the amount of time necessary to build or destroy mountain ranges [75, 76, 78, 92, 101] So intric ate are the interactions between climate, erosion, tectonics and topography that simple inputs can lead to complex outputs that cause surface processes such as climate and erosion to profoundly influence tectonic processes throughout the crust and even t he mantle [92] Unraveling the spatial variability and distribution of erosion throughout mountainous regions is therefore fundamental to understanding longterm evolution of orogenic belts (e.g., morphote ctonic, metamorphic, thermo mechanic, etc.) as well as patterns of long -term landscape development. Erosion by water is one of the most important mechanisms of exhumation of rocks in the tropical Andes of Colombia. Substrate stripping via erosion is usuall y accompanied by some form of soil removal [8, 66, 102104] T he velocity at which erosion takes place is related to various factors including climate, topography, substrate characteristics, and vegetation cover. Although it has been argued that modern sediment yield is not an appropriate way to reconstruct natural erosion rates due to the pervasive effect s of human activities on sediment production and transport [105] there seems to be consensus regarding the fact that fluvial networks show that erosion takes place at a higher pace in areas with steep topography. Sediment de livered by fluvial systems indicate that mountainous regions in tectonically active tropical settings tend to exhibit high erosion rates [98, 106110] This results from a combination of steep topography, high preci pitation rates and substrate instability induced by earth quakes. In a recent compilation of erosion rates worldwide, alpine -type topography is usually associated with erosion rates in the order of 1 mm/yr whereas cratonic regions erode at velocities of about 0.001 mm/yr [8]. Some of the highest long term (geologic) erosion rates have been reported at the Himalayan syntax es were erosion takes place at rates close to 10 mm/yr [92] whereas for the Andes, various studies

PAGE 24

24 have concluded that erosion rates at the climax of erosive pulses are in the order of 1 0.5 mm/yr and clos e to ~0. 0 2 mm/yr during intervals of tectonic quiescence [31, 32, 35, 111, 112] However, as further information on spatiotemporal patterns of erosion and exhumation is produced in the Andes, the spatial complexities become more apparent, mainly as a result of structural and climatic heterogeneities I t is important to continue to document these important processes along the entire Andean range, particularl y in its northern portion where information is notoriously scant [113] Anthropogenic Erosion W hen natural rates of soil pr oduction are overcome by human-induced erosion an imbalance t akes place in which soil degradation in various forms is the ultimate consequence. To some authors, changes in erosio n rates ( e.g., accelerated soil stripping ) are leading global society towards a severe environmental crisis [8, 58, 61, 114117] of a magnitude comparable to global warming. In Colombia loss of fertile soil and other associated environme ntal problems go still unnoticed and/or underestimated despite its evident severity. Nonetheless, w ater erosion o ccurring as sheet erosion and rill gully erosion is considered the major form of soil degradation in the Colombian Andes particularly since po st Colonial times [61, 118122] when a new ethnoecological paradigm was introduced whose main purpose was the rapid transformation of natural goods an services into cash -for the -crown, i.e., a lucrative approach to the interaction of humans wit h the landscape [123] Introduction of new cultigens, new animal species [124] and a completely different perception of the natural environment lead to environmental deterioration in the whole Andean territory ( Restrepo, unpublished; Popenoe, personal communication ). N atural g eom o rphic dynamics, and hence natural erosional process, can be drastically changed by human activities [5, 7, 105, 125133] Recent studies have demonstrated order of magnitude differences between modern (anthropogenic) and natural (geologic) erosion rates in

PAGE 25

25 tropical mountainous settings [134, 135] In areas under conventional agriculture, erosion rates fluctuate between ~1 and 100 mm/yr, far exceeding natural soil replenishment values [8, 115] Further, humans have been categorized as the most important geomorphic agent on Earth [4, 5, 7] and so me authors have advanced the idea that we may have entered the Anthropocene [136] The processes that determine the velocity and extent of water erosion are directly and indirectly af fected by land cover and land use. From the factors that are believed to control erosion rates (climate, topography, soil properties, vegetation, and land use), vegetation is one of the most important variables and one of the most susceptible to be changed by human action [105, 116, 118, 137, 138] Plants moderate hydrological activity and act as a barrier between climatic agents and the soil surface. Vegetation directly protects the soil surface by reducing raindrop impact and by slowing down runoff and increasing infiltration [66] It also improves soil structure via the physical binding of soil particles by ro ots, the generation of organic matter, and the increased biological activity [61, 66, 138, 139] Transformation of natural vegetation to agricultural systems leads to a deterioration of soil properties increasing the potential for soil losses due to erosion [59, 104, 118, 140] However, assessing substrate stripping and p redicting erosion trends has proven problematic and the topic remains hotly debated in the current scien tific literature [55, 141144] The application of different predictors yields results which are sometimes several order s of magnitude different [58, 108, 145] This is possibly related to the inherent complexity of sediment production, transport and deposition [146, 147] and to alterations of geomorphic dynamics due to human actions [74, 110] To better understand the impact of humans on modern geomorphic dynamics, it is important to increase our knowledge on both modern (anthropogenic) and l ongterm (natural) erosion rates.

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26 Assessing Erosion Rates in the Colombian Andes Modern Erosion The study of modern erosion rates in Colombia is marked by the lack of a systematic and unified approach. Most research has focused on the local scale trying t o reconstruct modern erosion under different vegetation covers [122, 148, 149] A transect approach across the Cordillera Central [149] south of the study region of this dissertation, showed that under undisturbed forest, normal Andean erosion rates are on the order of 0.1 mm/ y r while removal of vegetation to introduce agriculture and grazing can increase erosion rates by a tenfold. Sediment accumulation in dams within the Antioqueo Plateau conducted by repeated bathymetric studies in five dams [150] have been interpreted as basin wide erosion rates on the order of 1 mm/yr. The study by Surez de Castro and Rodriguez (1962) at the plot scale (10120 m2) and for periods close to a decade gave a verage soil losses as low as ~0.02 mm/yr on plots with a 53% slope (established shade coffee) and as hig h as ~6 mm/yr on plots with a 21% slope (yearly rotation of pasture and corn). Losses from bare plots with 21% slope ranged from 51 87 mm/yr. Similar results were reported in the Sierra Nevada de Santa Marta through a study of soil properties [118] At a larger spatial scale, coarse predic tions using the universal soil loss equation (USLE) [151] have provided figures for erosion potential values derived a t the regional similar to the ones just discussed. In relation to sediment load as an approach to infer erosion rate s, it has been estimated that the Magdalena has an average sediment yield in excess of 6 t/ha*yr [72, 152] Latter work shows that some sub -basins within the Magdalena, such as the Carare river have sediment yields in excess of 2000 t/km2*y [153] This value makes Magdale na the major contributor of sediment to the Caribbean Sea and Atlantic Ocean in South America. Both records suggest that erosion rates for the entire Magdalena basin are close to ~ 1 mm/ yr, a relatively high figure even for

PAGE 27

27 mountainous watersheds. If the fa ct that the Magdalena possesses extensive areas for temporary sediment storage and that the sediment delivery rations rarely approaches a value of 1, erosion rates for this populous watershed could be even higher. Some authors concur that the possible reas ons for such high rates are mainly related to human activities [152154] The Anthropocene is playing hard on society as environmental degradation and obliteration of our base of natural gods and services is on the rise [58, 155] Looked from a different perspective, these new period of Earths history also offers great opportunities as it exposes the scientific community to new and challenging intellectual proble ms. Put it in the words of Haff (2003): Today we stand on the brink of a change to the Earths surface that may be as profound and long lasting as the impact of land vegetation. Because of the short time -scales involved, and the fact that what will happen will affect us directly in many ways, it is in our interest to engage the study of t hese changes in a head -on way. However, a better understanding of the role played by our species in enhancing erosion rates implies accurate figures on both modern (anthr opogenic) and long -term (natural) erosion rates. Low temperature thermochronology in the AP reveals relatively low longterm erosion rates (0.02 0.008 mm/yr), despite its location in a tectonically active mountainous setting. This low pace of erosion is se veral orders of magnitude less t h an estimated modern substrate striping for the Magdalena basin and the AP [121, 150] (1 mm/yr). Long -term E rosion Studies of longterm erosional dynamics and associated exhumational response in the Northern Andes of Colom bia are rather scarce. M orphotectonic evolution of the Colombian Andes is poorly constrained [113, 156] The majority of studies that have tackled uplift and erosional exhumation have concentrated on the Cordillera Oriental and have applied tools such as stratigraphy and palynology [13, 157, 158] with a minor contribution from thermochronology

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28 [34, 159, 160] Pioneering sedimentstratigraphic studies aimed at unraveling orogeny in the Colombian Andes took place mainly in the Cordillera Oriental and between 19401960 [161, 162] leading to the recognition of discrete pulses of erosion that affected the entire Colombia n Andes from the Paleocene to Mio -Pliocene times [16] These periods of enhanced denudation were attributed to uplift and were named the orogenetic phas es Laramic (~60 57 Ma), Pre Andina I II (~54 40 Ma), Proto-Andina (~25 22 Ma), and Eu-Andina I IV (~14 2 Ma) [16] However, the magnitude of these event s, their varying efficiencies in removing crustal material, and the spatial distribution of erosion and exhumation throughout the orogen are themes that remain largely unconstrained. Diachronous uplift and exhumation are illustrated by sediment -stratigraph ic and pollen data that yield different timing of uplift -exhumation for the Eastern and Cordillera Central with the former experiencing major phases of uplift approximately in the last 12 Ma [15, 158, 163] while the latter was the locus of denudation since the late Cretaceous [16] Subsequent research in the sedimentary formations of the Magdalena Valley has confirmed this chronology [14, 17, 29] But space -specific information on uplift driven exhumation for the Colombian Andes contin ues to be scarce. Perhaps the only examples are a series of investigations in the Cordillera Oriental between Colombia and Venezuela with rather limited thermochronology datasets, that have suggested uplift in excess of 3 km [164] and major cooling episodes by fission -track data between 22 and 27 Ma [34, 160] More recent studies incorporating apatite fission track age and length data paleobotany and geological data, also in the Eastern Cordillera, suggest increased exhumation rates in the n ortheastern Andes at ~3 Ma and asymmetrical denudation of this range due to orographic effects [159]

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29 Major consideration has been given to the recent phase of uplift exhumation in the Colombian Andes, i.e., the Eu -Andina panes of Van der Hammen (1960), as the present configuration of t he Andes and most of the uplift has been attributed to uplift pulses during this interval [113, 156] Apatite fission track ages between Venezuela and Colombia cluster in two sets point to impor tant exhumation event s associated with the Eu -Andina phase [160] Similar ly, results for rapid cooling with apatite fission -track ages of 9 and 12 Ma have been reported for a small area of the Cordillera Oriental [163] Pliocene uplift has been extensively documented in the Sabana de Bogot by detailed stratigraphy, palynology, K/Ar and fission track dating [158, 165] It is now widely ac cepted that the final major tectonic uplift of the Cordillera Oriental in the Bogot area occurred between approximately 5 and 3 Ma [15, 113] However, the specific of other potentially important phases of uplift and exhumation in the Colombian Andes associated wit h major tectonic plate reorganization during the Eocene and Oligocene [18, 19] await to be documented. Data sets aimed at elucidating uplift and denudation patterns for the Cordillera Central are even scarcer Kroonenberg et al (1990) suggested that onset of uplift for the Nevado del Ruiz area occurred between 10 and 4 Ma, supporting their contingence on local stratigraphic and geochronology data. For the northern part of the Cordillera Central a set of restricted apatite fiss ion track ages are interpreted as reveling an Eocene cooling event driven by exhumation [166] In the Cordillera Oc cidental recent uplift phases are suggested by the presence of 11 Ma dioritic stocks exposed at elevations of ~4000 m [156] Despite the great lithologic and topographic potential of the Colombian Orogen (abundance of apatite rich igneous and sedime ntary units, deep intramontane valleys, presence of major elevated erosional surfaces, etc.), high resolution low temperature thermochronology studies implemented along vertical profiles have never been undertaken. Further, the highly

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30 complementary systems fission -track and (U Th)/He analysis have not been applied simultaneously, in fact, (U Th)/Ha dating has never been applied in the Colombian An des In this investigation the Cenozoic morphotectonic history of the northern tip of the Cordillera Central is constrained by simultaneously applying ap atite fission track and (U Th)/H e analysis along two separate vertical profiles covering ~2 km of paleocrustal depths in the AP the largest high elevation erosional surface in the Northern Andes The complementary nature of low temperature thermochronology data generated in this research permit to establish a compelling temporal framework for the episodes of exhumation in this portion of the orogen allowing to identify two pulses that are synchronous with well docum ented phases of increased plate convergence in the Eocene and Oligocene [18, 19] These major exhumation episodes are separated by intervals of tectonic -geomorphic quiescence in excess of ~15 Ma that coincide with periods of s low convergence rates between the Nazca and South American plates [18] implying that multiple phase of planation and relie f generation have been the norm in the Northern Andes and possibly in the rest of the orogen [167169] Pulses of denudation here reported are in agreement with the proposed model of orgenetic phases advances by Van der Hammen (1961) based on sedimentstratigraphic analysis and with more recent models of development of continental basins in the Magdalena [14] and Cauca [168] In addition to defining an accurate temporal framework, the magnitude of denudation is also well resolved by both systems indicating order of magnitude difference of denudation intensity between climax (0.2 mm/yr) and quiescence (0.040.008 mm/yr) Furt her, long -term figures for erosion define the first quantitative frame work for background erosion in the Northern Andes which may serve as the basis to establish a valid comparison between modern

PAGE 31

31 (anthropogenic) and longtern (natural) erosion rates in the region ; an important contribution to the understanding of man as a geomorphic agent. This Investigation in the Context of the Geological Discipline During the last two decades, for the scientific and societal reasons outlined above, studies of erosion hav e been gaining importance in geological and environmental sciences and have shifted from simply focusing on local, short -scale surface processes to broadening their scope to include a wide range of spati al ( local to regional to global ) and temporal (geolog ic to modern) scales [8, 9, 61, 62, 66, 69, 84, 86, 92, 109, 127, 133, 170180] Progress in assessing erosion at contrasting spatiotemporal scales is not only related to a change in philosophical scope [180 182] but also to the emergence of several low -temperature thermochronometric tools [54, 171, 183185] as well as other isotopic techniques such as terrestrial cosmogeni c nuclides [69] Assessing erosion at multiple scales permits increasing our comprehe nsion of mountain belt evolution and allows comparisons between natural (i.e., long term, geologic) and anthropogenically enhanced (i.e., short -term modern) erosion rates Combining data on erosion rates for such wide ranges of space and time allow s me to assessing important scientific problems on two main fronts: 1) reconstructing long -term exhumation, landscape evolution and morphotectonic response of the A P and 2) establishing the framework for a quantitative comparison between longterm (i.e., pre an thropogenic, natural, geologic) and modern (i.e., anthropogenic) erosion rates in the same geographic milieu These two aspects of my investigation fall under two main s cientific disciplines: geology ( particula rly geomorphology and tectonics) and geography Over the last decade, geomorphology has emerged as a fundamental line of research in Earth sciences. This is well supported by the resurgence of geologic literature on geomorphology related topics accompanied by revitalized efforts to tackle landscape ev olution

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32 in a more quantitative way [180, 186] In the words of Summerfield (2005b): These are certainly interesting times for geomorphology: from a discipline regarded by the great majori ty of Earth scientists as being, at best, of peripheral interest and having as its subject matter the understanding of a trivial property of the Earth (the morphology of its land surface), geomorphology has now become a prominent focus of research for thos e concerned with how the Earth works. Despite the fact that geomorphology is only a thin slice of the body of science, it has both scholar ly and practical value [181] and the field can contribute significant ly to the debate over the likely future of the Earths surface [7]. F urther, the last two decades have brought about the consolidation of a relatively new scientific field k now as t ectonic g eomorphology. As suggested by its name, this discipline resulted from the blending of t ectonics and g eomorphology two previously compe ting academic domains into one of the most interdisciplinary fields found today in the physical sciences. According to Burbank and Anderson (2001), very few fields easily merge traditionally unrelated and disparate topics such as seismology, climate chang e, geochronology, structure, geodesy, paleobotany, and geomorphology. Finally, the general consensus in Earth sciences is that surface processes are of first -order importance to improve our understanding of the functioning of Earths dynamic systems [54] Erosion can be a significant agent in active tectonic systems, particularly at larger spatial scales, therefore, interpretation of mountain belts past and present requires serious consideration of erosion as an important element of analysis [92] As stated by Pinter and Brandon (1997) a more profound comprehension of how erosional, cl imatic, and tectonic forces interact to shape mountains permits clearer insights into the Earths history.

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33 Methods Erosional Exhumation and Low Temperature Thermochronology Crustal rocks cool either because a thermal pulse wanes or a given parcel of rock a pproaches the surface via exhumation. Exhumation occurs by three processes: 1) erosion, 2) normal faulting and 3) ductile thinning [90] In areas without recent magmatism, near surface cooling occurs by either tectonic or erosional denudation. AHe and AFT thermochronology can effectively constrain the timing and intensity of erosional episodes at the regional scale and on timescales of ~105107 years [54, 86, 91, 171, 172, 174, 177, 184, 187, 188] This spatiotemporal domain is germa ne for understanding orogenic evolution (e.g., mountain ranges growth and decay cycles, erosional surface development and uplift, paleorelief, etc.) and tectonics climate/tectonics -exhumation feedback responses. This study encompasses one of the first rout ine applications of a newly advanced methodology for apatite fission -track dating by laser ablation, inductively coupled plasma mass spectrometry ( LA ICP -MS ) [189] and conventional (U Th)/He analysis [183, 190] The simultaneous application of conventional (U Th)/He dating and LA -ICP MS AFT analysis provides an opportunity to cross check the consistency of both datasets with regards to ongoing arguments about increased alpha retention due to radiation damage and discrepancies between AFT and He ages [191194] Apatite low temperature t hermochronology Apatite ([Ca5(PO4)3(F,Cl,OH)]) possesses various characteristics that make it suitable for (U Th)/He and fission track dating [171, 183, 187, 190, 192, 195197] including: euhedral character; appropriate U concentrations; relatively minor U zonation compared to the more complex zoning patterns of zircon [198, 199] allows optical examination for inclusions; common occurrence in all major rock groups (igneous,

PAGE 34

34 sedimentary and metamorphic); resistance to chemical weathering; ease of mineral separation, mounting, polishing and etching procedures; reproducible etching characteristics; and well characterized and reproducible response to elevated temperatures. However, it is important to note that various recent articles have raised concerns about complications in apatite H e and fission track dating related to increased He retention potentially resulting from accumulated radiation alpha recoil damage within the crystal lattice [192] and /or U zoning [198, 199] B ut the problems of He kinetics under alpha particle trapping seem to be better understood now [194] and have in fact lead to important new applications of AHe as apatites effective closure temperature to He evolves through time allowing its use in reconstructing complex thermal histories [200] Apatite fission track analysis (AFTA) and (U Th)/He dating (AHe) are well established low temperature th ermochronometers and offer highly complementary information in the lowe st temperature range available, i.e., ~40 110C [184, 190, 195, 196, 201] AFTA and AHe apparent ages provide a measure of long term erosion rates a nd landscape evolution [86, 89, 171, 172, 183, 184, 188, 190, 200, 202, 203] because both systems record the time at which a rock cools through the upper 3 km of the crust, the domain strongly influenced by surficial processes such as erosion [54, 90, 186] Simultaneous application of both methods and comparison of results are being increasingly used to validate and cross -che ck data sets. One of the most common approaches to LTTC of both AHe and AFT involves sampling near vertical profiles such as steep valley, cliffs, regional scarps, etc., or paleocrustal depths along faulted structures [171, 172, 174, 201, 204206] In addition to using surface samples collected from vertical profiles, it is possible to use samples from different depths in a borehole [207209] These approaches allow exploiting the spatial link between the samples in the vertical

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35 dimension of the crust, i.e., paleocrustal depths, so that lower elevations yield samples that were hotter and y ounger than the higher elevation sample s Application of the vertical profile approach led to the recognition of zones of partial annealing of fission tracks [210] and partial retention of He [211, 212] in the case of AFT and AHe respectively (Fi gures 1 2 and 13) These two zones are now commonly reco gnized as the PAZ (for AFT; ~60 110C) and PRZ (for AHe;~40 70 C). When exhumed, such thermal bands represent paleocrustal depths and permit inferring the timing of cooling and the magnitude of un ro o fing directly from the shape of the age -elevation rela tionship [171, 172, 174, 184, 187, 201] In addition, the distribution of confined track lengths provides further constrain t s on the reconstruction of denudation hist ories [86, 171, 187, 196, 197] The timing of a cooling event is usually inferred from the age -elevation relationship as a sharp inflection point, i.e., break in slope (Figures 1 2 and 1 3) The elevation of t he inflection point is used to constrain the amount of denudation. Further, the gradient of the various segments in the age -elevation profile is used to constrain the rate of denudation, and potentially the amount of rock uplift and surface uplift [91, 171, 190, 213] Apatite (U -Th)/He dating Over the last decade apatite (U Th)/He thermochronometry (AHe) has emerged as a fundamental tool for quantifying the cooling history of rocks as they pass through the upper 1 3 km of the crust [54, 183, 190] The low closure temperature of this thermochronometer (40 70C) has attracted the interest of geomorphologists and tectonicists because it is applicable to interdisciplinary studies in landform evolution, erosion dynamics, structural geology, and geodynamics [54, 83, 89, 91, 214217] Current revival of interest in dating minerals via 4He accumulation has been stimulated by an improved understanding of the behavior of He in various minerals, predominantly in apatite [194, 212, 218 220] This fact, in combination with the increasing development of robust

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36 analytical techniques [54, 190, 199] has made it possible to address important geologic questions, particularly in the domain of morphotectonics [183, 190, 201, 214, 217] Helium analysis is based on measurements of 4 resulting from the radioactive decay of 235U, 238U and 232Th. Thermo kinetics of helium diffusion of U, Th and He by mass spectrometry allow s calculati on of the clos ure time of the system [190, 194] Radiogenic helium found in various mineral phases is large ly produced by radioactive decay of U and Th (and their intermediate daughter products) to Pb. Minor contribution of radiogenic He results from the decay of Sm to Nd. AHe ages are calculated from the following equation [190, 218] : 4He = 8238U(e 238t 1) + 7235U(e 235t 1) + 6323Th(e 232t 1) + 147Sm(e 147t 1) (1) where 4He, 238U, 232Th, and 147Sm represent the amounts of such isotopes as measured in the sample by mass spectrometry, t is the accumulation time (retention) or radiometric age, and s are the radioactive decay constants for each isotope [190] Detailed reviews of AHe dating fundamentals, techniques and applications are available in t he literature [183, 190, 191, 221] Apatite fission track analysis. Unlike other geochronology methods, AFT ages are calculated f rom the number of linear tracks of structural damage in the crystal found over a given area, i.e., fission tracks, and the concentration of parent atoms of 238U in the same area. The use of fission track analysis as a geological dating tool was initially proposed in the early 1960s. However, over the past two decades, AFTA has experienced a major increase in its application to a variety of geological problems in settings that range from cratonic regions to active orogenic systems [31, 32, 35, 111, 160, 171, 177, 185, 187, 191, 195197, 207, 210, 222] This is partly due to advances in understanding the temperature dependence of fi ssion track annealing and of

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37 the information contained in fission track length distributions, accompanied by the development of robust analytical and modeling techniques [184, 187, 196, 203, 223226] AFTA provides detailed information on the low -temperature thermal histories of rocks, below ca. 120C. Such temperature sensitivity applies to the study of multiple geologic phenomena including long-term chronology and magnitude of continental exhumation structural evolution of orogenic belts, sedimentary provenance, thermal history modeling of sedimentary basins and hydrocarbon maturation, etc. [171, 177, 184, 185, 195] AFT dating reli es on the measurable amounts of the density and lengths of lattice defects (tracks) induced by spontaneous nuclear fission of 238U from which apatite ages and cooling histories can be extracted. When an atom of 238U experiences spontaneous fission (a proce ss that occurs with a very low probability) two positively charged fission fragments are created and repel each other traveling at high speed through the mineral lattice and knocking other atoms o ut of their sites Thus, the trajectories of the two fission fragments create a single, long and thin region of damage (~15 x 1 um) known as fission track. Such trails of damage do not occur at any preferred direction so that the orientations are random. Further, the fission process itself is intrinsic to the atom and is not dependent on the external environment, e.g., heat and pressure. An additional characteristic of fission tracks is particularly useful in constraining cooling exhumation histories. After track formation, radiation damage in the mineral lattice re mains unaffected over geologic time unless changed by environmental effects such as heat. Laboratory and field observations have shown that heating of an apatite grain triggers a progressive annealing of the radiation damage, which is proportional to time elapsed and amount of heat. The work of a number of researchers [192, 226233] has triggered a growing understanding of the qualitative and quantitative properties of fission track thermal annealing in apatite which is at the

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38 basis of the use of this system in thermal history modeling and hence exhumation chronologies [192, 193] Track length distributions are now routinely used in several modeling strategies to unravel thermal histories in multiple geological settings [203, 224, 234] In a rather summarize d version, unimodal, symmetrical track length distributions with high mean values ( e.g., ~14 15 um) and small standard deviations (e.g., 1 2 um) are characteristic of rapid cooling [171, 187, 235] On the contrary, slow monotonic cooling produce length distributions that, although unimodal, have lower means, higher standard deviations and are negatively skew ed. Samples that have experienced more complex thermal histories, e.g., heating to some degree and subsequent abrupt cooling followed by lesser amounts of heating tend to yield track length distributions equally complex, usually bimodal, with intermediate means. Traditionally, the external detector method (EDM) has been applied to dating apatite grains by fission tracks. ( For a description of the method see [171, 187, 236] A new approach to AFT dating by LA ICP MS is employed in this investigation [189] In brief, samples are prepared in a similar fashion as for the EDM so as to expose internal faces of the apatite grains in mounted specimens and mounts are etch ed to reveal spontaneous tracks which are then count ed over specific areas to derive track densities. However, 238U concentrations are not obtained through the long, tedious and radioactive involving process of inducing 235U f ission by neutron irradiation, re -counting tracks, etc. characteristic of the EDM. Instead, 238U and 43Ca are directly measured by LA ICP -MS on the surface where tracks for each individual grain analyzed are counted [189] It is assum ed that 43Ca in apatite is present in fixed s toichiometric amount and obtaining the ratio 238U/43Ca by LA ICP -MS analys is, here referred as P (uppercase Greek letter rho), is analogous to obtaining the ratio i/ d from the EDM. Thus, i = (238U/43Ca) for apatite

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39 grain i; 238U = background -corrected 238U cps (counts per second); 43Ca = background -corrected 43Ca cps. T he bas ic age equation is: i i s MS d d ig t,1 ln 1 (2) where subscript i refers to grain i ; ti = fission track age of grain i; d = total decay constant of 238U, MS = -calibration factor based on LA ICP -MS of fissiontrack age standards; g = 0.5 for internal grain surfaces upon which spontaneous fission tracks are counted; si = spontaneous fission track density for grain i which equals (Ns,i/i) where Ns,i is the number of spontaneous fission tracks counted over an area i derived by the sum of the spontane ous counts divided by the sum of the induced counts. Pooled ages are used in this investigation and they are obtained as the sum of the spontaneous counts divided by the sum of the induced counts (Galbraith and Laslett, 1993). Other ways to calculate ages include mean ages, determined by the arithmetic mean of the individual ratios of spontaneous to induced tracks counted in each grain, and central ages obtained as the weighted mean of the log normal distribution of single grain ages (Galbraith and Laslett, 1993; Galbraith, 2005). Post Crystallization Cooling and Petrolog y of the Antioqueo Plateau Spot U Pb and Hf isotopic analysis in m a gmatic z ircons Various physical and chemical characteristics of the mineral zircon ([Zr(SiO4)]), e.g., its refracto ry natu re resistance to physical and chemical weathering, relatively high U Th concentrations, and high abundance in a wide range of lithologies, make it highly appropriate for geochronology/ geochemistry and one of the most versatile tools for examining a wide r ange of Earth processes, e.g., crystallization age and magma source s of batholithic masses. Earliest proof of the potential use of LA -ICP -MS to perform in situ determination of 207Pb/206Pb on zircon with

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40 sufficient precision to be a practical tool for dati ng Proterozoic zircons took place in the early 1990s [237] During the second ha lf of the 1990s the technique started to be widely used to dating zircons by the Pb/U decay schemes [238241] This progress in the method was related to the development of ultraviolet laser ablation and high -sensitivity ICP -MS instrumentation. Today, LA ICP -MS U Pb dating is one of the cheapest, fastest, most reliable and most widely available techniqu es for in situ (spot analysis) U -Pb Dating [242] Zircon grains constitute a geochemical data repository of unparalleled quality [243, 244] Z ircon preserves a high quality r ecord of near initial Hf isotope ratios, which can be used both in provenance studies and as a petrogenetic indicator [245] In the recent years routine use of LA ICP -MS for in situ U -Pb dating in zircons has incorporated in situ determination s of within -grain isotopic variations of Hf [246, 247] Zircon contains relatively high concentrations of Hf (frequently at the percent to several percent level) which substitutes for Zr. Due to its high content, Hf isotopic composition is usually not significantly affected by in situ decay of the much less abundant 176Lu. Direct measurement of 176Hf/177Hf ratios by LA ICPMS in magmatic zircons is now being applied in investigations of the evolution of plutonic belts and provenance studies. In addition, spot LA -ICP MS determina tions of zircon age and Hf compositions allow studying the crustal evolutionary processes related to silicic magmatism such as magma mixing, crustal c ontamination and/or assimilation, etc. [248252] Zircon cathodoluminescence i maging (CL) Since the late 19th Century, it was recognized that some mineral species, including zircon, exhibit luminescence when bombarded with electrons [253] Cathodoluminescence imaging (CL) is a powerfu l tool to resolve micro -scale chemical heterogeneities even in compositionally complex zircons providing some of the best resolution to reveal internal structures [254, 255] Zircon crystals ubiquitously displa y fine -scale compositional zoning [243, 256, 257] that

