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Geomagnetic Field for the Past 5 Myr Recorded in Lava Flows from British Columbia, Patagonia, and Mexico

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GEOMAGNETIC FIELD FOR THE PAST 5 MYR RECORDED IN LAVA FLOWS FROM BRITISH COLUMBIA, PATAGONIA, AND MEXICO By VICTORIA MEJIA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Victoria Mejia

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I dedicate this dissertation to my parents, Teresita and German.

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iv ACKNOWLEDGMENTS I am very grateful to the chair of my supervisory committee, Dr. Neil Opdyke, for giving me the opportunity to participate in th is project and for his continuous support. I also thank the members of my supervis ory committee: Dr. Dwight Adams, Dr. Jim Channell, Dr. Mike Perfit, and Dr. David Foster, who were supportive and followed the progress of this investigation. I am especially grateful to all the researchers who collaborated with me in this study, particularly those who pa rticipated during field wor k, and from whom I received numerous suggestions and had important di scussions: Dr. Rene Barendregt who sampled the area of British Columbia; Dr. Harald Bhnel who did field work with Dr. Neil Opdyke and me in Mexico; Dr. Juan Francisc o Vilas who did field work with Dr. Neil Opdyke and me in Patagonia, and Dr. Joe Stoner who collected some samples in Patagonia. I also want to thank for comple menting this study by doing radiometric dating on the samples we collected, Dr. Brad Singe r, who dated samples from Patagonia, and Dr. Amabel Ortega-Rivera and Dr. James Lee who dated samples from Mexico. I wish to thank many geologists who opportunely and brie fly helped and shared with us their knowledge of the areas of study during field work, like Dr. Miguel Haller, Dr. Massimo D'Orazio and Dr. Fabrizio Innocenti, in Pata gonia and Dr. Jorge Aranda, in Mexico. I also want to express my gratitude to the numerous persons whose hospitality made field work abroad very pleasant.

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v Likewise, I wish to thank important sugge stions from paleomagnetists such as Dr. Joe Meert, Dr. Lisa Tauxe, Dr. Cat hy Constable and Dr Catherine Johnson. I am greatful to Dr. Kainian Huang and Ray Thomas for their assistance in the paleoamgentic laboratory at the Univiersity of Florida. I am grateful to the staff, faculty and graduate students of the Department of Geology for the team spirit lived during our every day work. I especially thank some graduate students (some of whom have gr aduated): Kusali Gamage, Sergio Restrepo, Jaime Escobar, Dr. Johan Guyodo, Helen Ev ans, Dr. Sharon Kanfoush, Dr. George Kamenov, Dr. Carlos Jaramillo and Dr. John Chadwick. I also want to thank my husband, Jorge Iv an Velez, for his moral support and for his patience, since he remained living in Colombia, and we c ould not share a lot of time together for several years. This study was mostly funded by the Na tional Science Foun dation. Additional funding to support myself was received from a graduate fellowship from the Florida Georgia Alliance for Minority Participati on (for one year) an d the McLaughlin Dissertation Fellowship (for one semester).

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT....................................................................................................................... xi CHAPTER 1 INTRODUCTION........................................................................................................1 Time Average Field (TAF) Models..............................................................................1 Paleosecular Variation (PSV).......................................................................................3 Paleomagnetic Datasets................................................................................................4 Data Quality..................................................................................................................5 Content of the Dissertation...........................................................................................6 2 PALEOSECULAR VARIATION OF BRUNHES AGE LAVA FLOWS FROM BRITISH COLUMBIA, CANADA.............................................................................7 Sampling and Geologic Description.............................................................................8 Laboratory Analysis....................................................................................................14 Data Analysis and Selection Criteria..........................................................................16 Results from British Columbia and Discussion..........................................................18 3 PLIO-PLEISTOCENE TIME AVERAGED FIELD IN SOUTHERN PATAGONIA RECORDED IN LAVA FLOWS...............................................................................25 Sampling and Sampling Area.....................................................................................27 Northern Sampling Area......................................................................................27 Pali-Aike Volcanic Field.....................................................................................28 Paleomagnetic Analysis..............................................................................................28 Data Analysis and Selection Criteria..........................................................................30 Radioisotopic Ages.....................................................................................................34 Northern Sampling Area......................................................................................34 Pali-Aike Volcanic Field.....................................................................................35 Results from Patagonia...............................................................................................36

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vii 4 PALEOSECULAR VARIATION AND TI ME-AVERAGED FIELD RECORDED IN LAVAS FLOWS FROM MEXICO......................................................................41 Geology and Sampling Area.......................................................................................41 Previous Paleomagnetic Studies in the TMVB..........................................................47 New Data....................................................................................................................48 Paleomagnetic Laboratory Work and Data Analysis..........................................49 Results from New Data.......................................................................................50 Compilation of Paleomagnetic Data...........................................................................56 Tectonic Rotations...............................................................................................57 Results from the TMVB.............................................................................................58 Conclusions from Mexico...........................................................................................60 5 PALEOINTENSITY...................................................................................................68 Laboratory Work........................................................................................................70 Data Analysis..............................................................................................................71 Linearity of the Arai Plot.....................................................................................72 PTRM Checks.....................................................................................................72 Quality Factors Established by Coe (1978).........................................................73 Results and Discussion...............................................................................................73 6 CONCLUSIONS........................................................................................................86 Recommendations for Future Studies.........................................................................86 Comparison with Recent Studies................................................................................88 LIST OF REFERENCES...................................................................................................94 BIOGRAPHICAL SKETCH...........................................................................................104

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viii LIST OF TABLES Table page 2-1 Paleomagnetic and age data of sites from British Columbia.....................................9 2-2 Statistical data among groups of sites from British Columbia.................................24 3-1 Paleomagnetic and age data of sites from Patagonia...............................................33 3-2 Statistical data among groups of sites from Patagonia.............................................39 4-1 New paleomagnetic and age data of sites from Mexico..........................................54 4-2 Statistical data among new sites studied in Mexico.................................................55 4-3 Compiled Late Pliocene Holocene paleomagnetic data from the TMVB.............61 4-4 Statistics of paleomagnetic data of late Pliocene to Recent age from studies..........67 4-5 Statistics of late Pliocene to Ho locene age results from compiled data...................67 5-1 Paleointensity results from the first set of samples..................................................77 5-2 Paleointensity results from the second set of samples.............................................79 6-1 Summary of recent secular variation studies ........................................................92

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ix LIST OF FIGURES Figure page 2-1 Location map showing sampling sites in British Columbia.......................................8 2-2 Geology of the Silverthrone volcanic field and sampling-site locations.................12 2-3 Geology of the Clearwater Wells Gray Park volcanic field and sampling-site.....15 2-4 Pair of Zijderveld diagrams show ing AF and thermal demagnetization..................17 2-5 Equal area projection of site-mean directions from British Columbia.....................18 2-6 Equal area projection of mean dir ections from several groups of sites....................20 2-7 Mean virtual geomagnetic poles ( VGPs) obtained from British Columbia.............23 3-1 Location map of southern Pa tagonia showing sampling sites..................................26 3-2 Examples of Zijderveld di agrams obtained from Patagonia....................................29 3-3 Equal area projection of site mean directions from Patagonia.................................31 3-4 Paleomagnetic results from Patagonia expressed in VGPs......................................32 3-5 Equal area projection of mean dir ections from several groups of sites....................39 4-1 Location of sampling areas in Mexico.....................................................................44 4-2 Location of compiled paleomagnetic sites ( 95 < 10o) from the TMVB...................464-3 Examples of Zijderveld diagrams from Mexico.......................................................51 4-4 Remagnetization circles and directions form samples of Site 17 (Mexico).............52 4-5 Equal area projection of paleomagne tic directions obtained from Mexico..............53 4-6 Equal area projection of paleomagne tic directional data compiled from.................59 4-7 Virtual geomagnetic poles (VGPs) from the TMVB...............................................61 5-1 Successful paleointensity result s of two samples from site 1-2...............................80

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x 5-2 Successful paleointensity result s of two samples from site 25-2.............................81 5-3 Successful paleointenity results fr om samples 25-7-6 (a) and 25-8-4 (b)................82 5-4 Successful paleointenity results from samples 14-1-5 (a) and 718 (b)....................83 5-5 Examples of rejected paleointensity results.............................................................84 5-6 Examples of rejected paleointensity results from of samples from site 25-3...........85 6-1 VGP scatter of studies in Table 6-1 (d iamonds) compared to model G (curve)..89 6-2 Magnetic inclination pl otted against latitude...........................................................90 6-3 Comparison of GAD and GAD plus 5% quadrupole, plus 7% octupole.................91

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xi Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy GEOMAGNETIC FIELD FOR THE PAST 5 MYR RECORDED IN LAVA FLOWS FROM BRITISH COLUMBIA, PATAGONIA, AND MEXICO By Victoria Mejia May 2005 Chair: Neil D. Opdyke Major Department: Geological Sciences Paleosecular variation (PSV) and time aver aged field (TAF) results recorded in lava flows younger than 5 million years are pres ented. The targeted areas of studies are several volcanic fields from British Columbia (mainly the Silverthrone, Garibaldi, and Wells Park volcanic fields), Southern Patagoni a (the Pali-Aike volcan ic field and Meseta Viscachas lavas), and Mexico (the TransMexican volcanic belt and several volcanic areas in San Luis Potosi). The purpose of this investigation was to obtain high quality paleomagetic data suitable to test the presence or absence of permanent non dipolar components of the field that have been interpreted from studies carried out with less rigor. The mean directions in the areas of British Columbia and Patagonia (roughly at 50o N and 50o S latitude) coincide with the expected geocentric axial dipole (GAD) at these areas. The presence of a quadrupolar component of the field is difficu lt to discard because it is expected to produce only about 1o shallower inclinations. The mean direction in the area of Mexico

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xii coincides with a GAD plus a 5% quadrupole. Th e VGP scatter in the three areas of study coincides with Model G. The asymmetry between the northern and southern hemisphere of the present magnetic field and particularly the 20o inclination anomaly rela tive to GAD in Patagonia, are not observed in the paleomagnetic data obtained, implying that the present field configuration is relatively recent. The result s confirm that axial components prevail in the time-averaged field.

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1 CHAPTER 1 INTRODUCTION The characteristics of th e present Earths magnetic field are well known from magnetic observatories, satellite airplane and ship data, but the history of the magnetic field blurs as we go into the past. The impl ementation of the magnetic compass in Europe by 1200 ultimately made possible historic reco rds of the magnetic field. Compilations of historic records dating back to about 1600 have been used to model the magnetic field (Jackson et al., 2000). The Earth's magnetic fi eld has also been modeled on other time scales such as the last 3000 years usi ng archaeomagnetic data derived from pottery (Constable et al., 2000) and in geologic times (the past 5 Myr) using paleomagnetic data derived from rock materials (e.g. Johnson and Constable, 1995). The analysis of these different kinds of records of the magnetic field has helped to understand both the time averaged field (TAF) and its secular variation, which applie d to the geologic past, is referred to as paleosec ular variation (PSV). Time Average Field (TAF) Models The TAF can be described as an expans ion of spherical harmonics, and, although a non-unique solution, the TAF is represented in most studies as mainly (about 95%) composed of a geocentric axial dipole term (G AD). Because of the de ficiencies in quality and spacial coverage of paleomagnetic data, it is still uncertain as to which other nondipolar terms best characte rize the paleomagnetic field. Besides the GAD (Opdyke and Henry, 1979), the most likely term present in the TAF is an axial quadrupole of about 5% of the magnetic field (e.g., McElhinny et al., 1996; Johnson and Constable, 1995;

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2 Hatakeyama and Kono, 2002). The quadrupole fi eld is regarded as more promising because of the consistency with which it is observed in paloemagnetic data (McElhinny et al., 1996). The quadrupole field was firs t observed by Wilson (1970) who noticed that it causes a far sided effect. That is, the virtual geomagnetic poles (VGPs) plot away from the pole with respect to the sampling area. The quadrupole field is observed in paleomagnetic datasets as consistent negative inclination anomalies (observed inclinations minus expected inclination fr om GAD) for the normal polarity data, which imply shallower inclinations than expected (positive or downwar ds) in the northern hemisphere and steeper inclinations than exp ected (negative or upwar ds) in the southern hemisphere. Likewise, the quadrupole field is manifested by consistent positive inclination anomalies in the reverse polarity data, which imply steeper inclinations than expected (negative) in the northern hemisphere and shallower inclinations than expected (positive) in the southern hemisphere. It is difficult to think of any source of error that could produce such pattern of inclination anomalies, which provides certainty that the axial quadrupole term is real and not an artifact. Another non-dipole term, the axial octupole, is expected to produce inclinations throughout all latitudes to be sh allower (or steeper if the term has a negative sign). It is therefore more likely to conf use the axial octupole with artifacts of the paleomagnetic data. Despite obtaining a signifi cant octupole term by analyz ing paleomagnetic dataset, some authors regard this term as an ar tifact (e.g., McElhinny et al., 1996) while others consider it real (Hatakeyama and Kono, 2002). Likewise the ex istence of persistent nonaxial terms, resulting in longitudinal stru ctures, has been suggested (e.g., Johnson and

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3 Constable, 1995; Johnson and Constable, 1997) nonetheless with a great deal of uncertainty due to insufficient data. Paleosecular Variation (PSV) PSV is most often analyzed by studying the VGP scatter relative to the axis of rotation among a group of paleomagnetic sites in a region. Paleosecular variation studies show that the value of VGP scatter increases with latitude. The pa leosecular variation model that best fit the observ ations is Model G (MacFadden et al., 1988). In Model G, VGP scatter is analyzed separately for the dipole and quadrupole family of spherical harmonics (i.e., spherical harmonic terms with Gauss coefficients in which degree (n) minus order (m) are odd or even numbers, resp ectively). According to this model the scatter due to the dipole family of harmonics varies with latitude and the scatter due to the quadrupolar family of harmonics is cons tant though out all lat itudes. The averaged VGP scatter of the present magnetic field fo r the northern and southern hemispheres is similar to Model G. Another observable of PSV is the dist ribution of the directions and VGPs. According to several studies (e.g., Tanaka, 1999; Tauxe and Kent, 2004), the distribution of VGPs is expected to be ci rcularly symmetric at all latit udes, while the distribution of direction is expected go from elliptical at lo w latitudes to circular at high latitudes. The Mu and Me parameter values of circularly symmetri c (or fisherian) dist ributions should be 1.207, and 1.094, respectively (Tauxe, 1998). These parameters are derived from the comparison of the declination and inclina tion distributions wi th the correspondent theoretical distribution of a Fisher distri bution. Tauxe and Kent (2004) have model the expected north-south elongation of the 95% c onfidence ellipses as a function of latitude.

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4 Paleomagnetic Datasets Two main archives of paleomagnetic data are available from different sources: sediments and lavas. PSV studies in which lava flows are used as recording materials are generally referred to as PSVL (acronym for pa leosecular variation in lavas). Lava flows, which were used for this study, have the abilit y to record the Earths magnetic field at the time they cool. The period of time in which a lava flow cools is so short (around a year) relative to our resolution of ge ologic time in general, that we can consider lava flows as archives of instant readi ngs of the paleomagnetic field. Lee (1983) compiled a paleomagnetic database from lavas of th e past 5 Myr. This database has been subsequently updated by Quidelleur et al. (1994), Johnson and Constable (1996) and McElhinny and McFadden (1997). After applica tion of different selection criteria the number of records in the renovated databases is not much greater than the 2244 records in Lees original compilation. The ch aracteristics of any of these datasets are far from being ideal; the main deficiencies are that (a) the sample distribution over the Earth is deficient and not uniform, (b) most studies have been undertaken using paleomagnetic procedures that are now obsolete and (c ) age control is limited. This study emerged upon realizing the need to improve the paleomagnetic dataset to better constrain time-averaged field (TAF) and secular variation models. The research project that emerged from this initiative, called the time averaged field initiative (TAFI), was undertaken with NSF funds by scientists fr om several institutions such as the Scripps Institute of Oceanography, University of Florida, University of Alaska, and University of Massachusetts. The areas of study were strategically chos en to improve the spatial and temporal coverage of the paleomagnetic datasets and tectonic stability was taken into

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5 consideration. For this reason, this study comprises only the pa st 5 Myr, a period of time in which significant influences of plate t ectonic movements are av oided. Most of the studies belonging to the TAFI initiative are located along th e backbone of the Americas, from the Arctic to the Antarctic. The target areas for this dissertation were several volcanic fields from British Columbia southern Patagonia, and Mexico. Data Quality The selection criteria for acceptance of paleomagnetic results are somewhat arbitrary, despite that data quality is a key issue for obtaining accurate non-dipole components of the TAF and secular variation estimates. Generally, for example, studies from individual areas in which blanket or no demagnetization have been applied are discarded and a maximum cut-off value of 95 (the 95% confidence cone around the mean direction) is determined. With in creasing attention placed on achieving very accurate results, recent paleomagnetic results from specific areas on Earth (e.g. Carlut et al., 2000; Johnson et al., 1998; Tauxe et al., 2000) have greatly superseded previous selection criteria. Tauxe et al. (2000) set up new selec tion criteria in which the 95 of individual sites is restricted to 5o (for comparison McElhinny and McFadden (1997) used 20o). Such selection criteria are attainable not just by discarding a lot more data but by implementing stringent laboratory and data anal ysis techniques that improve the accuracy of the results. Tauxe et al. (2000) applied seve ral strategies to obtai n paleomagentic data of improved quality. One of them emplaced in the laboratory is to increase the number or demagnetization steps that are often perfor med (from around 7 to 10 steps to around 15 to 18). This strategy enables us to resolve th e primary component of magnetization more accurately and to discard spur ious data more easily. Anot her strategy to improve the quality of the paleomagnetic data is perf ormed during data analysis. The principal

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6 component of magnetization is obtained from at least 5 points of the demagnetization curve (without anchoring to the origin) and the MAD that cannot exceed 5o. The measures of Tauxe et al. (2000) to achieve good data quality were followed in the studies described in this dissertation. These measures increase the expectati ons of obtaining data sensitive to non-dipole (5-10% of the Earth's field) components of the field. Content of the Dissertation Chapters 2, 3, and 4 contain the paleomagne tic studies from the three areas of study chosen for this dissertation: British Co lumbia, Patagonia and Mexico respectively. Results from the studies in British Columbia (chapter 2) and Patagoni a (chapter 3) have been published (Mejia et al., 2002, and Mejia et al., 2004, respectively) and results from the study of Mexico (chapter 4) are in the process of bei ng published (Mejia et al., 2004, submitted). Many segments, tables, and figures from these articles are found throughout the dissertation in the same or somewhat modifi ed way that they appear in the journals. In chapter 5 some results of pale ointensity analysis are presen ted. Finally, chapter 6 presents the conclusions and inspection of recent paleomagnetic studies of the same kind presented here. Part of the work of analyz ing recent studies has been already published (Opdyke and Mejia, 2004).

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7 CHAPTER 2 PALEOSECULAR VARIATION OF BRU NHES AGE LAVA FLOWS FROM BRITISH COLUMBIA, CANADA Paleomagnetic results from 53 sites from s outhern British Columbia (latitude 5051.5o N) are presented. Samples were taken fr om the Silverthrone Mount Meager, and Garibaldi Lake volcanic fields of the Garibaldi Volcanic Belt, as well as from the Wells Gray Clearwater volcanic fi elds of the Anahim Volcanic Belt and the Kelowna area field of the Chilcotin Plateau Basalts (Fi gure 2-1). These volcanic areas rest on metamorphic rocks and were produced in a va riety of tectonic settings: arc volcanism (Garibaldi Volcanic Belt), hot spot volcan ism (Anahim Volcanic Belt), and back arc volcanism (Chilcotin Plateau Basalts). Volcanis m in this area both predates and postdates Pleistocene glaciations. During the glacial and interglacial periods, se diments of glacial, fluvial, and lacustrine environments we re deposited along with volcanic products, including the lava flows that are the focus of this study. A rich variety of geomorphic shapes such as tuya volcanoe s, horns, glacial valleys, and moraines are a legacy of the glacial age. Many of the lava flows studied here are valley-filling basalts and display a wide variety of sub-aqueous and sub aerial textures. Age control was achieved by: (a) paleomagnetic sampling of previously studied la va flows, and (b) accepting the age of the volcanic unit given in the li terature. Based on these ages the sampled lava flows represent mostly the last 550 ka, although flows ranging from 550 to 760 ka were also sampled.

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8 Figure 2-1. Location map showing sampling site s in British Columbia (black dots) and their correspondent volcanic fields. Sampling and Geologic Description A total of 53 paleomagnetic sites were sampled by Barendregt during the summers of 1998 and 1999. Samples were taken using a hand held drill and oriented using magnetic compass and, when possible, sun compass (30% of the samples). From 8 to 10 cores were collected from all sites, each site corresponding to a single lava flow. Site location was determined using GPS. Most sites were accessed by road and short hikes were necessary. A helicopter was used to access two flows. Most of the sampled lava flows fill valleys and are exposed as a result of subsequent erosion. Other lava flows were sampled closer to volcanoes and are at a higher alti tude. Several sites ar e from a series of overlapping lava flows, generally from 3 to 9 in number. The lava sequences and their stratigraphic relationship ar e given in Table 2-1.

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9Table 2-1. Paleomagnetic and age data of sites from British Columbiaa Site U/ DG V.F. Site Site D I SCn/N K 95 VGP VGP 95 K AF/Age U (+/-) R Meth. ID L Lat. Long. Dir. Dir. Lat. Long. VGP VGP Th (ka) (ka) 1-1 MM 50.68 -123.48 23.2 53.2 10 6/10 642 2.6 66.1 2.5 3.3 410 Th 2.34 0.05 1 14C 1-2* MM 50.65 -123.44 23.1 56.2 10 10/10 110 4.6 68.8 357.1 5.7 73 Th 2.34 0.05 1 14C 1-3 GL 50.07 -123.04 41.3 77.5 12 11/12 350 2.4 63.6 273.7 4.4 109 Th 50 to 150 2 ST 1-4 U GL 50.07 -123.09 342.1 68.7 10 10/10 401 2.4 78.5 164.1 3.9 157 Th <34.2 3 14C 1-5 L GL 50.04 -123.11 323.1 77.0 10 10/10 502 2.2 65.6 199.5 3.9 153 Th <34.2 3 14C 2-1 U ST 51.62 -126.62 343.4 68.1 0 6/7 146 5.6 79.8 148.4 8.8 60 Th 150 2 K/Ar 2-2 ST 51.62 -126.62 46.0 56.4 0 7/7 327 3.3 54.7 329.3 3.9 241 Th 150 2 K/Ar 2-3 L ST 51.62 -126.62 311.5 74.0 0 6/7 21 14.8 62.7 180.2 26.2 7 Th 150 2 K/Ar 5-1 ST 51.62 -126.60 358.5 67.3 0 5/7 488 3.5 88.3 87.2 5.0 234 Th 12 4 14C 7-1 ST 51.59 -126.45 6.3 61.3 0 7/7 68 7.4 80.7 22.2 9.5 41 Th 150 2 K/Ar 8-1 ST 51.59 -126.45 5.1 56.1 0 7/7 523 2.6 74.7 37.7 3.4 322 Th 150 2 K/Ar 9-1 L a ST 51.61 -126.40 1.6 67.9 0 7/7 696 2.3 88.8 356.7 3.5 301 Th 150 2 K/Ar 9-2 ST 51.61 -126.40 356.2 72.8 0 6/6 102 6.7 82.3 217.5 11.5 35 Th 150 2 K/Ar 9-3 ST 51.61 -126.40 357.9 63.6 0 7/7 93 6.3 83.6 65.2 9.7 40 Th 150 2 K/Ar 9-4 a ST 51.61 -126.40 8.9 72.2 0 6/7 480 3.1 82.2 272.1 5.0 180 Th 150 2 K/Ar 9-5 U ST 51.61 -126.40 1.3 68.3 0 6/7 409 3.3 89.2 326.9 5.5 152 Th 150 2 K/Ar 10-1 ST 51.55 -126.35 12.6 68.9 0 8/8 206 3.9 82.1 311.8 6.2 80 Th 150 2 K/Ar 13-1 ST 51.59 -126.43 23.1 67.9 0 5/7 352 4.1 75.6 316.9 6.6 137 Th 150 2 K/Ar 14-1 ST 51.59 -126.43 13.0 68.0 0 7/8 254 3.8 82.0 320.7 5.9 107 Th 150 2 K/Ar 15-1 ST 51.59 -126.43 14.7 68.7 0 7/7 391 3.1 80.9 313.6 4.7 177 Th 150 2 K/Ar 16-1 U ST 51.40 -126.31 348.3 58.3 0 5/10 476 3.5 75.2 91.7 4.4 310 Th 400 100 2 K/Ar 16-2 L ST 51.40 -126.31 357.1 51.3 0 10/10 96 4.9 70.7 60.2 6.0 65 Th 400 100 2 K/Ar 18-1 CW 52.13 -120.23 5.7 69.2 1 8/8 151 4.5 86.4 309.3 7.3 59 AF300 300 5 K/Ar 19-1 CW 52.17 -120.22 23.6 71.6 0 6/10 576 2.8 75.6 303.3 4.4 231 AF300 300 5 K/Ar

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10Table 2-1. Continued U/ DG V.F. Site Site D I SCn/N K 95 VGP VGP 95 K AF/Age U (+/-) R Meth. ID L Lat. Long. Dir. Dir. Lat. Long. VGP VGP Th (ka) (ka) 21-1 U b CW 51.95 -120.08 15.4 85.8 0 9/12 898 1.7 60.0 244.4 3.4 232 Th 200 / 280 110 / 1506 K/Ar 21-2 L b CW 51.95 -120.08 78.9 88.0 0 8/11 155 4.5 52.5 246.2 8.8 40 Th 200 / 280 110 / 1506 K/Ar 22-1 U c CW 51.93 -120.03 26.2 73.5 0 7/10 610 2.4 73.6 292.3 4.2 203 Th 561 106 6 K/Ar 22-2 c CW 51.93 -120.03 20.4 76.2 11 9/11 459 2.4 73.9 273.7 4.3 145 Th 561 106 6 K/Ar 22-3 L c CW 51.93 -120.03 9.7 74.6 10 8/10 157 4.4 79.1 265.2 7.5 55 Th 561 106 6 K/Ar 23-1 CW 51.79 -120.01 312.0 66.4 9 3/12 291 7.2 59.9 164.0 10.3 145 Th 547 102 6 K/Ar 24-1 CW 51.77 -120.01 330.8 62.0 0 4/8 166 7.1 69.0 138.4 9.4 96 AF547 102 6 K/Ar 25-1 CW 51.73 -120.01 348.3 66.7 0 11/12 323 2.5 82.1 138.1 3.9 140 th 547 102 6 K/Ar 25-2 U CW 51.73 -120.01 336.3 69.3 0 10/10 596 2 75.6 164.5 3.2 222 th 547 102 6 K/Ar 25-3 CW 51.73 -120.01 329.7 64.0 0 9/10 665 2 69.3 145.3 2.8 329 th 547 102 6 K/Ar 25-4 CW 51.73 -120.01 359.6 68.4 0 10/10 290 2.8 89.6 141.8 4.6 112 th 547 102 6 K/Ar 25-5 CW 51.73 -120.01 341.9 70.0 0 8/10 814 1.9 78.8 168.9 3.0 338 th 547 102 6 K/Ar 25-6 d CW 51.73 -120.01 327.6 70.9 8 10/10 398 2.4 70.6 173.7 4.0 148 th 547 102 6 K/Ar 25-7 d CW 51.73 -120.01 324.8 66.9 0 10/10 239 3.1 67.8 158.6 4.9 98 th 547 102 6 K/Ar 25-8 e CW 51.73 -120.01 351.9 66.5 3 7/10 220 4.1 84.4 126.9 6.0 102 th 547 102 6 K/Ar 25-9 e CW 51.73 -120.01 350.3 66.5 0 10/10 237 3.1 83.3 133.5 4.4 122 th 547 102 6 K/Ar 25-10 L e CW 51.73 -120.01 350.6 65.1 0 11/11 134 4 82.8 120.3 5.7 65 th 547 102 6 K/Ar 26-1 L f CW 51.68 -120.05 350.5 68.7 10 8/10 401 2.8 84.1 157.6 4.5 152 th 350 90 7 K/Ar 26-2 f CW 51.68 -120.05 344.1 66.7 10 7/10 226 4 79.8 143.5 6.3 93 th 350 90 7 K/Ar 26-3 U f CW 51.68 -120.05 351.8 61.8 10 5/10 333 4.2 79.9 96.4 5.1 224 th 350 90 7 K/Ar 27-1 L g CW 51.67 -120.04 327.8 71.1 0 8/10 202 3.9 70.7 175.4 6.2 81 th 350 90 7 K/Ar 27-2 g CW 51.67 -120.04 314.2 66.0 10 10/10 101 4.8 60.8 162.9 7.3 45 th 350 90 7 K/Ar 27-3 U g CW 51.67 -120.04 311.1 69.2 0 10/10 169 3.7 60.9 172.0 6.0 65 th 350 90 7 K/Ar 28-1 CW 51.63 -120.13 332.169.3 9 9/10 200 3.6 73.0 165.5 6.0 74 th 350 / 500 90 / 50 6 K/Ar

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11Table 2-1. Continued Site U/ DG V.F. Site Site D I SC n/N K 95 VGP VGP 95 K AF/Age U (+/-) R Meth. ID L Lat. Long. Dir. Dir. Lat. Long. VGP VGP Th (ka) (ka) 29-1 L KA 49.83 -119.75 3.3 81.9 0 11/11 355 2.4 65.6 242.3 4.7 96 th 760 110 8 K/Ar 29-2 h KA 49.83 -119.75 345.9 77.6 0 10/11 821 1.7 72.0 221.8 3.1 241 th 760 110 8 K/Ar 29-3 U h KA 49.83 -119.75 343.3 76.4 0 9/10 744 1.9 73.1 214.8 3.4 226 th 760 110 8 K/Ar aSite identification number (ID) is composed of two numbers separated by a dash. Si tes with the same number before the dash usua lly belong to stratigraphic sequences. Asterisk is indicated in a site that was not cons idered for paleomagnetic calculations. Th e number after the dash indicates the posit ion of the lava flow in the sequence (upwards or downwards) U/L indicate the uppermost and lowermost lava flows of these sequences. DG refers to directi onal group (see text for explanati on). Volcanic field abbreviation s are: MM for Mount Meager; GL for Garibaldi Lake ; ST for Silverthrone; CW for Clearwater Wells Gray Park and KA for Kelowna Area. D and I are the mean site declination and inclination ; SC indicates the number of samples orie nted with sun compass in the site; n/N are the number of samples used for cal culation of site mean direct ion per number of processed samples. K is the dispersion parameter of directions (dir) or VGPs; 95 is the 95% confidence cone about the mean di rection (Dir.) or mean VGP; AF/Th represent mean direction results after AF or thermal demagnetization. Age is the age(s) obtained in previous studies from the flow or vol canic unit (see text). U is the uncertainty range of the age. R is the reference source on which a give n age is based: (1) is Read, 1990 and Hickson et al, 1999; (2) Green et al., 1988; (3) Green, 1977; (4) Roddick, 1996; (5) Hickson et al., 1995; (6) Hickson and Sou ther, 1984; (7) Hickson, 1986; (8) Church, 1980. Meth. is the method used to obtain the age (Str. = stratigraphic relationship).

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12 Figure 2-2. Geology of the Silverthrone vol canic field and sampling-site locations (triangles) (modified from Green et al., 1988). The si tes that fall outside the mapped area belong to the unit of ba saltic andesite flows (crosses). The Silverthrone volcanic field (figure 2) was studied by Green et al. (1988) who defined three units: (a) brecc ias overlying metamorphic rocks in angular discontinuity; (b) ryolitic, dacitic, and andesitic flows ove rlying the breccia, and (c) unconsolidated fluvial and volcano-sedimentary deposits re sting on deeply eroded flow surfaces. The main focus of sampling in this volcanic fi eld were the basaltic andesite flows which originated from numerous centers and filled the Machmell and Pashleth river valleys and are locally interstratified with sediments (uni t c). Sites 16-1 and 16-2 were sampled from older andesitic flows near Ki ngcome Glacier (unit b). An age of 0.4 0.1 Ma (K-Ar) was

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13 obtained by Green et al. (1988) at a nearby site. The rema ining 15 paleomagnetic sites from this volcanic field were obtained from the valley filling basaltic andesitic flows of unit c. The radiometric dates obtained for some of these basaltic flows place them in the last 150 Ka (Green et al., 1988) Site 5-1 was taken from a lava flow associated with a boulder covered in part by barn acles which yielded a 12 Ka 14C radiometric date (Roddick, 1996). Sites from this volcanic field are indicated in table 1 by the abbreviation ST. The Garibaldi Lake Volcanic Field wa s studied by Green et al. (1988) and comprises nine volcanic centers that have been active through the Quaternary. Three flows were sampled from the volcanic center that occupies the Cheakamus river valley. One of these flows (site 1-3) corresponds to a unit that locally contains pebbles striated in two directions suggesting that the unit predates the Fraser Gl aciation ice sheet, which has an age of 50 Ka. The other two sites (sites 1-4 and 1-5) correspond to lava flows that overlie the previously descri bed unit and underlie glacio-la custrine sediments with a 14C radiometric date of 34.2 Ka (Green, 1977). Sites from this volcanic field are indicated in Table 2-1 by the abbreviation GL. The Mount Meager Volcanic Field has b een active since the Pliocene (Green, 1988) and has several volcanic centers. The onl y lava flow that was sampled here was a dacitic lava flow, erupted from Mount Meager along with other volcanic products (volcanic ash and pyroclastic flows) and has a 14C radiometric date of 2350 yr BP (Read, 1990 and Hickson et al. 1999). This lava flow was sampled twi ce (site 1-1 and site 1-2), and although the range of uncerta inty of the directions at bot h sites overlap, the result of

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14 site 1-2 was discarded because of field obs ervations and site 1-1 was preferred. Sites from this volcanic field are indicate d in table 1 by the abbreviation MM. Lava flows from the Clearwater Wells Gray Park Volcanic Field (Figure 3) are exposed along the Clearwater and Murtle river basins. Hickson and Souther (1984) differentiated four volcanic assemblages that began forming 3 Ma ago: (a) an early glacial assemblage; (b) a deeply dissecte d, valley-filling assemblage composed of basaltic lava flows originati ng from different eruptive center s, and locally interstratified with sediments, which forms the main focus of sampling in this area; (c) a late intraglacial assemblage that includes ice-c ontact deposits, and (d) pos t-glacial pyroclastic cones and lava flows. Twenty-eight basaltic lava flows (sites 20-1 to 28-1) were sampled from assemblage (b). We assigned ages to these flows, ranging between 0.2 and 0.5 Ma, based on K/Ar ages obtained from nearby outcr ops which were studied previously (table 1). Sites 18-1 and 19-1 were obtained from eroded lava conduits near Ray Mountain Volcano dated at 0.03 0.03 Ma (K-Ar) (Hickson et al., 1995). Sites from this volcanic field are indicated in table 1 by the abbreviation CW. Lavas from the Kelowna area (Table 1) were sampled from eroded Pleistocene volcanic outcrops. Three overlapping lava flow s at Lambly Creek, near Kelowna, were sampled. A radiometric date of 0.76 0.11 Ma (K/Ar) was given by Church (1980) to these flows. Sites from this volcanic field ar e indicated in table 1 by the abbreviation KA. Laboratory Analysis Most of the directions derived from sun and magnetic compass readings coincided within few degrees (<3o). Directions from sun compass r eadings were used at four sites (sites 1-1, 1-2, 1-5 and 26-3) in which some of the directions derived from sun compass and magnetic compass differed 5o or more.