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41 demon strates zircons capacity to retain geochemically important trace and rare earth elements (REE) over a range of geological conditions [257] This attribute has resulted in the pervasive application of zircon geochemistry to a variety of Earth Science disciplines (e.g., Schrer and Allgre, 1982; Froude et al., 1983 ; Fedo [258] In this study, some crystals on which LA ICP MS was undertaken were studied by CL to reveal the spatial distribution of trace elements that were incorporated into the growing crystal at the melt/crystal interface at various stages of crystallizati on. Whole -rock m ajor -trace element, and radiogenic i sotop ic a nalysis W hole rock t race (e.g., REE) and major element analysis combined with ra diogenic isotope analysis (Nd, Pb) has a long tradition in petrogenetic studies and provide s valuable data for deci phering magmatic process (fractionation, contamination, crustal mixing, depth of magmatic chamber, etc.) tectonic setting and genesis of intrusive s and associated volcanics i n diverse geologic environments [45, 251, 259, 260] In this investigation, whole rock major and trace element analyses in conjunction with Nd, Pb isotopic analysis were conducted on the six samples from which zircon U -Pb ages and Hf isotopic data had already been derived (LA ICP MS) with the aim of further constraining the petrogenetic characteristics of the Antioqueo and Ovejas batholiths. Several st udies have shown that lead isotopic compositions of igneous rocks reflect the composition of the underlying basement so that lead isotopes can be used to map crustal domains [261, 262] constrain tectonic settings [263265] }, and study magma sources and magma mixing process between mantle and crustal sources reflecting distinct geological provinces [266] Similar information can be derived from Nd isotopic data [267, 268] Although the sample strategy, sample collection, preparation and analysis were mostly undertaken by the author at UF, the variety of analytical techniques deployed for this investigation was possible through the close collaboration with researchers from various

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42 universities and nonacademic institutions. G eorge Kamenov and M isty Stroud from UF assisted me with geochemical work presented in Chapter 2 (U -Pb and Hf spot analysis, and whole rock Nd, Pb, major and trace element analysis). At the University of Kansas, D anilel Stockli helped by providing permanent guidance in the intricacies of (U Th)/He dating. The assis tance of P aul O Sullivan and R ay Donelick in applying a novel methodology to apatite -fission track analysis was of paramount importance. They provided invaluable help both at UF during my trai n ing period in fission track dating and at their own facilities in Apatite to Zircon, Inc conducting analysis for a large number of my samples. All of these individuals actively participated in the discussion and analysis of t he data and are co authors in the three articles derived from this dissertation which are in t he process of being published. Thesis Outline T he main obj ectives of this study are organized chronologically from older to more recent geologic events and will appear in this manuscript in the following order: 1) assessment of the temporal framework for t he emplacement of the Antioqueo and Ovejas batholiths potential c onsanguinity of both intrusives, post -crystallization cooling history, and magmatic sources/generation/ mixing and geodynamic setting associated with this im portant petrogenetic province; 2 ) constrain s on the exhumation hi story, long term erosion trends, patterns of landscape evolution and morphotectonic development of the Antioqueo Plateau; and 4 ) generati on of quantitative data on long -term erosion rates for the region to establish backgr ound erosional process which permit preliminary comparisons against modern erosion rates This dissertation is divided into five chapters, three of which correspond to separate research articles which comprise the core of this study. The first chapter ( Cha pter 1, this introduction ), deals with the various subjects covered throughout the investigation particularly the matters that motivated the author to pursue this line of research, the scientific and societal

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43 importance of erosion at multiple spatiotempora l scales and the objectives and significance of the investigation. Very concisely, the most rel evant aspects of the study site (geology, geomorphology and tectonics) and the methods and various analytical techniques utilized in addressing morphotectonic e volution and process at various scales of space and time are presented The next three chapters correspond to the core of the investigation and are derived from the application of three different techniques that permit tracking important geologic events in space and time. Although the emphasis of this investigation is placed in low temperature thermochronology, the findings in these three chapters are presented in chronological order from oldest to youngest. Thus, Chapter 3 constitutes the first publishable thesis related article ( to be submitted, Journal of South American Geology) and includes formation age post crystallization cooling and petrogenetic constrains on the Antioqueo and Ovejas batholiths These intrusive bodies represent an important volume of granitic rock, constitute the largest calc alkaline suite in the Northern Andes and may ha ve exerted important controls on the geomorphic evolution of the AP including the initial phases of uplift driven buy crustal thickening However, formation age, mechanisms of magma genesis and magma source s, potential process of magma mixing during emplacement and post crystallization cooling trends are poorly constrained. This information is relevant in reconstructing the evolution of the petrogenetic suite und er scrutiny and also provides important clues about the initial phases of cooling and uplift related to crustal thickening and sustained rates of rapid convergence in the region during the Late Cretaceous The work in Chapter 2 is based on U -Pb dating and Hf isotopic spot analysis in zircon grains by La ICP -MS, complemented by whole rock isotopic, trace, and major element analysis.

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44 In Chapters 3 and 4 two separate applications of low temperature thermo-chronology (LTTC) along vertical profiles are presented In both Chapters, apatite grains are used to reconstruct the longterm erosional history of the Antioqueo Plateau by producing, analyzing and modeling thermochronology data in the systems (U Th )/He (Restrepo -Moreno et al., 2009; Chapter 3) and fission t rack (submitted, The J ournal of Geology Chapter 4 ). This part of the dissertation provides important data on the longterm (Cenozoic) erosional exhumation and morphotectonic evolution of the Antioqueo Plateau, which appears to be characterized by craton -like denudation rates. Findings reported here permit defining two discrete pulses of exhumation one in the Eocene and another in the Late Oligocene separated by a long interval of tectonic quiescence These exhumation events are synchronous with similar events throughout the Andes and the Caribbean implying continental -scale controls on erosional exhumation associated with documented changes in convergence between Farallon (Nazca) and South America, an outcome that has implications for regional geodynamic interpretations of Andean evolution. The dataset also allows a better understanding of the paleogeographic and morphotectonic evolution of the area while permitting a reconstruction of spatially averaged, long -term ( pre anthropogenic ) erosion rates that s erve as a quantitative benchmark against which modern erosion rates can be compared. Finally, Chapter 5 encompasses a set of general conclusions in regards to the three main subjects discussed throughout the dissertation, i.e., formation age and magma sour ces for the Antioqueo and Ovejas batholiths; long -term morphotectonic development, landscape evolution and exhumation history of the AP; and an initial comparison of modern (anthropogenically accelerated) and geologic (natural) erosion rates. As the closi ng section of Chapter 5 a set of

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45 potential lines of inquiry and further research topics that have emerged over the course of this investigation are proposed.

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46 Figure 1 1. Physiographic Units of Colombia. Andean Region (dark green): W=Western Cordille ra, C=Cordillera Central and E=Eastern Cordillera. Isolated ranges (light green): 1=Sierra Nevada de Santa Marta, 2=Serrana de Baud, 3=Serrana de La Macarena, and 4=Serrana de Chiribiquete. Plains (brown): CP=Caribbean Plains, EP = Eastern Plains (Or inoco Savanna and Amazon Forest), PP=Pacific Plain (Atrato, San Juan, and Patia valleys). Interandean Valleys (blue): M=Magdalena Valley; C=Cauca Valley. (Physiographic units derived from a GTOPO 30 digital elevation model).

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47 Figure 1 2. The Antioqueo Plateau a geomorphic feature carved on the Antioqueo Batholith. Red-black circles show the location of the sampled sites for thermochronology (apatite (U Th)/He and fission -track dating), geochronology (U Pb spot analysis in zircons), and geochemical iso topic analysis (major and trace elements, Hf, Nd, and Pb). The Medelln -Porce valley has deeply incised (1200 m) into the plateau (narrow green trough in the center of image) dividing the AP into a northern and southern portions. The area covered by the DE M falls approximately between geographic coordinates 7.5 4.5 76

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48 Figure 1 3 The concept of the partial annealing zone (PAZ) and partial retention zone (PRZ) in apatite fission track and (U Th)/He thermochronology. The symbols and the li ght grey shading delimit the crustal region over which tracks are partially annealed and/or He is partially retained. Dark gray shading represents the zone of complete FT annealing and/or He -diffusion. White band indicates the zone of track stability and H e retention over geologic time. At a time to in the pre -exhumation condition rocks above the PAZ and PRZ apatite grains register the age of an exhumation event. Subsequently, low relief topography characterizes the region as the Antioqueo Plateau develops Fission tracks are being annealed within the PAZ and He partially retained in the PRZ over geologic time. Apatite grains in rocks below the bottom of the PAZ and PRZ are recording an apparent age of 0. In the Modern Topography situation the PAZ/PRZ for merly developed appears as an exhumed or fossil PAZ/PRZ that can be sampled along the valley walls of the Medelln -Porce fluvial system. In the bottom of the figure, the Age -Elevation relationships for AFT (bottom left) and AHe (bottom right) clearly displ ay the patterns of apparent ages young toward the bottom of the profile and old toward the top expected from a vertical profile with negligible perturbation from thermal events or faulting (Adapted from Burbank and Anderson, 2001)

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49 Figure 1 4 V ertical profile approach to l ow temperature thermochronology in the Antioqueo Plateau Sketch illustrating the regional scarp carved by the Medelln -Porce fluvial system incising into the Antioqueo Plateau that was used to collect samples from the widest range p ossible of paleocrustal depths.

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50 CHAPTER 2 FORMATION AGE AND MAGMA SOURCES OF THE A NTIOQUEO AND OVEJAS BATHOLITHS, CENTRAL CORDILLERA, COLOMBIA Introduction Granitic plutonism is considered the principal agent of crustal differentiation and i s evidenced by the vast granite batholiths and erupted equivalents present in continental masses [249] Granitoid suites are an essential constituent of continental and orogenic belt evolution in subduction settings. Throughout the Andes, the Mesozoic and Cenozoic are recognized as period s of significant magmatic activity [44, 47, 269, 270] Calc alkaline batholiths constitute a substantial component of the lithosphere of the Andean margin [45, 271] A long orogenic parallel, Meso Cenozoic batholithic belt is thus a conspicuous feature of the western South American margin from Colombia to southern Chile and Argentina. The evolution of Andean magmatism is a result of a wid e variety of tectonic and geochemical processes [42, 269, 270, 272] Important changes in subduction geometry and kinemati cs have taken place over time and have induced significant variations in process es such as volcanism, mountain building, metamorphism, tectonics and seismicity [18, 19, 44, 46, 47, 273] The Middle to Late Cretaceous interval is recognized as a period of important plutonism in the Andes, which is possibly related to enhanced subduction triggered by a regional circum -Pacific superplume, which also produced episodes of deformation and uplift in the lower Cretaceous rocks throughout the Pacific basin [274] In addition, a major peak of Andean magmatism that occurred between 120 and 70 Ma has been identified [272] Variations in magmatic activity along strike are also related to changes in the characteristics of the subduction process which are partly related to variations in the rate o f convergence and age of the subducting slab through time particularly during the Cenozoic [42, 44, 47] Soler (1990) argues that despite the continuous subduction of Nazca (Farallon) beneath South America During the Late Cretaceous and the

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51 Cenozoic, calcalkaline magmatism has been episodic in nature whereby periods of voluminous magmatism can be associated with i ntervals of high convergence rates (>10 cm/yr). Collision of ribbon continents co nstitutes another possibility [275, 276] C ompositional diversity in Andean plutoni sm appears to be related to 1) differences in the composition of the source, 2) vari ability in melting condi tions, 3) complex chemical and physical interactions between maf ic and felsic magmas and 4) crustal contamination [44, 45, 249, 260, 270, 277280] Mesozoic p lutonic and volcanic events in the Colombian Andes are we ll expressed as large batholith s stocks, lava flows, and pyroclastic formations [21, 281] E nhanced magmatism in the Northern Cordillera Central during the Late Mesozoic led to the development of large intrusive bodies such as the A ntioqueo and Ovejas batholiths, hereafter AB and OB, respectively (Figure 2 1). The AB (7500 km2) is the largest intrusion and one of the broadest, most continuous lithologic units in the Colombian Andes. S maller plutons similar in composition and spatially related to the AB, such as the OB and the Uni n Cupola, are considered genetica lly related to the AB. A precise chronological and geochemical framework that would permit a better understanding of the timing, the source s, the geodynamic setting, and potential mechanisms for the development of s uch large plutons has not been completed. T his investigation was carr ied out with the objectives of 1) precisely defining the crystallization age of the AB and OB plutons 2) constraining magma source s and magma mixing process in the AB and OB, and 3) asse ssing the hightemperature portion of the post crystallization cooling path for these intrusives Accurate constrain t s in age formation and magma source s for the AB and OB are important to support our understanding of MesoCenozoic crustal evolution proces ses and the spatiotemporal distribution of magmatic activity in the Northern Andes a region that has traditionally lacked studies of that nature.

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52 Because magma addition has been invoked as a potential mechanism for crustal thickening and subsequent uplift -denudation, the information generated in this investigation is also important for elucidating the timing of potential crustal thickening through magmatic activity that may help explain the initial stages of uplift of the northern Cordillera Central during the Laramic O rogenetic phase at ca. 64 Ma [16] Magma addition has been suggested as one of the plausible mechanisms of crustal thickening and subsequent uplift for the Andes of Bol ivia and Peru [99] and may be an important aspect of the morp hotectonic evolution of the northern Cordillera Central given the magnitude of Late Cretaceous intrusions. T o constrain magmatic evolution of both intrusive bodies their plutonic archive was unraveled by simultaneously conducting in situ hafnium (Hf) isotope analysis on precisely dated magmatic zircons (spot U -Pb) and performing whole rock major, trace element a and radiogenic isotopic analyses (e.g., REE, Pb, Nd) Results of this investigation support 1) a massive, L ate Cretaceous, subd uctionrelated magm atic event, 2) crystallization and emplacement of the AB and OB over a relatively short interval of ca. 5 million years, 3) assemblage of both batholiths through rapid incremental steps by multiple injections, 4 ) significant magma mixing involving mantle a nd crustal source s as well as variations in the nature and depth of magmatic source s and 5 ) rapid post -crystallization cooling (~800 to ~300C) from 77 to 60 Ma,. Although the Late Mesozoic seems to have been a period of enhanced subduction throughout the circum -Pacific region [282] and this has been taken to imply increase d magmatic activity in these subduction zones in this study other potential mechanisms are considered (e.g., short lived extension through slab roll -back, flare ups, etc.) and evaluated in light of the geochemical and isotopic data here presented

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53 Study Site Large granitic Mesozoic batholiths of a nearly circular outcrop are intruded in the cordilleran axis of the northern portion of the Cordillera Central in the Colombian Andes. The area focus of this investigation is situated between longitude -latitude coordinates 5.7 7.7N and 74.6 76W (average elevation ~ 2500 m) and encompasses t he Antioqueo and Ovejas batholiths, two massive granodiorite intrusives In Colombia, the Andean chain is arranged in the form of three distinct and sub -parallel ranges separated by major intramontane depressions. From west to east these ranges are known as the Cordillera Occidental (Western Cordillera), the Cordillera Central (Central Cordillera) and Cordillera Oriental (Eastern Cordillera) The Cauca and Pata rivers occupy the depression between the Occidental and Central cordilleras, while the Magdalen a River valley runs between the Central and Eastern ranges (see Chapter 1, Figure 1 1). These three cordillera possess substantial geological differences that reflect the complex geologic evolution of this segment of the Andean range. Despite the geologic and geographic importance of the Colombian Andes, most studies have concentrated on the Central Andes of Bolivia and Peru. Geologic Setting The Cordillera Occidental and Pacific co a stal plains of Colombia have oceanic crustal affinity and are part of an is land arc, i.e., allochthonous terrane, accreted to the continental margin in the Cretaceous and early Cenozoic [21, 283] The Cordillera Occidental is composed of oceanic sequences of mafic vol canic rocks and marine sedimentary sequences (turbiditic deposits and ophiolites) of upper Cretaceous and Cenozoic age, intruded and covered by Cenozoic plutons and volcanic sequences [44, 284, 285] This oceanic domain prevails through western Colombia and is separated from an eastern continental domain by the Cauca Romeral fault system, which is a paleosuture zone that extends all the way to Ecuador [25]

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54 In contrast, t he Central and Oriental cordillera possess crust of continental affinity and an old crystalline basement. The C ordillera Oriental consists of continental Precambrian and Paleozoic metamorphic and igneous rocks overlain by Paleozoic to Mesozoic sedimentary sequences [21] The Cordillera Central encompasses a pre Mesozoic polymetamorphic core complex [286] intruded by Meso Cenozoic batholiths [21, 27] Cenozoi c to recent volcanics associated with subduction of the Nazca plate are restricted to the Cordillera Occidental and a segment of the Cordillera Central between 2S and 530N The study area includes a crystalline basement of low to medium -grade metamorphic rocks grouped as the Cajamarca Complex [287] and high -grade rocks of the El Retiro Group and Las Palmas gneiss [21, 288] Collectively, these roc ks comprise the so called pre Mesozoic to Mesozoic polymetamorphic complex of the Cordillera Central [286] In the early Cretaceous, shallow marine sedim entary sequences were deposited over the metamorp hic basement [289 291] Intense magmatic activity during the Mesozoic led to the emplacement of major cordilleran plutons that intruded the metamorphic complex as well as early Cretaceous sedimentary cover producing clear contact aureoles (Figure 2 1) Plutons emplaced during this era include Triassic ademelitic stocks (e.g., Amag, Honda, El Buey), Trias sic batholiths such as the AB and OB and ot her associated stocks [21] Late Eocene continental arc volcanic rocks cover portions of this province [284, 286] Finally, Late Neogene volcaniclastic sequences associated with modern volcanic activity further south in the Cordillera Central mantle the region [12, 292] (Figure 2 1 ). Mesozoic volcanic edifices associated with the emplacement of the AB and OB have been removed by a series of discrete exhumation pulses ( Paleocene, Eocene and Late Oligocene) that have been revealed by recent low temperature thermochronology

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55 studies [293, 294] and sedimentstratigraphic investigations [16, 295] At present, only small patches of the metamorphic basement, marine sequences, and Eocene volcanic rocks remain on top of the AB as roof pendant structures. The geologic province that encompasses the study site is considered part of the allochthon ous Taham terrane, affected by several tectono -metamorphic episodes related to the Hercynian orogeny in PermoTriassic times, a Devonian Carboniferous event, and the beginning of the Andean cycle in the Cretaceous [279, 283, 286] The Antioqueo and Ovejas Batholiths The AB is the only major pluton of any age in western Colombia that is not aligned with the predominant NS structural grain of the orogen. Along with other major granitic bodies of lesser extent (e.g., Yarumal Stock, Amalfi Stock, Pueblito diorite, Sabanalarga batholith, and various gabbroic units), the AB and OB are part of the final and most important episode of plutonism in th e northern Cordillera Central. Relatively early, the great regional extent and compositional homogeneity of the AB was noted [296298] More in -depth petrographic, geochemical, and cartographic studies were subsequently conducted and the denomination Batolito Antioqueo (Antioqueo batholith) was assigned to this lithologic unit [299] Further p etrographic and geochemical studies were undertaken in the early 1980s [300, 301] and were complemented by the work of Gonzlez (1993). Such studies have confirmed the compositional (chemical and mineralogical) and textural homogeneity of the AB. About 95% of the samples analyzed to date fall in the range granodiorite to quartz -diorite. Recent geochemical data on major elements [166] has confirmed previous ideas in relation to the composition and homogeneity of the AB and OB while providing valuable information about the subalkalic character of the entire mass. Contemporaneous and much smaller mafic units (< 2% of the total outcrop) are spatially and temporally associated with the AB and OB [21] which is a common feature of most cordilleran batholiths [271]

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56 Previous Thermochronology and Isotopic Analysis Radiometric studies of the AB and OB have not been carried out in a systematic way and several isotopic systems have been applied to this vast geologic unit. Such studies have yielded a rather wide age range from ca. 22 to ca. 98 Ma that have made it difficult to precisely constrain the time of crystallization, which, in turn, precludes interpreting the origin of such large igneous masses, the source s of magma involved, the mechanisms of magma generation and emplacement etc. in the contex t of Cretaceous geodynamics Biotite K/Ar ages (~300C) range from ~68 Ma to ~90 Ma [301303] Three biotite Rb/Sr ages (~350 C) from ~56 to 66 Ma are also reported [304] More recently two whole rock isochrone Rb/Sr yielded ages of ~82 and 98 Ma [305] Zircon fission track dates from ~49 to 67 Ma and apatite fission -track ages varying between ~49.1 and 28 Ma [166] in conjunction with apatite (U Th)/He and FT ages of ~2149 Ma [294] and ~3059 Ma [293] (Chapters 3 and 4) have been used to study the low temperature history and Cenozoic exhumation of these units. T his ample variation in apparent ages is explained by the wide range of closure temperatures of the thermochronometric systems employed (ca. 50400 C) and can be interpreted as cooling rather than formation ages. The R b Sr isochrone age of ~98 Ma was proposed as the best estimate of crystallization age for the Antioqueo batholith [166, 305] Whole rock RbSr ages are highly suspect and often er roneous due to mobility of the Rb-Sr system and incorrect assumption of a single Sr initial common source. Thus, the Rb -Sr whole rock ages obtained by rocks collected from a single outcrop to specimen-size are not always coincident with those of emplacemen t of granitic magma [306] Until the investigation here presented, no U -Pb zircon ages or other robust high temperature geochronometer has been used to determine the formation age of the AB or OB.

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57 Constraints on ma gma source s and magma mixing for the AB are limited to the study of Ordonez and Pimentel (2001) who proposed that the generation of such unusually large volume s of magma should be related to enhanced slab dehydration resulting from accelerated subduction r ates from ca. 110 96 Ma. Mixing of mantledepleted fluids and crustal components are suggested by 2.40 and +2.66 [305] Tectonic and Geodynamic Setting The Late Mesozoic -early Cenozoic tectonic and geodynamic setting of northwestern South America has been contr olled by macro -, meso and micro -scale tectonic process strongly associated with the subduction of the oceanic Farallon plate beneath the continental South American plate and with the evolution of the Caribbean plate [44, 47, 307311] The Andean region of Colombia has been traditionally considered as a mosaic of allochthonous terrains accreted to the autochthonous continental crust of the northwestern South American margin since the Paleozoic and until the Late Neogene [278, 283] Eastward displacement of the Farallon Plate relative to the Americas during the Mesozoic was characterized by subduction of oceanic crust beneath South America and associated volcanism/plutonism, deformation, and obduction/accretion of oceanic terrains in west ern Colombia during the Cretaceous [278] This predominantly oblique mode o f subduction induced large -scale transpression features throughout the northern Andes such as the well defined, right -lateral shear zones along the northern border of the North Ande an Block [44, 278, 312, 313] Patterns of subduction (e.g., convergence rates and direction, dipping of the subducting slab, etc.) have changed substantially both in space and time [18, 273, 282, 314] Variations in t he subduction parameters have exerted important controls on th e timing, magnitude and type of associated volcanism/plutonism since the Mesozoic [44, 47, 278] The Faral lon Plate, which was subducted along the south -western portion of the northern Andes and Central America during the Mesozoic and part of the

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58 Cenozoic, split as a result of thermal effects by the underlying Galapagos hot -spot at ~25 Ma B.P. This event led t o the modern configuration of regional tectonics as the Cocos and Nazca plates were created [20, 315, 316] and the vector fields were redistributed to establish a predominantly E -W interaction between Nazca and South America [25, 47] Such reorientation is responsible for the current style of deformation and seismicity in the region [23, 25, 317] At present, the Northern Andean region is located in the junction of four major tectonic plates, including Nazca, Pacific, Caribbean and South America, and two major, fault bounded lithospheric blocks know as the North Andean and the Pana ma Choc blocks [23 25] Intracontinental deformation is associated with inherited faults of large scale and both reverse and strike slip character [25] Orientation of topographic features and cordilleran grain/structure in the Colombian Andes is predominantly NS to NE -SW compatible with W E convergence directions between major plates, i.e., Nazca and South America. Crustal deformation is primarily associated with the collision of allochthonous terrains and interactions of s everal tectonic plates and micro plates. The Cordillera Central is bordered by reverse fault systems located along the foot hills (east and west) which are thought to be rooted beneath the range [23, 25, 26] The Cauca Romeral faults system to the west is associated with the Cauca river depression and has been activated since the Oligocene combining strike -slip and reverse movement [25, 26] To the east, the Palestina fault system marks the boundary of the C ordillera Central Th is range is asymmetric with a much steeper western flank along the Cauca I nterandean valley. The western margin has been uplifted by transpressive movement along faults dipping eastward that belong to the Cauca -Ro meral fault system. The less abrupt eastern flank along the Magdalena river valley

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59 is characterized by a west dipping reverse faults located along the foothill. Neogene transpression is right lateral in southern Colombia and left lateral north of 4N [23, 25] Methods M ultiple techniques are utilized in this investigation to understand the timing of crystallization, the magma sources, the magma mixing pro cesses, and the geodynamic environment that led to the development of the AB and OB. Those techniques include in situ U Pb dating, Hf isotopic analysis and c athodoluminescence imagi ng ( CL ) in magmatic zircon grains, whole rock major and trace element anal ysis, and whole rock radiogenic Nd and Pb isotopic analysis Z ircon grains form a highly robust phase in most geological environments and represent a geochemical data repository of unparalleled quality Integrated application of U Pb dating and Hf -isotope analysis to magmatic zircon grain populations offers a rapid means for constraining the age and magma source s/processes associated with the intrusion of batholithic masses [249 252, 318] Zircon is readily amenable to U -Pb radiometric dating methods [243, 244] Furthermore, zircon preserves a high -quality record of near -initial Hf isotope ratios, which can be used both in provenance studies and as a petrogenetic indicator [245] H f analysis is being increasingly applied to evaluate terrestri al crust mantle evolution [248, 249, 252, 319] Zircon imaging by C L is a powerful complementary tool in radiometric dating and isotopic analysis because it resolve s micro -scale chemical heterogeneities even in compositionally complex zircons providing some of the best resolution to reveal internal structures [245, 254] CL is now commonly used in most studies that incorporate single grain spot measurements in zircon [243, 319, 320] Finally, Whole rock major and t race element analysis combined with whole rock ra diogenic isotope analysis (Nd and Pb) has a long tradition in petrogenetic studies and provide s

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60 valuable data for deciphering magmatic process ( e.g., fractionation, contamination, crustal mi xing, depth of magmatic chamber, etc.) tectonic setting, and genesis of intrusive s and associated volcanics i n diverse geologic environments [45, 251, 259, 260] Sample Collection and Preparation Samples were collected along three vertical profiles (tw o in the AB and one in the OB) to derive geochronology, geochemical and isotopic information over the widest range possible of crustal depths into both batholiths Profiles were also selected to incorporate broadly separated areas in latitude and longitude from the center to the periphery of the batholiths. A total of six sam ples (2 3 kg) were obtained from the highest and lowest points of each profile. Although the majority of the samples were fresh (SR 001, SR 003, SR 9, and SR 41) relatively altered samples (SR 24 and SR 42) were also included as they are useful for spot U -Pb dating and Hf analysis in zircon; provided the grains are non -metamictic. Hand specimens correspond to mottled (black in -white), phaneritic, coarseto medium grain, hypidiomorphic, equigranular, granodiorite with varying amounts of quartz and feldspar as the felsic phases and mostly biotite and some hornblende as mafic minerals (Figure 2 2) Petrographic analysis indicate the approximate mineralogical composition consists of pla gioclase 4051% (Albite -Andesine), quartz 18 35%, hornblende 0 12%, biotite 6 12 %, K -feldspar 0 7%, clinopyroxene 0 0.5%. This mineralogy is consistent with previous petrographic results [21, 301] Plagioclase crystals are subhedral to euhedral and maximum size of 4 mm. They are usually twinned and contain poikilitic inclusions of accessories and opaques. Quartz occurs as inequigranular anhedral grains smaller than 2.5 mm. Hornblende is green, subhedral and exhibits poikilitic texture, enclosing accessories and opaques, it is partially altered to chlorite and has a maximum size of 3 mm. Biotites are subhedral to anhedral, grain size up to is dark brown, and also encloses opaque minerals. Alkali feldspar is minor, smaller than 2 mm, and is altered to

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61 sericite and clay minera ls. Alteration of hornblende and biotite to chlorite is relatively common. Sericitic alteration of plagioclase is also noticeable. Accessory minerals are mainly apatites, zircons and sphene. Small amounts of opaque minerals are also present. Zircon separat ion, grain description and c athodoluminescence For zircon recovery, the s amples were washed and weathered rims removed from the fresh specimens to eliminate potential surface contaminants. Samples were crushed, pulverized in a disk mill, sieved at 125400 and air dried. Zircon concentrates were obtained from the sand fraction (~2 kg) following a conventional protocol involving : a) Gemini water table for initial separation of heavy fractions, b) paramagnetic removal with hand magnet, c) density l iquid separation using tetrabromoethane (TBE ) and methylene i odide ( MeI ) d) separation based on magnetic susceptibility in a Frantz isodynamic e) further cleaning of the non-magnetic fraction in MeI, and f) final depuration by magnetic susceptibility, s ieving and hand picking. Zircon grains for the non magnetic fraction (0, 1.0 A) were optically screened using a petrographic microscope under transmitted (normal and polarized) and reflected light to monitor for inclusions. Abundant, completely euhedral, clear, unfractured zircons were obtained Most grains are transparent with a yellowish tone. Magnetic separation provided non -magnetic zircon populations that are virtually inclusion free, as confirmed by bin ocular microscope scanning. External crystal mor phology varies relatively little within sample zircon populations and even between samples. Euhedral, complete grains dominate most concentrates with a prevalence of the prismatic forms (101) terminated in pyramids. Most crystals are medium sized (60 equidimensional (i.e ., low elongation ratios). Some acicular cryst als pres ent in samples SR 41 and SR 42. Representative sets of non -magnetic fractions for the six samples plus a fraction of zircon standard FC 1 were placed along seven separate bands utilizing adhesive paper. Epoxy resin was