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15 Figure 2-3. Geology of the Clearwater Wells Gr ay Park volcanic field and sampling-site locations (dots) (modified from Hickson and Souther, 1984). Stepwise AF demagnetization was perf ormed on one sample per site. AF demagnetization was done using a Schonstedt AF demagnetizer and later a Dtech D-200 AF demagnetizer. All samples were thermally demagnetized in 14 to 21 steps utilizing a Schonstedt oven. The measurements were ma de in a 2G Cryogenic magnetometer in a shielded room at the University of Florid a. Generally good agreement between AF and thermal demagnetization was observed (Figures 2-4a to 2-4c) and the results from thermal demagnetization were used to calcula te the primary component of magnetization of most sites. However, AF demagnetization gave better results (F igure 2-4d) at sites 231, 18-1, 19-1, and 24-1 and was used for analys is of the latter three sites. Magnetic susceptibility was monitored during ther mal demagnetization, using a Sapphire susceptibility meter (except for samples from Clearwater-Wells Gray Park and Lambly

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16 Creek volcanic fields). The behavior of th e basaltic rock material during laboratory analysis was in general typical for subaeria l basalts. The NRM intensity varied between about 1 to 10 A/m, but was more commonly around 4 A/m. The principal component of magnetization was easily defined by removi ng the viscous remanent magnetization (VRM) in the first few demagnetization steps. In most cases a small (<5%) or no fraction of the NRM remained after the 570oC heating step, suggesti ng titanomagnetite as the magnetic carrier; however few sites stil l had >5% of the NRM remaining at 600oC (the highest temperature step app lied) which can indicate the presence of small amounts of hematite. Data Analysis and Selection Criteria In order to obtain suitable measurements of the paleomagnetic field and avoid spurious data that could blur real non-dipole contributions to magnetic field models, it is necessary to establish stringent procedures and selection criteria for statistical parameters. The data analysis procedures and selection cr iteria used in this study closely follow those proposed by Tauxe et al. (2000) and s upersede those of McElhinny and McFadden (1997). The direction of the primary compone nt of magnetization wa s obtained through principal component analysis (Kirschvink, 1980) using a segmen t of at least 5 points of the demagnetization curve direct ed toward the origin with a maximum angular deviation (MAD) 5o. Site-mean directions (Figure 2-5, Tabl e 2-1) were obtaine d applying Fisher (1953) statistics from at least 5 cores and selected for further analysis if 95 values were 5o (rounded to the nearest in teger). Based on thes e criteria, we selected 45 of the 52 flows sampled; that is, 7 sites were rejected (Figure 2-5).

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17 Figure 2-4. Pair of Zijderveld diagrams sh owing AF and thermal demagnetization (left and right side plots, respectively) on replicate samples from British Columbia. Good agreement between the two methods is shown in a, b and c. Difficult to interpret thermal demagnetization curves relative to AF de magnetization, with both methods still indicating the same general direction, is shown in d. Approximate intensity values in A/m. Secular variation analysis were made by calculating the angular standard deviation (Sb) of the virtual geomagnetic poles among th e selected group of s ites with respect to the Earths axis of rotation using the met hod described by Johnson and Constable (1996) in which the effect of within-site scatter is subtracted from the total scatter. The 95% confidence limits of the resultant scatter (a ngular standard deviat ion) were calculated using the method described by Cox (1969).

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18 Figure 2-5. Equal area projection of site-mean directions from British Columbia. Squares represent directions of lava flows 150 Ka and circles repr esent directions of lava flows 200 to 760 Ka old. Rejected si tes are shown with dots inside their respective symbols (squares or circle s). All inclination point downwards. Results from British Columbia and Discussion The directions of sites within volcanic fiel ds tend to cluster to gether indicating that their ages are closer in time compared to the sites from other volcanic fields. More precisely, it can be observed that you nger sites (1-1 to 15-1) that are 150 Ka and older sites (16-1 to 29-3) that are 200 to 760 Ka (table 1) show a distinct distribution of the paleomagnetic directions in time (Figure 2-5). The mean directions among the younger and older lava flows have circ les of confidence that barely overlap and only the mean direction from the group of younger lavas co incides with the GAD (Figure 2-6, Table 22). Given the age uncertainty of the lava flow s and the episodic nature of volcanism, it is uncertain to what extent has the paleomagnetic field been sampled in time. That is, it is not known which fractions, corresponding to peri ods of enhanced volcanism, of the total period of time being studied (mainly the last 560 Kyr) have been sampled.

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19 The mean direction of magnetization and the mean VGP of the 45 selected lava flows with 95 < 5o (Table 1) coincide at the 95% c onfidence level, with the expected direction of the GAD (Inc = 68.3o) in the area (mean latitude = 51.5o N) and the geographic north respectively (Figures 2-6 and 2-7). The directional distribution is fisherian while the VGP distri bution is not. Similar results are observed when considering all the lava flows (including results from the 7 la va flows that were re jected), but in this case, at the 95% confidence level, the mean direction almost falls away from the GAD and the mean VGP does not coincide with the Earths axis of rota tion. The shift in the direction obtained by taking into considerati on the previously dis carded sites produces a shift of the mean direction away from the GAD de spite the fact that 5 of the rejected sites belong to the group of younger flows that te nd to plot closer to the GAD. With a relatively small number of observations, it would be premature to argue that the application of selection criter ia yields better results. St udies that explain the possible underlying reasons supporting the use of selection criteria are necessary. The mean direction of the selected sites li es somewhat closer to the group of older flows, because they are represented by more than twice as many flows as the group of younger lavas, although that does not imply an uneven sampling in time since the group of older flows represents about twice th e period of time represented by the younger group (only the three flows from the Kelowna area are older than 560 Ka). This observation highlights the need for unif orm sampling through time. Results of sites obtained from lava sequen ces deserve a closer examination because they are also likely to incorporate a bias by over-sampling a particular period of time. In order to investigate this problem, mean di rections from contiguous lava flows having

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20 Figure 2-6. Equal area projecti on of mean directions from several groups of sites from British Columbia. Comparisons of the mean direction from the selected group of sites (gray-filled circ le), GAD (triangle pointing down) and IGRF (triangle pointing up) are shown in plot (a) wi th the mean direction among the younger group of lava flows ( 150 Ka) (gray-filled diamond) and the older group of lava flows (200 to 760 Ka) (striped diam ond); and in plot (b ) with the mean direction of all lava flows (square ) and filtered data (empty diamond). directions with overlapping circles of c onfidence were obtained, and were used to calculate an overall mean dire ction along with the remaining flows that pass the selection criteria. Mankinen et al. ( 1985) applied this method and identified directional groups

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21 among the flows having overlapping circles of confidence. The same terminology is used here, and flows conforming to directional gr oups are shown in Tabl e 2-1. This procedure produces a mean direction and mean VGP (Table 2-2 and Figure 2-7) that is very similar to the unfiltere d result, although 95 increases and the mean is slightly displaced toward the GAD. It is uncertain that the process of filtering the data benefits the quality of the results. The inclusion or exclusion of sites from the group of older or younger flows slightly shifts the balance between the two groups, drivin g the mean in one way or another. In this case more data points were taken out from the older group (13) than from the younger group (3), driving the mean (of filtered site s) toward the cluster of younger sites and toward the GAD (Figure 2-6). Instead, it is uncertain if contiguous lava flows with overlapping 95% circles of confid ence record a relatively stationary direction of the field rather than that those flows erupted in a very short period of time (Love, 2000). The validity of this concern is observed in the overlapping directions between flows 9-4 and 9-5 which are separated by a soil layer, t hus indicating a substantial period of time between the two flows (for this reason these two flows were not included in the same directional group, see Table 2-1) Several cases in which f iltering has been applied in lava sequences have also produced mean directional results that do not differ substantially from those obtained from the unfiltered data (McElhinny et al., 1996b; Szeremeta et al., 1999; Laj et al., 1999). In ge neral, it can be said that the relative low accumulation rate in the Garibaldi volcan ic belt (Sherrod and Smith, 1990) and the presence of multiple volcanic centers in th e Silverthrone and Wells-Grey Park volcanic fields helped to evenly sample the paleomagnetic field in time.

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22 The fact that the mean direction and mean VGP among the selected flows and filtered dataset coincide with the expected GAD and geograp hic pole, respectively, does not suggest a significant pe rsistent contributi on of the non-dipole components of the field. Particularly the fa r-sided effect (Wilson, 1970), although expected to be relatively small at this latitude, that would cause the VGP to plot further from the North Pole with respect to the sampling area, is not observed. Likewise, the small negative inclination anomaly (-2o to -4o) for the area obtained as a result of modeling Brunhes-age paleomagnetic records from lava flows (Johnson and Constable, 1995) is not present in the dataset. The corresponding angular standa rd deviation of the VGPs relative to the Earths axis of rotation (Sb) for the selected sites is 17.5o with lower (Sl) and upper (Su) 95% confidence limits (Cox, 1969) of 20.5o and 15.2o respectively (Table 2-2). These values are within the range expected from Model G (McFadden et al., 1988) of secular variation for that latitude (17.4o with upper and lower confidence limits of 19.3o and 15.6o respectively). Similar scatter results are obtained when considering all the flows and after filtering (Table 2-2). The agreement of the VGP dispersion of this study with the dispersion value predicted in Model G sugge sts that the secular variation has been adequately sampled. The mean VGP reported from a similar st udy (in terms of age, rock type and quality) further south in the Indian Heaven Volcanic Field of Wash ington State (Mitchell et al., 1989) plots, as in this study, in a near-sided position alth ough coinciding with the rotational axis at th e 95% confidence level (Figur e 2-7). The VGP scatter of 15.0o (with upper and lower limits of 17.9o and 13.2o) obtained in that study coincides less precisely

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23 than in this study with the scat ter expected from Model G at 46o N (16.6o with lower and upper confidence limits of 15.0o and 18.3o). Figure 2-7. Mean virtual geomagnetic poles (VGPs) obtained from British Columbia. The mean VGPs among selected sites (circl e), all lava flows (empty diamond), filtered data (gray-filled diamond), is s hown along with results (Mitchell et al. 1989) from Indian Heaven volcanic field (square).

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24 Table 2-2. Statistical data among gro ups of sites from British Columbiaa D I N K Dir DirVGP LongVGP Lat K VGP VGPSt Sb Su Sl O.G O.A Selected Sites 356.9 70.2 45 57 2.8 215.8 85.5 23 4.5 17.7 17.5 20.5 15.2 yes yes All Sites 354.7 70.0 52 57 2.6 199.6 85.1 23 4.2 17.8 17.5 20.2 15.4 yes no Filtered Sites 0.0 69.4 33 55 3.4 237.3 87.2 23 5.4 16. 9 16.7 20.0 14.3 yes yes Older Flows 349.5 70.7 31 65 3.2 191.0 82.5 25 5.3 18.0 17.8 21.6 15.2 no no Younger Flows 11.7 68.1 14 61 5.1 311.4 83.0 26 7.9 17.4 17.3 23.1 13.8 yes yes aAbbreviations for columns D to VGP (first to ninth) are as in table 1. Data of VGP scatter relativ e to the Earth's axis of rotation is given in columns: St (tota l scatter), Sb (corrected scatter), Su (upper 95% confidence limit of the scatter) and SL (lower 95% confidence limit of the scat ter). O.G./O.A indicates whether the 95% confidence limits () of the mean direction/mean VGP overlap the GAD/Earth's rotation axis.

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25 CHAPTER 3 PLIO-PLEISTOCENE TIME AVERAGED FIELD IN SOUTHERN PATAGONIA RECORDED IN LAVA FLOWS A paleosecular variation study from two areas of southern Patagonia (latitudes 51.5o to 52.5o S and latitudes 49.5o to 50.5o S) is presented in th is chapter (Figure 3-1). The data will help to fill a gap of paleoma gnetic data from high southern latitudes. The results that were obtained were compared with those predicted by existing TAF and PSVL models. The inclination anomalies depi cted by some TAF models for this area (JC95) as well as the expected scatter of th e virtual geomagnetic po les (VGPs) relative to the Earths axis of rotation according to Mode l G of secular variation (McFadden et al., 1988) are tested. Samples were obtained from and around Meseta Viscachas (northern sampling area) and from the Pali-Aike volcanic field (F igure 3-1), the southernmost among a series of Late Cretaceous Holocene alkali basal tic plateaus (Skewes and Stern, 1979) that extend east of the Patagonian Andes. Ramos and Kay (1992) have proposed that back-arc volcanism in southern Patagonia is the result of slab window formation in the mantle produced by the collision of th e Chile ridge with the South American plate. The tectonic environment in southernmost Patagonia is co mplicated by sinestral motion of the Scotia plate along the southwestern tip of Tierra del Fuego. Skewes and Stern (1979) suggest the presence of thermal or mechanical perturbations of the mantle related to the trenchtransform triple junction between South Americ an, Antarctic, and Sc otia plates, based on findings of ultramafic inclusions and chemical characteristics of the Pali-Aike basalts,

PAGE 38

26 that are indicative of a mantle origin and are not observed in other Patagonian plateau basalts. Figure 3-1. Location map of southern Patagoni a showing sampling sites and radiometric dates obtained in this an d previous studies. The ge neral location map to the left (modified from Skews and Stern, 1979) shows Upper Cretaceous to Quaternary alkali basaltic plateaus of Patagonia (red). The locations of Meseta Viscachas and the Pali-Aike Volcanic field (red-fill areas) are designated as MV and PAVF, respectively. The map to the right contains the location of radiometric (40Ar/39Ar) and/or paleomagnetic sampling sites (red-filled triangles pointing up). The sampli ng sites of previously obtained 40Ar/39Ar (Meglioli, 1992) and K/Ar (Mercer, 1976) radiometric dates in the area are shown by squares and triangles poin ting down, respectively. Results of radioisotopic dates (in Ma ) are within ellipses.

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27 The Meseta Viscachas and Pali-Aike basal tic flows are locally interbedded with tills. K-Ar and 40Ar/39Ar radioisotopic dates of these flows (Mercer, 1976; Meglioli, 1992; Singer et al., 2004) obtained to help depict the glacial history in Patagonia indicate primarily Pliocene Pleistocene ages. 40Ar/39Ar radioisotopic dates from 17 of the paleomagnetic sites were obtained by Dr. Br ad Singer (Mejia et al., 2004) which support and complement previous results. Sampling and Sampling Area Most of the samples (49 sites) were co llected in Argentina during February of 2000. Four sites were collected by Joe Stoner dur ing February of 1998 in the Chilean part of the Pali-Aike volcanic field. Each site repr esents an individual lava flow. Access to the outcrops was achieved by road and tracks. Short hikes were occasionally necessary. Normally 10 samples were collected at each site and oriented using a magnetic compass and sun compass, when possible. Sun compass declinations were obt ained for 75% of the collected samples and used for further calculations when they differed by 5o or more from the magnetic compass declinations. Two lava flows from the Chilean side of Patagonia were sampled twice and their data combine d. This way, the pairs of sites PA3 and PA6 (PA3-6) and sites PA4 and PA5 (PA4-5) were combined. Northern Sampling Area This region is adjacent to the Andes and consequently the relie f is high. Lava flows east of the Andes are now often exposed in cli ffs that have resulted from scarp retreat. Lava flows along these scarps outcrop in st ratigraphic sequences usually composed of several (more than 3) flows.

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28 Pali-Aike Volcanic Field The area sampled in the Pali-Aike volcanic field is predominantly flat and covered with Patagonian gravel. Some topography is created by lava flows and volcanic centers. We sampled lava flows, usually < 10 m thick, exposed in areas around and roughly along the eastern parts of the Rio Gallegos and Rio Chico valleys. In the area around Rio Gallegos, individual flows can be traced for a few kilometers, and up to three lava flows were sampled in stratigraphic order. Volcanic cones and eruptive centers are more eroded in the Rio Gallegos than in the Rio Chico area. In the Rio Chico area cinder cones (often aligned indicating fissure volcan ism) are very well preserved. Examples of these cinder cones are Cerro de los Frailes, Cerro Conventos, and Cerro Tres Hermanos. The geomorphologic differences between these two areas of the Pali-Aike volcanic field suggest that the lava flows that outcrop along Rio Gallegos are generally older than those that outcrop along Rio Chico, which is in agreement with re sults obtained from radioisotopic dating. Paleomagnetic Analysis Laboratory analysis was car ried out in the paleoma gnetic laboratory at the University of Florida. AF demagneti zation was done using a Dtech D-200 AF demagnetizer and thermal demagnetizati on using a Schonstedt oven. Magnetic measurements were made in a 2G Cryogenic magnetometer in a shielded room. Pilot sets of samples composed of one sample per site and three samples per site were run using stepwise (around 17 steps) AF and therma l demagnetization, respectively, to choose which method of demagnetization was more appropriate for each site. The general agreement between the directions obtained from AF and thermal demagnetization (Figures 3-2a and 3-2b) indi cates that any CRM acquired du ring heating does not alter

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29 significantly the primary direc tion of magnetization, possibly due to low field conditions within the oven. Figure 3-2. Examples of Zijderveld diagrams obtained from Patagonia. Figures 3-2a and 3-2b are AF and thermal demagnetizati on (left and right respectively) on replicate samples. Figure 3-2a s hows a case in which the thermal demagnetization curve is difficult to interpret and the AF demagnetization curve is not, while both methods still indicate the same ge neral direction. Figure 3-2b is an example of both AF and thermal demagnetization curves showing similar results a nd Figures 3-2c and 3-2d show AF demagnetization curves of sites affected by lightning. A pproximate intensity values are in A/m. Thermal demagnetization was the preferred procedure for processing all the samples from each site, except when th e orthogonal projections from thermal

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30 demagnetization were more difficult to inte rpret than those from AF demagnetization (Figure 3-2a) or when the site was affect ed by lightning (Figures 3-2c and 3-2d). The most distinctive symptoms of sites being aff ected by lightning are sc atter in directions and high intensity of the NRM (reaching around 20 A/m in some of the samples whereas the intensity of magnetization of the sites not affected by lightning is around 4 A/m). Twelve of our sites showed signs of being a ffected by lightning; therefore we applied AF demagnetization, which is the most successful method to remove the overprint caused by lightning-induced IRM. The remaining 20 sites that were treated using AF demagnetization were those in which the th ermal treatment produced a noisy orthogonal projection or rapid loss of mo st of the magnetization during the first few temperature steps, while results from AF demagnetizati on were clear. Other re searchers have also documented more successful AF versus ther mal demagnetization treatments on basaltic flows (e.g., Camps et al., 2001, and Szeremeta et al., 1999). Despite the preference to apply thermal demagnetization, AF demagnetiza tion was used more times (in 32 sites) than thermal demagnetization (in 21 sites). Data Analysis and Selection Criteria The procedures to obtain and process pa leomagnetic data were aimed at obtaining high quality results. We followed the procedures and selection criteria for data analysis used by Tauxe et al. (2000). That is, th e primary component of magnetization from individual samples was obtai ned using principal component analysis (Kirschvink, 1980) from a segment of at least 5 points of the orthogonal projectio n directed toward the origin and with maximum angular deviation 5o. Site mean directions we re calculated from at least 3 samples per site using Fisher (1953) st atistics and selected as successful when 95 values were 5o (rounded to the nearest integer).

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31 Figure 3-3. Equal area projecti on of site mean directions from Patagonia. The figure shows site mean directions of sites thought to be Plio-P leistocene in age (circles) and of Miocene or unconstrai ned ages (squares). Magnetic vectors pointing up and down are represente d by shaded and unshaded areas respectively; crossed sites are rejected sites because of the quality of the paleomagnetic data. The IGRF (blue tria ngle) is plotted as a reference point. A summary of the paleomagnetic results of all the sites is cont ained in Table 3-1 and the data plotted in Figures 3-3 and 34a. We obtained successful paleomagnetic results from most of the sites (except s ite PA1) that were treated using thermal demagnetization. AF demagnetization was applie d to 32 sites. Among the 12 sites treated using AF demagnetization, as an alternative for treating samples a ffected by lightning, 4

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32 sites had random directions (no results), 6 sites had 95 values > 5o, and only 2 sites had 95 5o, complying with our selection criteria Among the remaining 20 sites that were treated using AF demagnetization, only 4 sites did not pass the selection criteria. Figure 3-4. Paleomagnetic re sults from Patagonia expressed in VGPs. Normal and reverse sites are represented by shaded and unshaded areas respectively. (34a) Site-mean VGPs and (3-4b) Mean VGP from selected group of sites (square), normal sites (filled diamond) and reversed sites (empty diamond). Triangle indicates the position of the geomagnetic North Pole. Many of the sites that were rejected occ upy the periphery of th e overall distribution of paleomagnetic directions (Figure 3-3), which suggests that the applied selection criteria are successfully filtering out noise in the paleomagnetic data. Likewise the application of detailed stepwise demagnetizat ion seems to improve the quality of the data. Previous paleomagnetic analysis by Fleck et al. (1972) in a se quence of lava flows interbedded with tills from Cerro El Fraile (South of Lago Argentino), that used mild or no demagnetization techniques, rendered results with 95 values higher than 10o, that do not pass our selection criteria.

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33 Table 3-1. Paleomagnetic and age data of sites from Patagoniaa Site U/L Site Site Dec Inc SCN 95 K Th/LVGPVGP 95 K R.D. U (+/-)R Lat Long Dir Dir AF Lat Long VGP VGP (Ma) PT1 -51.74 -70.15 9 0/9 AF PT2 -51.74 -70.17 140.8 64.7 0 6/10 14.422 AFX-64.19.917.9 15 1.3 0.03 a PT3 -51.78 -70.23 357.1 -65.79 8/9 3.7231 Th 862635.5 103 PT4 -51.94 -70.42 142 60.5 0 7/10 3.2368 Th -62.426.44.1 216 PT5 -51.68 -70.19 196.7 66.0 0 5/10 4.7264 AF X-78.7189.97.4 107 PT6 -51.88 -70.66 217.7 59.0 0 10/10 1.41202Th -61.6189.41.8 721 8.67 0.15 a PT7 -51.89 -70.72 186.4 60.4 9 8/9 2.3605 AF -78.6134.43.2 302 1.14 0.02 a PT8 -51.87 -70.59 185.4 60.7 8 7/8 4 229 AF -79.2132.25.8 108 PT9 -51.84 -70.51 183.4 71 104/10 7.1167 AF X-85.4264.410.8 73 0.857 0.032 a PT10 -51.85 -70.52 172.9 76.5 1010/10 2.4391 Th -76.9302.84.3 129 PT11 -51.86 -70.52 4.5 -57.1119/11 3.4235 AF 75.7303.94.1 158 9.16 0.08 a PT12 -51.79 -70.28 184.6 60.3 103/10 12 107 AFX-79.2130.216.8 55 PT13 -51.78 -70.27 355.8 -71.71010/10 2.4400 Th 84.6134.84.1 142 0.486 0.096 a PT14 L -51.72 -70.15 200.5 68.4 119/11 4.3147 AF -77.4209.26.6 63 1.79 0.12 a PT15 -51.72 -70.15 170.4 54.4 1 5/11 3.3537 AF -71.983.84.2 326 PT16 U -51.71 -70.15 150.2 72.6 7 6/10 5.5149 AF -71.7346.38.5 63 PT17 -51.79 -70.00 100/10 AF X PT18 -51.83 -70.03 0 0/10 AF X PT19 -51.90 -70.05 356 -76.50 9/10 3.5222 AF 77.3117.26.2 69 0.69 0.05 b PT20 -51.87 -69.42 30.3 -37.66 5/10 8.779 AFX51.5339.96.2 155 PT21 -51.94 -69.57 352.2 -73 107/10 2.7513 AF 82140.74.6 175 0.32 0.02 b PT22 -51.94 -69.60 342 -65.7107/9 3.9244 AF 78211.35.6 116 0.165 0.046 a PT23 -51.91 -69.64 295.8 -81.80 7/10 4.5184 AF 56.4136.78.4 53 0.23 0.02 b PT24 -51.98 -69.73 351.6 -57.89 9/10 4.4140 AF 75.7263.45.1 102 PT25 -51.87 -69.39 43.2 -51.70 3/10 8.2226 AFX53.45.78.7 201 PT26 -51.88 -69.19 18.1 -73.31010/10 3.7174 AF 77.463.56.4 58 0.31 0.03 b PT27 -51.87 -69.17 10.4 -72 1111/11 3.6163 AF 81.866.25.8 62 0.31 0.03 b PT28 -51.84 -69.40 100/10 AFX PT29 -51.99 -69.85 350.7 -63.6106/10 5 179 AFX80.9242.77.2 87 0.665 0.168 a PT30 -50.55 -71.65 317.8 -75.89 8/10 2.8386 Th 64.4152.54.7 142 PT31 -50.52 -71.70 100/10 AF 15.42 0.17 a PT32 -50.52 -71.70 180.4 63.1 1010/10 2.1512 Th -84.11113 253 15.41 0.16 a PT33 -50.52 -71.70 49.2 49.1 1010/10 4.6113 Th -1.6329.35.8 69 PT34 -50.53 -71.61 297.9 -77.6106/10 9.749 AF 56.3144.615.2 20 PT35 -50.32 -71.22 44.2 -70.6106/10 4.5221 Th 63.546.37.2 87 PT36 U -50.32 -71.22 24.8 -63.79 8/10 2.9366 Th 72.812.14.2 179 PT37 L -50.32 -71.21 11.7 -66.5106/10 5.9131 AF 82.715.28.9 58 3.02 0.04 a PT38 -50.34 -71.23 34.5 -65.45 6/9 6.995 AFX67.525.210.7 40 PT39 L -50.33 -71.07 8.2 -74.2107/10 3.4312 AF 78.788.35.9 107 PT40 U -50.34 -71.07 6.3 -79.2109/10 4.6126 AF 71.1102.78.4 39 PT41 U -50.29 -71.13 38.4 -72.50 6/11 2.2912 Th 66.752.53.7 336 2.98 0.03 a PT42 -50.29 -71.13 27.2 -67.86 5/9 3.6462 AF 72.731.45.8 174 PT43 L -50.29 -71.12 1 0/9 AFX PT44 L -49.51 -72.13 179.3 52.8 1010/10 1.8694 Th -73.9105.82.1 554 4.08 0.12 a PT45 -49.51 -72.13 180 58.3 109/10 2.9308 Th -79.6108.43.8 181 PT46 U -49.51 -72.13 184.3 56.9 109/10 2.3495 Th -77.6123.73.2 254 PT47 L -50.01 -71.87 345.3 -66.38 7/10 3 408 Th 80.4199.64.3 199 3.02 0.03 a PT48 -50.01 -71.87 352.4 -74.11110/11 4.2130 Th 78.1126.97 49 PT49 U -50.01 -71.87 342.2 -67.4109/10 1.51129Th 79190.22.2 536 3 0.04 a

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34 Table 3-1. Continued Site U/L Site Site Dec Inc SCN 95 K Th/LVGPVGP 95 K R.D. U (+/-)R Lat Long Dir Dir AF Lat Long VGP VGP (Ma) PA1 -52.05 -70.02 31.9 -61.90 4/4 17.329 Th 6710.426.1 13 PA2 -52.05 -70.02 26.2 -51.96 5/8 4 358 Th 62.9344.34.7 267 PA3-6 L -52.02 -69.93 323.7 -62.01713/17 2.5271 Th 64.4205.23.5 139 1.12 0.01 a PA4-5 U -52.02 -69.93 315 -65.11917/19 1.8404 Th 61.41922.5 204 a U/L indicates the uppermost and lowermost lava flows of lava sequences; the asterisk in site 35 indicates that it is in-between the following two sites. Dec and Inc are the mean site declination and inclination; SC is the number of sun compass declinations obtained in each site; n/N is the number of samples used for calculation of site-mean direction per number of processed samples. K is the dispersion parameter of directions (Dir) or VGPs; 95 is the 95% confidence cone about the mean direction (Dir) or mean VGP; Th/AF represent mean direction results after thermal or AF demagnetization; R.D. is the 40Ar/39Ar radioisotopic date obtained for some sites; U is the uncertainty range of the radioisotopic date. R is the reference source on which a given radioisotopic date is based: (a) is this study and (b) is Meglioli, 1992. Sites PA1, PA2, PA3, PA4, PA5 and PA6 were labeled on the samples as BN1, BN2, SDL1, SDU1, SDU2 and SDL2 respectively. Radioisotopic Ages The radioisotopic dates that we obtained from 17 of the 53 paleomagnetic sites sampled indicate mostly Pliocene Pleisto cene ages. Mercer (1976) Meglioli (1992) and Singer et al. (2004) have obtained similar results (F igure 3-1 and Table 3-1). Northern Sampling Area 7 of the 20 sites in this area were date d. In all cases the magnetic polarities coincided with the ones expected from the ma gnetic polarity time scale (Cande and Kent, 1995). Flows PT31 and PT32 with reverse polarity were dated at 15.4 Ma. These two flows were not considered for the calculation of mean directions or mean VGPs, because their ages are out of the scope of this study. The nearby flows PT33 (a dyke with intermediate direction), PT34 and PT30 located close to the previously mentioned sites were not considered either, because of lack of age control. The isochron ages of the remaining dated flows in this area indicated either Gauss or Gilbert magnetic chrons (spanning from 2.98 to 4.08 Ma) in agreement with previously dated nearby sites (Mercer, 1976). Most of the sites that were not dated crop out in stratigraphic sequences

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35 in which at least one of the flows was date d. In these cases we assumed for the undated flows the magnetic chron obtained for the dated lava flow in that se quence, provided that the same polarity was observed among all lava sequences. The only stratigraphic sequence in which no lava flow was radioisoto pically dated was that composed of flows PT39 and PT40. Because of their normal polarity and closeness to flows PT41 (determined as Gauss) they probably corre spond to the Gauss polar ity chron. The nearby site PT38 is not discussed because paleoma gnetic results were c onsidered unsuccessful for this site. Pali-Aike Volcanic Field Ten of the 33 sites of this area were date d. The radioisotopic da tes that we obtained range from 9.15 Ma to 0.165 Ma. This age range is similar to that indicated by previous radiometric dates (Mercer, 1976; Meglioli, 1992 and Singer et al., 2004) in this volcanic field (Figure 1). The radioisotopic age obtai ned for site PT11 was checked and confirmed by a second measurement using whole rock mate rial (Table 3-1). The initial result was preferred for being more precise and th e product of a measurement of ground mass material. Site PT24 corresponds to a lava fl ow that has not been covered by soil that according to Skewes and Stern (1979) represents the most recent volcanic activity in the Pali-Aike volcanic field th at took place 5000 to 10000 yr B.P., based on anthropologic studies (Bird, 1938). In all but one case the magnetic polarities that we obtained for the dated lava flows were in agreement with thos e expected from their ages, according to the magnetic polarity time scale (Cande and Kent, 1995). Despite its lack of coincidence with the magnetic polarity time scale, the normal polarity of site PA3-6 (isochron age of 1.12 0.01 Ma) does coincide with the Punaruu Even t (Singer, 1999) with a recalculated age of 1.12 0.01 Ma (Singer et al., 2002).