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62 used to enclose the grains in to one single mount which was ground and polished to expose internal surfaces of zircon grains. The zircon mo unt was washed in 4X distilled water and further cleaned in an ultrasonic bath with diluted HNO3 for 5 minutes to remove impurities at the surface prior to laser ablation and mass spectrometry analysis. Conventional Cathodoluminescence (CL ) procedures were followed to obtain images of zircon populations, reveal internal structures associated with chemical zonation, and select analyzed grains to monit or the type of zoning, if any, and identify potential inherited cores alterations, recrystallizations, and/or metamorphic absorptions [243, 255] Growth zoning is easily captured by CL [243, 321] CL imaging was conducted at, USGS Ion Probe Lab, Stanford, CA. The zircon mount was gold coated and placed into the CL system. Grey -scale CL imaging was performed using a Hitachi S 2250N scanning e lectron microscope. The photomultiplier detector operates in the broad green regio n of the electromagnetic spectrum (~490 525 nm) producing grey -scale images over that spectral range. CL spectra were collected at 15 kV with an Acton Research ARC 150 Spectr ograph (146 mm focal length) equipped with an 1800 groove/mm holographic grating connected to an Olympus BH 2 microscope. CL emission was detected using a Princeton Instruments TEA/CCD 576EMUV 576 X 384 charg e -coupled device (CCD) detector 2. CL shows that all of the gr a ins analyzed posess well dev eloped growth zoning (Figure 2 3 ) refle cting compositional variations. Zoning in t he majority of crystals analyzed (>98%) consists of repeated, fine to medium -scale concentric bands (~2 inosity around a relatively dark euhedra l core. No alteration patterns (e.g., feathery textures, botroids, etc.) were found in any of the mounted grains imaged by CL indicating that all of the crystals are suitable for isotopic analysis (i.e., no Pb leachi ng). None of the zircons exhibited inherited cores,

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63 a feature confirmed by the homogenous U Pb ages obtained from all of the grains analyzed (see U -Pb results below) Three grains in samples SR 43 and two in SR 003 display sector zoning. A combination of o scillatory and sector zoning has been reported in previous studies of inter mediate to felsic suites [257, 322] and are thought to be modifications of oscillatory zoning by recrystalization (i.e., patchwork replacement of zoned by unzoned zircon). P atches of unzoned zircon appear to develop after completion of primary crystallization, and are accompanied by loss of U, Th and Pb and the removal of oscillatory zones [323] Such grains were avoided for U -Pb dating. Whole rock sample preparation Sample rock chips (~1 cm in diameter) were washed to remove surface impurities and dried with an air compressor. Chips were further crushed using a small jaw crusher with steel plates, a riffle to split the sample s, and pulverization in a 99.8% pure Al2O3 planetary ball mill. The powders were treated differently depending on the analysis to be performed For major element analysis a minor amoun t of Al was added to the sample. Trace element and radiogenic isotopic a nalyses were carried following the procedure employed by Kamenov et al (2008) Specifics for ea ch sample preparation protocol are inclu ded in separate sections below. Radiogenic isotopic analyses were conducted at the Department of Geological Sciences of t he University of Florida. Approximately 0.05 gr of sample powder was dissol ved for 4 6 h in hot (801003). Neodymium and Lead isotopes were separated using standard chromatographic methods in a clean laboratory. A small volume of the rock solution is loaded into the column, washed into the resin bed carefully with eluent, and then washed with more eluent until a fraction is collected when the desired element is released from the resin. The preliminary separation of Sr and REE was performed in a cation exchange quartz glass column packed with Mits ubishiTM cation

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64 exchange resin elluted with dilute 3.5 M HCl and 6N HCl. The rare earth elements ( REE) are separated as a group on the cation exhange column eluted with 6N HCl acid. Separation of Sm and Nd from the REE group wa s accomplished by using a Hexyil di -ethyl hydrogen phosphate (HDEHP) -coated Teflon powder (stationary phase) with 0.18N HCl acid (Richard et al., 1976). The column was washed with 1.7N HCl to remove co-existing elements such as Ca and Mg. Ba, which negativ ely impacts the analysis of Nd isotopes, was removed by washing the column with 2N HNO3. The resultant solution was preserved in sealed Teflon vials for subsequent chemical and isotopic analyses. U/Pb and Hf Spot Analysis in Zircons Fifteen zircon grains pe r samples were analyzed to produce a total of 90 U -Pb data for the six individual sample s from the AB and the OB. In addition, five U -Pb -dated dated crystals per sample were examined for Hf isotopes for a total of 30 analyses. Spot U Pb dating and Hf isotopic analysis by laser ablation mass spectrometry were performed at the Department of Geological Sciences of the University of Florida using a Nu Plasma (Nu Instruments, UK) laser ablation, multi -collector inductively coupled plasma mass spectrometer ( LA -MC ICP -MS ) equipped with three ion counters and 12 Faraday detectors The LA -MC -ICP MS used possesses a specifically designed collector block for synchronized acquisition of 204Pb (204Hg), 206Pb, and 207Pb signals on the ion-counting detectors and 235U and 238U si gnals on the Faraday detectors. This arrange has been previously described in detail [324] Analysis protocol followed in this investigation has been previously described [267, 320, 325, 326] For U Pb dating, the m ounted zircon grains were ablated using an attach ed New Wave 213 nm ultraviolet laser. A mix of Ar and He carrier gas (1 L/min Ar, 0.5 L/min He) was used for sample transport into the mass spectrometer. The laser was set at 4 Hz pulse frequency, 40% -peak

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65 background zero measurement s were performe d for 20 seconds on the blank He and Ar gases with closed laser shutter. This zero was used for online correction of isobaric interferences, particularly from 204Hg which is largely derived from the argon gas. Following blank acquisitions, sample ablation proceeded for 2530 seconds as a way to minimize the depth of the ablation pit and hence reduce large elemental fractionation. Sample analyses were bracketed by 2 analyses of FC 1 Duluth Gabbro zircon standard for every 10 unknown zircons. The U -Pb isotop ic data were acquired using the Nu Instruments Time Resolved Analysis (TR A) software. G rai ns were ablated combining centers and rims. The TRA software allows for isotopic ratios to be calculated from the desired time segment of data, which avoids variation s due to grain defects or surface contamination. Raw isotopic data obtained from the LA -MC ICP -MS were imported into a Microsoft Excel spreadsheet where they were corrected for instrumental drift and mass bias. Data reduction is based on the FC 1 zircon s tandard, dated at 1099.3 0.3 Ma (207Pb / 206Pb = 0.0762, 207Pb / 235U = 1.9428 and 206Pb / 238U = 0.1850) [327] as well as being dated more recently at 1099.0 0.7 and 1099.1 0.5 Ma [328] Co mmon Pb correction was applied in the above Excel spreadsheet using the 207Pb/206Pb correction s previously outlined [329] Representative age errors based on the long -term reproducibility of FC 1 were 2% for 206Pb=238U 207Pb=206 n, two other approaches to common Pb reduction correction have been employed following the approach by Jackson et al (2004), namely, selective integration of time resolved signals (as described above) and Tera -Wasserburg diagrams [330] Previous studies have concluded that, for young zircons containing a significant amount of common Pb, plotting of data on Tera Wasserburg diagrams constitutes the most effective method of evaluating and correcting for common Pb [242] Tera -Wasserb urg concordia diagrams were generated using Isoplot/Ex Version 2.4 [331] Errors on individual analyses are

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66 based 95% confidence. Tera -Wasserburg diagrams are very useful in identifying common Pb. This type of diagram plots 238U/206Pb on the x axis versus 207Pb/206Pb on y axi s. The ratio 207Pb/206Pb is especially susceptible to the presence of any common lead, particularly in young zircons. Samples containing common Pb will plot above the concordia line, and can either be excluded or used to construct a regression line to dete rmine a concordia intercept age [242] The same N u Plasma MC ICP -MS and New Wave 213 nm laser devices described above were used for the laser Hf isotopic analysis of accurately U -Pb dated grains Laser beam was set for ablation on s Typical a blation times were on the order of 30 s econds Measurements were made in static mode on Faraday detectors acquiring 180Hf, 178Hf, 177Hf, 176Hf, 175Lu, 174Hf, and 172Yb simultaneously. Isobaric interference corrections during analyses were performed with on -line Lu and Yb, using 176Lu/175Lu=0.02653 and 176Yb/172Yb=0.5870, both ratios within the range of published values. All isotopic ratios were corrected for mass -bias using 178Hf/177Hf=1.46718. Multiple analyses of FC 1 zircon standard performed during the sample analyzes yielded 176Hf/177Hf = 0.282168 ( 0.000016, 2 n=24), indistinguishable from liquid analyses of this standard (176Hf/177Hf=0.282174; 0.000013, 2 ). The potential of FC -1 as an Hf isotopic standard has been previously assessed [332] finding it suitable f or LA ICP -MS applications. Measured and mass -bias corrected 176Lu/177Hf ratios were used to calculate initial 176Hf/177Hf ratios. Overall, due to the very low Lu/Hf ratios, the difference between the present -day measured and calculated initial 176Hf/177Hf ratios in most cases is less than 1 epsilon unit figures were calculated using the 176Hf/177Hf = 0.282843 as the most reasonable value for Bulk Silicate Earth Reference [333]

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67 Whole Rock Geochemical and Isotopic Analysis Six whole rock samples for major elements and trace elements (including REE) were analyzed. Major elements were measured by XRF spectrometry. An Element 2 ICP -MS was used for trace elements. A "Nu Plasma" MC ICP MS for Nd and Pb radiogenic isotopic analyses following a method for high -precision isotop ic analyses of small samples [325] Major and trace element a nalysis Major element analyses were conducted at Geo -Labs, Ontario, by X ray fluorescence in a wave length dispersive Philips PW 2400 XRF spectrometer. Samples were first run for loss on ignition (LOI) and then 700 mg of powdered sample were thoroughly mixed with 4200 mg Spectroflux 100 (Dilithiumtetraborate [Li2B4O7]) and melted to a glass disc on which XRF analysis were performed. Analytical errors for major elements are ~1% (excluding Fe and Na ~2%) and for trace elements ~5%. For the calibration of major and trace element determination approximately 40 reference materials were used including a selecti on of internationally accepted geochemical reference samples from the US Geological Survey, the National Research Council of Canada, the International Working Group Analytical standards of minerals, ores and rocks, and the Geological Survey of Japan (F or more detail about specific analytical techniques the reader is directed to www.mndm.gov.on.ca ). Whole rock samples were analyzed for trace element concentrations using and Element2 HR ICP -MS at the Department o f Geological Sciences of the University of Florida. The analyses were performed in medium resolution, with Re and Rh used as internal standards. Quantification of the results was done by external calibration using a combination of USGS rock standards. Nd i sotopic analysis A small volume of the final sample solution containing the analyte ions was aspirated in the plasma through a DSN 100 desolvation nebulizer Nd isotope measurements were performed

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68 with the Nu -Plasma Time -Resolved Analysis (TRA) software op erating with 0.2 s integration time, resulting in acquisition of 300 ratios per minute. TRA was used in static mode, simultaneously acquiring 142Nd on low 2, 143Nd on low 1, 144Nd on Axial, 145Nd on high1, 146Nd on high2, 147Sm on high 3, 148Nd on high 4 and 150Nd on high 5 Faraday detectors. M easured 144Nd, 148Nd, and 150Nd beams were corrected for isobaric interference from Sm using 147Sm/144Sm = 4.88, 147Sm/148Sm = 1.33, and 147Sm/150Sm = 2.03. All measured ratios were normalized to 146Nd/144Nd = 0.7219 using an exponential law for mass -bias correction. Baseline was measured by electrostatic analyzer ( ESA ) deflection of the beam. Replicate analyses of JNdi 1 and La Jolla Nd standards were undertaken for every 4 5 samples yielding a good measure of the longterm external reproducibilities with mean v alues of 0.512107 ( 0.000021, ) and 0.511856 (0.000020, ), respectively. To further evaluate the analytical protocol, t hree separate dissolutions of USGS SRM BCR 1 were prepared and analyzed for Nd isotopes together with the samples The mean value of 143Nd/144Nd for th e analyses of BCR 1 was 0.512645 (0.000011, ), identical to the recently published value of 0.512645 ( Weis et al. 2006). Pb isotopic a nalysis T he same Nu Plasma MC -ICP -MS procedures descried above were utilized for Pb isotopic analyses applying the Tl n ormalization techn ique (Kamenov et al. 2004). Lead isotope data are relative to the following values of NBS 981: 206Pb/204Pb = 16.937 (0.004, 2 ), 207Pb/204Pb = 15.490 (0.003, 2 ), and 208Pb/204Pb = 36.695 (0.009, 2 ). Results Major Elements Rel e vant ma jor element d a ta for the Antioqueo and Ovejas batholiths is presented in T able 2 2 (data includes geochemistry from this study and from Saenz (2003)). Rock

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69 classification was determined by plotting total alkalis -silica diagram (TAS) adapted for plutonic rocks [334] All samples fall in the sub alkali field, wit h a prevalence of intermediate to acid compositions between the fields of diorite and granite exhibiting relatively minor compositional ranges of SiO2 between ~69 60% (Figure 24 ). Geochemical results are in agreement with the estimated petrographic composition and are similar for other plutonic regions in the Andes [44, 45] The SiO2 vs. K2O relationship defines the suite as a typical medium -K calc alkaline series (Fig ure 2 5 ). This granitoid suite display fairly small scatter of oxide variations relative to SiO2. Negative linear correlations indicate normal differentiation trends of decreasing Al2O3, FeO(total), CaO, MgO, and TiO2, with increasing SiO2 (Figure 2.6). P2O5 also defines a negative, linear correlation but much more scattered. Slightly positive correlations with SiO2 are found for K2O and Na2O (Figure 2.6). Magnesium numbers, usually fluctuating around 47, indicate relatively high degree of differentiation. Zircon U -Pb and Hf Zircon ages for the AB vary between 711.2 Ma and 771.7 Ma In the case of the OB U Pb ages yielded two values at 721.4 Ma and 771.7 Ma. D iffe rences in ages are significantly greater than associated errors at 1 sigma and are interpre ted as real formation ages. A summary of U -Pb ages is presented in Table 2.1. When plotted against elevation, U -Pb ages show no correlation (Figure 2 7 ). Tera -Wasserburg diagrams indicate minimal perturbations of U Pb ages by common lead. Only one of the 90 zircon grains analyzed exhibits a n ellipse that is clearly above the cluster of ellipses and the concordia line probably pointing toward a high common 207Pb /206Pb ratio (sample SR 43). The rest of the analysis plot in relatively compact clusters provid ing precise ages (Figure 2 8) All of the U -Pb ages display excellent reproducibility and errors are usually below 1%.

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70 2.61 and +5.30 (average of 1.77). Summarized Hf data is presented in Table 2 1 Sample SR 001 is the only one to record slightly 0.53 to 2.72) while the rest display values be tween approximately 1 and 5 42 exhibits the largest range in variation (2.2 units) while sample SR 9 shows the narrowest range (0.88 units). Pb and Nd Isotopic Compositions Lead isotopic compositions expressed as rations of 206Pb /204Pb display little variability between 18.861 and 19.079, while 207Pb /204Pb values range from 15.612 to 15.682. Such figures define a relatively narrow field and plot within the characteristic Pb/Pb dom ain of the Northern Volcanic Zone (Figure 2 10). Neodymium isotopes range from 0.51254 to 1.83 to 3.04) i.e., right around previously reported values for the Northern Volcanic Zone [269, 271] Hafnium and Neodymium isotopes share a similar systematic and display a high positive correlation (r = 0.72) when plotted together (Figure 2 11). Pb and Nd values indicate that these samples do not contain old Precambrian crustal components. Further, Pb isotope ratios plot between the orogen growth and the upper -crus t curves [265] thereby indicating a significant crustal component (Figure 2 10). Nd and Hf isotope data, in agreement with the Pb isotope results, also indicate incorporation of crustal component (Figure 2 1 1). Rare Earth Elements Six sa mples of whole rock granodiorite were analyzed for rare earth elements ( REE), whose abund ances in ppm are summarized in Table 3 1. REE values (Figures 2 12 and 2 13) are normalized to primitive mantle according to specific val ues previously published [337] Light REE (LREE) are enriched in all samples and most converge at a value of about 60 70. This trend is typical of calk alkaline suites. Heavy REE (HREE) patterns are generally flat and fluctuate between 2 and 17. The portions of the patterns from Hf to Yb is also relatively flat. Eu

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71 anomalies, although not pronounced, clearly show two populations of data, one with negative Eu anomalies (samples SR 003 and SR 41) and a nother one with positive Eu anomalies (SR 9, SR 42, SR 001). Fractionation is also quite variable (e.g., low for Sr 24 1.4 and large for Sr 42 18). HREE patterns, although flat, show variation between samples. Discussion and Interpretation Major Elements T he AB -OB is a plutonic suite dominated by granodioritic composition with hypidiomorph granular phases. Major element analyse s show variations in composition from diorite to granite suggesting differentiation. Most samples possess medium -K calc alkaline com position, which is a primary feature of the NVZ in the Andes [45, 269271] Samples with the highest degree of differentiation fall in the subalkalic, calc alkaline high -K which is a consequence of significant crustal contamination This explanation is further supported by the characteristic trend in the SiO2 vs. major oxide Harker diagrams for Al2O3 FeO (total), MgO, CaO, and TiO2 (Figure 2 6). This characteristics are similar to those described in fractionating systems [45]. Decrease in MgO, FeO (total) and CaO as SiO2 increases is consistent with the removal of early -forming plag ioclase, plus olivine and/or pyroxene from the cooling liquid. MgO and FeO (total) are incorporated into the typically early -forming mafic minerals. CaO may have been removed by a calcic plagioclase, a clinopyroxene or both. Rock suites that are related by fractional crystallization and unmodified by significant crustal contamination define coherent linear trends on Harker diagrams [271, 334] Therefore, the relatively scatter observed in the Harker diagrams may indicate contributions from the crust (e.g., magma mixing, crustal contamination) in addition to fractional crystallization during pluton formation. This is also supported by the variations in the isot ope data, as presented below. Presence of hydrous magmatic minerals points to high H2O contents in the magmas. Textures in which pyroxene is surrounded by hornblende, which is in

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72 turn intergrown with biotite, also reflect the gradual increase in the concentration of H2O in the melt throughout crystallization. Zircon CL, U -Pb and Hf Predominant CL patterns found in all samples correspond to the typical oscillatory zoning, which is considered as evidence for a primary magmatic origin [256] and characterizes zircons developed in intermediate to acidic plutonic rocks [243, 322] Relative to U -Pb dating, the predominant compositional zonation is interpreted to reflect igneous growth zoning such that concordant or near -concordant analyses of multi grain zircon fractions yield true crystallization ages. Compositional variation of U and Pb evidenced in CL does not undermine the utility of zircon in U/Pb dating because, despite differences in absolute amounts of both elements, all zones possess identic al ratios of U and Pb and thus yield the same U Pb age [256] Variations in U -Pb ages from 711.2 Ma to 771.7 Ma for the AB and OB are thus interpreted as crystallization ages of the respective plutons implying relatively rapid incremental assemblage for both batholiths near the Campanian-Maastrichtian transition. U -Pb formation ages are more consistent than previous Rb/Sr results [305] which were generated through an isochrone from samples collected over very large distan ces that violate the assumption of a common initial Sr. This is supported by the Hf isotopic data including multiple samples from the plutons indicating that there is a possibility that samples in this large batholitic mass are not necessarily from a singl e source further complicating Rb/Sr isochrone results. These results are consistent with a recognized Mid Cretaceous pulse of magmatism in the Andean range [44, 272] The Campanian Maastrichtian transition is considered as a period of rela tively rapid high angle convergence, a situation that may have lead to enhanced magmatism [44, 47] Gradual assemblage through discrete pulses of large batholithic masses has been reported else where [249, 338]

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73 In addition to constraining formation ages for the AB, U Pb zircon data derived in this study provide relevant infor mation regarding the early post crystallization history. The AB cooled relatively rapidly following intrusion/ crystallization at ca. 75 Ma from ~800C (U -Pb zircon closure) to ~300C (K/Ar biotite closure) at ca. 60 Ma with an average rate of ~ 53 C/Ma (Figur e 2 14). This rate can be interpreted as in situ cooling after AB shallow emplace ment into old and cool met amorphic roc ks of the Cajamarca Complex in the core of the Cordillera Central. Between 60 and 50 Ma cooli ng rates decreased to about 9 C /Ma where cooling was probably controlled by exhumation related to the Laramic Orogenic Phase [339] Low temperature thermochronology by zircon and apati te fission track data [166, 293] as well as a patite (U Th)/He [294] indicate that cooling rates slowed since the Early Eocene to less than 4C/Ma and are related to gradual erosion of the Antioqueo Plateau punctuated by short duration-rapid cooling e pisodes due to distinct erosional exhumation events that climaxed during the early Eocene and Late Oligocene/E arly Miocene [16, 293] and characterized by cooling rates between 1015C/Ma. 3 to +5, which reveals that the magma sources for AB and OB were a blend of upper crustal and mantle derived materials. Hf isotope data imply 1) mixing of mantle and crustal sources and 2) ju xtaposition of zircons that were crystallizing in slightly different environments as to be able to record a progression of crustal assimilation. T although considerable, is substantially less than that reported for other petrog enetic provinces (e.g., Australia, +10 to 10 [249] Major element data and petrographic analysis for the AB and OB indicate significant compositional variations but do not point to any clear relationship between variations in composition and Hf values where more felsic phases such as granodiorites and granites are

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74 marked by more negative (crustal) values while more mafic phases are associated with more positive (mantle) values. Hf results are in agreement with REE data, further indicating a combination of mantle and crustal sources. Sample compositional diversity probably resulted from variations in the composition of the source s, uneven melting conditions, complex chemical and physical interactions between mafic and felsic magmas, and crusta l contamination. Similar processes have been reported in other petrologic studies of batholithic be l ts in the Andes and elsewhere [45, 260, 269, 340, 341] Hf variability conflicts with previous Rb/Sr ages for the AB because changes in Hf in time and space imply heterogeneities of the different magmatic sources involved in the genesis of this petrogenetic province calling into question the validity of assumptions made in Rb/Sr dating [342] This fact, combined with the consist ency of U -Pb ages found throughout zircons from all of the samples imply that the range 7275 Ma is a closer to the formation age than the previously reported 98 Ma. Subtle changes in composition reported for the AB and OB (ranging from diorite to granite) and structures such as the lack of foliation, roof pendants, low degree of internal faulting, and clear contact aureole developed in the metamorphic and sedimentary hosting rocks suggest emplacement in the epizone, i.e., 2 4 km [343] Acicular crystal populations in samples SR 41 and SR 42 also point to fast cooling at epicrustal levels. According to Corfu et al., (2003), this ratio points to crystallization velocity with elongate crystals being common in rapidly crystallizing magmas and high level granitoids and gabbros. Trace Element and Radiogenic Isotopes Multi -ele m ental variation diagrams (Figures 2 12 and 213) s how the typical signatures of continental margin plutonism (i.e., subduction related) including: Typica l enrichment of Large Ion Lithophile E lements ( LIL E) and other i ncompatib les up to 10 time the c hondrite value, and a

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75 negative Nb anomaly, which indicate magma generation in a subduction setting with an important input of continental lithosphere P atterns for REE also show the characteristic Y and HREE depletions of slab melts REE patterns range from LREE enriched (SR 42, SR 001) to relatively flat (SR 24 and SR 41). Chondrite -normalized (cn) REE patterns are relatively flat [(La/Yb)cn=5.1 7.8; (Tb/Yb)cn=1.2 1.8 ]. Despite the relatively flat trends for HREE some variation is a pparent between samples even for specimens within the same topographic profile demonstrating differences in the source Lower values of HREE indicate that the residue is enriched in minerals that retain Er, Yb, etc. so that the magma generated was depleted in such elements. At greater depth, where mineral phases such as garnet are stable, selective incorporation of the HREE over the LREE during fractional crystallization is a viable mechanism affecting REE slope significantly. However, the HREE portion of m ost cur ves is usually flat implying that garnet, which strongly partitions among the HREE, was not in equilibrium with the melt at the time of segregation. In other words, most of the REE patterns are characterized by relatively low Tb/YbN ratios (1.0 1.5 ) ruling out substantial amounts of garnet as a fractionating phase. Instead, the residues were probably dominated by amphibole and plagioclase, and, possibly pyroxene. A negative Eu anomaly indicates that plagioclase was a residual phase involved in the f ormation of the rocks. These residual phases could reflect crustal source for the magmas, further supported by the high CaO/Na2O content. Enrichment of incompatible and LREE elements (Figure 2 13), and slight depletion of HFSE, suggest a crustal source for the parental magmas Inflection between Sm and Eu plus the negative correlations between Sr and both SiO2 and K2O versus a positive correlation CaO/Sr is also consistent with differentiation and plagioclase fractionation.

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76 HREE slopes are intermediate betw een the medium and high k series, displaying wide spectra of HREE relative to the convergent values of LREE. This is more common in high-K series where differences in SiO2 led to spread of HREE and yet maintain convergence of the LREE with marked enrichment of LREE relative to HREE (overall positive slopes). These trends are similar to those reported for other granitoids in the Cordillera Central (Vinasco et al., 2006) and felsic units in isolated massif s in northern Colombia (Cardona et al., 2006). LILE s such as Sr, K, Rb, Ba are enriched (~80 100 rock/Mantle ratio, Figure 2 13) and behave differently from the High Field Strength E lements (HFS E ) particularly Yb which show nearly primitive mantle like concentrations (rock/ m antle ratio of 1 7). T his high LIL E /HFS E pattern is now recognized as a hallmark of subduction zone magmas. Because the LIL E and HFSE elements are all incompatible and behave similarly in solid -melt exchange, the decoupling of these two groups, and enrichment of the LILEs is frequently explained by the participation of H2O rich fluids in the genesis of subduction zone magmas. Aqueous fluids are an important component of subduction zone petrogenesis. Since LREE are more incompatible and more mobile than the MREE and HREE in hydrous fluids the LREE enrichment in the samples can be explained by metasomatic enrichment fr om a hydrous fluid phase ( Gregoire et al. 2001; Franz et al. 2002). The relatively flat HFS E element pattern, usually between 1.0 and 12, means the concentration of these ele ments is higher than primitive mantle and may suggests that the source of continental margin magmas is indeed affected by the subducted oceanic crust, the lithospheric wedge, the sub-continental cru stal root, and the upper crust. H igh contents of K, Rb, Ba, and Pb indicate the involvement of crustal material in magma generation Negative Nb anomaly in spider diagrams (Figure 2 13) are now considered a characteristic of subduction related magmas ( magmatic arcs ) and have been interpreted in various ways. The

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77 low Nb concentrations can be attributed to the presence of a residual Nb -bearing mineral. Nb behaves similarly to Ti, so rutile, ilmenite, sphene, and hornblende are potential scavengers of Nb and Ta (Morris and Hart, 1983; Saunders et al., 1991). Immobil e HFS E element concentrations are similar to those of primitive mantle, and probably reflect overall mantle source characteristics, while the LILE element concentrations reflect the more water soluble components from the slab. Normalized spider diagrams, i n which continental arc magmas also show the decoupled LILE/ HFSE pattern (high LILEs K, Rb, Sr, Ba, low HFSE Yb) and the Nb trough, constitute a set of features that is now accepted as a characteristic of subduction zone magmas. AB and OB data clearly disp lay these trends supporting the idea that continental arc magmas are probably created by similar processes as island arc magmas, including LILE enrichment of the mantle wedge via aqueous fluids derived from dehydration of the altered oceanic crust of the s ubducting slab and subducted sediments. Enrichment in the NVZ and SVZ lavas is similar to that of island arcs while enrichment in the CVZ is considerably greater. Pearce (1983) attributed the enriched central hump shape on normalized spider diagrams to i ntraplate enrichment (including both LILE elements and more mobile HFSE elements, such as Ce and P), by analogy with intraplate basalts. He suggested that the enriched sub -continental lithospheric mantle (SCLM) was responsible for the enriched pattern in t he source of the CVZ magmas, which overlie thicker and older crust. He noted that such a component, containing elevated Ta, Nb, Zr, and Hf, is present in many continental arc magmas in amounts higher than found in island arcs, and proposed that it may be a good indicator of the involvement of SCLM in magma genesis. Davidson et al. (1990, 1991) and Wrner et al. (1994) argue that the trace element (and isotopic) trends correlate with crustal thickness, and conclude that, instead of the lithospheric mantle, t he thickened crust is responsible for the enrichment patterns

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78 The crustal sources and their geochemical signature s can be quite variable, however, and difficult to interpret. In this context, variations of REE patterns trough time in the samples presented in this study may reflect slight changes in crustal thickness with time allowing for consideration of crustal thinning, i.e., extension through slab roll back, as a potential mechanism of magma generation via decompressive melting. Further, differences in Hf values within the AB and OB can be attributable to variable input of juvenile magma controlled by modifications of the compressional regime such as it has been suggested fro the Peruvian Andes [248] Negative Eu anomalies developed in these relatively high -SiO2 batholiths are similar to those in other continental margins and can be related to the removal of plagioclase [45, 271] Medium -K and high -K calc alkaline series, which represent the majority of samples analyzed for the AB suite are progressively more LREE enriched. LREE enri chment is similar to that of other highly incompatible elements, such as K, and can be accomplished by low extent partial melting of a primitive mantle. An alternative explanation (e.g., Thompson et al., 1984), suggests that the mantle source for continent al arc magmas is a heterogeneous mixture of depleted primitive mantle and enriched OIB mantle types further contaminated by crustal materials. Whatever the case may be, it seems that more than one source with different incompatible element concentrations i s involved reflecting variable depletion, and perhaps enrichment. Negative Eu anomalies may also suggest an origin of the granodiorite suite from enriched lithospheric mantle sources. Trace element and isotopic data support the hypothesis that AB and OB, a s most of the products of Andean volcanism, have distinct characteristics. S pread values of HREE that characterizes NVZ suggest difference in the source s with some deep and garnet -scavenged HREE and some more surficial and hence, ric h er in HREE, i.e., clos er to, or above, a value of 10