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36 The northern part of the Pali-Aike vol canic field, along Gallegos river has two flows with radioisotopic dates 8.67 Ma (PT6 and PT11) that occupy relatively high elevations and are remnants of old lava fl ows. Similar ages were obtained by Meglioli (1992) around 80 km west of these sites (Fi gure 3-1). These two flows were excluded for the calculation of mean dire ctions or mean VGPs in order to focus the analysis on Pliocene-Pleistocene lava flows. All the othe r sites that were dated in the Pali-Aike volcanic field are less than 1.79 Ma. The remain ing sites that were not dated in this volcanic field were considered for paleoma gnetic analysis. A Pliocene-Pleistocene age was assumed for some of the undated flows by: (a) inferring a similar age of that of a dated flow within the same lava sequence (s uch is the case of flows PT15 and PT16 that are assumed to have a similar age of flow PT14); (b) inferring a young age when the sampled lava correspond to a volcanic stru cture (such is the case of flow PT3, PT10, PT20 and PT25) and (c) inferring a simila r age of a previously obtained nearby radioisotopic date (Figure 3-1) like in the case of the flows related to cinder cones that crop out roughly along Rio Chico (sites PT 21, PT23, PT26 and PT27). A relationship of sites PT5 and PT19 with previous nearby radiometric dates of 2.1 Ma and 0.69 Ma, respectively (Meglioli, 1992) is more difficu lt to determine due to the relative greater distance and the complex stratigraphic relati ons of the lava flows in the area. The previously mentioned two flows along with the undated flows PT4, PT8 and PT12 were considered for calculation of m ean directions and mean VGPs, despite that there is some possibility that these flows are older than Pliocene in age. Results from Patagonia No attempt was made to filter the data of serial correlation in lava sequences, considering the contention of Love (2000) that this procedure can be inadequate due to

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37 the possibility of slow secular variation of the magnetic field rather than fast accumulation rate of lava flows. However, the only flows in lava sequences that have overlapping circles of confidence are flow s PT45-PT46, PT39-PT40 and PA3-6 PA4-5. Table 3-1 summarizes mean directions and mean VGPs among groups of sites calculated using the statistical methods of Fisher (1953). Accordi ng to the selection criteria previously described we obtained 38 successful paleomagnetic results out of the 53 sites that were studied. Excl uding the sites that had Miocen e ages or were likely to have that age, we calculated mean directi ons among 33 sites. The mean direction (D = 358.7o, I = -68.2o, 95 = 3.5o) and mean VGP (Lat = 88.5o, Long = 141.3o, 95 = 5.4o) among the selected group of sites coincide at the 95% confidence level with the expected direction of the GAD ( 68.1o) and axis of rotation respec tively (Figures 3-4b and 3-6a and Table 3-2). Likewise the mean direct ion and mean VGPs among the normal, reverse and all the results (without cons ideration of selection criter ia, N = 41) coincide at the 95% confidence level with the GAD and axis of rotation respectivel y (Figures 3-4b and 3-6a and Table 3-3). The normal and reversed groups of sites pass the reversal test (McFadden and McElhinny, 1990) with a "B" classification. The VGP scatter with respect to the Earth's axis of rotation (traditionally used as indicative of s ecular variation, Table 3-2) that we obtained from th e selected group of results is 17.1o (within-site scatter considered, e.g. Johnson and Constable, 1996) with upper and lower 95% confidence limits of 20.6o and 14.6o (Cox, 1969). This value is in clos e agreement with that predicted by Model G (McFadden et al., 1988) of 17.4o (with upper and lower confidence limits of 19.3o and 15.6o) for that latitude. Scatter values from high latitudes compatible with those expected from Model G have been also recently obtained from lavas younger than 5 Ma

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38 of British Columbia (Mejia et al., 2002) Deception Island (Baraldo et al., 2003), and Possession Island (Camps et al., 2001). However th is has not been the case in the recent study from Patagonia (around 47o S) by Brown et al. (2004) in which scatter is substantially higher than that expected from Model G. The IGRF of the year 2000 fo r the studied area is Dec = 13.5o and Inc = -48.1o and the GAD for this same area is Dec = 0o, Inc = -68.1o. Therefore the present inclination anomaly in the area is 20o. Such anomaly reflects the pronounced non-dipole structure of the present field in South America. The pres ent inclination anomaly in the area of study is greater than the inclination anomaly of any of our sites th at passed the selection criteria (Figure 3-3). This observation suggests that the present inclination anomaly is among the greatest that has occurred in the area at least during non-transiti onal states of the magnetic field. The mean directions of the normal and reve rse data of this study were compared to ranges of directions corresponding to the dec lination and inclination anomalies (obtained mean values minus those expected from GAD) resulting from the TAF models (Figure 36b) obtained by JC95 for the past 5 Ma, th at are based on normal (LN1 model) and reverse (LR1 model) data sets derived from lava flows (Johnson and Constable, 1996). The agreement between the values mode led by JC95 for the Patagonia area and our results (Figure 3-6b) is faci litated by large 95 % confidence ranges among our normal and reverse mean directions. However, the depart ure from the more ubiquitous negative and positive inclination anomalies (for normal and re verse data respectively) depicted in the TAF models of JC95 is not clearly observed in our data set. Only the inclination anomaly of .7o (63.4o 68.1o) that we obtained from the revers e sites closely agrees with the -4o

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39 Figure 3-5. Equal area projection of mean directions from seve ral groups of sites. (Figure 3-5a) Comparison of the mean direction of the selected group of sites (shaded square) and the selected pl us rejected group of sites (empty square) with the GAD (triangle pointing down). (Figure 3-5b) comparison of the mean direction from normal (gray-filled diamond) and reverse (empty diamond) data with the GAD (tria ngle pointing down) and ra nges of inclination and declination in the area of study (shaded quadrilaterals) as modeled by Johnson and Constable (1995). Table 3-2. Statistical data among groups of sites from Patagoniaa Group Dec Inc N 95 K FishVGP VGP 95 K Fish O.G.O.A. St Sb Su Sl Dir DirDir Long Lat VGPVGP VGP Sel 358.7 -68.2 33 3.5 51 yes141.3 88.5 5.4 22 yes yes yes 17.3 17.1 20.6 14.6 Sel + Rej 3.3 -67.3 41 3.6 40 yes24.5 88.2 5.3 19 yes yes yes 18.9 18.6 21.9 16.6 Normal 359.3 -70.6 22 4.3 53 yes113.8 85.5 6.8 22 yes yes yes 18.2 18.0 22.6 14.9 Reverse 177.7 63.4 11 6.1 57 yes91.3 -84.6 8.7 28 yes yes yes 16.3 16.1 22.4 12.6 aSel and rej are selected and rejected groups of sites. Abbreviations for columns Dec, Inc, Dir, K Dir, VGP Long, VGP Lat, VGP, K VGP are as in table 3-1. Fish Di r/Fish VGP indicate whether the distribution of the directional/VGP data is fisheria n. O.G./O.A indicate whethe r the 95% confidence limits () of the mean direction/mean VGP overlap the GAD/E arth's rotation axis respectively. Data of VGP scatter relative to the Earth's axis of rotation is given in columns: St (total scatter), Sb (scatter corrected for within-site scatter), Su (upper 95% confidence limit of the scatter) and Sl (lower 95% confidence limit of the scatter). to -6o inclination anomaly range obtained for the Patagonia area by JC95. But at the same time, the declination anomaly of this same group of sites of D = 2.3o (177.7o 180o) is

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40 quite distant from the range of -10o to -12o of the model. Detecti ng true departures from negative and positive inclination anomalies (for normal and reverse data respectively) would be important, because they would re present a contribution opposite to the axial quadrupole term (that causes the so called far-sided effect seen in VGP plots (e.g. Wilson, 1970)) that is nevertheless e xpected to be small close to 55o latitudes such as in this study. Such axial quadrupole term (g2 0) contribution becomes zero at these latitudes because of the shape of the wave of the Le gendre polynomials of this spherical harmonic term.

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41 CHAPTER 4 PALEOSECULAR VARIATION AND TIME -AVERAGED FIELD RECORDED IN LAVAS FLOWS FROM MEXICO The Trans-Mexican Volcanic Belt (TMVB) wa s targeted for investigation in order to increase the number of paleomagnetic sites with reverse directions since less than 25% of the sites from previous paleomagnetic studi es in Mexico showed reverse polarity. This chapter presents results from 13 paleomagnetic sites from an area west of Mexico City and 7 sites from an area of dispersed monoge netic volcanism in the state of San Luis Potosi, accompanied by seven 40Ar/39Ar radiometric dates. The time averaged field is usually compiled from lavas that are 0 to 5 Myr in age. This time range is used in order to avoid the data being compromi sed by plate tectonics or true polar wander. This assumed safe period of time is invalid where fast moving plates carry the lavas across lines of latit ude (Yamamoto et al., 2004), or where rotations of continental blocks occur over a shor t time (e.g., Luyendyk, 1990). Tectonic rotations appear to be a particular problem in the TMVB (Ruiz-Martinez et al., 2000) which lead us to restrict the analysis of the paleomagnetic field in this area to the past 2 Myr. In this study our results from the TMVB were analy zed along with data compiled from previous studies, thus updating the comp ilation by Bhnel et al. (1990). Geology and Sampling Area The Trans-Mexican Volcanic Belt (TMVB) crosses Mexico in east-west direction between around 19o to 21o north latitude. This volcanic belt has developed since the Miocene and both polygenetic and monogene tic volcanoes have contributed to its

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42 formation. The tectonic setting most often proposed to explain the volcanism in the TMVB is subduction of the Rivera and Co cos plates under the North America plate (Pardo and Suarez, 1995), however others have interpreted rifting unrelated to subduction, based on several characteristics su ch as the oblique position of the TMVB relative to the subduction plate boundary and the presence of lavas with composition similar to oceanic island basalts (OIB ) (Marquez et al., 1999; Verma, 2002). The oblique subduction beneath the TMVB is accommodated by a transtensive tectonic regime, shear and rifting that pr oduces two fault systems of E-W and N-NW directions (Ferrari et al ., 1994; Alaniz-Alvarez et al., 1998). Volcanism in the TMVB, closely related to faulting (Johnson and Harris on, 1990), is concentrat ed in several areas along its length. The paleomagnetic sites obta ined by us (Figure 4-1) or compiled from the literature (Figure 42) that were analyzed in this study are from the following areas: In the western TMVB, the paleomagnetic si tes compiled are mostly of Pleistocene to Holocene age and closely related to ma jor fault systems. Most of the sampled areas are the Jalisco Block, the Chapala region, Valle de Santiago Volcanics, Rio Grande de Santiago Volcanics and the Michoacan-Guanajuato Volcanic Field (MGVF). The MGFV has numerous monogene tic volcanoes and is located at the intersection of a tect onic triple junction (Johnson and Harrison, 1990). In the central TMVB, surrounding the basin of Mexico, we obtained data from the volcanic ranges Sierra de las Cruces, Sie rra Santa Catalina, Sierra Nevada, Sierra de Rio Frio and the Sierra Chichinautzi n. All the ranges around the Mexico basin, except Sierra de las Cruces, were consider ed of Pleistocene to Holocene age (Nixon et al., 1987). Sierra de las Cruces, as described by Osete et al. (2000), gets younger toward the south with ages ranging from 3.7 to 0.29 Ma (late Pliocene Pleistocene). When radiomet ric dates were not available, we assumed the age for some sites from Sierra de las Cruces, as expected from the areas of Gauss to Brunhes age, mapped by Osete et al. (2000) The volcanic ranges around the basin of Mexico are mostly composed of stratovolcanoes. However the Sierra Chichinautzin is a volcanic field that consists mainly of numerous monogenetic volcanoes. The exposed lavas from this volcanic field are very young, probably less than 100 Ka old (Urrutia-Fucuchauchi and Martin del Pozzo, 1993). In the eastern TMVB, many of the pale omagnetic sites compiled are from the Altiplano Area. In the northern part lavas are mostly Miocene to Pliocene in age

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43 (Ruiz-Martinez et al., 2000; Cantagrel and R obin, 1979). In the southeastern part of the Altiplano, the shield volcano Cofre de Perote and the st ratovolcano Citlaltepetl (Orizaba Peak), are surrounded mainly by monogenetic volcanoes that reach lower elevation. These monogenetic volcanoes ar e of Brunhes age (Negendank et al., 1985). Other volcanic areas in the eastern TMVB where paleomagnetic studies have been made are Los Tuxtlas (PlioPleistocene) and Palma Sola Massif (Miocene Pleistocene). Thes e two areas are often not considered to belong to the TMVB (e.g., Cantagrel and Robin, 1979).

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44 Figure 4-1. Location of sampling areas in Me xico using geologic base maps (Carta geologica 1:250000 Estados Unidos Mexi canos, 1983). Figure 4-1a shows the sampling area in the central part of the TMVB. Sampling sites (black dots), volcanoes (filled triangles), and areas of upper Tertiary andesites (crosses) are represented. Figure 4-1b shows the samp ling area in the state of San Luis Potosi. Sampling sites (filled triangles) a nd areas of Quaternary basalts (dots) are represented

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Figure 4-2. Location of compiled paleomagnetic sites ( 95 < 10o) from the TMVB plotted on neotect onic map (modified from Ego and Ansan, 2002). MB is Mexico Basin, Abbrev iated for some volcanic ar eas are: Jalisco Block (J B), Tepic-Chapala Rift (TCHR), Rio Grande de Santiago Volcanics (RGS), Chapala Region (CHR), Michoacan-Guanajuato Volcanic Field (MG), Valle de Santiago Volcanics (VS), Acambay Graben (AG), Sierra de las Cruces (SC) Sierra Chichinautzin (SCH), Sierra Santa Catalina (SSC), Sierra Rio Frio (SRF), Sierra Neva da (SN), Altiplano Area (AA), Los Tuxtlas Volcanic Field (LT) and Palma Sola Massif (PSM). Abbreviations for some volca noes are: Toluca Volcano (Tol), Iztaccihuatl Volcano (IZ), Cofre de Perote Volcano (CP), and Orizaba Peak, i.e. Citlal tepetl Volcano (PO). Site loca tions have different symbols depending of their reference number (as they appear in Tables 5 and 6). Empty squares represen t site-locations of ref. 8 (dark green), ref. 11(light green), ref 5 (light blue), Watkin s et al. (1971) (black), ref 17 (magenta), ref 16 (grey), ref 2 (dark blue). Filled squares represent site-locations of ref 14 ( light brown), ref 3 (magenta), ref 4 (yellow). Circles represen t site-location of this study (dark blue), ref 12 (light green), ref 13 (dark green), ref 9 (gray), ref 15 (light blue), ref 10 (brown) and ref 7 (magenta).

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46

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47 Previous Paleomagnetic Studies in the TMVB One of the earliest most important paleomagnetic studies from the TMVB is by Mooser et al. (1974) in the central TMVB, surrounding the basin of Mexico. That study was made primarily for magnetostratigraphic purposes, and the standard paleomagnetic techniques of the time were applied. That is stepwise AF demagnetization was applied to one sample per site (out of 5 to 10 field-or iented cores). The remaining samples of the site were then blanket demagnetized at a field judged to be most appropriate based on the data from the only sample of the site that had several demagnetization steps. The data reported include 187 sites of igneous rocks, both volcanic and plutonic, from several stratigraphic groups (Miocene-Pl eistocene) from the TMVB. Several subsequent paleomagnetic studies in the TMVB were made during the late 70s and 80s. Although paleomagenetic technique s similar to those applied by Mooser et al. (1974) were used, secular variation was al so analyzed. Most of those studies were made in the Sierra Chichinautzin (e.g. Herre ro-Bervera and Pal, 1977; Herrero-Bervera et al., 1985) and the nearby Izta ccihuatl Volcano (Steele, 1971; Steele, 1985), both of Brunhes age. The only available paleomagntic study from the 80s on mostly PlioPleistocene lava flows from the eastern part of the TMVB (Altiplano area and Palma Sola Massif) is by Bhnel and Negendank (1981) who report results from 53 paleomagnetic sites. These rocks were demagnetized in more detail (at least 3 st eps), but no principal component analysis (PCA) was applied to determine remanent directions. H. Bhnel (unpublished data, 2005) has enhanced the results from this preliminary study by applying PCA and constraining the age of the lava flows based on new data. These data are reported in this paper.

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48 During the 90s up to the present, pale omagnetic studies on lavas increased, extending from east to west along the TMVB. In these studies modern procedures expected to produce data of improved quality have been applied. In particular, all the samples have been step-wise demagnetized using AF or thermal demagnetization and principal component analysis has been used to determine remanent directions. Most of the recent studies have been dedicated to studying specific volcanic areas and age interpretations have been made based on radiometric dates and magnetic polarity. Recent paleomagnetic data has been obtained on very young lava flows (< 40 Ka) along the TMVB. These lava flows belong mos tly to the Chichinautzin Volcanic field (e.g., Morales et al., 2001; Gonzales et al ., 1997) and to the MGVF (Gonzales et al., 1997). Bhnel and Molina-Garza (2 002) have compiled paleom agnetic data for secular variation analysis for the pa st 40 Ka. In their study results of the same flows from different studies have been taken into consideration and averaged. New Data Our sampling within the TMVB took place in th e southern part of the Sierra de las Cruces, that limits the basin of Mexico City to the west as well as around the Nevado de Toluca Volcano (Figure 4-1a). We also sample d lava flows away from the TMVB in an area of about 20000 km2 of the state of San Luis Poto si (Figure 4-1b) where dispersed monogenetic basaltic volcanoes occur (Ara nda-Gomez et al., 1993). This study reports for the first time paleomagnetic data from this area. Because of the scarcity of paleomagnetic data from reversely magnetized flows, one of our goals was to focus our sampling on reverse flows. We attempted to determine in the field which flows were reverse by taking rock-blocks from the outcrops while roughly keeping track of their orientation and measuring with a portable flux-gate vector

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49 magnetometer the sign of the variation in ma gnetic moment, parallel to the north, that occurred as the rock-blocks were positioned closer or away from the magnetometers sensor; when the intensity of magnetization in creased, the lavas were interpreted to have normal polarity and vice-versa. In order to avoid sampling in areas that had been tectonically affected (e.g., Ru iz-Martinez et al., 2000; Osete et al., 2000), we sampled in areas of the TMVB thought to be of Pleistocene age, as we ll as off the TMVB in an extensive area in the state of San Luis Po tosi (Figure 4-1b). Sampling was carried out during May of 1999. Sampling sites were chosen to be representative of a single lava flow and the location established using GPS. At least 10 cores a few cm long were drilled in each site using a portable gasoline-power ed drill. Samples were oriented using magnetic compass and sun compass when possible. Paleomagnetic Laboratory Work and Data Analysis Laboratory paleomagnetic analysis was c onducted at the University of Florida. Magnetic measurements were made in a 2G cryogenic magnetometer in a shielded room. Thermal demagnetization was done using a Sch onstedt oven and AF demagnetized using a Dtech D-200 demagnetizer. The minimum nu mber of demagnetization steps used during thermal demagnetization was 14 and, during AF demagnetization, 13. High scatter and strong intensity of the natural remanent magnetization (NRM) was observed on sites 1 through 5, indicating that these sites were affected by lightning. Samples from these sites were then AF demagnetized since this is the preferred method to remove the isothermal remanent magnetization (IRM) acquired by lightning (Schmidt, 1993). All the remaining samples were subjected to st epwise thermal demagnetization on all the samples and AF demagnetization on one re plicate sample. Subsequently replicate samples of 9 sites were subjected to st epwise AF demagnetization after obtaining