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79 (Figure 2 12).The AB and OB as part of a NVZ setting show greater degrees of differentiation (expressed as the overall slope of the trend or La/YbN ratio) than SVZ, a condition that is possible to attribute to variance in bot h differentiation and contamination assimilation by a thicker crust. (For a defnintino of modern volcanic zones in the Andes i.e., NVZ, CVZ, and SVZ, the reader is reffered to Winter (2001), Wrner et al (1992), Davidson et al (1991), and Harmon et al (1 984)). Radiogenic Pb and Nd Lead isotope signatures are consistent with mixing of Pb from the mantle and crustal reservoirs. Such signatures are consistent with plutonic masses in the CVZ and NVZ, and very distinct compared to the SVZ, which further indica tes the involvement of a thick crustal filter suggest mixing of mantel an d crustal components (Figure 211) A Feasible Geodynamic Setting Our data support the hypothesis of a Mesozoic subduction related magmatic arc. The presence of calc alkaline magmatism along continental margins is most probably related to the subduction of ocenic lithosphere [44, 47, 271] Changes in factors such as rate and angle of convergence, age of the subducted plate, subduction angle, and interaction with hotspot traces exert a major control magmatism and tectonism along active margins (Pilger 1984; Badham and Halls, 1975). For Pilger et al. (1984) the predominant control to magmatism in the Andes of northwestern South Amer ica is related to the changes in the approach angle of the oceanic plate (F a rallon Nazca). Reorientations in plate motions have been well documented in South America [18, 19, 311] Su ch reorientations have produced significant changes in the approach angle of the interacting plates strongly affecting the tectono -magmatic evolution of the common plate boundary. From ~120 to 70 Ma a tectonic regime with relatively rapid NW -SE subduction (~10 -

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80 15 cm/yr) at a large angle (~ 80 in the northern Colombian Andes [44, 311] Is this latter scenario of s lab melting and induced melting of the asthenospheric wedge with varying degre e s of differentia tion and crustal contamination assimilation the most feasible one to explain the early evolution of the AB and OB. Alternatively, g ranodioritic to granitic calc alkaline magmas can be generated by dehydration melting of fertile portions of the continental crus t at temperatures above 780C (e.g., Wolf and Wyllie, 1994; Rapp and Watson, 1995; Kemp et al., 2007; Patino-Douce and McCar thy, 1998 and references there in). Temperatures in excess of 780C within the continental crust require unusual tectonic circumstanc es, such as slow exhumation allowing for long -time heating [340] Alternative source s of heat that would promote lower crust melting include advective input of heat from the mantle (Bergantz, 1989; von Blanckenburg et al., 1998). Occurrence of mafic enclaves, i.e., dioritic to gabbroic rocks, in more felsic hosts or as separate intrusive bodies suggests a direct chemical input from the mantle. This is p roposed here as a potential mechanism given the fact that extremely high temperatures, above ~ 1100C, must be reached to generate large amount s of magma by H2O under -saturated partial melting of mafic crustal source rocks (e.g., Rapp and Watson, 1995; Pati noDouce and McCarthy, 1998). Age constrains and geochemical signatures in this study support the contention that granitoids are chemically complex and seldom reflect characteristic or single uniform sources Hybrid source s and changes through time of particular source s and mechanisms of magma generation are probably a common feature of most batholithic belts. This variability in composition is often exacerbated by assimilation during ascent. In the case of mantle -derived magmas, an imprint of crustal a ssimilation is often isotopically detectable. In the case of crustal melts it is difficult, if not impossible, to assess the extent of assimilation processes, because

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81 numerous crustal inputs could occur at any point from the source to the level of emplacem ent. Some of these processes are reflected in the Hf and REE datasets here presented T he crust possesses important characteristics that may play pivotal roles in the generation and evolution of continental arc magmas in various ways, including but not lim ited to: 1) m agmas generated in the mantle wedge or subducting slab must pass through the broad layer of sialic and incompatible element -enriched cr ust, an efective filter, before reaching the surface with the potential for considerable crustal contamina tion of r ising liquids; 2) t he low density gradients between crust and mafic to intermediate magmas may significantly retard the upward movement of the latter thereby promoting enhanced assimilation, and/or differentiati on in stagnated plutonic bodies; 3) a ssimilation can be further enhanced by a combination of the relatively low melting point of the continental crust and the heat released from magma generation in the subduction zone, ultimately leading to partial melting of continental crust and concomitant addition of large s ilicic components to the system; and 4) the sub -continental lithospheric mantle (SCLM) may be quite different than the lithosphere beneath the ocean basins could have been anchored to the overlying continent since its formation that th e SCLM may have locally enriched during the longterm s tagnation beneath the continent. If any or all of the considerations listed above are true continental arc volcanics, on average, are expected to be more evolved and enriched than corresponding island arc volcanics and this is the case for all the samples presented in this investigation. Partial melting of the mantle lithosphere posibly along with melting of the subducting slab are considered as the mechanisms responsible for triggering the initial ma gma surge. This is followed by complex process of fractional crystallization, assimilation, magma mixing, etc. that reproduce the spectrum of igneous rock types usually found in convergent margins. This is not

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82 new as the generation of basaltic magma from t he mantle is considered a critical first step in m agma genesis in orogenic belts [44, 45, 47] The thick silica and aluminum rich crust, and possibly an enriched and heterogeneous mantle, make the continental arc the most complex igneous/tectonic environment on Earth Moreover, all of the enriched elements in the continental arc patterns are soluble, and likely to be concentrated in fluids. The most feasible source s for H2O in subduction zones would be water contained in the sediments and hydrated oceanic crust of the subducted slab. These fluids, enriched in LILE elements scaven ged from the crust, can both lower the melting temperature of the mantle wedge and concentrate LILEs in the resulting hydrated magma thus playing a paramount role in magma genesis t. Such a model is also consistent with the common occurrence of hydrous mi nerals found in suites like the AB and OB and the explosive nature of arc volcanism, which are all common features of dormant (Western and northern Cordillera Central) and active (southern Cordillera Central) magmatic arcs in the Colombian Andes. M ix ed so urces are characteristic of subduction zones and geochemical trends are very distinct compared to island arcs. Along continental arcs where volcanism has ceased to provide a renewed surface cover of accumulated lavas and pyroclastics, e.g., AB and OB, eros ion, has gradually exposed this remarkable granitic suite. Conclusions The AB and OB batholiths in the Cordillera Central of Colombia are a typical medium -K calc alkaline, granodioritic suite of with relatively high degree of differentiation. The medium -K calc alkaline composition is a primary feature of the Northern Volcanic Zone of the Andes. Both intrusives possess geochemical and isotopic characteristics typical of continental margin, subductionrelated plutonism. Radiogenic isotopic composition and Hf analysis in zircon indicate varying but significant contribution form the crust. This is expected as the relatively

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83 thick crust acts as a filter increasing upper crust signals. REE patterns also support similarities with the NVZ (important crustal componen ts) as well as differentiation and fractionation. U -Pb ag es range from ca. 71 to 77 Ma and are the best estimates of formation age for this granodiorit e suite. P lutons were emplaced near the transition Campanian-Maastrichtian and not in the Cenomanian as previously proposed. Variations in ages between different zones of both plutons represent specific times of emplacement of discrete magmatic injections corroborating the idea that large pl utonic masses are formed incrementally [344, 345] The relatively short span however, implies rapid assemblage thou gh multiple consecutive pulses. The geodynamic milieu was one in which convergence between Farallon and South America was accel erated and the angle of convergence was high promoting enhanced magmatism. Changes in the compressional regime may exert an important control in the relative amounts of juvenile material and degree of interaction with the crust. G eochronological and geoche mical isotopic data in this study suggest that the AB and OB possess significant chemical and isotopic variations that imply complex histories of multiple intrusions from different sources. Intrusions have taken place over a relatively short period, ca. 7 Ma each carrying a slightly different geochemical signature indicating variations in the source and different magma mixing/contamination processes and leading to a rather quick process of assemblage through incremental additions. This large batholith was assembled in the epizone through incremental pulses. Emplacement in a semi rigid lithospheric block can be a feasible explanation for the circular, discordant outcrop and the negligible structural perturbation evidenced by this rather continuous unit. Post -crystallization cooling was rapid (800 300C) over the period ca. 75 to 60 Ma and mainly controlled by waning of the thermal perturbation introduced by emplacement in the epizone in

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84 cold metamorphic rocks from the Paleozoic Cooling from ~ 60 to present has been constrained by previous LTTC studie s indicating slow rates controlled by erosional exhumation triggered by tectonic input in various discreet pulses on the Eocene (ca.40 50 Ma), the Late Oligocene early Miocene (ca. 25 20 Ma), and Pliocene Present ( Restrepo -Moreno et al, 2009; Restrepo Mor eno et al., submitted ).

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85 Table 2 1 Sum mary of m ajor e lemen t d ata for the Antioqueo and Ovejas batholiths Major element amounts are given in % wt. SR = samples in this study, S = sa mples referred in Saenz (2003). Rock types are QdGd = Quartz diorite granodiorite, D = Diorite, G = Granite, GD = Granodiorite. S ample condition varies from fresh (F) to weathered (W). Values for Fe2O3 and FeO are derived from total FeO. Mg# = [(MgO)]/ [100*(MgO/40.3114)/(MgO/40.3114+Fe2O3/175.7+FeO/71.8464)]. Samples SR 001, SR 003, SR 9 and SR 41 are belong to the Antioqueo Batholith. Samples SR 24 and SR 42 belong toe the Ovejas Batholith. All tha smaples fmor Saenz (200 3) belong to the Antioqueo B atholith. Sample SR 001 SR 003 SR 9 SR 24 SR 41 SR 42 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 S 11 S 12 Al 2 O 3 15.95 15.5 16.55 19.2 16.01 15.71 16.07 17.67 16.09 14.91 17.29 16.68 17.75 17.23 16.9 15.54 13.52 12.8 CaO 3.82 5.09 4.25 0.04 5.78 3.6 5.63 4.39 2.57 3.97 6.39 5.65 5.93 4.31 5.8 5.37 3.92 0.6 9 Fe 2 O 3 t 3.01 4.98 4.07 5.34 6.77 3.48 6.65 7.19 1.96 4.17 6.78 5.79 6.29 4.99 6.32 5.79 4.31 1.34 K 2 O 2.34 2.27 1.89 1.63 2.11 1.28 2.45 1.66 1.8 3.36 1.6 1.81 1.9 1.59 2.11 2.39 3.76 5.37 MgO 1.11 2.14 1.39 1.13 2.56 0.95 2.84 3.27 1.04 1.54 2.82 1.95 2.53 1.43 2.22 2.38 2.74 0.37 MnO 0.06 0.09 0.1 0.24 0.13 0.07 0.11 0.13 0.04 0.07 0.12 0.12 0.11 0.13 0.12 0.1 0.07 0.02 Na 2 O 3.46 3.39 3.79 0.04 3.01 3.59 3.13 2.2 5.47 2.99 3.76 3.27 3.26 3.64 3.27 3.05 2.55 2.15 P 2 O 5 0.09 0.1 0.16 0.06 0.15 0.15 0. 13 0.15 0.09 0.09 0.2 0.15 0.15 0.16 0.17 0.12 0.09 0.02 SiO 2 64.51 64.8 67.12 61.03 62.44 68.84 61.39 58.76 70.51 67.63 59.55 63.38 60.73 65 61.76 63.95 67.02 75.8 TiO 2 0.3 0.52 0.38 0.66 0.69 0.45 0.62 0.68 0.27 0.45 0.72 0.56 0.63 0.41 0.69 0.57 0.47 0.12 LOI 1.93 0.33 1.02 10.41 0.65 2.23 0.73 3.71 0.46 0.53 0.56 0.62 0.87 1.28 0.57 0.62 1.02 0.86 Total 96.5 99.2 100.7 99. 8 100.3 100.3 99.8 99.8 100.3 99.7 99.8 99.9 100.2 100.1 99.9 99.9 99.5 99.5 Fe 2 O 3 1.51 2.49 2.04 2.67 3.39 1.74 3.33 3.60 0.98 2.09 3.39 2.90 3.15 2.50 3.16 2.90 2.16 0.67 FeO 1.35 2.24 1.83 2.40 3.05 1.57 2.99 3.24 0.88 1.88 3.05 2.61 2.83 2.25 2.84 2.61 1.94 0.60 Mg# 50.11 53.92 48.19 36.56 50.73 42.64 53.77 55.33 59.10 50.14 53.11 47.84 52.28 43.83 48.89 52.82 63.39 42.92 Ro ck Type Qd Gd Qd Gd Qd Gd D D Qd Gd D D G Qd Gd D Qd Gd D Qd Gd D Qd Gd Qd Gd GD S. Cond F F F W F W F F F F F F F F F F F F

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86 Table 2 2 U Pb ages and present for the Antioqueo and Ovejas batholiths. [(176Hf/177Hf/ 0.282843) 1]*10000 using Patchett et al. (2004) value of 176Hf/177Hf = 0.282843 equivalent to the mean of carbonaceous chondrites. Ages for each set of samples represents the average calculated age from the analysis of 15 zircons per each s ample. Numbers to the right of the sample name (1 though 5) indicate the number of the analysis undertaken for Hf isotopic analysis. Samples SR 001, SR 003, SR 9 and SR 41 are belong to the Antioqueo Batholith. Samples SR 24 and SR 42 belong toe the Oveja s Batholith. All tha smaples fmor Saenz (200 3) belong to the Antioqueo B atholith. Sample U Pb Age (present) Geographic Coordinates (decimal degrees) Profile Elevation (m) SR 9_1 1.87 SR 9_2 1.56 SR 9_3 75 1.2 1.41 6.46 N/ 75.37W Matasanos 2370 SR 9_4 1.06 SR 9_5 0.99 SR 41_1 1.30 SR 41_2 0.92 SR 41_3 71 1.2 2.93 6.36N / 75.59W Matasanos 1380 SR 41_4 1.94 SR 41_5 1.27 SR 24_1 3.85 SR 24_2 3.39 SR 24_3 72 1.4 4.13 6.39N / 75.60 W La Garca 2500 SR 24_4 4.49 SR 24_5 4.80 SR 42_1 3.01 SR 42_2 5.30 SR 42_3 77 1.7 3.61 6.34N / 75.58W La Garca 1520 SR 42_4 3.15 SR 42_5 3.75 SR 003_1 0.81 SR 003_2 1.34 SR 003_3 77 1.7 2.30 6.76N / 75.12W P orce 1070 SR 003_4 1.41 SR 003_5 2.12 SR 001_1 0.53 SR 001_2 1.70 SR 001_3 75 1.5 2.72 6.86N / 7 5.18W Porce 0760 SR 001_4 2.01 SR 001_5 2.61

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87 Table 2 3 Whole rock trace element concentrations for the Antioqueo a nd Ovejas batholiths Trace element shown in ppm (Rare Earth Elements included). Isotopic compositions displayed as ratios. Sample SR 001 SR 003 SR 9 SR 24 SR 41 SR 42 Elem e nt Li 17.96 29.74 47.26 13.24 31.40 13.42 Sc 5.94 13.80 9.46 12.67 21.71 3 .70 Ti 3553.84 5895.10 5381.24 6729.04 7947.29 4767.78 V 54.57 115.18 62.01 69.12 148.94 32.46 Cr 8.28 10.87 6.84 17.82 11.21 13.31 Co 242.94 74.36 116.66 22.94 67.79 24.72 Ni 7.87 9.53 7.54 6.95 7.21 5.72 Cu 15.78 49.17 18.87 6.56 19.93 5.65 Zn 35. 84 50.81 72.12 92.91 85.25 65.99 Ga 15.45 16.56 16.37 23.44 17.82 17.02 Rb 58.54 93.19 47.50 23.90 88.68 45.84 Sr 233.45 259.26 312.66 4.52 224.29 250.66 Y 7.27 19.88 11.01 7.56 30.02 5.44 Zr 42.60 24.21 31.99 16.71 21.46 7.66 Nb 4.48 9.60 11.56 6.5 5 10.78 5.88 Cs 0.10 2.88 1.16 1.32 4.17 1.79 Ba 755.52 597.88 741.95 361.15 484.36 353.83 La 10.69 9.12 8.47 3.37 7.35 10.71 Ce 19.58 22.14 17.23 7.91 20.38 19.73 Pr 2.24 3.09 2.12 1.23 3.32 2.21 Nd 8.42 13.53 8.76 5.52 16.01 8.04 Sm 1.58 3.25 1.91 1.46 4.25 1.34 Eu 0.80 0.83 0.75 0.57 0.99 0.74 Gd 1.51 3.22 1.90 1.66 4.46 1.23 Tb 0.22 0.52 0.30 0.31 0.77 0.17 Dy 1.18 3.13 1.76 1.96 4.66 0.89 Ho 0.25 0.66 0.37 0.44 0.99 0.19 Er 0.69 1.97 1.07 1.35 2.90 0.53 Tm 0.10 0.30 0.17 0.21 0.45 0.08 Y b 0.67 1.97 1.15 1.49 2.98 0.51 Lu 0.10 0.31 0.18 0.23 0.46 0.06 Hf 1.53 1.12 1.04 0.91 1.09 0.27 Ta 1.04 5.00 6.66 0.87 5.30 1.45 Pb 6.07 6.79 8.68 11.25 6.86 7.46 Th 5.15 5.70 2.66 1.78 1.59 1.27 U 1.44 1.22 1.39 0.97 2.93 0.76

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88 Table 2 4 Lead and Neodymium isotopic compositions for the Antioqueo and Ovejas batholiths Isotopic compositions displayed as ratios, values for epsilon Sample SR 001 SR 003 SR 9 SR 24 SR 41 SR 42 Isotopic Ratio 206 Pb /204 Pb 19.079 19.001 18.913 18.861 19.074 18.735 207 Pb /204 Pb 15.682 15.640 15.638 15.618 15.644 15.612 20 8 Pb /204 Pb 38.952 38.820 38.666 38.578 38.625 38.545 208 Pb /206 Pb 2.042 2.043 2.044 2.045 2.025 2.057 207 Pb /206 Pb 0.822 0.823 0.827 0.828 0.820 0.833 143 N d /144 N d 0.51254 4 0.5127 47 0.5127 08 0.51281 4 0.51272 2 0.51279 4 Nd 1.83 2.13 1.37 3.43 1.64 3.04 Figure 2 1. Geologic and structural map of the study site. Red circles indicate sampling locations. AB = Antioqueo Batholith, OB = Ovejas Batholith. Adapted from Gonzlez (2001) and Tapias et al. (2006).

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89 Figure 2 2 Color photographs of sampl es in their hand specimen. White color is a combination of plagioclase feldspar and quartz, black color is predominantly biotite and hornblende.

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90 Figure 2 3 Photomicrographs of grey -scale CL imaging of typical zircons from the Antioqueo and Ovejas b atholiths Internal structures are predominantly oscillatory zoning. LA ICP -MS analytical spots appear as perfectly circular craters. Deep and narrow spots (40um) correspond to laser ablation session s for U -lt from laser ablation session s for Hf analysis.

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91 Figure 2 4 Classification of the Antioqueo and Ovejas batholith based on their SiO2 vs. Na2O + K2O concentrations. Diagram includes samples for the Antioqueo batholith analyzed in this study and by Saenz (2003) (red circles and gray diamonds respectively) and the Ovejas batholith (orange circles).

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92 Figure 2 5. Sub alkalinity degree for the Antioqueo and Ovejas batholiths. Dotted lines define the fields for the different series (after [335, 336] ). Diagram includes samples for the Antioqueo batholith analyzed in this study and by Saenz (2003).

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93 Figure 2 6 Maj or element Harker variation diagrams for the Antioqueo and Ovejas batholiths. FeO(t) = Total Fe.

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94 Figure 2 7. U Pb Age Elevation relationship and range of previously reported ages for the A ntioqueo and Ovejas batholiths. Black triangles, squares and d iamonds show the U Pb ages found in this study for the Matasanos, Garcia and Porce value of the U Pb ages obtained for all the samples. Horizontal dashed line at the bottom ind icates the range of ages reported in the literature [299, 301, 302, 304, 305]

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95 Figure 2 8 Tera Was serburg diagrams Total of ellipses per s ample = 15. Ellipses represent 2

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96 Figu re 2 9 U Pb Age

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97 Figure 2 10. Pb/Pb correlation diagrams for the Antioqueo and Ovejas batholiths. Fields for the different modern volcanic zones of the Andes are displayed for comparison according to various sources [261, 262, 269271]

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98 Figure 2 11. ship for the Antioqueo and Ovejas batholiths. Hf isotope composition of magmatic zircons plotted as a function of whole rock Nd isotope composition at the time of crystallization. Error bars represent 2 SEM. CC = Continental crust; DM = Depleted mantle.

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99 Figure 2 12. Primitive mantle normalized R are Earth Elements patterns (REE) for the Antioqueo and the Ovejas batholiths. Samples SR 001, SR 003, SR 9 and SR 41 belong to the Antioqueo Batholith, samples SR 24 and S R 42 correspond to the Ovejas Bath olith. Yellow shade shows the field for REE patterns found in the Northern Volcanic Zone of the Andes [271]

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100 Figure 2 13. Multi element plot for the Antioqueo and the Ovejas batholiths. This REE Spider Diagram is normalized to the primitive mantle [337] Samples SR 001, SR 003, SR 9 and SR 41 belong to the Antioqueo Batholith, samples SR 24 and S R 42 correspond to the Ovejas Batholith.

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101 Figure 2 14. Post -crystallization cooling path for the Antioqueo Batholith. Age errors are true numerical errors ( Ma). a. U -Pb zircon (this study), b. Rb/Sr [305] c. Rb/Sr biotite [304] d. K/Ar biotite [303] e. K/Ar biotite [302] f. K/Ar biotite [27] g. K/Ar biotite [301] h. FT zircon and apatite [166] i. FT and (U Th)/H e apatite [293, 294]

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102 CHAPTER 3 L ONG TERM EROSION AND EXH UMATION OF THE ALTI PLANO ANTIOQUEO, NORTHER N ANDES (COLOMBIA) F ROM APATITE (U TH)/HE THERMOCHRONOLOGY Introduction Defining the morphotectonic evolution of the Andean Cordillera is critical for understanding the tectonic processe s driving uplift, climatic influences of the topography, the source of sediment to oceans and continental sedimentary basins, and changes in paleogeography [14, 43, 113, 346, 347] Despite its importance in the geodynamic evolution of a geo logically complex region, the Northern Andes in Colombia remain poorly studied compared to many segments of the Andes to the South. Little is known about the morphotectonic evolution of some major provinces, including the Antioqueo Eastern Massif [27] which is a polymetamorphic complex [286] intruded by large Mesozoic calcalkaline plutons of the Antioqueo and Ovejas Batholiths [21] Geomorphologically, the Antioqueo Eastern Massif encompasses the Altiplano Antioqueo [12, 28] hereafter the Antioqueo Plateau (AP), w hich is the largest high elevation erosional surface in the Northern Andes. The AP is preserved as a wide, slightly dissected surface extending for tens of kilometers, locally and deeply incised by the Medelln -Porce fluvial system ( Figure 3 1 and 33 ). A better understanding of the longterm landscape development and erosional exhumation history of the AP is needed for defining the Cenozoic morphotectonic and uplift history of the Northern Andes, the paleogeographic evolution of the region, and the relationships between erosion and tectonics [14 16, 24, 43, 113, 158] The findings of this study also have implications for refining morphotectonic response to variations in convergence rates between Nazca (Farallon) and South American plates during the Cenozoic [18]

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103 Elevated erosional su rfaces affected by deep and localized fluvial incision, such as the AP (Figure 3 1 and 3 3), constitute an important source of morphotectonic information, retrievable from low temperature thermochronologic data. Such surfaces, usually referred to as relict surfaces, have played an important role in the reconstruction of the morphotectonic evolution of active [97, 348, 349] and passive continental margins [350] Elevated, relict erosional surfaces, indicate that portions of an orogen have not achieved equilibrium between erosi on and tectonics [83, 96, 97] It is generally thought that these surfaces develop by planation close to base level [351] and are subsequently uplifted via tectonic processes to their present position [12, 96, 167] The nonequilibrium character of these d enudational features implies lagged response to tectonic forcing, which allows their use for reconstructing regional, long -term erosional exhumation as well as patterns of surface/rock uplift and topographic evolution [83, 216] Cenozoic uplift and erosion in the northern Cordillera Central is recorded by sedimentary units (e.g., associated litho facies and fluvial paleoflows) in the Middle Magdalena Valley to the east [14, 16] the Lower Magdalena Valley to the north [16, 29] and the Cauca valley to the west [168] The traditional interpretation suggests that since the latest Cretaceous the eastern flank of the Central Cordillera has operated as the source area for synorogenic detritus to the Middle and Lower Magdalena Valleys [14, 29] The region was also considered as a source of terru ginous material for the Terciario Carbonfero de Antioquia during the Oligo-Miocene (Grosse, 1926), a coal bearing sedimentary sequence later renamed Amag Formation (Gonzlez, 2001), situated in the Cauca valley.Cenozoic uplift in the Northern Andes of Colombia has been also invoked as the cause of substantial paleogeographic changes that took place in the region,

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104 particularly, the reorientation reorganization of the Orinoco and Amazon fluvial systems [43] Thermochronological evidence from apatite (U Th)/He analysis from two vertical profiles (Figure 3 3) in the AP that reveal the er osional exhumation history of the plateau with implications for the timing of tectonic pulses of the Andes and the topographic evolution of the range during and after uplift. Study Site Geological and Physiographic Overview From west to east, the Colombian Andes are composed of three main mountain belts known as Cordillera Occidental, Cordillera Central, and Cordillera Oriental. These Cordillera are separated by two inter -Andean depressions occupied by the Cauca and Magdalena rivers. The Magdalena River in the east and the Cauca Rivers in the west enclose the AP (Figure 3 1). The Magdalena River yields ~1000 t/km2yr of sediment to the delta which is the highest sediment yield of any large river in South America along the Caribbean and Atlantic coasts [121] Such sediment yield is roughly equivalent to denudation rates on the order of ~1 mm/y. S imilar figures have been reported for the AP based on a temporal series of bathymetric surveys in five reservoirs in the AP [150] Augmented erosion in the Colombian Andes has been attributed to anthropogenic activities, e.g., deforestation, agriculture, and development of infrastructure (Restrepo and Syvitski, 2006). However, quantitative data regarding long-term, background erosi on rat es for the area are inexistent. The three major cordilleras belong to two distinct domains that are separated by the Romeral Fault system (Figs. 1 and 2). The western province consists of accreted oceanic crust and is exposed in the Cordillera Occide ntal and the lower western flank of the

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105 Cordillera Central The eastern province is underlain by continental basement and is exposed throughout most of Cordillera Central and Cordillera Oriental [25, 281] The Cordillera Oriental is a fold andthrust belt that consists of polydeformed, continental Precambrian and Paleozoic metamorphic and igneous rocks The Cordillera Central is characterized by modern volcanic activity related to the subduction of the Nazca plate and a basement consisting of a Mesozoi c -Cenozoic plutonic arc. This study focuses on a cordilleran segment located from 545 to 7 latitude North without modern volcanism. Within the study area (Figure 3 2), the Cordillera Central encompasses a pre -Mesozoic to Mesozoic polymetamorphic complex [286] composed of low to medium -grade metamorphic rocks of the Cajamarca Complex [287] and high grade metamorphic rocks of the El Retiro Group and Las Palmas Gneiss [21] Lower Paleozoic or Precambrian age has also been proposed for this metamorphic core [290] The metamorphic basement is covered by the unmetamorphosed shallow marine to epicontinental sedimentary sequences San Luis [289] San Pablo, La Soledad [290] and Abejorral [291] that are Early Cretaceous in age (132 127 Ma). Metamorphic and sedi mentary units were intruded at a shallow crustal level (~4 km) by Meso Cenozoic batholiths, including the Batolito Antioqueo and Batolito de Ovejas, and overlain by Late Eocene continental arc volcanic rocks [284, 286] Finally, Late Neogene volcaniclastic sequences associated with modern volcanic activity in the Cordillera Central mantle the region [12] The northern portion of the Cordillera Central is considered to be part of an exotic terrane, the Terreno Taham which was affected by Devonian Carboniferous, Permo T riassic (Hercynian), and Cretaceous tectono metamorphic events [283, 286, 352]

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106 The Antioqueo and Ovejas bathol iths are texturally and compositionally homogenous. Approximately 95% of the samples collected from this units have been classified as quartzdiorite granodiorite [166, 301, 353] Previously published thermochronologic data from the Antioqueo Batholith and other associated plutons include biotite K/Ar ages, which rang e from 683 Ma [301] to 906 Ma [354] three biotite Rb/Sr ages from 567.4 to 669.2 Ma [304] ; two whole rock Rb/Sr ages of 828 and 9827 Ma [305] ; zircon fission track dates from 49.12.5 to 67.12.1 Ma and fourteen apatite fission -track ages varying between 49.11.2 and 28.71.5 Ma [166] The l arge spectrum of ages can be explained by the wide range of closure temperatures of the thermochronometric systems employed and should be interpreted as cooling rather than formation ages. Although the RbSr age of 9827 Ma has been proposed as the best e stimate of crystallization age for the Antioqueo batholith [305] recent zircon U Pb spot analyses (LA ICP MS) give ages between 711.2 and 771.7 Ma suggesting that formation occurred close to the transition Campanian-Maastrichtian [355] The Antioqueo Plateau The AP is a lowrelief geomorphic domain, which app ears to be a >5000 km2 relict surface located in the northernmost portion of the Cordillera Central [12] It is flanked in the east and west by the Magdalena and Cauca river valleys, respectively. Mean elevation in the AP is ~2500 m and it possess subdued internal topography characterized by rolling hills with local relief <40 m and slopes <10. Its overall geometry resembles an asymmetric, truncated pyramid with its steepest margin corresponding to the Cauca River canyon (Figure 3 1 and 3 3). The AP is dominated by a dendritic drainage network and developed over gently rolling terrain where most hill slopes are mantled by saprolitic horizons up to 100 m