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50 relatively high within-site scatte r and realizing that many of t hose sites were also affected by lightning (Figure 4-3). The e ffects of lightning on those site s were not as obvious as in sites 1 to 5 since fewer samples per site had very high NRM intensities and the scatter of NRM directions was smaller. The use of AF demagnetization successf ully decreased the values of the 95% confidence angle around the site mean direction ( 95) obtained from thermal demagnetization but the changes in site -mean directions were rather small, and did not exceed 4.8o. The demagnetization curves of individua l specimens sampled in our study were analyzed by determining directions trendi ng to the origin (lines) as well as remagnetization circles (planes, Figure 4-4) when direct lines could not be obtained (Kirschvink, 1980). The quality criteria with which the components of magnetization were calculated were those suggested by Tauxe et al. (2000): at least 5 points of the demagnetization curve and a maximu m angular deviation (MAD) < 5o. Site-mean directions (Table 4-1, Figure 4-5a) were determined from at least 3 samples and calculated using Fisher (1953) statistics or, when it was applicable, the method for combined analysis of remagnetization circ les and direct observa tions by McFadden and McElhinny (1988a). As quality control for the anal ysis of the results, the cut-off values of 95 10o (used by McElhinny and McFadden, 1997) and 95 5o (used by Tauxe et al., 2000) were considered. Results from New Data As shown in Table 4-1, 15 out of 20 samp led sites yielded coherent paleomagnetic results (Figure 4-5a); no mean direction of magnetization could be interpreted from the remaining 5 sites. The sites collected in the area of San Luis Potosi are so affected by lightning that only 4 out of the 7 sites from that area have interpretable results. All,

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51 Figure 4-3. Examples of Zijderveld diagrams from Mexico. The pairs of zijderveld plots in Figures 4-3a and 4-3b, and Figures 4-3c and 4-3d represent, respectively, thermal and AF demagnetization curv es on replicate samples, showing a better definition of the primary compone nt of magnetization on the AF than on the thermal demagnetization curves. Approximate intensity values are in A/m. except three sites from the San Luis Poto si area, meet the conditions of having 95 10o (the same sites meet the condition of having 95 5o and a dispersion parameter k > 100, the last being a selection cr iterion introduced by Tauxe et al., 2003). The homogeneity of the data obtained is disrupted by the presence of two sites with normal polarity and one site with intermediate direction (thus our ai m of collecting samples with reverse polarity was unsuccessful in these cases), as well as th e considerable latitudi nal differences of the two areas of study that imply significant diffe rences of the GAD values with which we compare our results (GAD inclination is around 34.9o in the TMVB versus 40.0o in the

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52 state of San Luis Potosi). Th erefore we grouped our results as presented in Table 4. The group of reverse sites from the TMVB is th e most homogeneous. The statistical results among all the groups of sites are very similar (T able 4-2), for this reason only the results corresponding to the selected group of sites were plotted (Figure 4-5b). All the mean directions overlap both with GAD and GAD pl us a 5% quadrupole (GAD + Q5), and the inclination anomalies are very low, indicati ng a closer approach to GAD than to the GAD + Q5. Likewise all the mean VGPs coincide wi thin the errors limits with the axis of rotation. The VGP scatter of the results we obtained from the TMVB is around 10o which is low compared to the value expected from Model G (13.5o) but in all cases it coincides within the uncertainty range (Table 4-2). Tectonic rotations are not observed in this dataset. Figure 4-4. Remagnetization circles and direc tions form samples of Site 17 (Mexico). Figure 4-4a is an equal area pr ojection showing an example of demagnetization steps (circles) obtai ned from sample 17.5. Filled circles represent the steps used to calculate the remagnetization circle (arc). Embedded is the respective zijderveld plot for the sample. Figure 4-4b shows the remagnetization circles (arcs) and dire ctions (circles) used to calculate the site-mean direction of magnetization (t riangle). Arcs represent portions of planes pointing down.

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53 Figure 4-5. Equal area projec tion of paleomagnetic direct ions obtained from Mexico. Figure 4-5a represents the site-mean dire ctions of sites collected in the TMVB (circles) and SLP (squares) areas. Crossed out sites are sites that were rejected. Figure 4-5b represents the m ean directions among the selected sites (diamond) surrounded by the 95% confidence angle ( 95). Black and blue triangles are the expect ed GAD and GAD plus a 5% quadrupole expected in the area. New 40Ar/39Ar dates obtained by Dr. Amabel Or tega-Rivera and Dr. James K. W. Lee (Mejia et al., unpublished data, 2005) fo r seven of the sampled paleomagnetic sites (Table 4-1) range from 1.28 0.54 Ma to 4.14 0.37 Ma. Results from 5 sites from Sierra de las Cruces (9, 12, 13, 19 and 20) sugge st late Pliocene Pl eistocene ages, that are consistent with previous radiometric dates by Osete et al. (2000) and Mora-Alvarez et al. (1991) in that area. A resu lt from a site in the Toluca volcano region (site 18) suggests a Pleistocene age that is consistent with ra diometric dates obtained by Cantagrel et al. (1981). The oldest age obtained is for site 6, from the San Luis Potosi area (4.14 0.37 Ma), which correspond to the Pliocene epoch.

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54Table 4-1. New paleomagnetic and age data of sites from Mexicoa Site Site Site Dec Inc SC n/N Dir Dir Th/ L VGP VGP VGP VGP Age U (+/-) Lat Long 95 k AF Long Lat 95 k (Ma) (Ma) 1 21.270 -100.520 --------7/11 0/12 --------AF x ----------------2 22.837 -99.878 346.7 23.5 5/9 5/10 37 5 AF x 136.2 75.5 25.7 10 3 22.660 -99.903 46.8 25.2 10/10 4/10 13 51 AF x 353.5 44.8 11.2 68 4 22.260 -100.564 351.4 24.1 8/10 5-5/10 11.8 ----AF x 121.2 77.3 --------5 22.306 -100.610 --------9/9 0/10 --------AF x ----------------6 22.820 -101.914 189 -37.1 8/10 8/10 2.8 396 Th 180.3 -81.3 2.8 407 4.14 0.37 7 22.834 -101.889 --------7/7 0/10 --------AF x ----------------8 18.976 -99.648 187.5 -36.4 4/11 8/11 4.8 134 Th x 158.1 -82.9 4.6 148 9 19.309 -99.305 182.8 -33.7 0/7 7/10 4.3 196 Th 187.3 -87.2 4.6 172 1.28 0.54 10 19.322 99.301 184.5 -37.7 0/0 5/10 4.5 291 AF x 146.8 -85.3 4.2 333 11 19.291 -99.261 9.9 29.4 2/9 8/9 4.6 148 Th 9.6 80 4 195 12 19.279 -99.278 161.2 -43.1 0/3 10/10 3.9 153 Th 13 -71.8 4.2 134 1.43 0.17 13 19.267 -99.292 166.9 -47.8 1/10 8/10 3.7 230 Th 32.2 -74.6 4.3 170 2.29 0.58 14 19.199 -99.250 185.7 -42.1 3/10 6/10 4.1 270 AF x 125.1 -82.6 3.9 297 15 19.230 99.272 182.2 -47.7 0/0 3/10 2.6 2308 AF x 92.4 -80.2 2.9 1759 16 19.197 -99.260 --------2/10 0/10 --------AF x ----------------17 19.156 99.806 181.4 -16.1 0/0 4-4/11 4.8 ----AF x 252.9 -79.0 --------18 19.168 -99.805 177.7 -16.7 3/9 5/9 2.5 968 Th x 272.6 -79.1 2.3 1129 1.49 0.51 19 19.301 -99.375 186.1 -24.4 0/0 6/10 3 503 AF 217.5 -81.2 2.7 638 1.69 0.21 20 19.339 -99.362 182.6 -41.3 0/0 7/10 3.4 307 AF x 108.6 -84.9 2.9 428 2.61 0.52 aDec and Inc are the site-mean declination a nd inclination; SC is the number of sun co mpass declinations obt ained in each site; n/N is the number of samples used to calculate th e site-mean direction (when two numbers sepa rated by a dash they indicate the number of lines and planes used in calculation of great circles) per the number of processed samp les. K is the dispersion paramenter of d irections (Dir) or VGPs; 95 is the 95% confidence cone about the mean direction (Dir) or mean VGP; Th/AF represent whether the paleomagnetic technique applied for the resu lt reported was Thermal or AF demagnetiza tion; the sites affected by lighting are p oints out with an x under L; R.D. is the 40Ar/39Ar radiometric date obt ained for some sites; U is the un certainty range of the radiometric date reported as 2(Mejia et al., unpublished data, 2005).

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55Table 4-2. Statistical data among new sites studied in Mexicoa Group of Sites Dec Inc N Dir 95 Dir k VGP Long VGP Lat VGP 95 K VGP O.G. O.Q O.A. Inc Dec St Sb Sl Su All sites 180.4 -34.3 14 6.2 42 178.3 -89.2 5.2 59 yes Yes Yes 1.32 0.40 10.7 10.5 8.4 14.0 Selected (95 < 10o) 181.7 -35.2 13 6.4 43 149.5 -88.3 5.3 63 yes Yes Yes 0.13 1.70 10.5 10.3 8.2 13.9 Selected (from TMVB) 181.1 -35.0 12 6.9 40 132.0 -88.8 5.6 60 yes Yes Yes -0.09 1.10 10.6 10.4 8.2 14.3 Selected ( TMVB and Reverse) 180.2 -35.5 11 7.5 39 90. 1 -88.8 6.0 60 yes Yes Yes -0.60 0.20 10.7 10.5 8.2 14.6 aAbbreviations for columns Dec, Inc, N, Dir Dir k, VGP Long, VGP Lat, VGP VGP k are as in Table 4-1. O. G./O.A indicate whether the 95% confidence limits () of the mean direction/mean VGP overlap the GAD/Earth's rotation axis respectively. O.Q. indicates whether the 95% confidence limits () of the mean direction overlaps the GAD plus a 5% quadrupole. Data of VGP scatter relative to the Earth's axis of rotation is given in columns: St (total scatter), Sb (scatter corrected for within-site `catter), Su (upper 95% confidence lim it of the scatter) and Sl (lower 95% confidence limit of the scatter).

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56 Compilation of Paleomagnetic Data The data compilation includes paleomagnetic sites from lava flows ranging from Pliocene to Recent age. We excluded sites with a 95% c onfidence angle around the site mean direction ( 95) 10o (rounded to the nearest integer) and results from studies in which no demagnetization techniques were appl ied. Table 4-3 lists the sites that were used for TAF and secular variation analysis, after excluding the sites that appear to be affected by rotations. The compiled dataset was obtained from the published literature and the unpublished reanalysis of data initially presented by Bhnel and Negendank (1981) (H. Bhnel, unpublished data, 2005). Included in th e dataset are the compiled paleomagnetic sites from the secular variation study by B hnel and Molina-Garza (2002) for the past 40 Ka. Efforts were made to bring the dataset in to a homogeneous state. When not reported, VGPs were calculated. Some of the compiled paleomagnetic studies dont report specific site coordinates, and they had to be determin ed from maps. Site coor dinates were used to show the spatial distribution of sampling sites (Figure 4-2) and help determine which lava flows might have been sampled in several studies. The coordinates of the paleomagnetic sites studied by Mooser et al. (1974) were measured by relocating them on geologic maps (Carta geologica de la Re publica Mexicana, Scal e 1:50000) using the location descriptions and location maps provi ded in the paper. Site coordinates were measured from the location maps contained in the studies of Alva-Val divia et al. (2001), Urrutia-Fucugauchi and Rosa s-Elguera (1994), Soler-Arech alde, Urrutia-Fucugauchi (2000), Uribe-Cifuentes and Urrutia-Fucugauchi (1999), Steele (1971) and Watkins et al.

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57 (1971). The precise locations of the sites st udied by Herrero-Bervera (1986) in Sierra Chichinautzin could not be obtained because in sufficient data was provided in the paper. The possibility of compiling paleomagnetic results from a lava flow that has been studied several times was taken into consid eration, and when noti ced only one of the results was included in the dataset. This is th e case of the well studied Xitle lava flow (e.g. Urrutia-Fucugauchi, 1996). In this partic ular case, only the paleomagnetic result reported by Bhnel and Molina-Garza (2002) was used. Tectonic Rotations Previous paleomagnetic studies from th e western TMVB where tectonic rotations have been interpreted were examined. The r eanalysis of these results (Table 4-4) was done using our selection criter ia and excluding sites of Plio cene age (only 7) that are more likely to be affected by tectonic ro tations. As can be deduced from Table 4-4, evidence of tectonic rotations in each of the studies is limited. The mean direction often coincides with the GAD because the number of sites (N) in each of these studies is low (usually < 10), which leads to relatively high 95 values. There is however a general tendency toward negative declination anomalies ( D). The nature of the observed rotations is difficult to interpret and beyond of the scope of this study. Most of the discarded data are located in areas of inte nse faulting and local tectonic rotations are likely. Tectonic rotations would also be cons istent with the obse rvation by Johnson and Harrison (1990) that the wester n TMVB is more intensely aff ected by recent faulting than the central TMVB. However, because of the small number of paleomagnetic sites and the overall low quality of the existing results (40% of the sites with 95 <15o have 95 > 7.5o), we think that more studies area needed in this area to study tectonic rotations. The studies

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58 that show obvious declination anomalies were not considered for further analysis. The only sites from the western TMVB that were us ed for TAF and secular variation analysis are the ones studied by Maillo l et al. (1997) and by Bhnel and Molina-Garza (2002). Ruiz-Martinez et al. (2000) interpreted about -10o counterclock rotations in the central and eastern TMVB among 28 sites of Pl iocene age. Included in this analysis were sites from the Altiplano, Palma Sola Massif a nd from Sierra de las Cruces. Similarly negative declination anomalies are obtained among these sites and/or eight additional sites of similar age from the Sierra de la s Cruces, Los Tuxtlas volcanic field and Palma Sola Massif. Therefore all these sites were excluded from the final analysis. Results from the TMVB The mean direction of magnetization was calculated for the normal, reverse and combined data (Figure 4-6a). Sites with very low VGP latitude, usually interpreted to be representative of a transitional or a re versing field, were excluded from these calculations. The cut-off value of VGP la titude was calculated using the method by Vandamme (1994), in which the VGP cut-off va lue is a function of the VGP scatter as calculated from Model G (McFadden et al., 1988b). The VGP cut-off value calculated that way depends on the sampling latitude. VGP cut-off values were calculated for each site and ranged from 60.4o to 60.8o. The mean direction among the sites wa s compared both to the GAD and to the GAD plus a 5% quadrupole (GAD+Q5). A sma ll quadrupolar component of this order has been proposed in several models (e .g., Johnson and Constable, 1995; Hatakeyama and Kono, 2002) and, as argued by Merrill (2003), is the only non-dipole term significantly different from zero in the TAF. The mean direction among the normal polarity sites (within the 95% confidence level) is close but signifi cantly different from

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59 GAD and coincides with the GAD+Q5. The m ean direction among th e reverse polarity sites is not significantly diffe rent from GAD or GAD+ Q5 (T able 4-5, Figure 4-6b). The normal and reverse mean directions pass the reversal test with an A classification (McFadden and McElhinny, 1990). The mean di rection of the overa ll results doesnt overlap the GAD but coincides with GAD+Q5 (Table 4-5). The coincidence with GAD+Q5 of the overall mean direction originat ed from the sites of normal polarity that outnumber the reverse polarity sites by 101 (N is 144 / 43 for normal/reverse polarity sites). Figure 4-6. Equal area projecti on of paleomagnetic directiona l data compiled from the TMVB using a selection criterion of 95 < 10o, after excluding sites with transitional directions or that appear to be affected by rotations. Filled (or crosses)/open symbols indicate downw ard/upward directions. Figure 4-6a shows site mean directions. Figure 4-6b shows the mean directions, for normal and reverse data, of the groups of sites in Figure 4-6a with the 95% confidence ellipse (Tauxe, 1998). Blue triangles represent the expected GAD and orange squares represent the ex pected GAD plus a 5% quadrupole.

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60 The distribution of site-mean directions is not fisherian, but elongated in a northsouth direction. This is consistent with pr evious studies (e.g., Tanaka, 1999; Tauxe and Kent, 2004) in which the north-s outh elongation is expected to decrease with latitude. As modeled by Tauxe and Kent ( 2004) the elongation (zeta/eta) at the area of study should be 2.5. The elongation of the 95% confidence ellipses (Tauxe, 1998) of the normal and reverse mean directions in th is study (Figure 4-7) are 1.4 a nd 2.3, respectively. Therefore, only the elongation of the confid ence ellipse of the reverse m ean direction is close to the value expected from the secular vari ation model by Tauxe and Kent (2004). The values of VGP scatter among the normal and reverse polarity sites (Table 4-5) coincide with the value expected from Model G (13.5o). The mean VGP of the sites plots slightly toward the far side of the pole rela tive to the sampling area (Figure 4-7). This is the so called far-sided effect (Wilson, 1970) th at results from the quadrupole portion of the field. When the criterion of 95 5o is applied to the dataset the mean statistical values are similar to those using the selection criterion of 95 10o. Conclusions from Mexico The data obtained in this study increa se the number of reverse direction paleomagnetic data from the Trans-Mexican Volcanic Belt (TMVB) by about 25%. After avoiding the troublesome influence of rotated sites, the mean direction among selected compiled sites of late Pliocene to Holocene age (< 2 Myr) reveals a clear coincidence with the GAD plus a 5% quadrupole. Such small quadrupole contribution would be increasingly less detectable at higher latit udes. The VGP scatter is best described by Model G of McFadden et al. (1988).