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107 thick. Inselbergs (e.g., bornhardts and tors) are conspicuous, sporadic geomorphic features throughout the AP and in some cases create local relief >300 m. To the north and northeast the AP descends in a series of step like, less extensive erosional surfaces into the lowlands of the Lower Cauca and Magdalena Rivers. The internal low -relief for the AP is interrupted by steep regional escarpments and deeply incised gorges (Figure s 3 1 and 3 3). In some localities, steep topographic margins expose up to 3200 m of crustal section with slopes in excess of 50. Most of the planar topography of the AP coincides with the Antioqueo Batholith, one of the largest (7600 km2) and most continuous lithological units in the Cordillera Central. The prominent topographic features within the AP, like the Las Baldas Range and Pramo de Belmira in the western margin of the plateau, are held up by the more resistant metamorphic units (Figure s 3 1 through 3 3). About 150 km s outh of the study area, the plateaus low relief rapidly transitions into a more rugged portion of the Cordillera Central dominated by active volcanoes. This part of the orogen, in clear contrast to the AP, has geometry that bears resemblance to a symmetri c pyramid terminated in higher summits reaching up to ~5000 m. The AP has been interpreted to have developed as an erosion surface near sea level in Paleocene time and that was uplifted in discrete episodes during the Cenozoic, and modified by fluvial acti vity combined with the unique weathering properties of granitic rocks in a tropical setting [12, 297] While this is the prevailing view, the APs morphotectonic history is still poorly constrained, and the age of the surface is strongly debated. Despite the fact that exhumation and paleoelevational history of the AP remain virtually undocumented, remnants of shallow, marine sedimentary sequences formed

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108 during an early Cretaceous marine transgression point to the origin of the region as a lo w eleva tion, low relief domain. No other elevated low relief erosional surfaces as extensive as the AP have been reported in other parts of the Northern Andes. Nevertheless, similar geomorphic features of lesser extent referred to in the literature as elevated p lanation surfaces (Coltorti and Ollier, 1999), elevated plateaus (Wdowinski and Bock, 1994), high altitude paleosurfaces (Kennan, et al., 1997), and erosion plateaus (Ollier and Pain, 2000) are found in Ecuador, Peru, and Bolivia. Quantitative data that constrain erosion rates, landscape evolution, and uplift history of these important geomorphic features is still scarce. Tectonic Setting Orogeny in the Northern Andes is primarily driven by the collision of allochthonous terrains and interactions bet ween several tectonic plates and micro plates [23, 25] This has resulted in heterogeneous deformation compared to the Central Andes [23, 25, 26, 356] Most authors agree that the regional tectonic history ha s been dominated by the subduction of two oceanic plates, Nazca and Caribbean, beneath continental South America and by interactions of two lithospheric provinces, namely, the North Andean and the Panama Choc blocks [23 26, 356] (Figure 3 4 ). The Nazca plate is undergoing rapid steep subduction along the length of the Colombia Ecuador trench at a rate of ~54 mm/a, and is responsible for the volcanism in the Cordillera Central [25, 26] The Caribbean plate is being subducted slowly from the northwest at ~20 mm/a, at a shallow angle, and without an associated volcanic arc [23, 26] During Paleocene until Miocene, the collision of the Caribbean plate with South America resulted in uplift in the Cordillera Central [357] Starting in the Neogene time, subduction was blocked in the

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109 collision area between the Panama Choc block and the Western Cordillera of Colombia and this ongoing collision is producing deformation in the three cordilleras [23, 25] Modern stress regimes are accommodated by major structures within the North Andean Block, which is part of the South American plate [25] and within the Panama Choc block [23, 24, 356] ramo faults separate the North Andean Block proper from the South American Plate (Figure 3 1 and 3 4).The North Andean Block is escaping rigidly at 62 mm/a to the northeast with the South American craton acting as a rigid buttress w hile the Panama Choc block is in active collision at a rate of ~25 mm/yr [26] Finally, the area that encompasses the AP is characterized as a lithospheric unit of high rigidity [22] Two major variations in the rate of convergence between the N azca (Farallon) and South American plates have been reported for the Eocene and Oligocene [18, 19, 42, 315] These changes may have determined morphotectonic, deformational and magmatic trends in the whole Andes [36, 47] Collision of the Panama Choc block is considered as the trigger of the modern topographic configuration of the Northern Andes and h as been invoked as the cause of the most recent phase of uplift and exhumation in the region (Eu Andina Phase of Van der Hammen, 1960), as well as de driver of the Late Miocene Pliocene uplift of the Western and Eastern cordilleras that has produced most o f the present day structural relief [14, 15, 24, 25, 156] The Central Cordillera may represent a crustal -scale, positive flower structure [14, 25] The age of major deformation of the Central Cordillera at the latitude of this study is pre -middle Eocene, as evidenced by the widespread Middle Magdalena Valley unconformity [14] Morphotectonic evolution of the Central Cordillera has exerted a

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110 major control on the tectonostratigraphic development of the Magdalena and protoEastern Cordillera basins [14] Elevated areas in the Central Cordillera were a source of detrital material for basins on both sides of this range. Good examples ar e the Late Cretaceous to present sedimentary sequences in the Magdalena Basin to the East [14, 29] and the continental, coal rich Amag Formation with >1,500 m of sediments deposited on the west side of the Cordillera Central [21, 168] Apatite (U -Th)/He Thermochronology AHe dating is a well -established thermochronometer with temperature sensitivity between 40C and 80C [183, 190, 358, 359] a range that is lower than that of any other routinely used thermochronometer. Assuming geot hermal gradients of 2030C and mean annual surface temperature of ~15 C, this temperature range is equivalent to depths of ~1.2 3 km. AHe analysis has been used in diverse tectonic settings to investigate several geologic processes in the upper crust, in cluding landscape and topographic development (Clark et al., 2005;House et al.,1998), orogenic exhumation in collisional orogens (Reiners et al., 2003),uplift/erosion intranspressive environments (Spotila et al., 2001), development of rifted margins and es carpments (Persano et al., 2002), and footwall exhumation in extensional settings (Stockl i, 2005; Stockli et al., 2000). AHe thermochronometry of samples collected from near -vertical elevation profiles enables study of the timing, magnitude, and rate of co oling of rocks as they traverse the upper 1 4 km of the earths crust, the realm strongly influenced by upper -crustal tectonic and erosional processes [54, 90, 91, 183, 359] Improved reconstructions of erosional exhumation by AHe are achieved where (1) geothermal gradients have remain ed relatively stable through time [360] ; (2) denudation rates are not excessively high (> 5 mm/y) as to promote significant he at advection in the upper crust [361, 362] ; (3) pre -

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111 existing topography was not rough i.e., topographic amplitude >3 km, topographic wavelength < 20 km at denudation rates > 0.5 mm/y [363] ; and 4) surface relief amplitude has not changed significantly since the rocks cooled through the closure temperature, as this has a strong effect on the slope of the age -elevation relationship and can lead to overestimation/underestimation of exhumation rates [364] Although AHe can be used to reconstruc t exhumation histories even when these criteria are not met, uncertainties associated with these four factors are minimized if a vertical profile for sample collection is available and if samples are collected over a large range of elevations. Extended ver tical profiles allow a more thorough analysis of features displayed in age elevation profiles such as exhumation events, helium partial retention zone (He -PRZ), and paleogeothermal gradients, which permit better constraints on denudation [190, 201, 364] The AP possesses geologic, morphologic and tectonic characteristics that make it a perfect scenario to undertake AHe low temperature thermochronology studies along vertical profiles. Methods The AHe techniqu e is based on the accumulation of 4He ( particles) resulting from the radioactive decay of the parent isotopes 235U, 238U, 232Th, and, 147Sm. Thermokinetics of helium diffusion and constrained so t hat measurements of 4He and its parent isotopes by mass spectrometry allows calculation of the systems closing time [190, 208, 220] Measurements were made by degassing multigrain aliquots through laser heating and evaluating 4He by isotope -dilution gas source mass spectrometry, followed by determination of U, Th, and Sm on the same crystals by isotope -dilution ICP -MS. Mean (U Th)/He ages were

PAGE 112

112 calculated on the basis of 3 5 apatite replicate analyses Propagated errors for He ages based on the analytical uncertainty associated with U, Th, and He measurements are 4% (2 ) for laser samples [190] Nevertheless, a 6% (2 ) uncertainty for all samples is reported based on the reproducibility of replicate analysis of l aboratory standard samples [201] AHe analyses were conducted on samples from two separate elevation profiles located ~40 km apart in the central portion of the AP. The La Garca and Matasanos Porce profiles are situated on a regional scarp develo ped on the northern flank of the Medelln/Porce River canyon. Samples were collected at elevation intervals of ~70 m, spanning the largest possible range of paleocrustal depths (~2 km) in the exposed structural relief along this margin of the AP (Figure 3 3). Apatite concentrates were obtained through the conventional method of heavy liquid and magnetic susceptibility separation. Nineteen samples yielded good quality inclusion -free, euhedral, non -fractured apatite with >65 m in the prism width, suitable fo r AHe analysis [183, 190] Apatite (U Th)/He age determinations were carried out at the University of Kansas using laboratory procedures described in Farley (2002), House et al. (2001), Reiners et al. (2003) Stockli et al. (2000). Radiogenic He was analyzed using a fullyaut omated mass spectrometry system consists of a Nd:YAG laser for total He laser extraction, an all -metal ultra -high -vacuum extraction line, a precise volume aliquot systems for 4He standard and 3He tracer for isotope, a cryogenic gas purification system, an d a Blazers Prisma QMS 200 quadrupole mass spectrometer for measuring 3He/4He ratios Three to five multiple grain aliquots per sample were prepared following the standard protocol available at the University of Kansas (U Th)/He Laboratory. Euhedral

PAGE 113

113 crystal s to be dated were hand picked from apatite separates using a high -power (180x) stereo zoom microscope under reflected and transmitted light and screened for inclusions. To enhance discrimination of inclusions, all of the grains selected for He mass spectr ometry were further examined under the microscope by immersion in alcohol. Only transparent, inclusion -free, euhedral, unfractured apatite grains with similar shape and size (average diameter ~80 10 m) were chosen. Selected crystals were digitally measu red and geometric parameters were used to characterize the mor phometry of each -ejection corrections. Uniform grain size minimizes differences in He diffusion behavior (Farley, 2000). Sizes >6070 m require low correction factors for He ages and increase accuracy (Farley, 2002). After careful se lection and screening, apatite crystals were placed into 1 -mm Pt tubes, which were then loaded into copper sample holders with 36 sample slots. Samples were degassed at ~ 1050 C for 5 minutes and analyzed for 4He, followed by a second extraction (He re -ex traction) to ensure complete degassing and to monitor He release from more retentive U and Th -bearing inclusions in analyzed apatite. Once 4He measurements were completed, samples were dissolved in HNO3 and spiked with mixed 230Th -235U -149Sm tracer for is otope dilution ICP -MS analysis of U, Th, and Sm. Procedures were performed at Isotope Geology Laboratory of the University of Kansas (IGL -KU). Each aliquot was analyzed for U, Th, Sm and selected REE using a Fisons/VG PlasmaQuad II+XS ICP -MS. Precision and sensitivity of the instrument allow isotopic analyses with RSD <1%. Concentrations of 147Sm were close to zero for all samples. Th/U rations can be used to monitor for the presence of Th rich phases such as monazite [215] Normal Th/U ratios for monazite are between 5 20[365] The mean Th/U

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114 for all of the replicates combined is 0.9 and most values are bellow 1 so none of the analysis had to be excluded. Alpha ejection was corrected using the method of Farley and others (1996), modified by Farley (2002). Standard 97MR 22 apatite, a well -characterized plutonic sample from British Columbia (4.5 Ma) (Farley et al., 2001) was analyzed with each batch of unknown sample in order to monitor system per formance and check analytical accuracy. Replicate analyses of 97MR Analytical uncertainties for the University of Kansas (U Th)/He facility are assessed ~6% -MS uncertainties. Results U, Th and H e concentrations, Ft correction for grain dimensions and alpha ejection corrected He ages are reported in Table 1 3 for the La Garca and Matasanos -Porce profiles. All individual AHe sample aliquot analyses show excellent reproducibility. Similarity in the age -elevation relationships for both profiles indicates consistency of the data sets and allowed us to combine AHe data into one composite vertical transect (Figure 3 5). Most aliquots yielded ages with standard deviations < 2.0 Ma. Only one sample, SR 11, yielded an aliquot with irreproducible 4He concentrations (see data supplement) and AHe age of 1088.8 Ma. This was likely caused by parentless 4He hosted by mineral or fluid U and/or Th -bearing micro -inclusions, which is the most common reason for irrep roducible ages [183, 190, 201] Small, acicular mineral inclusions were found in some resin -mounted transparent crystals from sample SR 11 when analyzed under high magnification. Spontaneous fiss ion track distributions in other apatite grains emission corrections, which

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115 assume homogenous U and Th distributions [190] can be considered adequate for the population of grains analyzed. AHe ages in the La Garca vertical transect increase systematically with elevation from 26.6 1.3 Ma at the bottom of the scarp to 46.7 2.4 Ma at the level of the plateau. Almost identical results were obtained in the Matasanos transect, with samples ranging in age from 22.81.1 Ma to 48.92.4 Ma (Figure 3 5 and Table 3 1). Variations in AHe ages with elevation in this study are positively correlated and resemble theoretical He retention vs. depth curves [211] and age -elevation relationships obtained for other He vertical -profile studies [89, 201] Positive correlation in our age -elevation profiles, similarity in slope between previous a patite fission track data [166] and modern topography of the AP suggest that relief changes have been negligible s ince Eocene time, therefore, topographic corrections by the admittance ratio [89, 364] to our data are not required. Further, the combination of U and Th concentrations in our apatite grains (average 19 ppm) and previous apatite fission track ages reported for the same geologic province (2645 Ma, Saenz, 2003) fall within the figures recently proposed as the range over which excessive He retention due to radiation damage, and concomitant increases in AHe ages should be negligible [192] Even though these are general guidelines, they enhance our confidence that the He kinetics we used in interpreting our data are a reasonable assumption, so that non e of our AHe apparent ages are potential overestimates of the time of cooling through the He partial retention zone. The age -elevation relationship for all data is characterized by three segments (Figure 3 5). Segment 1 between 760 and 1600 m displays AHe ages from ~22 to ~26

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116 Ma, exhibits very low data dispersion (R2=0.98), and has a slope of 0.24 km/Ma. Segment 2 at sample elevations between 1500 and 2000 m shows a broader range of AHe ages from 25 42 Ma (R2<0.2) and a lower slope than the other two segme nts (0.02 km/Ma). Segment 3 at elevations >2000 m is characterized by apparent ages from ~41 to 49 Ma, and data scatter higher than in segment 1 but lower than in segment 2 (R2<0.5) and a slope of 0.07 mm/yr. Discussion and Interpretation Our AHe results c omplement apatite and zircon fission track thermochronology from the Antioqueo Batholith and associated plutons providing additional detail to constrain the exhumation history of the region during Cenozoic time. The AHe data reveal a marked cooling event at ca. 23 Ma (Figure 3 5). We interpret the almost invariant He ages of segment 1 as representing a rapid exhumation pulse starting at ~25 Ma. Segment 2 of the age -elevation profile is interpreted to be part of the lower segment of the Oligo-Miocene He -PRZ and may represent a period of tectonic quiescence lasting ~17 million years. The change in slope at ~41 Ma (~2000 m) is not as well defined as the one at 25 Ma (1500 m). It is probably indicative of an exhumation event that was less significant than the O ligocene epis ode. Apatite fission -track data [166] although not derived from vertical profiles, provide additional insight to evaluate the AHe change in slope between segments 2 and 3 at ca. 41 Ma. All of the apatite fission -track apparent ages are equal or older than AHe ages with a mean age of ca. 46 Ma (Figure 3 6). The samples show unimodal fissiontrack length po in Eocene time [166] This finding corroborates our interpretation that the segment 3 of the age -elevation plot records an Eocene exhumation event. The average AHe age for

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117 segment 3 (~41 Ma) is a minimum age for exhumation as these samples resided in the lower temperature part of the He PRZ when exhumation slowed. Nearly concordant AHe and apatite fission -track ages over crustal section of ~ 1500 m (Figure 3 6) suggest that rocks were exhumed from temperatures above the apatite fission track partial annealing zone (>110C) to below most of the AH e PRZ. This suggests that rocks within this zone cooled ca. 50C in 5 million years, which, at a gradient of 25 C, implies unroofing on the order of 2 km, i.e., ~0.4 mm/yr. Basal conglomerates in the Magdalena and Cauca basins deposited at this time also suggest significant erosion of the AP during the Eocene (e. g., Gmez, 2005; Grosse, 1926). The AHe results are consistent with interpretations that kilometer -scale uplift and exhumation in the Northern Andes, as well as in other portions of the Andes, occurred in discrete pulses in the Cenozoic [14, 16, 40, 366] These exhumation events occurred d uring or just after rapid rates of convergence between the Farallon and South America (~150 mm/yr) documented for the Middle Eocene and the Late Oligocene [18] suggesting a relationship between o rogeny and subduction dynamics. The break in slope between segments 1 and 2 of the age -elevation plot (Figure 3 5) defines the bottom of He -PRZ established towards the end of the qui escence period between ca. 45 and 25 Ma when erosional processes lead to the development of the protoAP. This pre -early Miocene He -PRZ was exhumed at ca. 25 23 Ma. The base of this fossil He -PRZ (~80C paleo -isotherm) is at a modern depth relative to the present AP surface of ~1000 m. Before exhumation rates increased at ca. 25 Ma, the break in slope was at a depth of about 2.6 Km, assuming a geothermal gradient of 25 C/km and a mean surface temperature of 15 C. This implies rock uplift of approximately 1 km (Figure 3 -

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118 7). Based on the assumed paleogeothermal gradient and the temperature difference across the He -PRZ (~ 40 C), the top of the pre Miocene He-PRZ is probably at ~3100 m, which is the elevation average in the Las Baldas Range on top of the AP (Figure 3 3). If our assumptions for the geothermal gradient and pre -existing topography are correct, only about 9000 m of plateau have been removed by erosion in the last 25 Ma, yielding a low erosion rate for an active orogen (~0.04 km/Ma). Erosion in th e summits of the Las Baldas Range (e.g., Pramo de Belmira) must have been very limited, i.e., less than 200 m in the last 25 million years (~0.008 km/Ma) and such surfaces probably represent a position closer to a true relict of the original plateau. O ur interpretation implies very contrasting efficiencies in erosion on the granitic units of the Antioqueo Batholith (~0.04 km/Ma) relative to the metamorphic units in Las Baldas Range (0.008 km/Ma) where less than 200 m of crustal material (and in some cases <100 m) have been removed since Late Oligocene. The spatially restricted remnants of Early Cretaceous marine sedimentary strata present in the area are portions of the overburden removed by erosion during the development of the present AP since 25 Ma as well as during previous exhumation pulses in the Eocene and Paleocene. The low values for background erosion rates derived from our study are in agreement with denudation rates for subdued relief and soil mantled regions suggested by other authors (Ahn ert, 1970; Summerfield, 1991) supporting the idea that the AP is a relict surface that has not adjusted to modern erosional conditions of the orogen. Such rates are in clear contrast relative to proposed figures of modern erosion in the region (Giraldo, 2005; Restrepo and Syvitski, 2006). The discrepancy may be explained by the intensification of erosional process due to

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119 anthropogenic influence in the Magdalena basin, a problem not addressed in detail in this investigation, but that certainly warrants furth er consideration. Chronology and intensity of exhumation pulses of the AP documented in this study coincide remarkably well with the timing and relative magnitude of the orogenic phases Proto Andina (24 Ma) and Pre -Andina II (43 Ma) proposed for the Northern Andes of Colombia by Van Der Hammen (1960) based on a stratigraphic and paleobotanical study. Further, these phases of erosion occurred at the same time as the discrete orogenic events in Peru known as the Incaic in the Eocene, and The Quechua 1 in the Early Miocene [36] The Late Oligocene-early Mioene exhumation event recorded in our study is consisten t with documented erosional phases for the Peruvian Andes and the Bolivian Eastern Cordillera, where Gregory Wodzicki (2000) reported an erosive phase at ca. 22 Ma that removed ~2 km of overburden. For the Eastern Cordillera of the Andes of central Peru ap atite fission track data record two exhumation pulses during the Neogene at ca. 21 Ma and between 12 Ma and the Present, which removed about ~ 4 to 6 km of overburden in the past 30 million years [35] Similarly, an exhumation pulse at ~40 Ma was reported by Benjamin (1987) in his s tudy of fissiontrack thermochronology for the Bolivian Andes, whereas analogous timing for Eocene cooling events associated with erosion have been indicated for the Sierras Pampeanas in Argentina [32] and northern Bolivia [30] Rapid exhumation events in mountain ranges in the Caribbean region [37] have also been recognized for the Late Eocene and Mid to Late Oligocene. This evidence suggests that Cenozoic exhumation pulses in the Northern Andes and t he Caribbean are controlled by continental -scale tectonics. The trigger of these accelerated pulses of erosion may be related to well documented changes in convergence rates between the Nazca (Farallon)

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120 and the South American plates [18, 19, 315] which are in turn related to major reorganization of the plates such as the one caused by the break up of the Farallon Plate into Nazca and Cocos plates. The period of relative t ectonic quiescence between the two exhumation pulses provides enough time for the development of an erosional surface on which the Terciario Carbonifero de Antioquia of Oligocene -Miocene age was deposited [168] Our results are also in agreement with the tectonostratigraphic development of sedimentary sequences in the Magdalena Inter Andean depressions [14, 29] corroborating that the Cordillera Central was an elevated massif that functioned as a source of sediment to major sedimentary basins during the Paleogene and Neogene. Late Paleogene Neogene sedimentary sequences found in the AP (Sanchez and Parra, personal communication) imply that fluvial activity and concomitant obliteration of the original erosional surface in the AP have been in operation throughout the Neogene. AHe data in this study do not record post -Middle Miocene exhumation events, i.e., the Eu -Andina orogenic phase of Van der Hammen (1960), which has been invoked as the phase responsible for the modern configuration of the Andean range and which has also been recorded in many massifs throughout the Andean range. The fact that the Eu Andina phase is not verified by AHe (this study) and previous fission track data [166] constitutes compelling evidence that the AP is in geomorphic disequilibrium. The modern expression of the AP developed through a long history (Figure 3 8) of multiple phases of erosional surface formation at low elevation ( peneplanation) beginning with its emergence in the Santonian Early Masstrichtian [16] The Eocene and Oligocene -Miocene exhumation events here reported w ere initiated after kilometer -scale

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121 orogenic uplift accompanied by fluvial incision and erosion that removed part of the cover of the Antioqueo Batholith including the majority of the Cretaceous marine formations. Although there seems to be an agreement i n regards to the timing of the major phases of surface uplift -denudation for the Andes, paleoelevation of the region remains for the most part undocumented. Gregory -Widowsky (2000) suggested that the Colombian Andes was at ~40% of its modern elevation by 4 Ma. Similar results, e.g., uplift of the Colombian Andes concentrated in the last 10 Ma and intensified in the Pliocene, were obtained by Van der Hammen et al., (1973) and by Hooghiemstra (2006) from studies of pollen and macrofossil vegetation assemblage s from the Cordillera Oriental. We suggest that the age of the most recent peneplain in the AP is Miocene. During the Pre -Andina (Eocene) and Proto -Andina (Oligo Miocene) phases the area experienced kilometer -scale uplift followed by erosion surface development (relief obliteration) during intervals of tectonic quiescence. The actual elevation of the AP was attained with the onset of the Eu -Andina phase, particularly the sub phases II through IV (12 Ma to present) when the erosion surface raised from close to sea level (e.g., 500 m) to ca. 3600 m. However, reliable paleoelevation data for the Central Cordillera is scant so the question about the progression of elevation within the context of orogenic pulses and uplift phases in the region have only been addr essed indirectly, particularly in relation to the evolution of depocenters in the Magdalena Basin and its evolution from a foreland basin to an Interandean basin during the Cenozoic [14, 29] Deep incision in the AP along the MedellnPorce and R o Grande rivers is probably a recent feature developed during surface uplift in the Pliocene. Modern

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122 geomorphic expression of the AP is the result of recent fluvial processes controlled by nick point propagation. The closest point to a relict Mesozoic surf ace may be found at the summits of Pramo de Belmira, although glacial erosion during Paleocene -Holocene glacial periods may have removed some of it (~100 m). The saprolitic cover of the AP is absent above 3300 m elevation where the landscape is dominated by remnants of glacial l andforms and truncated summits. The similarity in the age versus elevation plots for both profiles (Figure 3 5) is interpreted as indicating exhumation of the entire AP as a discrete unit. Our results support the hypothesis that the Northern Cordillera Central is part of a structurally coherent crustal block within the orogen [22] The rigidity of the Northern Cordillera Central block, where the AP is located, may have controlled the nearly circular outcrop pattern of the Antioqueo batholith. Cross -sections presented by Cortes and Angelier (2005) show a major structural zone at ~7N/75 77W with high angle, reverse faults flanking the AP (Palestina and Romeral fault systems in the east and west respectively). These structures are inherited from the basement and bound the AP block. Rigidity of the block and bounding faults may have controlled the structural continuity of the area as well as the mode of emplacement and overall shape of the Antioqueo batholith, thus imposing litho -st ructural control in the geomorphic evolution of the AP. Reactivation of these major crustal structures during periods of enhanced convergence may have facilitated uplift of the AP as a coherent block. The Cenozoic uplift and exhumation history of the north ern Cordillera Central of Colombia resulted from plate tectonic reorganization and directly affected the morphotectonic evolution of the AP as well as the development of the sub-Andean fold -

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123 and thrust belt and the fill and tectonostratigraphic evolution of the intramontane and foreland basins to the east (Magdalena, Llanos Basin) and west (Cauca). This is particularly clear at the end of the Oligocene when the Farallon plate broke up into the Nazca and Cocos plates [18, 42] Augmented convergence rates induced synchronous tectonic and geomorphic activity on the overriding plate (i.e., uplift, exhumation, deformation, shortening, etc.) from Argentina to Colombia. A similar effect was also observed during an earlier phase of rapid convergence in the middle Eocene. Additional control to the more rece nt morphotectonic history in the area is related to the west-to -east motion of the Caribbean plate and the collision of the Panama Choc block at ~10 Ma, which closed the Panama isthmus between 3.7 and 3.4 Ma [24] triggering the most recent orogenic phase in the region and giving the Colombian Andes its present topogr aphic configuration [156] The main phases of uplift of the Northern Andes led to important paleogeographic events [43] and to erosion of a large volume of sediments resulting in thick basal sandstones and molasse successions in the sub -Andean basins [14, 16] Age control for two of these major periods is accurately provided by our AHe data fitting well the spatiotemporal framework for the evolution of the Colombian Andes in the context of continental tectonics. Conclusions AHe thermochron ometry of samples collected from two elevation profiles spanning ~2 km of exhumed crustal sections reveal the long-term erosional exhumation of the AP. Sampled profiles exhibit similar behavior in the age elevation relationship. AHe ages increase with elev ation from ca. 22 Ma at the bottom of regional scarps to ca. 49 Ma on top of the AP. A marked inflection point in age versus elevation data at ca. 25 Ma defines the bottom of the exhumed post Oligocene He -PRZ.

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124 Elevation invariant ages below ca. 25 Ma recor d the onset of rapid exhumation and surface uplift of the AP that led to river incision. A subtle change in slope within the He PRZ, ca. 41 Ma, corresponds to a less intense, exhumation related cooling episode. Previous AFT data in the AP, although limited and not derived for samples over the same profiles, allows to further define this Eocene exhumation pulse. Exhumation pulses identified coincide remarkably well with climax of the Proto -Andina (~23 Ma) and Pre Andina (`43Ma) orogenic phases previously pro posed for the Colombian Andes, and are synchronous with tectonically driven exhumation events reported for the Peruvian, Bolivian and Argentinean Andes, and for orogenic systems in the Caribbean. These pulses are correlated with variations in the rates of convergence between Nazca (Farallon) and South America previously documented for the Middle Eocene and the Late Oligocene suggesting continental -scale controls on uplift and denudation throughout the Andean range. During the Pre -Andina and Proto -Andina pha ses the area experienced kilometer -scale uplift followed by erosion surface development (relief obliteration) during intervals of tectonic quiescence. AHe results are in agreement with tectonostratigraphic data in the Magdalena and Cauca basins and with pr oposed scenarios for paleogeographic evolution in the Northern Andes during the Eocene and OligoMiocene. Similarity between AHe profiles indicates the whole AP was uplifted and exhumed as a coherent structural block, corroborating previous evidence for the rigidity and coherence of this crustal domain in the Northern Andes. Strength of this block in the Northern Central Cordillera and bounding faults may have controlled the structural continuity of the area as well as the geometry of emplacement of the Ant ioqueo

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125 batholith imposing litho -structural control in the geomorphic evolution of the AP. Reactivation of these major crustal structures during periods of enhanced convergence probably facilitated uplift of the AP as a coherent block. AHe data indicate th at the age of the most recent peneplain in the AP is Miocene. The actual elevation of the AP was attained with the onset of the Eu -Andina phase, particularly the sub-phases II through IV (12 Ma to present) when the erosion surface raised from close to sea level (e.g., 500 m) to ca. 3600 m. Nevertheless, paleoelevation data for the Central Cordillera is virtually inexistent so that the issues of the evolution of elevation within the context of orogenic pulses and uplift phases in the region are still unresol ved. Pulses of exhumation are separated by interludes of tectonic quiescence of durations in excess of 15 million years, spans during which low -standing erosional surfaces can develop. Modern configuration of the exhumed PRZ implies that only ~ 9000 m of p lateau have been eroded in the last 25 Ma, i.e., ~0.040.02 km/Ma, which is a low erosion rate for an active orogen. Erosion in the summits of the Las Baldas Range is even lower, i.e., ~0.008 km/Ma. This implies very contrasting efficiencies in erosion on the granitic units of the Antioqueo Batholith relative to the metamorphic units in Las Baldas Range. Such low values for background erosion rates are in agreement with denudation rates for subdued relief and soil mantled regions a finding that support ing the idea that the AP is a relict surface that has not adjusted to modern erosional conditions of the orogen. Such rates are in clear contrast relative to proposed figures of modern erosion in the region, i.e., 1 mm/yr. The discrepancy may be explained by the intensification of erosional process due to anthropogenic influence in the Magdalena basin. A more thorough

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126 comparison between modern (anthropogenic) and long -term (natural erosion rates constitutes an important line of inquiry not addressed in deta il in this investigation, but that certainly deserves more consideration

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127 Table 3 1. Summary of (U Th)/He results for Matasanos and La Garca vertical transects. The row marked as A/G indicates the number of aliquots (A) and the number of grains (G) per aliquot analyzed for each sample, for instance, in sample SR 9 a total of 3 aliquots were analyzed and each aliquot was prepared using two to three grains. Average mass of aliquots run for each sample is indicated in column Mass (mg). 147Sm concentration s (ppm) in all samples analyzed were virtually 0 or below the detection limits. Sample Age Std A Mass U Th U/Th He Ft RawAge Radius Elev. LatN/LonW number (Ma) (Ma) Dev G (ppm) (ppm) (ncc/mg) (Ma) (um) (m) Transect SR 9 43.6 2.2 1.6 3/3 2 2.8 17.5 9.5 1.8 76.5 0.70 30.5 52.1 2370 6.46/75.37 M SR 11 43.4 2.2 2.5 3/2 1 2.8 13.5 7.7 1.8 54.0 0.70 30.4 53.1 2230 6.46/75.37 M SR 15 48.9 2.4 3.9 5/2 4.6 24.8 3.4 7.3 112.3 0.73 35.7 57.6 2100 6.45/75.37 M SR 19 40.7 2.0 2.8 5/3 2 4.0 17.7 8.0 2.2 54.0 0.73 29.7 57.5 1990 6.45/75.36 M SR 6 33.7 1.7 2.5 3/3 9.4 20.3 29.5 0.7 82.3 0.72 24.3 56.0 1710 6.43/75.37 M SR 2 36.6 1.8 0.8 3/2 4.0 8.4 10.8 0.8 34.5 0.70 25.6 51.7 1520 6.47/75.38 M SR 40 31.4 1.6 2.5 4/2 3.6 44.4 55.4 0.8 156.7 0. 71 22.3 53.0 1500 6.41/74.44 M SR 41 25.1 1.3 3.5 3/2 3.7 42.5 56.7 0.7 115.6 0.70 17.6 52.0 1380 6.41/75.41 M SR CC3 24.2 1.2 2.6 3/2 8.0 18.7 39.7 0.5 63.4 0.80 19.4 79.0 1070 6.76/75.12 M SR CC2 23.9 1.2 2.8 3/2 3.1 29.0 18.2 1.6 67.0 0.70 16.7 51.5 1000 6.80/75.14 M SR CC1 22.8 1.1 1.5 3/2 3.6 19.0 15.0 1.3 39.7 0.70 16.0 52.4 760 6.86/75.18 M SR 26 46.7 2.3 3.4 3/3 2 5.7 14.1 15.3 0.9 76.1 0.75 35.0 62.5 2350 6.38/75.59 G SR 31 42.9 2.1 1.1 3/3 5.6 14.9 15.1 1.0 68.2 0.71 30.5 54.0 21 70 6.37/75.59 G SR 32 41.3 2.1 0.5 3/2 4.3 13.1 13.9 0.9 57.6 0.70 28.9 51.8 2110 6.38/75.60 G SR 45 45.7 2.3 1.5 4/2 4.1 8.3 9.7 0.9 42.0 0.71 32.4 53.5 2015 6.36/75.59 G SR 46 40.8 2.0 2.7 3/3 9.2 10.7 11.3 0.9 49.2 0.74 30.2 60.0 1900 6.36/75.5 9 G SR 48 32.2 1.6 1.8 4/3 1 4.6 14.7 15.3 1.0 49.1 0.70 22.5 51.8 1710 6.35/75.58 G SR 44 26.6 1.3 3.8 3/2 3.5 16.7 15.9 1.1 45.3 0.69 18.4 50.6 1640 6.34/75.58 G

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128 Fig ure 3 1. Shaded relief map of the Northern Andes, Colombia. The Andean mou ntain chain in Colombia is divided into three separate ranges: Cordillera Occidental (Western Range), Cordillera Central (Central Range), and Cordillera Oriental (Eastern Range). The Antioqueo Plateau in the northern Central Cordillera is part of the Anti oqueo Eastern Massif, which is bounded by major fault zones R=Cauca Romeral fault system to the west and P=Palestina fault system to the east. The Guaicramo (G) and Bocon (B) faults define the limit between the Andean Block and the autochthonous South A merican Plate (craton).

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129 Figure 3 2. Geology of the study site. Area dominated by the polymetamorphic Cajamarca Complex (Pz), small patches of marine sedimentary sequences early Cretaceous in age (Kisp, Ksls, Kissl) and Late Cretaceous calk alkaline i ntrusion of the Antioqueo Batholith and associated stocks (Ksta, dotted grey). Other units include: Q=Quaternary fluvial and alluvial deposits; Ngc+gas=Neogene sediments from Combia Formation (volcaniclastic continental) and Amag Formation (fluvial); Ngr o=Mesa Formation (detrital, continental). Ks vb=Volcanic Barroso Formation (volcanic, marine) and Ksu=Urrao Formation (clastic -chemical, marine) belonging to =metamorphic rocks of Neoproterozoic age. The Cauca Almaguer Faul t (paleosuture) represents the boundary between the Eastern Continental Province (Central Cordillera) and the Western Oceanic Province (Western Cordillera). (Geologic information was adapted from Gonzlez, 2001; Tapias et al., 2006).

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130 Figure 3 3. Three d imensional representation and simplified topographic/ geologic cross section across the Antioqueo Plateau. The flat portion of the plateau has an average elevation of ~2500 m. MP=Medelln Porce fluvial valley, CG=Cauca Gorge (CG). The CG coincides with the Cauca Romeral (Cauca -Alm aguer) fault system Sampled profiles indicated with black and open small circles. Pramo de Belmira within Las Baldas Range, ~3600mof elevation. (Shaded relief DEM was generated from GTOPO30 elevation data. Geologic information was obtained from Gonzlez, 2001 and Tapias et al., 2006.).

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131 Fig ure 3 4 Modern tectonic setting. Generalized map of tectonic plates and crustal blocks around the Northern Andes of Colombia. Arrows and numbers indicate, respectively, the direction and velocity of pla tes motion. (Adapted from Corte s and Angelier, 2005; Coates et al., 2004; Taboada 2002; Duque Caro, 1990; and Trenkamp et al., 2002).

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132 Figure 3 5. Apatite (U Th)/He Age elevation Relationship Relation between apparent apatite (U Th)/He ages and elevation integrated for La Garca (black squares) and Matasanos He PRZ marked by inflection point at ~1500 m. Upper boundary of the He -PRZ appears to be beyond t he maximum elevation sampled. Slopes and R2 values of the regression lines by segments appear in front of the segments symbol (S1, S2, and S3) for the two exhumation pulses discussed in the text. Block arrows in gray indicate the duration of the Pre -Andina II and Proto -Andina orogenic phases of Van der Hammen (1960).

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133 Fig ure 3 6. Re lation between apparent (U Th)/He and fission -track ages in apatite. Error bars are -Andina Phases I and II (Van der Hammen, 1960 ). (AFT ages from Saenz, 2003).

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134 Fig ure 3 7. Simplified sketch illustrating the extent of the crustal section removed since c a. 25 Ma (dotted area). Left panel represents crustal conditions at the onset of major exhumation 25 million years ago. Right panel is the actual condition with remnants of a Miocene surface that has begun to be obliterated by uplift related denudation dur ing the most recent Andean phase (Eu -Andina II IV, 12 MA to present). Deep and localized incision of the Medelln and Porce rivers is shown. Rock uplift discussed in the text is represented by the displacement of point A to A. Bottom of He -PRZ represents the paleodepth at which A He ages were zero before onset of rapid exhumation at 25 Ma. PB=Pramo de Belmira, AP=Antioqueo Plateau.

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135 Fig ure 3 8. Simplified cartoon of cross -sections depicting the paleogeographic and geologic evolution of the Northern Andes at the latitude of the study site. (a) HauterivianConiacian: the ancestral Central Cordillera was covered by a shallow sea and marine

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136 sedimentary units were being deposited. The ProtoWestern Cordillera was part of the sub ducting plate separated from the Central Cordillera by the ancestral Romeral Fault System. (b) Santonian -Maastrichtian: Increased convergence rates lead to the magmatism of the Antioqueo Batholith, a shallow (epizonal) intrusion emplaced into PaleozoicPr ecambrian crystalline basement and lower Cretaceous marine sequences (contact metamorphism). Initial phase of uplift in the region. Central Cordillera emerges leading to a regional unconformity and becomes source of sediment to eastern and western marginal basins (Proto Magdalena and Proto Cauca basins respectively). Progradation to the east begins in the eastern margin of the Central Cordillera. Subsidence and sedimentation are prevalent over the region of the Proto Eastern Cordillera. (c) Paleocene, Laram ic Phase, 60 Ma: Generalized coastal deposition took place, inversion of the eastern margin of the Eastern Cordillera basin is now pronounced. Central Cordillera continued to be gradually uplifted. Initial phases of uplift of the Western Cordillera begin, the Tahami terrain is now accreted. Reactivation of vertical movement along the Cauca Romeral and Palestina fault systems. (d) Lower to Middle Eocene, Pre -Andina Phases I -II, 54 45 Ma: climax of pre -Andean Orogeny in Colombia. No deposition taking place in most of Colombia. Exposure of the future Middle Magdalena Valley as depocenter. (e) Oligo-Miocene transition, Proto -Andina Phase, 24 21 Ma: Major phase of uplift across the Colombian Andes, uplift on the three cordilleras. Amag formation begins to devel op in the Cauca depression. Western and Eastern cordilleras still low standing. Localized subsidence in the Cauca and Magdalena depocenters. Nazca plate subduction begins after splitting of the Farallon plate. (f) Miocene to Present, Eu Andina Phases I -IV, 180 Ma: Major uplift and initial inversion of the Eastern Cordillera by reactivation of inherited Jurassic Cretaceous steep and deeply rooted normal faults. Amag formation is folded and the Cauca valley is narrowed by indentation of the Panama Choc blo ck, Colombian Andes attain the actual topographic configuration. All of overburden over the Antioqueo Batholith on the AP has been removed.

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137 Figure 3 8 Continued.

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138 CHAPTER 4 FURTHER CONSTRAINTS ON LONG TERM EROSIONAL EXHUM ATI ON OF THE ALTIPLANO ANTIOQUE O, NORTHERN ANDES, COLOMBIA, BY LA ICP MS APATITE FISSION TRAC K ANALYSIS Introduction Meso Cenozoic geologic evolution of the Andean chain appears to be strongly determined by subduction dynamics between the South American and Nazca (Farallon) plates [18 20, 36, 40, 47, 315] In the northern Andes (Colombia, Ecuador and Venezuela), the additional influence of the Caribbean plate, exotic terrains, and major lithospheric blocks caused differences in deformation, uplift and exhumation relative to areas of the Andes to the south [25, 26, 113, 164, 356, 367] Effort has been devoted to understanding the timing, magnitude, spatial distribution, and nature of uplift and erosional denudation in the context of orogenic development in the Andean Range [16, 3032, 35, 36, 38, 39, 113, 349, 368, 369] The majority o f studies, however, were concentrated in the Central Andes of Peru Bolivia, with little attention given to the Northern Andes. Geochronology, low temperature thermochronology and isotopic studies have been completed in the Central Andes, particularly aroun d the Central Andean Plateau [31, 38, 216, 370, 371] but few investigations have been carried out in the Northern Andes of Colombia. Tectonic forcing is considered to have been the driver of orogenesis and concomitant erosion and exhumation pulses in the Andes throughout the Cenozoic [16, 36, 39, 346] It has also been proposed that climate dom inated over tectonic uplift as the controlling mechanism for continental erosion in tropical South America since the Middle Miocene [347] and worldwide, particularly during the past 5 10 million years [51, 52] Nevertheless the chronology of Cenozoic denudation for sectors of the Northern Andes is poor. This precludes an understanding of the relative roles of tectonic and climate in modulating erosion dynamics and exhumation in this important region. Recent apatite (U Th)/H e data from the Antioqueo Plateau (AP)

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139 demonstrate the utility of highelevation erosional surfaces for studying interactions between continental -scale tectonics and erosion [294] particularly the synchronous char acter of well documented increased convergence between Nazca (Farallon) and South American plates in the Oligocene and a marked pulse of exhumation in the AP ~2523 Ma. Erosion is a primary mechanism of exhumation, and hence cooling, of deeply buried crust al material [90] Spatial patterns of bedrock cooling ages in mountai n ranges can be used to map variations in erosion rates and potential relationships with geomorphic or structural features, tectonic input, climatic gradients, and/or kinematic models for material flow through orogenic wedges [54] Low temperature thermochronology is an effective tool to quantify rock uplift and exhumation [54, 171, 183185] With temperature sensitivity between ~110 and 60 fission track analysis (AFTA) enables constraint of the movement of a rock parcel relative to a shallow thermal reference frame (upper 4 km of crust). AFTA, which involves dating and examination of fission track length distributions, can constrain the timing and intensity of erosional episodes at the regional scale and on timescales of ~105107 years [86, 91, 171, 172, 174, 177, 184, 187, 188] This spatio temporal domain is germane to unders tanding key aspects of orogenic evolution (e.g., mountain range growth and decay cycles, erosional surface development and uplift, paleorelief, etc.) and tectonics -climate/tectonics -exhumation feedback response times [54, 81] In this investigation, apatite fission track data from vertical profiles in the AP are used to further constrain patterns of longterm erosional e xhumation. The objective is to improve the chronology and understand the magnitude of exhumation in the AP during the Cenozoic, with emphasis on a potential exhumation pulse incompletely pinpointed by previous AHe data [294] Integration of highresolution AFT and AHe data is applied to constraining landscape evolution

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140 of the AP by documenting periods of significant crustal thermal change due to denudational cooling, and intervals during which the landscape remained relatively stable both geomorphically and tectonically, thus allowing planation surfaces to develop. Analysis of two vertical profiles permits identification of important relationships between changes in convergence rates and patterns of erosional exhumati on in the Colombian Cordillera Central. It also provides quantitative data on long -term erosion rates for the region during periods of both enhanced tectonic activity and relative quiescence. Study Site The study was undertaken in the northern segment of the Cordillera Central, Colombia, a portion of the orogen that encompasses an anomalously large, high -elevation erosional surface, i.e., the AP [12] The distribution and extent of relict landscapes can shed light on the environments in which they formed, making them an integral component of stratigraphic and morphotectonic reconstructions [83, 169, 348, 350, 372374] Quantifying erosional and kinematic evolution of orogenic plateaus, however, has been limited by insufficient age constraints on their erosional and deformational histories. Furthermore, most investigations have focused on large plateaus such as the Himalayas and the Bolivian Altiplano [216, 349, 370, 375, 376] Nearly all mountain ranges in active tectonic settings, e.g., the Andes, are characterized by significa nt relief, rugged topography, and high erosion rates [62, 98, 108110] making them a focus of scientific study [82] Throughout the Colombian Andes, large amounts of detrital material in modern fl uvial systems such as the Magdalena River [121] along with the thick, continental Cenoz oic sedimentary formations [14, 16, 168: Reyes, 2002 #60] indicate that this mountain belt has been subject to intense erosional exhumation by fluvial activity. Orogenic response to tectonic and/or climatic pe rturbations via erosional exhumation often displays spatial

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141 and temporal differences within a single orogen due to geomorphic, lithologic, structural, and/or climatic controls [38, 62, 99, 106] The steep morphology of a tectonically active mountain range can be abruptly interrupted by extensive, high-elevation erosional surfaces, i.e., elevated plateaus. Such features are usually interp reted as paleosurfaces (relict landscapes) and are believed to represent portions of an orogenic system that has not fully adjusted to present erosional conditions [377] thus producing marked gradients in the s patio temporal distribution of erosion/exhumation. Lagged geomorphic response may allow relict landscapes to become decoupled from modern tectonic conditions and therefore become passive markers of vertical displacement of the earths surface. Such surface s also preserve information about longterm erosion dynamics and adjustments of the geomorphic system to past tectonic forcing [83, 96] Morphotectonic and lithologic characteristics of the AP are exploited in this study to conduct the first high resolution AFTA along two separate vertical profiles in the Northern A ndes and to infer the geomorphic history of this portion of the Andes in response to tectonic input. Geomorphologic Considerations The AP is the largest high -elevation, erosional surface in the Northern Andes [12, 28] This plateau has a broad elevated surface (>5000 km2) of subdued topography (slopes <10; local relief <60 m) that is dominated by transport -limited fluvial erosion. The AP is in obvious contrast to the mountainous rouged topography typical of the tropical Ande s of the Occidental and Oriental cordilleras and southern segments of the Cordillera Central whose geomorphic evolution is landslide -dominated [28] Higher elevations and relatively steep relief within the AP domain are restricted to a small range in the western edge of the plateau roughly coinciding with a belt of Paleozoic metamorphic rocks (F igures 4 1 to 42). Most of the surface is mantled by thick horizons of deeply weathered granodiorite (gibbsite and kaolinite as final weathering products) with occasional, outstanding outcrops of intact bedrock in the form of tors, rock-

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142 castles, and insel bergs characteristic of etch topography [12] The plateau topography is abruptly interrupted in the west and northwest by the deep, mountainous relief of the Cauca River canyon, approximately coinciding with the Cauca Romeral fault system, with drops in elevation of ~25003000 m over a few km. To the east, the AP terminates as a series of surfaces th at step down into the wider Magdalena valley, towards the Palestina fault system. Southwards, the AP transitions into the typical sierra like topography of the southern Cordillera Central marked by volcanic summits [12, 378] (Figures 4 1 and 43). The Medell n -Porce River has incised the central portion of the AP creating a relatively deep canyon with differences in elevation of >1300 m from the plateau top to the valley bottom. Perched fluvial systems are common throughout the regional scarps that define the boundaries of the AP relative to the Medellin -Porce River. Structurally, the plateau belongs to a discrete crustal block, slightly tilted to the southeast [12, 28] with minor internal perturbation [22] and bounded by maj or, sub -vertical crustal structures associated with the Cauca -Romeral and Palestina fault systems (Figures 4 2 and 4 3) to the west and east, respectively [22, 23, 27] As a disequilibrium domain, the AP is clearly discernible from typical equilibrated regions of the Colombian Andes around it and displays geomorphic features that resemble a relict landscape, indicating that the region has not adjusted to new morphotectonic boundary conditions. Such features include subdued hill slope and channel gradients [12] dominance of transport -limited fluvial erosion [28] and relatively low long-term erosion rates, i.e., 0.02 0.008 mm/yr [294] Morphotectonic evolution of the AP and potential lithologic control by the Antioqueo Batholith have attracted the attention of geologists since the 19th century [12, 27, 296298, 378, 379] Following a classic Davisan approach [380] it was proposed that this relict surface developed near sea level and was subsequently uplifted to its present position [12, 28]

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143 The definition of relict surface in this study is restricted to low -relief portions of the landscape within the fluvial network. This differs from earlier works because it is assumed that relief on upland surfaces is set by fluvial ero sional processes responding to a common base level set by major trunk streams. No remnants of the primordial (possibly Paleocene) surface sensu stricto remain in the AP because the landscape was denuded during multiple Cenozoic exhumation events [294] Relatively little is known about the chronology of uplift and exhumation pulses, the monotonic or punctuated nature of such events, the non-equilibrium character of the surface, and the morphotectonic evolution of this area in the context of regional geodynamics. Previous results of AHe analysis in the region highlight the episodic nature of denudation and a strong response to tectonic input [294] AFT data presented here refine our knowledge of this decoupled geomorphic domain and constrain the Eocene exhumation pulse, incompletely resolved by AHe thermochronology [294] Geological Considerations The Colombian Andes represent the northern termination of the Andean belt. At ~ 1.5N latitude, the mountain chain flares out into three ranges: the Cordillera Occidental, Cordillera Central and Cordillera Oriental (i.e., western, central and eastern cordilleras). The Cauca River depression separat es the Occidental and Central cordilleras, while the Magdalena River valley marks the boundary between the Central and Oriental cordilleras (Figure 4 1). The present trident pattern of the Colombian Andes and its diverse lithologic and tectonic configurati on have resulted from a complex geologic evolution since the Proterozoic, marked by: 1) interaction of major tectonic plates, microplates, crustal blocks, allochthonous terrains, island arcs and ridges [278, 283, 316, 367, 381] ; 2) marine transgressions and regressions [289, 290, 382] ; 3) magmatic/volcanic pulses and tectometamorphic events [44, 47, 286] and 4)

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144 removal of crustal material via erosion and subsequent sedimentation in marine and continental environments [14, 17, 168] Two distinct geologic domains are separated by the Cauca -Romeral Fault system (Figure 4 2), which is a Lower Cretaceous suture zone [278, 284] between oceanic blocks and the autochthonous margin of South America [278, 281] The western province, comprising the Cordillera Occidental and lower western flank of the Cordillera Central, consists of accreted oceanic crust. The eastern province, which encompasses most of the Cordillera Central and the entire Cordillera Oriental, is characterized by continental basement [25, 281, 284] The Cordillera Occidental is composed of allochthonous oceanic sequences of basic volcanic rocks and marine sediments of Upper Cretaceous and Cenozoic age, intruded and covered by Cenozoic igneous rocks and volcanic sequences [284] The Cord illera Oriental is a fold and thrust belt of polydeformed Precambrian and Paleozoic metamorphic and igneous rocks overlain by thick Paleozoic to Mesozoic sedimentary sequences [25, 284] The Cordillera Central is dominated by PaleozoicProterozoic me tamorphic rocks intruded by major granitic plutons of Permian to Late Cretaceous age and overlain by Late Neogene to Quaternary volcanic rocks. Segments of the Cordillera Central south of the study site are characterized by modern volcanism related to the subduction of the Nazca plate representing a Mesozoic Cenozoic plutonic arc [21] (Figure 4 2). The northern segment of the Cordillera Central (Figure 4 1) is considered part of an exotic terrane [283, 383] affected by several tectono -metamorphic episodes related to the Herc ynian orogeny in PermoTriassic times, a Devonian Carboniferous tectono -metamorphic event, and the beginning of the Andean cycle in the Cretaceous [279, 286, 352] Predominant lithologic units within the study site include a Paleozoic -Mesozoic polymetamorphic complex, elongated Jurassic batholiths, small outcrops of sh allow marine sedimentary sequences of early Cretaceous

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145 age, and extensive Late Cretaceous plutons (Figure 4 2). The polymetamorphic complex [286] is comp osed of low to medium -grade metamorphic rocks of the Cajamarca Complex, and high grade metamorphic rocks of the El Retiro Group and Las Palmas Gneiss [21, 287] Isolated patches of possible Precambrian rocks wi thin the Paleozoic basement have also been mapped [290] The lower Cretaceous marine transgression that cove red the crystalline basement is represented by fossiliferous sedimentary sequences such as the San Luis and San Pablo [289291] Metamorphic and sedime ntary units are intruded by large, non-elongated, epizonal, calk alkaline intrusions, such as the Antioqueo and Ovejas batholiths [27] that are the target lithologies of this investigation (Figure 4 2). Cenozoic sedimentation in the Cauca and Magdalena basins is continental and strongly coupled to uplift -exhumation of the Cordillera Central since the Paleocene [14, 16] All sedimentary sequences underwent Miocene faulting and ample folding. The Cauca and Magdalena basins exhibit Quaternary deposits with variable thicknesses and degree of consolidation and with evidence of neotectonic activity [384] Late Neogene volcaniclastic sequences associated with modern volcanic activity in the Cordillera Central mantle the region [12, 292] Available geochronology for the Antioqueo Batholith displays a wi de range of ages, from about 20 to 100 Ma. Such variation is attributable to the variety of closure temperatures of the geochronometers employed. Data include biotite K/Ar ages which range from 683 Ma to 906 Ma [301, 354] biotite Rb/Sr ages from 56 to 66 Ma [304] ; two whole -rock Rb/Sr ages of 828 and 9827 Ma [305] ; zircon fission track dates from 49.12.5 to 67.12.1 Ma, fourteen apatite fission track ages varying between 49.11.2 and 28.71.5 Ma [166] and apatite Helium ages from 48.92.4 to 22.81.1 [294] The most widely accepted crysta llization age is ~98 Ma [305] However, recent zircon U -Pb thermochronometry better constrains the age of Late Cretaceous

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146 magmatism (LA -ICP -MS on 90 zircons from the Antioqueo and Ovejas batholiths), yielding crystallization ages of 743 Ma [355] Tectonic Considerations The Colombian Andes are situated at the jun ction of four major lithospheric plates: South America, Nazca, Caribbean and Cocos [25, 26] Cenozoic regional tectonics have been dominated by the subduction of two oceanic plates, Nazca (Farallon) and Caribbean, beneath continental South America, as well a s by interactions of two lithospheric blocks known as the North Andean and the Panama Choc (Figure 4 3) [23, 356] Significant changes in convergence between Nazca (Frallon) and South America have been reported, notably two increases in the Cenozoic: ~50 44 Ma when rates reached ~165 mm/yr at the equator, and ~2419 Ma with maximum convergence of ~110 mm/yr [18, 19, 42] Variations in the patterns of subduction, e .g., convergence rate and direction, dipping of the subducting slab, etc. have exerted important controls in the timing, magnitude and type of associated uplift, exhumation, magmatism and deformation on the overriding plate throughout the Andes [18, 19, 36, 44, 47, 314] The Nazca plate is currently being subducted beneath South America along the Colombia Ecuador trench at a rate of ~54 mm/yr, while Caribbean subduction takes place from the northwest at ~20 mm/yr, at a shallow angle, an d without an associated volcanic arc [23, 25, 26] The North Andean Block is escaping rigidly in a NE direction at ~6 mm/yr and the Panama Choc block is moving to the east relative to South America as a rigid indenter in active collision at a rate of ~25 mm/yr [26] The Panama Choc block has played a crucial role in the morphotectonic evolution of the region for the last 12 Ma, producing most of the present structural relief [24, 25] narrowing the northern portion of the Cauca Valley that led to the development of the deep Cauca canyon [384] and driving the Late Miocene -Pliocene orogenic phase in the Colombian Andes [16, 24, 156]

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147 These relative motions of plates and blocks generated a complex stress field and heterogeneous deformation in which crustal shortening takes place in a complex mosaic of tectonic blocks, each with its own uplift history [156, 160, 385] Transpressional deformation has caused a mixture of strike -slip and vertical displacements along major crustal structures throughout Cenozoic time. The former is responsible for differential uplift of discrete portions of the orogen, while the latter led to the development of pull apart basins, particularly along the Cauca -Romeral fault system [384, 386] These processes are central to the interpretation of thermochronology datasets in the AP because the co rdilleran segment of this study behaves as a discrete coherent block bounded by two major, inherited crustal structures: the Cauca Romeral fault system to the west and the Palestina Fault system to the east (Figures 4 2 and 43). These sub -vertical reverse fault systems are located along the foothills of the Cordillera Central and are thought to be rooted beneath the range [23, 25] The Cauca Romeral faul t system coincides with the Cauca River depression and combines strike -slip and reverse movement. The Palestina fault system marks the east boundary of the Cordillera Central. This portion of the Cordillera Central is asymmetric, with a much steeper wester n flank along the Cauca Inter -Andean canyon. Neogene transpression is left lateral north of 4N [23, 25] The western margin has been uplifted by transp ressive movement along faults dipping eastward that belong to the Cauca -Romeral fault system [25] Lithospheric strength of this block is a potential controlling factor on the low internal distortion and the circular (i.e., not elongated) shape of the Antioqueo Batholith [22] a situation that may have imposed litho-structural control in the development of the AP [355] Uplift and Denudation in the Colombian Andes It is generally accepted that the Cenozoic and Quaternary are associated with major geomorphic and tectonic events that have had a marked influence on the present topographic and paleogeographic configuration of the Northern Andes [16, 43, 113, 156, 158] Uplift and

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148 concomitant denudation throughout the Andean chain [31, 32, 36, 40, 111] seem to be spatiotemporally related to periods of accelerated convergence [18, 19, 294] However, morphotectonic evolution of the Colombian Andes is poorly constrained [113, 156] Most studies of uplift and erosional exhumation concentrated on the Cordillera Oriental and applied chiefly stratigraphy and palynology, with a minor contribution from thermochronology [13, 34, 157160] Pioneering sediment -stratigraphic studies aimed at unraveling Andean orogeny were done in the Cordillera Oriental between 19401960 [16, 161, 162] and led to re cognition of discrete pulses of erosion that affected the entire Colombian Andes from Paleocene to MioPliocene times [16] Periods of enhanced denudation were attributed to uplift and were named orogenetic phases Laramic (~60 57 Ma), Pre -Andina I II (~54 40 Ma), Proto-Andina (~25 22 Ma), and Eu -Andina I IV (~14 2 Ma) [16] The magnitude of these denudational events, their varying efficiencies in removing crustal material, and the spatial distribution of erosion and exhumation throughout the orogen remain largely unconstrained. Geological variations and structural complexity in the Colombian Andes should determine a heterogeneous uplift -exhumation history of discrete crustal blocks that can be uplifted and eroded at different rates and at different times. Diachronous uplift and exhumation are illustrate d by sediment -stratigraphic and pollen data that yield different timing of uplift -exhumation for the Eastern and Central Cordillera, with the former experiencing major phases of uplift in the last ~12 Ma [15, 158, 163] while the latter experienced denudation since the Late Cretaceous [16] Subsequent research in sedimentary formations of the Magdalena Valley confirmed this chronology [14, 17, 29] But spatially -specific information on the timing and magnitude of uplift driven exhumati on remains scarce.