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61 Figure 4-7. Virtual geomagnetic poles (VGPs) from the TMVB using a selection criterion of 95 < 10o after excluding sites with transitiona l directions or that appear to be affected by rotations. Figure 7a shows site VGPs. Filled/open circles represent normal/reverse polarities. Figure 7b shows the mean VGP, for the combined normal and reverse data, su rrounded by the 95% confidence ellipse (Tauxe, 1998). Table 4-3. Compiled Late Pliocene Holocene paleomagnetic data from the TMVBa TMVB Site Lat Long Dec. Inc. N 95 k P VGP VGP R Area Long Lat AA ALJ (25) 19.08 -97.52 350.4 33.4 5 10.3 56.1 N 168.8 80.9 2 AA ALM (1) 19.59 -96.78 351.9 26 7 3.6 283.9 N 137.3 80.3 2 AA ATZ (3) 19.00 -97.30 357.4 44.8 6 5.5 149.2 N 245.2 82.2 2 AA BAN (4) 19.61 -96.94 358.4 20.9 8 2.4 516.4 N 93.3 81.1 2 AA COA (5) 19.65 -96.92 4.3 24.9 9 2.6 389.5 N 50.2 82.3 2 AA CON (6) 19.60 -96.88 331 49 6 5.7 136.1 N 200.2 61.8 2 AA COZ (7) 19.43 -96.90 1.3 33.4 9 2.6 401.3 N 36.0 88.3 2 AA CPC (8) 19.50 -97.16 344.5 19.8 9 3.6 206.7 N 143.2 72.4 2 AA CSB (11) 18.90 -97.40 351.6 22.5 4 7.8 139.9 N 132.2 79.2 2 AA CSC (12) 18.90 -97.40 9.5 -3 6 8.5 63.4 N 57.0 67.6 2 AA FAL (29) 19.46 -96.75 4.1 22 5 5.9 167 N 56.7 81.0 2 AA GIL (34) 19.30 -97.51 336.9 39.5 6 3.5 361.3 N 184.7 68.2 2 AA MAC (15) 19.53 -96.91 354.5 26 6 8.7 60 N 126.2 82.2 2 AA MIC (31) 19.61 -97.09 8.5 31.7 8 4 195.5 N 8.4 81.6 2 AA NAO (16) 19.67 96.87 353.1 25 8 6 85. 5 N 129.3 80.7 2

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62 Table 4-3 (continued) TMVB Site Lat Long Dec. Inc. N 95 K P VGP VGP R Area Long Lat AA NED (17) 19.29 -97.41 15.4 57.5 8 9.1 37.6 N 294.8 66.9 2 AA OLL (18) 19.62 -97.03 1.3 21.6 7 6.9 76.3 N 74.3 81.5 2 AA RIO (30) 19.49 -96.80 4.2 20.3 10 3.1 233.2 N 58.4 80.1 2 AA SDA (13) 18.90 -97.40 350 38.8 7 6.7 79.8 N 192.1 80.2 2 AA SLF (21) 19.30 -97.31 16.2 51.1 7 4.8 156.6 N 26.9 70.8 2 AA SSS (28) 19.13 -97.61 15 15.7 7 7 76 N 27.7 71.7 2 AA TEO (22) 19.40 -96.97 358.3 44.2 10 2.4 406.9 N 249.8 83.3 2 AA VIG (23) 19.63 -97.08 348.2 33.5 8 3.4 271.7 N 168.3 78.8 2 AA GV+LF (21&26) 19.28 97.30 11.9 48.4 11 4.3 111.8 N 142.0 75.2 2 AA CPA+CPB (9&10) 19.50 -97.16 350.1 29.8 15 1.6 641.6 N 135.9 79.9 2 AA P+PN (27) 19.01 -97.27 0.5 31.2 15 2.4 248.6 N 70.2 87.8 2 AA ACT (33) 19.50 -96.59 179.8 -19.5 6 4.8 193.2 R 264.6 -80.5 2 AA MIO (32) 19.60 -97.09 178.8 -33.4 7 3 398 R 303.1 -88.2 2 AA PES (54) 19.35 -96.80 160.6 -39.1 9 2.3 491.3 R 5.1 -71.7 2 AA TAT (45) 19.66 -97.14 182.5 -27.9 11 3.2 208.6 R 236.4 -84.6 2 AA SGV (20) 19.25 -97.37 338.6 -10.8 8 8.1 47.4 T 125.1 57.2 2 AA Toxtlacuaya 19.40 -96.90 345 34.4 26 1.8 248 N 173.6 75.8 4 AA Jalapa 19.65 -96.96 7.8 27.8 8 3.8 218 N 25.1 81.1 4 AA La Joya 19.59 -96.99 356.6 23.2 3 5.8 450 N 107.1 81.8 4 CHR La Primavera 20.66 -103.46 4.7 24.8 7 7.7 63 N 45.4 81.1 4 CR Colima 19.25 -103.53 1.4 33.7 23 9 12 N 17.6 88.4 4 JB MAS-1 20.55 -104.87 1.9 -3.6 8 3.7 221.2 N 70.2 67.6 8 JB MAS-5 20.51 -104.76 340.7 20.1 9 4.7 122.5 N 139.5 68.8 8 JB MAS-6 20.57 -104.76 356.8 38.2 8 2.7 422.4 N 182.8 86.9 8 JB MAS-7 20.58 -104.88 3.3 35.3 5 5.7 181.2 N 3.8 86.7 8 JB MAS-8 20.53 -104.85 359.5 47.4 9 2.6 397 N 252.0 82.0 8 JB MAS-9 20.50 -104.79 344.6 55 9 7.1 54 N 216.5 69.8 8 JB MAS-10 20.54 -104.72 345.7 18.8 9 8 42.4 N 129.1 72.4 8 JB MAS-13 20.46 -104.86 14 18.8 9 1.7 878.3 N 21.8 72.7 8 JB MAS-15 20.47 -104.76 347.9 65 8 2.4 529.1 N 237.7 61.7 8 JB MAS-20 20.82 -104.97 2.6 34.6 10 3.4 198 N 20.8 87.0 8 JB MAS-21 20.81 -104.93 352.8 31.5 10 7.8 39.3 N 137.3 82.2 8 JB MAS-2 20.51 -104.89 180.9 -50 9 3.1 280 R 79.4 -79.7 8 JB MAS-4 20.49 -104.88 173.3 -20.5 9 3.1 284.9 R 289.2 -78.2 8 JB MAS-12 20.42 -104.85 179.8 -49 10 2.3 442.5 R 74.1 -80.5 8 JB MAS-16 20.45 -104.76 168 26.4 8 2.2 635.9 T 275.1 -53.6 8 LT TS 18.46 -95.16 1.6 18.2 10 5.2 86 N 1.4 74.9 3 LT TP 18.44 -95.08 358.9 37.1 7 6.3 79 N 242.8 87.3 3 LT SRG6 18.23 -94.86 173.9 -30.4 6 7 72 R 338.3 -83.9 3 LT SRG8 18.38 -94.96 172.2 -42.6 9 4.3 166 R 38.2 -80.3 3 MGVF Paracutin 19.47 -102.25 10.7 37.8 6 4.4 238 N 336.1 79.8 4 MGVF El Jabali 19.45 -102.11 12.5 34.3 6 2.7 597 N 348.8 78.2 4 MGVF La Mina 19.71 -101.42 339.7 58.2 6 4.6 213 N 220.5 64.0 4 MGVF El Pueblito 19.82 -101.92 3.6 39.9 5 5.1 227 N 306.9 85.6 4 MGVF El Metate 19.54 -101.99 82 41.5 5 4.4 301 T 327.5 14.8 4

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63 Table 4-3 (continued) TMVB Site Lat Long Dec. Inc. N 95 K P VGP VGP R Area Long Lat MGVF El Huanillo 19.67 -101.98 42.2 10.7 3 5.3 540 T 1.4 46.6 4 SC 11 19.29 -99.26 9.9 29.4 8 4.6 148 N 9.6 80.0 1 SC 8 18.98 -99.65 187.5 -36.4 8 4.8 134 R 158.1 -82.9 1 SC 9 19.31 -99.30 182.8 -33.7 7 4.3 196 R 187.3 -87.2 1 SC 10 19.32 -99.30 184.5 -37.7 5 4.5 291 R 146.8 -85.3 1 SC 12 18.98 -99.65 161.2 -43.1 10 3.9 153 R 13.0 -71.8 1 SC 13 19.31 -99.30 166.9 -47.8 8 3.7 230 R 32.2 -74.6 1 SC 14 19.20 -99.25 185.7 -42.1 6 4.1 270 R 125.1 -82.6 1 SC 15 19.23 -99.27 182.2 -47.7 3 2.6 2308 R 92.4 -80.2 1 SC 19 19.30 -99.38 186.1 -24.4 6 3 503 R 217.5 -81.2 1 SC 20 19.34 -99.36 182.6 -41.3 7 3.4 307 R 108.6 -84.9 1 SC 13 19.29 -99.25 5.1 36.1 7 4.8 162 N 341.1 85.1 9 SC 26 19.31 -99.20 11.5 53.8 7 5.3 129 N 292.7 71.8 9 SC 28 19.22 -99.27 9.1 10.3 7 5.5 120 N 47.3 73.4 9 SC 6 19.48 -99.26 179.5 -18.1 6 9.4 52 R 263.5 -79.8 9 SC 8 19.35 -99.38 173.2 -53.7 6 8.4 64 R 59.9 -73.9 9 SC 11 19.32 -99.32 174.2 -61.2 7 4.8 157 R 69.9 -66.5 9 SC 14 19.27 -99.29 164.8 -39.5 8 5.8 94 R 5.8 -75.5 9 SC 16 19.23 -99.28 194.3 -41 6 6.3 113 R 150.5 -76.0 9 SC 20 19.19 -99.23 173.5 -53.8 6 6 128 R 61.2 -73.8 9 SC 27 19.30 -99.26 192.3 -46.7 6 6.8 98 R 130.9 -75.8 9 SC 18 19.17 -99.27 148.2 -64.8 8 8.6 42 T 44.7 -52.1 9 SC CR 19.52 -99.42 27.6 41 9 3.1 275.9 N 336.8 64.1 12 SC TO2 19.32 -99.32 362.2 32.2 9 4 170.4 N 31.8 87.2 12 SC TO4 19.32 -99.33 352.1 45.6 8 3 337.7 N 218.9 79.4 12 SC TO1 19.28 -99.40 167.3 -26.7 10 4.2 130.5 R 329.5 -76.8 12 SC TO3 19.27 -99.30 177 -53.9 10 5.4 80.1 R 71.3 -74.6 12 SC ST2 19.51 -99.48 145.3 -34.6 5 7 121.4 T 355.6 -57.3 12 SCH Xitle 19.36 -99.17 0.6 34.4 113 0.8 263 N 29.9 89.3 4 SCH Tres Cruces 19.10 -99.50 338.5 53 15 2.6 216 N 211.8 66.0 4 SCH Cima 19.10 -99.18 354.6 40.8 7 5.1 139 N 211.9 83.4 4 SCH Maninal 19.22 -99.21 359.1 33.7 5 5.8 175 N 128.5 88.8 4 SCH Cuautl 19.17 -99.42 342.6 16.6 6 4.2 255 N 140.6 70.0 4 SCH Texontle 19.22 -99.47 353.3 64.4 7 3.4 318 N 250.5 62.4 4 SCH XA 19.20 -99.20 343.8 22.1 5 4.9 242 N 146.9 72.6 6 SCH P 1 19.20 -99.20 5 23.7 6 7.9 72.4 N -296.3 81.6 6 SCH P 2 19.20 -99.20 3 25.7 8 7.1 62.2 N -287.5 83.6 6 SCH P 3 19.20 -99.20 355.9 29.9 8 6.8 67.3 N 132.5 84.9 6 SCH P 4 19.20 -99.20 357.6 28.4 8 8.7 41.2 N 111.0 85.3 6 SCH TEU 1 19.20 -99.20 338.7 17.1 7 4.2 199.8 N 147.2 66.9 6 SCH TEU 2 19.20 -99.20 358.2 22.4 5 7.8 95.4 N 93.8 82.3 6 SCH 0Z 19.20 -99.20 352.8 23.1 4 4.5 411.8 N 125.9 80.1 6 SCH ACO 19.20 -99.20 357.6 32.6 7 6.7 81.9 N 138.5 87.3 6 SCH JU 19.20 -99.20 323.2 22.4 7 3.5 289 T 164.5 53.8 6 SCH CHI 11 19.30 -99.20 16.1 36.1 12 2.9 229.7 N -14.6 74.8 7 SCH CHI 1 19.10 -99.10 357.6 26.7 8 6.5 72.7 N -253.9 84.5 7

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64 Table 4-3 (continued) TMVB Site Lat Long Dec. Inc. N 95 K P VGP VGP R Area Long Lat SCH CHI 2 19.00 -98.80 6.1 12.9 14 2.8 205 N -304.9 76.2 7 SCH CHI 3 19.10 -98.80 330.3 46.6 12 1.8 576.7 N -165.4 61.5 7 SCH CHI 4 19.03 -99.03 342.7 11.6 9 4.1 160.2 N -224.9 68.6 7 SCH CHI 5 19.20 -99.02 345.4 18.6 11 4.2 118 N -221.4 72.9 7 SCH CHI 12 19.10 -99.20 6.4 15.9 12 9.2 21 N -309.6 77.4 7 SCH 16 19.27 -99.16 3.7 35.4 3 10.1 150 N 345.4 86.5 9 SCH 17 19.26 -99.17 4.1 33.8 6 8.65 61 N 1.2 86.0 9 SCH 21 19.22 -99.16 1.1 36.1 8 6.8 68 N 312.5 88.7 9 SCH 24 19.20 -99.14 0.3 35.9 6 8.7 60 N 282.9 89.2 9 SCH 34 19.03 -99.20 352.8 41.8 7 2.7 489 N 209.1 81.6 9 SCH 35 19.02 -99.17 8.3 9.8 8 7.4 50 N 49.9 73.8 9 SCH 10 19.25 -99.07 356.9 34.9 6 7 92 N 172.0 87.1 9 SCH 34 19.03 -99.20 352.8 41.8 7 2.7 489 N 208.3 81.7 9 SCH 5 19.21 -98.92 346.5 38.3 8 5.2 114 N 183.8 77.1 9 SCH 8 19.25 -99.03 352 23.9 6 4.5 219 N 131.0 79.8 9 SCH 36 19.01 -99.13 346.6 36.1 7 6 101 N 176.9 77.4 9 SCH 1 19.19 -98.80 353.3 24.1 8 5.1 118 N 125.3 80.6 9 SCH 4 19.13 -98.82 23.8 21.5 5 9.4 67 N 7.1 65.6 9 SCH 7 19.25 -99.01 353.3 23.7 9 8.3 39 N 124.3 80.5 9 SCH 9 19.20 -99.01 10 17.3 7 10.3 36 N 36.9 75.7 9 SCH 23 19.23 -99.15 1 38.5 8 3.5 259 N 282.9 87.5 9 SCH 25 19.17 -99.15 352.6 15.4 5 7.9 95 N 113.7 76.4 9 SCH 27 19.14 -99.16 1.9 17.6 10 9.6 26 N 70.5 79.5 9 SCH 31 19.06 -99.24 16.2 25.3 8 9.2 37 N 9.8 73.3 9 SCH 37 19.03 -99.26 338.4 47.8 8 10.1 31 N 200.7 68.2 9 SCH 28 19.11 -99.18 358.5 33.4 7 3.8 253 N 137.3 88.3 9 SCH 28 19.16 -98.76 178.6 -43.3 7 3.7 274 R 69.4 -83.8 9 SCH 38 19.03 -99.29 46.4 12.2 6 8.8 59 T 0.8 43.0 9 SCH JB 19.19 -99.17 10.4 17 8 3.9 198 N 35.2 75.6 10 SCH JD 19.03 -99.27 13.8 10.8 8 3 353 N 34.4 70.9 10 SCH JE 19.04 -99.31 4 23.1 8 3.3 277 N 51.7 82.0 10 SCH JH 19.22 -99.27 342.7 21.5 8 2.9 371 N 148.3 71.6 10 SCH JJ 19.10 -99.18 352.8 33 13 1.9 498 N 163.7 83.1 10 SCH JL 19.20 -99.25 358.8 45 10 4.2 131 N 152.9 82.4 10 SN Tetimpa 19.05 -98.45 352.6 38.6 8 3.9 201 N 194.1 82.6 4 SN 16 19.21 -98.78 359.4 13 7 4.8 178 N 84.0 77.4 9 SN 19 19.26 -98.66 356.6 31.9 7 6.8 79 N 140.5 86.2 9 SN 20 19.26 -98.64 3.8 35.9 7 7.6 64 N 340.6 86.4 9 SN 21 19.23 -98.67 7.4 38.1 7 7.1 73 N 332.6 82.7 9 SN 29 19.17 -98.63 355.5 -8 7 7.2 71 N 92.6 66.4 9 SN 47 19.13 -98.66 13.8 31.2 4 9.4 96 N 359.0 76.7 9 SN 53 19.10 -98.60 356.1 39.8 8 4.2 178 N 215.0 84.9 9 SN 24 19.20 -98.64 7.8 21.4 3 8.9 192 N 36.8 78.9 9 SN 55 19.07 -98.68 352.7 18 7 7 76 N 117.1 77.8 9 SN 56 19.07 -98.68 358.3 35.5 7 4.9 153 N 186.0 88.3 9 SN 14 19.25 -98.72 177.6 -28.5 7 4.2 209 R 291.1 -85.3 9