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149 Investigations in the Cordillera Oriental between Colombia and Venezuela, despite limited thermochronology datasets, suggest uplift in excess of 3 km [164] and major cooling episodes by fission track data between 22 and 27 Ma [34, 160] More recent studies in the Cordillera Oriental incorporated apatite fission track, paleobotany, sediment -stratigraphy and other geological data, and also suggest increased exhumation rates in the northeastern Andes at ~5 3 Ma [2 95] and asymmetrical denudation of some segment of the range due to orographic effects [159] Major consideration has been given to the recent phase of uplift -exhumation in the Colombian Andes, i.e., the Eu -Andina panes of Van der Hammen (1960). The present topographic configuration of the Andes has been attributed to uplift -exhumation pulses during this interval [113, 156] Apatite fission track ages between Venezuela and Colombia cluster in two sets, ~19 14 Ma and 7 4 Ma, pointing to important exhumation events of the Eu -Andina phase [160] Similarly, results for rapid cooling with apatite fission track ages of 9 and 12 Ma and unimodal track-length distributions (mean track lengths of ~14.5 m) were reported for a small area of the Cordillera Oriental [163] while apatite fission track and vitrinite reflection data in the Middle Magdalena valley indicate km -scale uplift a nd ~3 km of erosion-related cooling at ca. 5 Ma [295] This Pliocene uplift has been documented in a core from the Sabana de Bogot, Cordillera Oriental at ~2500 masl by detailed stratigraphy, palynology, K/Ar and fission track dating [158, 165] It is now widely accepted that the final and major tectonic uplift of the Cordillera Oriental in the Bogot area occurred between ~5 and 3 Ma [15, 113] Data elucidating uplift and denudation patterns for the Cordillera Central are scarcer. Kro onenberg et al. (1990) suggested that onset of uplift for the Nevado del Ruiz area occurred 104 Ma, supporting their contention using local stratigraphic and geochronology data. For the northern part of the Cordillera Central, a set of restricted apatite fission track ages were

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150 interpreted as revealing an Eocene cooling event driven by exhumation [166] but the magnit ude of unroofing and duration of the event are undocumented. This AFT database is insufficient for a reliable denudation chronology because of the few samples dated, the low amount of confined natural fission tracks examined, the low number of samples mode led, and the poor spatial distribution of samples, which were not collected over vertical profiles. In the Cordillera Occidental, recent uplift phases are suggested by 11 -Ma dioritic stocks exposed at ~4000 m. Low Temperature Thermochronology and Erosional Exhumation During the last two decades, studies of erosion have shifted from a simple focus on local, short term surface processes to looking at a broader range of spatiotemporal scales [8, 9, 61, 62, 66, 69, 84, 86, 92, 109, 127, 133, 170180] Progress in assessing erosion at multiple scales is not only related to a change in philosophy [180 182] but also to the development of several low temperature thermochronometric methods, particularly apatite fission -track (AFT) and U Th/He (Ahe) dating [54, 171, 183185] Due to its low closure temperature range (~60110 thermochronology enables determination of rates of rock uplift and denudational exhumation from shallow crustal depths (upper 4 km) [54, 81, 86, 182, 196] and provides measures of long term erosion rates and landscape evolution directly from rock samples [86, 89, 171, 172, 183, 184, 188, 190, 195, 196, 200, 202, 203] AFT low temperature thermochronology can constrain the timing and intensity of erosional episodes at the regional scale and on timescales of ~105107 years [86, 91, 171, 172, 174, 177, 184, 187, 188] The technique can be used to understand tectonics -climate/tectonics -exhumation feedback response times [54, 81] and thus orogenic evolution. Apatite Fission Track Analysis Apatite fission track is a well established low temperature thermochronometer [197] Interest in this thermochronologic system stems from the fact that AFT is more than a dating

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151 technique [86, 171, 187, 195, 387, 388] Apatite fission track analysis (AFTA) can be applied not only to rock dating, but to the examination and mode ling of track length distributions, thus defining the thermal history of the samples greatly complementing the chronological constraints provided by AFT dating [86, 171, 187, 196, 197, 225] AFT thermochronology is based on the accumulation of ionization damage produced along the repulsive paths left by atomic fragments resulting from decay of 238U. Fission of 238U causes disruption of the mineral lattice, expressed as microscopic linear features (16 m long and 0.1 m wide) known as fission tracks [389] The nearly invisible tracks are revealed by polishing an internal crystal surface and acid -etching so that the tr acks can be counted and measured by optical microscopy. The number of tracks in an apatite grain is mainly a function of 238U concentration in the grain (ppm) and the time over which tracks have been retained, so that the number of tracks per unit area provides a measure of geologic age [171, 187, 196, 197, 389391] When exposed to temperatures >110C and for geological times of ~107 yr, the tracks in apatite grains are progressively repaired through a process of thermally activated annealing to the point that tracks disappear completely, reducing AFT ages to zero [196, 229 233, 388, 390, 392, 393] Annealing below 60 C is insignificant [230] Incomplete annealing of fission tracks occurs in the interval between 60 and 110C, a thermal horizon known as the partial annealing zone (PAZ). The PAZ was recognized in deep boreholes and along vertical profile s [210, 230] Ages across the PAZ vary with depth and increasing temperature. Since new tracks form continuously throughout geol ogic time, the AFT age and the distribution of track lengths in an apatite sample reflect the integrated thermal history of the host rock [171, 232] Thus, while fission track density provides a measure of the time elapsed since the crystal last cooled sufficient ly to retain

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152 structural damage, fission -track lengths record the temperature history that the crystal experienced over that time interval. Apatite Fission -Track Analysis in the Antioqueo Plateau A common approach to AFT analysis involves sampling vertical profiles such as steep valleys, cliffs, and regional scarps or exploratory boreholes [171, 172, 174, 188, 204206, 230] This allows exploitation of the spatial relation among samples in the vertical dimension of the crust, in which lower elevations yield samples that were hotter and younger than their higher elevation counterparts. Further, features such as the PAZ are often recognized in the ageelevation relationships of vertical profiles. When exhumed, the PAZ represents paleocrustal depths and permits to infer the timing of c ooling and the magnitude of unroofing from the shape of the age -elevation relationship [171, 172, 174, 184, 187, 201] The time of a cooling event is usually represen ted by an inflection point (break in slope) in the age -elevation plot. The gradient of segments in the age elevation profile constrains the rate of denudation, and potentially the amount of rock and surface uplift [91, 171, 172, 174, 190, 213] Further, the elevation of the inflection point is used to constrain the amount of denudation after cooling. Reliable vertical profiles for AFT are achievable where heat flow conditions and the geothermal gradient have been stable, where topography has not experienced major changes after samples cooled through the AFT closure temperature, and where erosion has no t been high enough to promote major heat advection into the upper crust [93, 360, 362, 364] In this i nvestigation, geomorphic, lithologic and structural characteristics of the AP were exploited to do AFTA along two vertical profiles. Data were utilized to complement the Cenozoic cooling history of the AP and to evaluate the response of this elevated plate au to orogenic pulses in the Northern Andes and Caribbean [16, 37] particularly during the Eocene [294] Characteristics of the AFT system, such as higher closure temperature and track -length

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153 distributions can be used in conjunction with AHe data to develop robust reconstructions of time temperature paths experienced by a rock parcel during erosional exhumation. The double AFTA profile strategy als o permits confirmation of the coherent response to uplift denudation of the AP as a structural lithospheric block. Finally, AFT data establish a long -term erosion rates into the Paleogene, against which modern erosion [1 50, 153] can be compared. Although this later issue is not covered in detail in this investigation, estimation of long term erosion rates is a first step in assessing the role of humans as geomorphic agents, a topic that has great significance scientifica lly and socially [6, 7] Methods Sample Collection and Preparation A total of 22 granodioritic samples, 2 3 kg each, were collected for AFT analysis. These included granodiorite and diorite specimens from the Antioque o and Ovejas batholiths retrieved at elevation in tervals of ~100 m along the La Garca and Matasanos Porce vertical profiles. These vertical transects are ~40 km apart (Figure 4 1). Sample locations (x, y, z coordinates) were determined with 1:25,000-scale topographic maps, a hand -held GPS unit, and a ba rometric altimeter. Accuracy was ~5 m horizontally and ~10 m verticaly. Weathering rims were removed with a rock saw to a depth of ~2 cm. Samples were reduced to sand -sized particles using a jaw crusher and pulverizer, and sieved to select particles between 300 and 60 m. Apatite concentrates were obtained using standard gravimetric (water table and heavy liquids) and magnetic (Frantz Isodynamic) mineral separation techniques. Apatite grains were enclosed in flat, square epoxy mounts (~3 x 15 x 15 mm) and cured at 45 to reveal internal apatite grain surfaces and polished. Etching to reveal tracks was accomplished by immersion in 5.5 N HNO3 at 21 C (1 C) for 20.0 s (0.5 s). Sample preparation was done at the University o f Florida.

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154 Age Measurements by LA -ICP -MS Although the external detector method (EDM) has been routinely applied to date apatite grains by fission tracks [171, 187, 236] a new approach, using LA -ICP -MS is employed in this investigation [189] In brief, samples were prepared in a similar fashion as for the EDM. However, 238U concentration was obtained by direct measurement of 238U and 43Ca by LA ICP MS on the surface where tracks for each individual grain were counted. It has been showed that 43Ca in apatite is stoichiometrically fixed, and the 238U/43Ca ratio, here referred to as P (Greek capital letter rho), can be considered as analogous to the ratio i/ d (i.e., induced track densities/track density in a dosimeter) from the EDM [189] The goal of LA -ICP -MS in AFT dating is to determine the 238U concentration from the ratio of 238U/43Ca in apatite grains and calculate cooling ages. The method of spot laser analysis for 238U and 43Ca employed here analogous to the one described in Donelick et al. (in review) is based on an ablation scheme with the laser beam centered at a fixed point on the area where the tracks were counted Details of data acquisition parameters used for LA ICP -MS apatite fission track dating are summarized i n Table 4 1. Spot analysis has advantages over EDM, including rapid set up, grainto grain uniformity from target grain to target grain, reliable estimates of the surface uranium concentration, derivation of potentially useful depth-dependent information regarding uranium concentration zoning, minimal damage to the polished and etched surface of target grains, and ability to work with small target grains [189] To obtain the best estimate of relative U concentration at the analyzed surface, a 3rd-order polynomial was fitted to the 238U/43Ca ratio versus analysis point at each spot using routine LFIT Pr ess [394] .The 238U/43Ca value is the y covariance matrix. This approach requires determination of an initial, primary zeta calibration

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155 factor of the desired precision during a single, extensive LA ICP MS session ( MS). For each subsequent LA ICP -MS session, a secondary zeta calibration factor, related directly to the primary factor, is determined. A total of 50 scans for 238U and 43Ca were used to acquire high precision data, while minimizing time spent at each target grain. A s can represents a single cycle of signal counting in the mass spectrometer to obtain counts for each studied isotope. Of these 50 scans, ~10 were run while the laser warmed and was blocked from hitting the target grain. During this interval, background coun ts were collected. Once the laser began to ablate a target grain, ~5 10 scans were required to attain full signal status. These scans represent the transition between background and full signal. For most grains, scans 21 50 were at full signal. Full -signa l analyses were corrected by subtracting the mean background signal. The initial full -signal scans are most important for use in AFT analysis because the spontaneous tracks counted form within ~10 m of the analyzed mineral surface. Plots of 43Ca cps were used for rapid assessment of instrument instability and data quality over the entire session. Comparing the background to uncorrected for -background full -signal ratios helped identify sources of instability in the 43Ca cps. For details on the methodology of apatite fission track dating by LA ICP -MS see Donelick et al (in print). Fission -track age of apatite grains were calculated using the ratio of the number of fission tracks in the grain to the amount of 238U in the same area of the grain [181]. The radia tion decay equation used incorporates the LA ICP -MS zeta calibration factor [189] Age calculatio ns for all samples involved LA ICP -MS dating of the Cerro de Mercado apatite standard (30.60.3 Ma), Durango, Mexico with a MS zeta calibration factor of MS= 14.5149 and 1 MS=0.2625. This zeta calibration factor is determined for each sample during each LA ICPMS session by analyzing the U:Ca ratio in grains from a standard at the beginning and end of each session

PAGE 156

156 [189] Durango pooled ages yielded a value of 31.40.8 Ma. Fissiontrack ages of individual apatite grains were measured at the University of Washington using the LA -ICP -MS. Tracks were counted at 2000X magnification under un-polarized ligh t at Apatite to Zircon, Inc by P. OSullivan. Track Length Measurements Irradiating apatite grains with 252Cf -derived fission fragments can yield a 20-fold increase in the number of available fission tracks for length measurement, enhancing confined fissio n track detection and analysis [395] The second of two polished grain mounts for each dated sample was irradiated with ~107 tracks/cm2 fission fragments from a 50 ci 252Cf source in a vacuum chamber (activity of 252Cf in July, 1996). Irradiated grain mounts were im mersed in 5.5N HNO3 for 20.0 0.5 seconds at 21 1 C to reveal horizontal, confined fission tracks and track lengths were measured. Only natural, horizontal, and confined tracks, with clearly visible ends, were measured for length and angle to the c axis. F ission tracks were viewed under unpolarized light at 1562.5x magnification (100x dry objective, 1.25x projection tube, 12.5x oculars). Length and crystallographic orientation of each fission track was determined using a projection tube and a digitizing tab let calibrated with a stage micrometer and interfaced with a personal computer. Precision of each track length was about 0.20 m, while the precision of each track angle to the crystallographic caxis was about 2 degrees. Approximately 130 fission tracks per sample were measured. Annealing Kinetics and Dpar Parameterization Variations in chemical composition affect the kinetics of fission-track annealing so that tracks are preferentially retained in chlorinerich, relative to fluorine -rich apatites [230232, 396] Thus, apatite fiss ion track ages and total etched fission track lengths are strongly correlated

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157 with the composition of their host apatite grain in samples that have experienced significant residence time at temperatures >70 C [397] It is therefore critical to include kinetic information to generate valid inte rpretations of apatite fission -track data. Separating apatite grains into kinetic populations enhances the reliability of the analysis and increases the thermal information derived as multiple kinetic populations simultaneously constrain the thermal histor y of the sample [226228] In this investigation, the parameter used to quantify solubility is termed Dpar. Commonly specified in microns, Dpar refers to the mean fission track etch pit d iameter parallel to the crystallographic c axis for each apatite grain [196, 227] Fission tracks in apatite grains exhibiting the smallest D par values usually anneal more quickly than fission tracks in apatite grains having larger Dpar values. In an apatite grain having a Dpar value near 1.50 m (a typical fluorine rich apatite), fission tracks generally do not survive geological heating above ~100 C. In an apatite grain having a Dpar value near 3.00 m (a typical chlorine rich apatite), fission tracks may survive geological heating >150 C [227] Age and track -length measured apatite grains were classified for annealing kinetics using Dpar. Between one and four Dpar values were recorded and a mean Dpar value was determined for each grain measure d for age or track length. Dpar values were used for modeling the time temperature paths with HeFTy [224] and AFTSolve [225, 234] Fission -track Modeling The grain age distribution and component ages, t rack length distribution and grain composition proxy data (Dpar) were combined to constrain a samples thermal history with inverse modeling. AFT data was modeled using AFTSolve [225, 234] and HeFTy [224]. AFTSolve uses search methods to quantify the range of statistically acceptable and better thermal histories for a sample that 1) adheres to user defined constraints and 2) matches the

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158 measured AFT data. For samples in this AFT study that possess previous AHe data [294] HeFTy was utilized to derive the maximum amount of information possible from thermochronometric data through forward and inverse modeling. An openended model with minimal constraints was used in itially and restrictions directed by the successive modeling results and relevant geologic data, such as stratigraphic age, the samples pooled or component AFT age(s), and AHe age(s) were subsequently imposed. A multi kinetic annealing model was used, allo wing for modeling of multiple kinetic apatite populations with Dpar [226] For the initial general model, 15,000 simulations were run with a controlled random search (CRS) technique for each sample. All age and track length data were modeled as one kinetic population projected to the c axis to effectively remove the problems of anisotrop ic track length reduction [228] Default initial track lengths were implemented as derived from Dpar. Calibration parameters for AHe ages were those of Durango apatite [220] Results LA -ICP -MS apatite fission track data and analytical results are presented in Table 4 1. AFT data were derived from a sufficient numbers of grains (n>20 for age, n>100 for track length) and uranium concentrations to produce statistically sound age populations [192, 196] otherwise stated. Age spread was determined using the Chi -square test, which indicates the probability that all grains counted belong to a single population of ages [387] All apatite fission track data passed the Chi -square test at 5%, implying that variations in fission track age are due to inherent variability of the fission decay process and analytical conditions. AFT a ges are much younger than formation ages of the sampled plutons [355] so that differences in grain ages are a result of thermal annealing due to denudational cooling that occurred long after batholith emplacement into relatively cool country rock and at shallow crustal levels near the Ca mpanian -

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159 Maastrichtian. The majority of the samples AFT ages are also older (or concordant) with previous AHe ages [294] with the exception of three samples ( SR 24, SR 26, and SR 29), which are discussed below. Dpar measurements show rather homogenous values and hence similar annealing kinetic characteristics for all of the samples along both profiles so that age differences are not related to compositional differences [226] Most of the Dpar values for both profiles low representative of the samples in this study, and less retentive than apatites having a Dpar value Samples between ~1640 and 2500 m were analyzed for t he La Garca profile. Along this altitudinal range, apatite fission track ages vary between 35.5 2.4 and 48.9 3.0 Ma. Considered 4). Average mean track length is ~14 m with standa rd deviation of ~1.5 m. Mean track lengths display a weak correlation with age (r=0.32), with older samples tending to exhibit longer track lengths than younger samples. This pattern, as discussed latter, is more clearly displayed when samples SR 24, SR 2 6, and SR 29 are excluded from the analysis, when the Matasanos -Porce profile is considered alone, or when the two profiles are analyzed simultaneously. Samples from elevations of ~700 to 2500 m in the Matasanos Porce profile were analyzed. Fission track a ges range between 30.4 3.6 and 47.7 3.1 Ma. Ages are essentially concordant at ositive correlation with age (r=0.91) so that older samples showing longer track lengths than their younger counterparts (Figure 4 5).

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160 Fission track data for both profiles display similarities with respect to age and confined track length distribution (Fig ures 4 4 to 4 7; Table 4 2). Such similarities are reflected by the nearly identical range in single grain ages (~49 30 Ma), the narrow and unimodal track -length tarck length and age (i.e. longer tracks with older grains and shorter tracks with younger grains). The age -elevation relationships are consistent with previous A He in both transects with the exception of sample SR 26 (38.2 Ma, 2350 m), which yielded and AFT age younger than its AHe counterpart. The profile dates exhibit a positive correlation between age and elevation (r=0.73 for Matasanos Porce and r=0.52 for La Garcia). Because the region experienced negligible changes in topographic parameters, except for the localized incision of the Medelln Porce River, it is not necessary to correct AFT ages using the admittance ratio (Reiners et al., 2003). In addition, U a nd Th concentrations, the crystallization age of the plutons sampled, and the range of apatite ages fall within the figures over which negligible discrepancies between AHe and AFT systems have been reported [192] Interpretation and Discussion AFT da ta from the upper portion of both profiles (i.e., 17002400 m) are fundamentally 4). Age elevation relationships of the Matasanos Porce profile record rapid cooling between ~50 40 Ma followed by a period of stability until at least 30 Ma. The two intervals of exhumation and quiescence are defined by regression lines through two sub sets of the data (samples above and below ~1600 m, excluding anomalously young AFT data, see Figure 4 4) whose intersection yields an inflection point at ~41 Ma. This slope change may mark the upper boundary of the A -PAZ (60

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161 Eocene time. The La Garca profile shows largely concordant ages and an interval of rapid cooling between ~4540 Ma (Figure 4 5). The Eocene cooling event occurred long after intrusion of the Late Cretaceous Antioqueo and Ovejas batholiths [355] AFT data indicate that all analyzed samples resided at paleotemperatures in exces s of the AFT closure temperature (>110 thr ough the apatite PAZ. In addition, Apatite fission track modeling (AFTSolve) for the majority of the samples shows a marked cooling event between ~50 43 Ma (Figure 4 7). Minimum dates for initiation of cooling for the 22 samples modeled converge at ~43 Ma Furthermore, HeFTy models indicate that samples were cooled in the Eocene through the apatite fission track PAZ and the apatite Helium PRZ, i.e., from >120 8). Concordant AFT and AHe ages for both profiles over a crustal section of ~1500 2000 m imply that rocks were exhumed from temperatures >110120C to below the AHe closure temperature 6070C (Fugures 4 4 and 45). T hat is to say, rocks cooled ca. 50C in 5 million years, which, at a gradient of 25 C/km, implies unroofing of ~2 km, or ~0.4 mm/yr. This erosion rate is comparable in intensity to the Oligocene -Miocene pulse previously identified in the region by AHe ana lysis and about an order of magnitude higher than average, longterm denudation rates [294] Three AFT samples from higher elevations in The La Garca profile SR 24 (37.7 Ma, 2500 m), SR 26 (38.2 Ma, 2350 m), and SR 29 (35.5 Ma, 2250 m) are offset in the age elevation relationship towards younger ages relative previous AHe profiles [294] and AFT ages for their elevation range (Figure 4 4). Sample SR 26, dated by both methods, shows an AFT age

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162 significantly younger than a previously reported AHe date (46.7 2.3 Ma). This discrepancy cannot be solely explained by rapid cooling of this crustal section during erosional exhumation. Even though SR 24 is weathered, grain quality is sim ilar for the three samples. A possible way to explain the presence of these outliers might be through a vertical displacement along a structure that would have uplifted these samples relative to the rest of the profile. There is, however, no evidence for faulting within the profile and maps of the area report no structures in this portion of the AP. In addition, previous AHe profiles yielded a consistent trend in the age -elevation relationship for this portion of the profiles. Excessive retention of He is quite improbable given the age of the plutons and the concentrations of U and Th in apatite crystals from this suite [294] Thus, transect samples likely belong to the same structural block. Furthermore, crustal rig idity and structural coherence of this lithospehric block was indicated in previous studies [22] An important aspect of the interp retation of AFT data revolves around the time of rapid data set (Figure 4 6) shows a positive correlation (r=0.91 for The La Garca profile) between samples with the longest tracks (between 14.0 50 Ma). Conversely, samples with the shorter mean track lengths (13.7younger ages (30 35 Ma). This relationship is indicative of a group of samples that coole d rapidly at ~50 45 Ma from temperatures > ~110 which some of the lower samples in the profile remained at annealing temperatures of ~65 85 resulting in reduced ages and track lengths. This is further supported by the AFT Solve and HeFTy modeling results (Figures 4 7 and 4 8). Modeling of AFT data from samples with the highest mean track lengths yielded earlier cooling times between ~50 45 Ma, whereas younger

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163 samples remained at higher temperatures, exper iencing partial annealing and recording later cooling. Slightly shortened tracks at lower elevations suggest that after rapid cooling in the Middle Eocene there was subsequent cooling. However, none of the samples record this younger event in detail. A maj or phase of Oligocene uplift and denudation of the AP is clearly displayed by AHe data [294] but is not resolved by AFT (FIgura 4 4). Previous AFT and ZFT low temperature data led to the conclusion that the region did not experience uplift and denudation during the Oligocene [166] However, AHe ages [294] which are sensitive to lower temperatures, resolve two Andean Orogenic Pulses for the Colombian Andes (Pre -Andina II 42 Ma and the Proto -Andina 23 Ma [16] ) and allow refuting the previous conclusion. In other words, the higher closer temperatures of both AFT and ZFT do not enable resolution of the younger part of the cooling history. In addition, the vertical profile approach in this st udy shows a flattening of the age elevation relationship (break in slope) from ~45 to 30 Ma (Figure 44), that is consistent with previous AHe profiles [294] This interval coincides with a period of tectonic quiescence related to a decrease in the rate of convergence between Farallon and South America [18, 19] Such periods of relative tectonic calm provide enough time for full development of erosional surfaces on which Oligocene sediments of the Terciario Carbonfero de Antioquia (Amag Formation) were deposited [168] The Eocene pulse, poorly delineated by the AHe dataset [294] is clearly recorded by AFT both in magnitude and timing, coinciding well with the proposed timing of the Pre -A ndina phase [16, 162] Combined AFT and AHe data indicat e that the extent and rates of this major exhumation pulse were much greater than what is evident from AHe data alone.

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164 In interpreting the AFT dataset, an Eocene geothermal gradient of ~25C/km and a mean surface temperature of 15C were assumed. This figu re is reasonable for subductionrelated continental margins with a thick continental crust and absence of modern volcanism [86] such as this non -volcanic segment of the Andes. If such assumptions are correct, the depth to the top of the pre Eocene PAZ (60 11). The break in slope of the age vs. elevation relationship is at ~1600 m or approximately 1000 m below the mean elevation of the AP. Thus, the amount of section removed since the Eocene exhumation pulse is ~1.2 km for the AP as a whole, but only ~100300 m for the highest areas such as the summits of the Belmira range. This implies spatially averaged, long term erosion rates of <0.03 mm/yr. AFT results and previo us sediment stratigraphic studies [14, 16] as well as a lack of regional -scale Cenozoic extensional tectonics in the northern Cordillera Central [25, Kellogg, 1984 #724] imply that this regional cooling phase of Eocene age was mainly due to erosional exhumation by fluvial systems that predate the modern Magdalena and Cauca rivers. Cenozoic cooling even ts associated with erosional exhumation, constrained by AFT in this study and by AHe [294] are synchronous with the Pre -Andina and Proto -Andina orogenic phases [16] Voluminous amounts of detrital material produced in the Eocene Late Oligoceneare reflected in sedimentary sequences with basal conglomerates deposited over major regional unconformities in the Magdalen a [14, 16] and Cauca [168] basins to the east and west respectively. The combined exhumation of the Eocene and Oligocene events removed most of the ~4 5 km of estimated overburden on the Antioqueo Batholith. Diachronous uplift -exhumation in the Cordillera Central was a major determinant in the evolution of the proximal Middle Magdalena and distal Llanos basins to the east of the AP. The northern Cordillera Central experienced episodic pulses of uplift and ex humation since the

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165 Paleocene (Laramic, Pre-Andina and Proto -Andina phases [16, 294] ) relative to the Cordillera Oriental that was uplifted during the Neogene, (Eu -Andina phases [16] ). AFT data presented here and previous AHe ages [294] agree with recently propo sed mechanical models for the Middle Magdalena basin [14] that indicate an Early Eocene foreland basin coupled with Cordillera Central uplift, a Middle Eocene basin characterized by erosion -exhumation of the Cordillera Central, and marked basin tilting to the east triggered by enhanced Cordillera Central uplift during the early Neogene. Cycles of uplift and tectonic quiescence, previously suggested for this segment of the Colombian Andes [14, 16, 168, 294] and the p ossibility of km -scale uplift episodes were quantitatively confirmed the preset AFT study. That is, the timing and magnitude of uplift and denudation were explicitly derived from our low temperature thermochronology dataset from the Cordillera Central. AFT results correlate well with previous paleogeographic and sediment stratigraphic studies [16, 43, 295] Increased cooling by fluvial erosion in the AP may have been caused by enhanced uplift driven by tectonic process associated with accelerated convergence between the Farallon and South Ame rican plates (Figure 4 10), documented for the Eocene [18] The Eocene pulse here identified is synchronous with exhumation events of a similar magnitude that have been reported in low -temperature thermochronology studies elsewhere in the Andes [30 32, 35, 36, 39, 111, 112, 167, 366, 368, 370, 398] and in the Caribbean region [37] Eocene to Miocene pulses of uplift and denudation in the Colombian Andes can be placed in a broader regional framework. Tectonic upheavals duri ng the Paleogene and Neogene are also referenced for other Andean locations. Paleocene to Eocene phases of exhumation have been reported in some sectors of the Ecuadorian Andes [39] Eocene (Incaic) and Miocene (Quecha 1 3) orogenic phases are w ell characterized in central and northern Peru [36] In the Peruvian

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166 Andes, apatite fission track analys is of the Huachon granite points to two periods of erosional cooling during the Neogene, at ~22 Ma and between 12 Ma and the present, with 46 km of unroofing [35] Similarly, Benjamin (1987) used low temperature thermochronology (LTTC) to constrain the Tertiary exhumation and uplif t history of the eastern Cordillera and Altiplano of Bolivia, revealing discrete pulses of uplift in the intervals 5 15 Ma and 25 45 Ma. Onset of Andean deformation and exhumation in the Sierras Pampeanas (Argentina) is represented by a cooling event durin g the Late Paleocene middle Eocene, followed by westward migration of the exhumation front during the Late Miocene Pliocene [32] Eastward migration of the locus of maximum denudation identified for several portions of the Andes [32, 35, 399] may also apply for the Colombian Andes, where the eastern cordillera seems to be the focus of intense erosion from the Miocene to the present. In the Colombian case, however, indentation of the Panam a Choc arc may imply concentration of denudation towards the active margin (Cordillera occidental) in the Neogene and exposure of Miocene plutons supports this contention [156] P ulses of Paleogene uplift and exhumation throughout the Andes display a clear correlation with increased rates of convergence between Farallon (Nazca) and South America [18] Although fairly well documented by paleontology, geomorphology, thermochronology, stable isotope paleo altimetry, and sedimentary stratigraphy [15, 30, 113, 158, 216, 349, 371, 375] the most recent Late MiocenePliocene orogenic phase in the Andes has been harder to explain, as it does not coincide with a period of enhanced convergence between Nazca and South America [18, 19, 80] Local mechanisms, such as crustal delamin ation [375] lateral flow of low er crust [216] and sequential stacking of basement thrusts [30, 111] as in the Bolivian case, or structural relief due to accretion -controlled crustal deformation by the Panama -Choc Block in the Colombian case [25] have been invoked to explain recent phases of uplift. However,

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167 synchronicity of this major episode of Neogene uplift denudation, documented by low temperature thermochronology studies from Chile and Argentina to Colombian and Venezuela, and correlation with periods of enhanced plate converg ence also suggest a major tectonic control of continental scale. Important climatic controls (enhanced erosion via aridization) have also been invoked to explain periods of rapid uplift and exhumation in the Andes [347, 369] and elsewhere [51, 52] The relative roles of climate and tectonics in sculpting mountainous landscapes remains strongly debated [50, 52, 97, 106, 400, 401] However, as the database on exhumation chronology of the Andes grows, important continental -scale controls are expressed differently for specific portions of the orogen. Perfect matches between exhumatio n pulses and increased convergence rates between Farallon (Nazca) and South America during the Eocene and the Late Oligocene, in Colombia, the rest of South America, and the Caribbean, point to increased erosion brought about by tectonically -driven uplift. Nevertheless, a climatic contribution to increased erosion in the Eocene (Molnar and England, 1990) cannot be ruled out, and further evidence of Eocene climate trends in the Northern Andes of Colombia should be sought. Several studies proposed a prevalent role of Caribbean subduction in the Eocene geologic evolution of the Northern Andes [33, 164, 313, 381] Caribbean subduction [164] has been invoked to explain stratigraphic unconformitie s and tight faulting and folding, tectonostratigraphic features related to rapid regional uplift (Early Eocene tectonic phase ~53 Ma, and Middle Eocene Caribbean Orogeny ~45 Ma) Eocene magmatism in the Sierra Nevada de Santa Marta [402] shown by plutons dated ~50 Ma by the K/Ar method, has also been ascribed to subductionrelated processes in the Caribbean d omain. Calcium carbonate compensation depths in marine sedimentary sequences now exposed in the San Jacinto Belt (northern Colombia) and

PAGE 168

168 the proto Cordillera Occidental suggest that this area also experienced several km of uplift [367] in the Eocene. However, evidence of Eocene phases of erosional exhumation in the rest of the Andean range, in addition to coincidence of enhanced convergence between the Farallon and South American plates, points to larger -scale, continental contr ol that needs to be considered for the Neogene Topography and low temperature thermo -chronometry can be used to define tw o parts of the landscape of dissimilar age and geodynamic significance. They are: 1) an old, slowly eroding highland of subdued relief that is associated with periods of sustained lower relief and elevation from ~60 50 Ma and from ~4025 Ma, and 2) younger rapidly incised river gorges and regional scarps that cut across and around the AP during the Cenozoic, particularly during the latest Eu Andina phase. Combined AFT and AHe data support low elevation of the range in the Paleocene, followed by two well re corded events of uplift and denudation across the plateau, as well as local fluvial incision at ~50 45 Ma and ~2420 Ma. The AP has had a cyclical geomorphic evolution since emergence of the region after the end of the Cretaceous marine transgression in th e Paleocene. Multiple erosional surfaces have been obliterated throughout the Cenozoic. The landscape probably evolved by tracing a rapidly advancing weathering front (etch surface) that had time to re -develop during periods of tectonic quiescence, given the fast rates of chemical weathering proposed for granodiorite under humid conditions [403] Modern geomorphic configuration of the AP reflects enhanced uplift during the most recent Mio -Pliocene Eu -Andina tectonic phase. In these later stages, erosion has concentrated along a few alluvial valleys (e.g., Medellin Porce River) creating localized incision of >1500 m. Differential erosion created numerous perched valleys and varying local base levels that permitted development of smaller planar surfaces within the AP. Hanging valleys impose

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169 fundamental control on how the landscape evolves [404] Nick point propagation ultimately determine s the extent and velocity with which the fluvial system encroaches into the plateau to degrade the planar morphology. This process is potentially enhanced as convergence of the Panama Choc block continues to maintain uplift in this portion of the Andes. P erpetuation of uplift, fluvial incision, and nick -point propagation will ultimately obliterate the planar topography of this morphotectonically odd segment of the Colombian Andes, leading to the development of typical mountainous topography (e.g., steep, v -shaped valleys), and leaving only small, discontinuous remnants of the erosional surface. Two important phases of tectonic upheaval and exhumation in the Colombian Andes [16, 162] are not captured by this or previous thermochronology studies [166, 294] The Eu -Andina phase from Miocene to present [16] considered as t he strongest and most recent pulse of uplift for the Northern Andes [34, 113, 156, 158, 162, 349] is not resolved by AFT or AHe. This may be a consequence of the disequilibrium character of the AP and its lagged geomorphic response to tectonic input relative to other portions of the orogen. In addition, AFT and AHe do not document the initial stages of uplift and exhumation of the Northern Cordillera Central, i.e., the Laramic phase [16] However, remnants of Hauterivian -Albian shallow marine sequences in the Cordillera Central, followed by a transition to continental deposition in the Maastrichtian, with clastic material o f Cordillera Central affinity in several Late Cretaceous Early Tertiary sedimentary formations, imply the establishment of this portion of the Andes as a relatively elevated massif about 65 Ma ago [14, 17] Zircon fission -track data with ages varying between ~50 67 Ma [166] probably point to this initial phase cooling via erosional exhumation that led to the development of the first erosional surface of Paleocene age. Subsequent phases of increased te ctonic convergence interrupted by long periods of tectonic quiescence led to a succession of

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170 planation surfaces, the youngest of which is Miocene, according to AHe ages [294] following the denudational pulse in the Oligo -Miocene transition. The Neogene geomorphic evolution of the AP was strongly determined by the Eu -Andina orogenetic phase (Mio -Pliocene). Removal of the saprolitic cover is documented by major inselbergs (tors, rock castles) that indicate a stage of etch surface development [405] Erosion, however, has been focused on a few fluvial systems that cut deeply into the plateau morphology. Large differences in erosion resistance created numerous perched valleys and local base levels that permitted dev elopment of smaller planar surfaces within the AP. Shifts between rapid and slow convergence determined orogenic exhumation in the Andes and the morpho -tectonic evolution of the AP. However, relatively lagged response of the AP and low erosion rates relati ve to other portions of the orogen confirm that the AP is in morphotectonic disequilibrium. This is the result of many factors, for instance, lithospheric strength and structural integrity of the area which may have controlled the mode of emplacement and geometry of the Antioqueo Batholith (circular pattern) at the center of the AP, surrounded by more resistant metamorphic units, thus providing litho -structural control to the development of the AP. AFT and AHe double profiling also confirm the behavior o f the AP morphotectonic domain as a rigid and coherent portion of lithosphere. Erosion rates in the humid tropics differ widely between tectonically active and inactive areas [406] Long term erosion rates for mountainous environments are between 0.1 and 2 mm/yr [8, 186] Longterm erosion rates in the AP are ~0.02 mm/yr, while denudation at the climax of orogenic phases incre ased to ~0.2 mm/yr. Longterm denudation is low compared to rates in other sectors of the Colombian Andes and the rest of the Andean range to the south. Exhumation rates during the climax of exhumation are comparable to rates reported for the

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171 Bolivian and Peruvian Andes [30 32, 38, 111] but are about an order of magnitude slower than rates in Taiwan, Nanga Parbat, the Greate r Himalaya of Nepal, and the Southern Alps of New Zealand [186, 407] A comparison of results from this AFT study with recent thermochronology data in the Cordillera Oriental [159] confirms that neighboring regions in the same orogenic system subjected to similar large-scale tectonic input experienced long term erosion rates that differ >100 -fold. Within the same tectonic setting, crustal blo cks may undergo different exhumation histories [174] Low erosion rates i n the AP result from its morphote ctonic disequilibrium. Conclusions Apatite fission -track along two vertical profiles within the AP complements previous AHe data and constitutes the first high resolution record of thermochronologic information in the Colombian Andes. AFT analysis identifi es a major cooling event from ~50 -40 Ma, associated with erosional unroofing >2 km and exhumation rates of ~0.4 mm/yr. This erosional pulse is much stronger than previously indicated by AHe thermochronology alone. A period of tectonic quiescence ~41 to 30 Ma was also recorded. Long term erosion rates derived from the vertical profiles are ~0.01 mm/yr. Pulses of exhumation in the AP defined by low temperature thermochronology are synchronous with similar phases in the rest of the Andean orogen and the Caribbean. The match between pulses of exhumation and increased convergence rates between Farallon (Nazca) and South America during the Eocene and the Late Oligocene (in Colombia, the rest of the Andes and the Caribbean) points to continental -scale tectonic cont rols on denudation. Shifts between rapid and slow plate convergence determine orogenic exhumation and the morphotectonic evolution of the AP and the Andes in general. Multiple episodes of planation and relief development occurred during the Cenozoic in the AP region.

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172 The lagged response of the AP and low erosion rates relative to other portions of the orogen confirm that the AP is in morphotectonic disequilibrium. The AP functions as an independent morphotectonic domain embedded in a rigid and coherent lit hosphere block. A preliminary comparison between long term erosion (i.e., natural, 0.04 to 0.008 mm/yr; Restrepo -Moreno et al, 2009 and Restrepo -Moreno, submitted) against modern erosion rates (i.e., anthropogenic, 1 mm/yr, Giraldo, 2005) reveals a 23 ord er of magnitude increase. Such disparity can be interpreted as being related to anthropogenic perturbations of natural geomorphic dynamics.

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173 Table 4 1. Instrumentation, operating conditions, and data acquisition parameters used for LA ICP -MS apatite fissi ontrack dating. ICP MS operating conditions Instrument Finnigan Element II Magnetic Sector ICP MS Forward power 1.25 kW Reflected power <5 W Plasma gas Argon Coolant flow 15 l/min Carrier flow 1.0 l/min (Argon) 0.8 l/min (Helium) ICP MS da ta acquisition parameters Dwell time 10 msec per peak point Points per peak 4 Mass window 5% Scans 50 Data acquisition time 30 sec Data acquisition mode Electronic scanning Isotopes measured 43 Ca and 238 U Laser operating conditions Laser type New Wave UP213 (Nd: YAG) Wavelength 213 nm Laser mode Q switched Laser output power 8 J/cm Laser warm up time 6 sec Shot repetition rate 5 Hz Sampling scheme Single spot, 16 m

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174 Table 4 2. Summary of apatite fission track data. Apatit e fission -track age, length and Dpar data for the Matasanos (M) and the La Garca (G) profiles. Reported dates are pooled FT ages 2 MS (secondary zeta calibration parameter for LA ICP -MS) as proposed by Donelick et al (in revi ew). Numbers in [ ] indicate number of grains analyzed. (MS = Mass spectrometry, FT = Fission track, MTL = Mean track length, STDV = Standard deviation, SE = Standard error, bkg:sig = Background to signal ratio). Sample [no. grains] Dpar ( m) Ns (tracks) Area analyzed (cm2) ( ) (cm2) 1 ( ) (cm2) MS 1 MS 43 Ca (bkg:sig) 238 U (bkg:sig) Q Chi Square (%) Lat/Lon (Dec Deg) Elevation (m) Pooled FT Age (Ma) MTL 1SE) STDV MTL Durango [492] 1.52 3091 2.3x10 2 7.1x10 4 1.3x10 6 14.5149 0.263 2.565x10 2 2.166x10 3 0.97 31.4 0.8 14.59 0.08 0.98 Unknowns SR 001 M [25] 1.54 232 9.2x10 4 3.9x10 5 1.5x10 6 15.4613 0.423 2.12x10 2 2.87x10 3 0.83 17.4 6.857/75.181 760 46.23.7 14.18 0.11 1.23 SR 003 M [25] 1.34 531 8.5x10 4 1.2x10 4 2.1x10 6 15.3485 0.418 2.42x10 2 1.20x10 2 0.30 27.0 6.758/75.115 1000 34.81.9 13.67 0.13 1.5 SR 2 M [22] 1.28 72 7.6x10 4 1.6x10 5 3.4x10 7 15.2356 0.414 2.46x 10 2 6.61x10 2 0.48 20.7 6.424/75.379 1520 35.34.3 13.35 0.15 1.75 SR 3 M [25] 1.22 247 1.1x10 3 5.3x10 5 1.6x10 6 15.1228 0.410 1.20x10 1 2.73x10 1 0.13 31.8 6.426 75.376 1704 35.02.6 13.70 0.12 1.39 SR 5 M [20] 1.19 83 6.9x10 4 2.0x10 5 5.9x10 7 15.0 099 0.405 2.78x10 2 1.12x10 2 0.30 21.7 6.427 75.371 1630 30.43.6 13.49 0.18 1.47 SR 6 M [24] 1.36 622 1.1x10 3 1.1x10 4 2.3x10 6 14.8971 0.401 6.66x10 2 8.46x10 2 0.01 40.2 6.433/75.374 1710 41.12.1 13.85 0.12 1.32 SR 9 M [23] 1.41 485 9.6x10 4 7.9x10 5 1.6x10 6 14.7842 0.397 2.68x10 2 3.83x10 3 0.02 37.6 6.463/75.374 2270 45.12.5 13.96 0.13 1.52 SR 11 M [24] 1.39 333 8.6x10 4 5.1x10 5 1.0x10 6 14.6714 0.392 2.81x10 2 6.85x10 2 0 75.7 6.462/75.370 2230 47.73.1 13.99 0.12 1.41 SR 15 M [24] 1.38 925 1.1x10 3 1.5x10 4 2.9x10 6 14.5585 0.388 3.87x10 2 8.48x10 2 0 67.6 6.452/75.369 2100 45.52.1 14.03 0.11 1.32 SR 19 M [25] 1.47 303 8.7x10 4 4.6x10 5 9.0x10 7 14.4456 0.384 7.35x10 2 1.98x10 1 0.91 15.3 6.449/75.361 1990 47.23.1 14.23 0.13 1.45 SR 24 G [10] 1.46 134 2.6x10 4 2.6x10 5 1.1x10 6 14.3666 0.381 3.22x10 2 2.53x10 3 0 33.4 6.386/75.598 2400 37.73.8 13.96 0.19 1.7 SR 26 G [22] 1.48 264 6.9x10 4 4.9x10 5 1.2x10 6 14.2877 0.378 1.53x10 1 2.88x10 1 0 91.0 6.382/75.595 2350 38.22.7 14.25 0.12 1. 41 SR 29 G [23] 1.49 293 7.4x10 4 5.8x10 5 1.4x10 6 14.1748 0.373 3.25x10 2 8.43x10 3 0.06 33.4 6.381/75.591 2250 35.52.4 13.81 0.14 1.59 SR 31 G [25] 1.49 280 7.7x10 4 4.8x10 5 1.0x10 6 14.0619 0.369 4.21x10 2 6.69x10 2 0.09 33.6 6.380/75.593 2170 41.22.8 13.97 0.13 1.47 SR 32 G [24] 1.39 405 8.8x10 4 5.8x10 5 1.3x10 6 13.9491 0.365 3.18x10 2 2.89x10 2 0 114.0 6.379/75.589 2110 48.93.0 14.02 0.12 1.41 SR 35 G [22] 1.42 215 7.5x10 4 3.8x10 5 6.9x10 7 13.8362 0.360 1.09x10 1 1.69x10 1 0.10 29.7 6.374/ 75.579 1900 39.33.0 13.52 0.14 1.62 SR 40 M [24] 1.47 835 9.1x10 4 1.4x10 4 2.3x10 6 13.7234 0.356 3.84x10 2 2.82x10 2 0.14 30.2 6.406/75.435 1500 41.41.9 13.85 0.13 1.44 SR 41 M [25] 1.48 1101 1.1x10 3 1.8x10 4 3.8x10 6 13.6105 0.352 1.73x10 1 3.85x10 1 0 99.6 6.413/75.411 1380 40.51.8 13.92 0.13 1.45 SR 44 G [25] 1.44 347 9.8x10 4 5.6x10 5 1.0x10 6 13.4977 0.347 3.27x10 2 1.26x10 2 0 59.0 6.345/75.579 1640 42.02.6 13.93 0.10 1.18 SR 45 G [22] 1.42 269 9.4x10 4 4.2x10 5 1.0x10 6 13.3848 0.343 6.55x 10 2 7.59x10 2 0.03 34.9 6.359/75.590 2015 42.43.0 13.85 0.13 1.49 SR 46 G [24] 1.56 242 9.6x10 4 4.0x10 5 7.5x10 7 13.272 0.339 3.44x10 2 4.72x10 2 0.02 38.2 6.357/75.588 1900 39.82.9 13.82 0.12 1.42 SR 48 G [24] 1.51 255 8.6x10 4 4.2x10 5 1.0x10 6 13 .1591 0.334 6.24x10 2 1.19x10 1 0.08 33.1 6.353/75.581 1710 39.62.8 13.70 0.15 1.75

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175 Figure 4 1. Regional geomorphology. CO= Cordillera Occidental (Western Cordillera), CC=Cordillera Central (Central Cordillera), COr=Cordillera Oriental (Eastern Cor dillera). Red circles indicate the location of the elevation profiles sampled. Mean elevation of the AP is ~2500 m. Elevation of the small range on the west margin of the AP is ~ 3600 m. Bottom of the Cauca River is ~600 m. Bottom of the Medellin River ~12 00 m. (DEM generated with ERMapper from GTOPO 30 elevation data.)

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176 Figure 4 2. Geology of the study site. Target lithology is granodiorite plutons of the Antioqueo and Ovejas batholiths (AB and OB). Location of sampled profiles the La Garcia (west) and Matasanos Porce (east) marked with red circles. CA -F = Cauca Romeral (Cauca -Almaguer) fault system (paleosuture).

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177 Figure 4 3. Tectonic Setting. Study region delimited by the red square. Major structural features in Colombia: RF= Romeral fault sys tem, PF=Palestina fault system, UF=Uramita fault, GF=Guaicramo fault, SBF=Santa Marta Bucaramanga fault, BF=Bocon fault, OF=Oca fault. Major tectonic plates and two lithospheric blocks (PC=Panama Choc and Andean) are accompanied by arrows and numbers that indicate direction and velocity of motion relative to the South American Plate. Black triangles indicate zones of the Cordillera Central with active volcanism. (Faults are compiled form Taboada et al. (2000) and Tapias et al. (2006); plate velocity vect ors from Trenkamp, et al. (2002); digital elevation model (DEM) derived from GTOPO 30 database).

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178 Figure 4 4. Age -elevation relationship. Low temperature thermochronology age data and elevation from the Antioqueo Plateau. Black triangles are AFT data for the Matasano Porce profile. Black diamonds are AFT data for The La Garca profile. Segments S1 and S2 are regression lines (least square) through subsets of the data explained in the text. Red line encloses AFT data anomalously young relative to previous AHe ages. AHe data (gray squares) from Restrepo -Moreno et al. (2009) for the same profiles are displayed for comparison.

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179 Figure 4 5. AFT Age vs. Elevation Relationship for The La Garca profile. At 2 virtually concordant between ca. 1200 and 2600 m and are centered at ~41 Ma, indicating rapid exhumation at this time.

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180 Figure 4 6. A ge vs. mean track length relationship for the Matasanos -Porce profile. All samples in the profile included (r=0.91), MTL= mean track length.

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181 Figure 4 7. Representative Time -temperature paths modeled with AFTSolve. Models by (Ketcham et al., 2000; Ketc ham and Donelick, 2001) were run by Restrepo-Moreno and OSulivan. Results show permissible thermal histories in the left panel (a) and measured (bars) versus best -fit model (solid line) track length distributions in the right panel (b). Thermal histories shown are acceptable (dark gray), good (light gray) and best (GOF) between the model results and the data for both the age and MTL data are reported at the base of each thermal history.

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182 Figure 4 8. Relevant HeFTy Time temperature paths. Paths modeled with HeFTy [224] by Restrepo -Moreno. Results show permissible thermal histories in the left panel and measured (bars) versus best -fit model (so lid line) track length distributions in the right panel. Thermal histories shown are acceptable (dark gray), good (light gray) and best -fit (black line). Goodness of fit (GOF) between the model results and the data for both the age and MTL data are repor ted for each thermal history.

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183 Figure 4 9. AFT and AHe time temperature paths. Black dotted lines show periods of rapid cooling-exhumation. Gray dotted arrows indicate spans that are unconstrained by AFT or AHe. The hachured balloon on the bottom lef t shows the temperature and age of emplacement of the Antioqueo and Ovejas batholith. AFT data from this study, AHe from RestrepoMoreno et al. (2009). Age of intrusion of the Antioquenio batholith and associated Ovejas Batholith from Restrepo et al. (2007).

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184 Figure 4 10. Longterm spatially averaged erosion. Simplified sketch illustrating the extent of the crustal section removed by erosion since ca. 45 Ma (light gray, dotted area). Upper boundary of the APAZ (upper dotted line defining the band in dark gray). PB=Pramo de Belmira, AP=Antioqueo Plateau. Present day topography defined by curved black solid line.

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185 Figure 4 11. Timing and magnitude of Cenozoic changes in convergence between the FarallonNazca and South American Plate. Onset and termin ation of orogenic phases found in this study and in previous investigations (Restrepo-Moreno et al., 2009) are represented by gray block arrows labeled Pro (Proto -Andina) and Pre (Pre Andina) between shaded bars delimited by dotted lines (Van der Hammen, 1 958, 1961). Figure modified from PardoCasas and Molnar (1987).

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186 CHAPTER 5 GENERAL CONCLUSIONS AND FURTHER RESEARCH Antioqueo and Ovejas Batholith Age and Petr ogenensis An extensive geochemical and isotopic study was carried out in the Antioqueo and Ovejas batholiths (AB and OB respectively) with the aim of constraining the age, magma sources -mixing process and geodynamic environment. Geochemical and isotopic analysis allows to define the AB and OB as a typical medium -K calc alkaline plutonic suite, m ainly granodioritic in composition and developed though continental margin magmatism (i.e ., subductionrelated). The AB and OB were emplaced in the Maastrichtian and not in the Albian as previously proposed. Structures such as roof pendants, septum, copulas well defined contact aureoles, lack of foliation and faulting, etc. suggest that these plutons are post -tectonic and were emplaced in the epizone. Epizonal emplacement and rapid cooling are supported by zircon grain morphology. This mass was assembled in crementally through a succession of magmatic injections over a relatively short span from ~72 to 77 Ma. Geochemical signatures (e.g., REE, Pb, and Nd) are similar to magmatic products for the North Andean Volcanic Zone indicating that at the time of the A B and OB genesis, the geodynamic setting was very simlaar to the one foud today in the Northern Andes. However, geochemical and isotopic differences between samples are notable (e.g., different slopes for the REE patterns, variable values for eHf, etc.). T his, along with clear magmatic zoning indicates different source s, different depths of equilibration and disparate degrees of fractional crystallization during the various pulses. Overall, and this is also the case for most igneous products in continental arcs, there is a marked influence of crustal contamination expressed in the high values of LIL E hi gh LREE and Hf. Similarity in age and geochemical isotopic signatures for both plutons is indicative of consanguinity.

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187 Potential mechanis ms for magma production and emplacement are difficult to precise. Opting for one particular model to categorically claim that these batholiths were formed through crustal thinning and decompression due to extension, through a flare up a -la Ducea [338] by enhanced dehydration of the oceanic slab triggered by accelerated subduction or by enhanced thermal activity and exchange between the mantle and the crust is may not be appropriate. Perhaps a combination of mechanisms ac ting in concert can be invoked to explain the development of this large petrogenetic province. Whatever the mechanism, the AB and OB constitute a large mass of igneous rock and its emplacement at the end of the Cretaceous may have contribute to crustal thi ckening and hence to the initial pulses of uplift and denudation such as the Weak Laramic Phase (Van der Hammen, 1958). Further sampling of the AB and OB can assist us in better constraining the variation information ages of these plutons. However, the act ual results obtained in this investigation suggest that this is a robust estimate of formation age and provides an excellent age signature of magmatic zircons for the petrogenetic province under scrutiny. In conjunction with Hf data and CL imaging here gen erated, this database can be used to undertake provenance studies in sedimentary formations within the Cauca and Magdalena depressions to the east, west and north. In addition, this strategy can be extended to other smaller, plutonic units within the area of interest (more acidic and mafic) to better understand Late Cretaceous magmatism in the northern Andes Low Temperature Thermochronology A detailed, high resolution low temperature thermochronology study has been undertaken in the Antioqueo Plateau, nort hern segment of the Colombian Cordillera Central. The combination of AHe and AFTA provides a very coherent dataset that reveal s a two -phased erosion history for the AB and OB. Pulses of uplift and exhumation here recorded are in

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188 remarkable agreement with m ajor exhumation events defined for the northern Andes. However, the most recent phase of uplift and exhumation in the region, known as the Eu-Andina Phase (12 Ma to present and recognized as the driver for the most intense period of uplift and structural relief generation in the Colombian Andes is not recorded by either AFT or AHe. This confirms the hypothesis that the AP is a relict surface in morphotectonic disequilibrium with the rest of the cordilleran system and with a lagged response to uplift and ex humation. Longterm, spatially averaged erosion rates in this unusual geomorphic domain are similar to those of cratonic regions (0.020.008 mm/yr) suggesting that the AP resembles a portion of a shield pending at high elevation in the middle of a cordille ran domain bordered by step topographic terrain to the west (Cauca gorge and sierra like Western Cordillera), the east (Magdalena Depression regional scarp), north (end of the Central Cordillera, Cauca river regional scarp), and south (sierra like portions of the Central Cordillera). AFT and AHe data along two vertical profiles suggest that t he AP has responded as a coherent discrete crustal block to Cenozoic tectonic input. The region has been elevated from near sea level in the Paleocene to its actual ave rage 2500 m as a consequence of intermittent pulses of tectonicdriven uplift end exhumation. Several phases of planation, i.e., erosional surface development, have taken place during intervals of relatively slow convergence between Farallon (Nazca) and So uth American plates AFT and AHe data confirm that uplift -exhumation pulses in the Eocene and Oligocene in the Antioqueo Plateau have been important in the paleogeographic evolution of Northern South America and the tectonostratigraphic development of sedimentary basins to the east (Magdalena and Llannos basins) and to the west (Cauca basin). Spatiotemporal coincidence between pulses of exhumation in the AP (and elsewhere in the Andes and the Caribbean region) with periods of enhanced convergence between t he Farallon

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189 (Nazca) and the South American plates point to important, continental -scale tectonic controls on erosion rates. Additional information can be extracted from the AP region by implementing a horizontal transect approach that would allow to: 1) te st whether apparent tilting of the plateau to the southeast is a tectonic feature of if it i s related to fluvial activity; 2) evaluate the degree of internal relief through time with an approach similar to the one employed by House et al. {House, 1998 #39; House, 2001 #297}. Well characterized AHe ages achieved in this investigation can be further used, in combination with U -Pb/Hf in zircon, to study provenance of detrital material to the Amag formation and to constrain variations in the elevation of mo dern source s of sediment. Finally, all line of evidence suggest that modern erosion rates (i.e., anthropogenic; 1 10 mm/yr) are 2 4 orders of magnitude greater than long term (i.e., natural or pre anthropogenic; 0.020.008 mm/yr) correlatives. This implies loss of soil (end regolith in already degraded areas) at a magnitude far greater than the tolerance thresholds for such process accepted worldwide. Soil erosion is in itself a major environmental problem that is further amplified by the deleterious effect of excessive sediment transport and deposition along waterways and into strategic water ecosystems such as rivers, lakes, and oceans. Today, coral reefs, estuaries, lakes, riverine ecosystems, etc. are facing major problems due to accelerated sedimentatio n. In addition, the potential to store water and generate electricity through the establishment of artificial reservoirs is c ritically hampered by high rates of substrate removal by water. .

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221 BIOGRAPHICAL SKETCH Sergio Andres Restrepo Moreno (alias Yiyo) was born in Fort Collins (Colorado, USA), by the mountains in North A merica, in s pring 1969. This happened while his father Miguel Angel was conducting his MSc in plant physiology at Colorado State University and his mother was totally engaged in the not less-demanding and rather inspiring chores associated with rising kids (Camilo and Clara ). Sergio was prematurely taken to Medelln (Antioquia, Colombia) in the winter of 1970, right to the middle of the Northern Andes. From a mountain range to the next sort of thing! A year after, his younger brother, Jose Miguel, was born and with that a playing crew that has lasted for almost four decades was complete. Sergios wonderful great parents contributed to the warmth of the family niche. In 2000 Sergio married his great friend, Isabel, and a year and a -half after their wedding, D ecember 29th 2001 to be precise, Sergio and Isabel (and everyone else in the now larger family) were already enjoying the company of the beautiful Luna. A bit afterwards Iti Miguel came along to further the level of happiness of the three families involved: Isas, mine, Isas, and mine Inti Miguel was born on July the 10th 2004. Night -winter girl and day -summer boy. What else can one ask? In 2003, Sergios alias was changed for Papa Yiyo as Luna, his daughter, learned to identify dad and Yiyo as the sa me thing. Inti Miguel quickly followed Luna in their interesting conversations and, of course into the new nickname given to Sergio In other words, Luna and Inti have awarded Sergio one of the greatest titles so far, that of a loving close father. It is in the context of that spiritual wealth that Sergios academic career has developed. Sergio has been in the schooling system (some have opted to call it the educational system) since age 4. Among his most fascinating experiences in that realm are his first days of kindergarten at a small preschool facility by the Sin river at the Universidad de Cordoba were his father, rejected from the educational system in Medelln for having a long beard during the times of

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222 communism erradication ended up teaching for a couple years, and the company of one of the first teachers nun Bernardita in the Los Sagrados Corazones Elementary School in Medelln suburbs. Sergio never new why after having conditional matriculation for 5 straight years he graduated with honors fro m his high school days in 1986 and then joined as an undergraduate student the program in Geology at Universidad Nacional de Colombia, the University system that had unjustly persecuted his father several decades into the past and re appointed hi m again. P olitics some say. Anyway, Sergio graduated from Universidad Nacional without any honors in December 1994. S oon after Sergio had to flee as he and his younger brother were facing life threats in a time in Medelln town were a life threat by any of the seve ral hundred gangs in town was worst t h an a death penalty in the more civilized United States. Maybe they are just the same stuff. Two years on the road were very meaningful educationally speaking although he was willingly deprived of any formal schooling work whatsoever. Sergio remember s reading two of his first books in English: Like Water for Chocolate which inspired him to get finally immersed in a new language, and Deschooling Society (Ivan Illich) which he had already read in Spanish at age 16 or so a nd who taught him prematurely, to look at the compulsory educational system from which he is still part with profound criticism. Back in Colombia Sergio has worked as a volunteer academic advisor CIER, a local NGO, a local dream, in a project to revive non compulsory educational systems in impoverished rural villages in the mountains of Colombia. Trying to spread the news that wisdom is already within them and not in fancy educational institutions is hard work against the media, against the government, against those who have made the maintains of Colombia (and the valleys, and the rivers, and the beaches, and the air) a scenery of war.

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223 He somehow regained his interest in academic work (it may have to do with his early encounter with fossil in Colombia or with the devotion placed in using rocks to get fruit, scare detractors, etc.) without changing his impression of the academic system as a whole, and joined the Department of Geological Sciences on a fellowship in 1999. In 2000 he quit academic work to go back to Colombia and be with Isabel in the west coast of Colombia during her semester as a rural doctor in a nation of war and bloodshed. Few days before her arrival, Isabel had to conduct autopsies on more than 80 bodies of young soldiers that the governm ent assured were fighting a good war for a good cause mainly to defend the interest of my nation. Although l ate to support her through those painful h ou rs by the Pacific Ocean Sergio managed to engage her into a marriage project that took only 2 week s to be organized. In spite of being called molasses by one of his best friend in Gainesville, Richard Barclay, this quick and smooth marriage is proof that Sergio can actually do some things very f ast. Not a dissertation though! Finally, Sergio was admitt ed to candidacy at UF in the Spring of 2004 and graduated, witho ut honors, in the Spring of 2009, 40 years after his wondrous birth day this time away from the mountains of the US and of Colombia and away (only geographically) from his first family