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65 Table 4-3 (continued) TMVB Site Lat Long Dec. Inc. N 95 K P VGP VGP R Area Long Lat SN 17 19.21 -98.74 183.5 -24.5 7 4.5 178 R 232.9 -82.8 9 SN 27 19.18 -98.77 170.2 -30.1 7 5.2 135 R 334.9 -80.2 9 SN SM7 19.29 -98.68 356.5 38.2 13 3.9 112.7 N 205.6 86.1 13 SN SM8 19.28 -98.72 6.2 34.6 10 5.3 84.4 N 352.7 84.1 13 SN SM11 19.06 -98.38 5.8 16.7 10 7 48.4 N 52.7 78.1 13 SN SM13 19.09 -98.59 2.1 32.9 13 3.9 116 N 21.4 87.7 13 SN SM14 19.09 -98.62 0.9 19.7 9 3.9 117.5 N 75.7 81.0 13 SN SM15 19.14 -98.65 4.4 32.4 11 5 84.1 N 10.8 85.6 13 SN SM17 19.19 -98.80 355.2 24.4 11 4.6 98.6 N 117.6 82.1 13 SN SM16 19.21 -98.74 180.8 -16.5 11 2.4 357.2 R 257.0 -79.2 13 SN IZT 82 19.11 -98.64 356.1 39.8 8 4.2 178 N 215.4 85.0 15 SN IZT 29 19.21 -98.66 7.4 38.1 7 7.1 72.6 N 332.3 82.7 15 SN IZT 30 19.22 -98.65 356.6 31.9 7 6.8 79.4 N 141.6 86.3 15 SN IZT 31 19.22 -98.64 3.8 35.9 7 7.6 64.3 N 339.4 86.4 15 SN IZT 10 19.14 -98.65 7.4 28.5 6 4.8 199.3 N 19.7 81.9 15 SN IZT 13 19.13 -98.65 355.9 31.5 7 2.5 569.1 N 143.2 85.6 15 SN IZT 78 19.14 -98.66 1.3 28.3 7 3.9 243.2 N 64.4 85.7 15 SN IZT 11 19.14 -98.65 3.6 23.5 7 4.7 168.4 N 54.3 82.3 15 SN IZT 18 19.13 -98.64 17.3 52.9 7 7.5 65.6 N 305.1 69.0 15 SN IZT 21 19.14 -98.63 352.4 32.1 7 5.3 131.7 N 158.9 82.6 15 SN IZT 24 19.14 -98.63 355.3 21.3 7 6.7 82.4 N 111.2 80.7 15 SN IZT 133 19.19 -98.66 10.2 25.5 6 4.7 207.5 N 20.4 78.6 15 SN IZT 27 19.19 -98.66 354 30 6 7.6 79.6 N 144.0 83.5 15 SN IZT 32 19.22 -98.65 356.7 30 7 9.3 43.2 N 127.2 85.6 15 SN IZT 23 19.14 -98.63 0.3 24.6 7 9.2 43.8 N 78.6 83.7 15 SN IZT 79 19.13 -98.66 13.8 31.8 4 9.4 96.1 N 357.7 76.7 15 SN IZT 84 19.15 -98.65 355 -8 7 7.2 70.5 N 92.6 66.4 15 SN IZT 20 19.13 -98.64 22.7 60.8 7 4.8 158.4 T 296.9 60.3 15 SN IZT 25 19.14 -98.63 27 61.5 6 3.7 327.6 T 299.6 57.3 15 SRF 2 19.50 -98.81 177.1 -32.8 8 8.6 43 R 320.8 -86.8 9 SRF 3 19.50 -98.82 180.3 -62.3 6 7.9 74 R 81.7 -65.9 9 SRF 4 19.39 -98.87 176.7 -27.9 8 8.2 47 R 296.3 -84.5 9 SRF 6 19.32 -98.87 129.2 70.5 7 4.8 162 T 287.8 -4.3 9 SRF 7 19.32 -98.79 144.5 78.6 6 6.8 98 T 273.8 1.1 9 SRF SM1 19.33 -98.78 358.3 33.8 10 4.7 108.8 N 144.4 88.2 13 SRF SM5 19.34 -98.69 343.7 34 11 7.8 35.1 N 171.4 74.6 13 SRF SM4 19.32 -98.73 191 -32 10 2.5 389.1 R 180.2 -79.4 13 SRF SM3 19.34 -98.71 324.6 36.3 11 4.8 93 T 178.8 56.7 13 SSC 41 19.35 -99.09 348.9 32.8 7 7.8 61 N 164.3 79.4 9 SSC 44 19.33 -98.97 9.7 35.1 8 6.4 77 N 349.4 80.9 9 SSC 42 19.40 -98.97 168.7 -7.7 6 9.5 51 R 297.8 -71.0 9 SSC 47 19.32 -98.91 183.2 -10.6 5 7.1 117 R 248.1 -75.7 9 SSC 51 19.24 -98.87 176.8 -24.5 8 4.9 131 R 287.2 -82.9 9 SSC 52 19.37 -99.03 175.4 -12.6 7 3.4 316 R 280.6 -76.3 9 SSC 53 19.32 -98.74 176.7 -27.9 8 8.2 47 R 296.8 -84.5 9

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66 Table 4-3 (continued) TMVB Site Lat Long Dec. Inc. N 95 K P VGP VGP R Area Long Lat TCHR Ceboruco 21.14 -104.50 360 36.6 7 3.2 361 N 75.5 89.2 4 TOL 17 19.16 -99.81 181.4 -16.1 8 4.8 0 R 252.9 -79.0 1 TOL 18 19.17 -99.81 177.7 -16.7 5 2.5 968 R 272.6 -79.1 1 aAreas in the TMVB are: Altiplano Area (AA), Jalis co Block (JB), Los Tuxtlas Volcanic Field (LT), Michuacan-Guanajuato Volcanic Field (MGVF), Sierra de las Cruces (SC), Sierra Chichinautiz (SCH), Sierra Nevada (NS), Sierra Rio Frio (SRF), Sierra Santa Catalina (SSC), Tepic-Chapala Rift (TCHR) and Nevado de Toluca (TOL). Site is the nomenclature of th e site as it appears in the compiled study; Dec, Inc, are the site-mean declination and inclination; N is the number of samples used to calculate the site-mean direction, 95 is the 95% confidence cone about the mean direc tion, K is the dispersion paramenter; P is the polarity: normal (N), reverse (R) or transitional (T); R is the reference used. References are: (1) This study, (2) Bhnel (unpublished data, 2005); numbers in parent hesis indicate the correspondent site-identifications as they appear in the preliminary study by Bhnel and Negendank (1981), (3) Alva-Valdivia et al. (2001), (4) Bohnel and Molina-Garza (2002), (6) Herrero-Berv era et al. (1986), (7) Herrero-Bervera and Pal, (1977), (8) Maillol et al. (1997), (9) Mooser et al. (1 974), (10) Morales et al. (2001), (12) Osete et al. (2000), (13) Ruiz-Martinez et al. (2000) (15) Steele (1985) and Steele (1971).

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67Table 4-4. Statistics of paleomagnetic data of late Pliocene to Recent age from studies in the western TMVBa Group of Sites Ref. Dec Inc N Dir 95 Dir k Fish O.G. O.Q D St Bohnel and Molina-Garza (2002) 4 3 38.2 7 9.4 42 yes yes yes 3 13.0 Delgado-Granados et al. (1995) 5 348.6 28.3 15 9.4 17 yes no no -11.4 20.8 Maillol et al. (1997) 8 356.3 37.4 14 9.9 17 no yes yes -3.7 15.8 Nieto-Obregon et al. (1992) 11 354.4 42.6 9 8.6 37 yes yes no -5.6 15.1 Soler-Arechalde and Urrutia-Fucugauchi (2000) 14 349 36.4 4 19.7 23 yes yes yes -11 19.1 Uribe and Urrutia (1999) 16 354.5 39.6 8 8.9 9 yes yes yes -5.5 15.1 Urrutia-Fucugauchi and Rosas-Elguera (1994) 17 344.9 30.6 5 6.6 137 yes no no -15.1 17.6 All Sites with D > 5o (N + R) 350.4 34.8 41 4.6 25 yes no no -9.6 17.2 All Sites with D > 5o (N) 351.7 35.2 30 5.5 24 no no no -8.3 16.9 All Sites with D > 5o (R) 167 -33.8 11 9.3 25 yes no no -13 18.6 a Ref. is a reference number used in Figure 4-2. The meanin g of the remaining columns is as indicated in Table 4-2. Table 4-5. Statistics of late Pliocene to Holocene age results from compiled dataa Group of Sites Dec Inc N Dir 95 Dir k Dir Fish VGP Long VGP Lat VGP 95 K VGP VGP Fish O.G. O.Q O.A. Inc Dec St Sl Su Selected Data (N + R) 358.8 31.6 187 2 29 no 119.7 87.8 1.6 42 no no yes no -3.5 -1.2 12.73 11.9 14.1 Selected Data (N) 359 30.7 144 2.2 29 no 107.6 87.3 1.8 41 no no yes no -4.4 -1.0 12.95 12.0 14.1 Selected Data (R) 178 -34.5 43 4.1 29 no 11.8 -88 3.2 46 no yes yes yes 0.6 -2.0 12.11 10.7 13.9 aAbbreviations of most columns as in Table 4-2. Fish indicates whether the site directions or VGPs have a fisherian distribution

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68 CHAPTER 5 PALEOINTENSITY Previous chapters have dealt with paleom agnetic directional data (i.e. declination and inclination) from lava flows. In this ch apter paleomagnetic intensity results obtained from some samples of the same lava flow s plus other samples described below are presented. In addition to directi ons, paleointensity values are necessary to fully define the magnetic vector at a paleomagnetic site. Obta ining paleointensity data from lava flows has been rather unsuccessful in most studies and the success rate is usually around 20%. Several laboratory procedures have been designed to obtain paleointensity data. The Thellier method (Thellie r and Thellier, 1959) as modi fied by Coe (1967) has been the most commonly applied. These methods rely on the assumption that NRM acquired during cooling of the igneous rock is proportiona l to the intensity of the Earths magnetic field in which they cool. In principle, these methods are essentially simple: they consist in comparing the NRM lost through thermal dema gnetization with a TRM acquired at the same temperature in a known magnetic field applied in the laboratory. The results of these experiments are therefore regarded as absolute. However other assumptions and conditions on which these methods rely are di fficult to meet which result in a rather low rate of success of the experiment. One of the conditions of th e Thellier method (and its m odifications) is that the magnetic characteristics of th e rock dont change significan tly after the NRM acquisition. It is often the case that mineralogic change s take place in-situ through the geologic time as well as during heating of the samples in the laboratory, and therefore the rocks no

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69 longer magnetize proportionally to the field in which they were initially magnetized in nature. When heating in air, oxidation of the magnetic carri ers can occur, leading to an increase in the capacity of the sample to magnetize. To get around the problem of alteration during heati ng of the samples, one of the primary controls devised in the Thellier method, is to measure the NRM lo st and the TRM acquired in the laboratory field (pTRM) at several increasing temperatur e steps. This allows using the portion of lower unblocking temperatures that are free of mineralogic changes (for example titanomagnetites, depending on their composition, can oxidize at temperatures as low as 300o C). If no mineralogic changes of the magnetic carriers occur during heating the relation between NRM lost and pTRM gained at each temp erature step is expected to be linear. When chemical alteration occurs during heat ing this relationship is no longer linear. Flattening of the corresponding plot (Arai pl ot) is often observed when oxidation takes place. One method to detect chemical alterati on is to perform the so called pTRM checks (Thellier and Thellier 1959) which consists on perfor ming additional measurement of pTRM after having taken the sample to a hi gher temperature. If chemical alteration occurs at higher temperatures the pTRM from the pTRM check should be significantly different from the pTRM initially obtain ed. Additionally, as found by Dunlop et al. (2005), pTRM check can also be unsuccessful when using multidomain (MD) materials, even if no chemical alteration takes place. When MD materials ar e studied, interpreting the steep portion of a curved Arai Plot with successful pTRM checks, that is followed by a shallow portion of the curve with unsucce ssful pTRM checks, as a linear segment suitable for paleointensity calculations leads to misleading results. Therefore, in practice, two slope or curved Arai plots (due to chem ical alteration or MD behavior, respectively)

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70 are difficult to distinguish from each other a nd paleointensity determination should not be made in these cases. Because of the need of several temperature steps in the Thellier experiment and its modifications, a second condition for the reliabi lity of the results is that the blocking temperature spectrum coincides with the unbl ocking temperature spectrum, this condition is called reciprocity. This condi tion is met in the case of ro cks with single domain (SD) magnetic carriers, but it is not met in th e case of pseudo single-domain (PSD) and multidomain (MD) minerals and the resulting Arai plots are usually concaved down. Ideally a paleointensity result could be derived from the NRM pTRM of the highest and lowest temperature steps, however chemical altera tion often occurs at high temperatures. The lava flows studied here have been formed under sub-aerial conditions. Subaerial lava flows are relatively suitable for pale ointensity studies. The process of deuteric oxidation usually occurs when cooling during the formation of the lava flows is a key process by which titano-magnetites become mo re stable magnetic carriers. Submarine basalts that are formed virtually without deuteric oxidation, are rather unsuitable for paleointensity studies. An exception to this statement is the glassy portion of submarine basalts (SBG) which as a result of disequi librium during quenching have very low Ti titanomagnetites (Zhou et al., 2000) and ar e therefore excellent materials for paleointensity studies. In addi tion to lava flows, paleointensity results obtained from obsidians are presented. Such ma terial to my knowledge has not been used previously for paleointensity analysis. Laboratory Work Paleointensity experiments were run in the paleomagnetic laboratory at the University of Florida in a shielded room using a MMTD80 thermal demagnetizer special

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71 for paleointensity experiments. Measurements were made in a 2G Cryogenic magnetometer. Two sets of 36 and 31 samples, respectively, were chosen from the set of samples from British Columbia, Patagonia a nd Mexico. In addition to those, other samples run were: some belonging to the set of samples from Australia collected and studied by Opdyke and Musgrave (2004). Othe r samples were not oriented obsidians most of them from New Mexico and Ariz ona donated by Steve Shackley, lava flows donated by Jeff Gee for inter-laboratory comp arison of results, and recently extruded basaltic flows from Hawaii collected by Dr. Neil Opdyke. Replicates of samples previously run for directional studies, were se lected based on their st ability of remanence in an attempt to improve the su ccess rate of the experiment. One set of samples was run using the Thel liers method as modified by Coe (1967), and the second set of samples was run usi ng an additional modification to the Coe method proposed by Aitken et al. (1988). The Co e method consists of a series of stepwise double heating and cooling under 0 field and kno wn field conditions (in this case 50 T). The method also involves performing pTRM checks. The modification to the Coe method by Aitken et al. (1988) consists in running the inn-field step pr ior to the off-field step. In theory the Aitken method has the advant age over the Coe method of allowing the detection of mineralogic alteration. The temp erature steps used to apply the Thellier experiment were determined based on the behavior observed during thermal demagnetization for initial directional analysis. Data Analysis Arai plots and Zijderveld plots from the o ff-field steps of the Thellier experiment were made in order to interpret the results. Th e analysis of the Arai plots in conjunction with the zijderveld plots is important to ensure that the segment of unblocking

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72 temperatures used to calculate a paleiontensity value trends to the or igin. The analysis of Zijderveld plots is also important to obs erve any changes in the direction caused by magnetomineralogic changes during heating. Paleointensity data was interpreted from some samples that displayed linearity in a portion of the Arai plot and had good pTRM checks. However no interpretation was made in the cases of curved Arai plots (sugge sting MD magnetic part icles) or scattered data. There is, however, a lack of consen sus about the standards of acceptance of paleointensity results. The fo llowing paragraphs explain quality criteria that have been used by other authors and that were taken into consideration to anal yze the data presented in this chapter. Linearity of the Arai Plot A way of assessing linearity of the NRM-TR M segment chosen for paleointensity calculation established by Coe (1978), is th at the slope of the segment chosen for paloeintensity calculations can not differ by more than 20% with the slope of any subsegment that is at least half of the se gment chosen for paleointensity calculations. PTRM Checks One of the main quality criteria is the coincidence of pTRM checks (Coe, 1967) with the initial measure of pTRM. Coe (1967) did not set up any percentage of difference between the two values that coul d be significant, thus leavi ng this matter to a judgment that one could call by eye. Pick and Tauxe (1993) using SBG established that pTRM checks should coincide within 5% of th e initially measured pTRM. However this standard is rarely met when using materials su ch as lava flows. A va riety of alternatives have been implemented to analyze pTRM ch ecks (Juarez and Tauxe (2000), Selkin and Tauxe (2000) and Tauxe and Staudigel (2004)). In this study the approach by Tauxe and

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73 Staudigel (2004) was used, in part because it appears to be more practical. These authors defined a parameter called DRATS (pTRM di fference ratio sum) which is the sum of the differences between the initially measur ed pTRM and the pTRM checks, normalized by the maximum pTRM used for paleointensity calculations. By doing this, the negative and positive differences between initial pT RM and pTRM checks tend canceled out, which according to Tauxe et al. (2004) is advantageous because it gives relevance to trends indicating mineralogic changes rath er than to scatte r brought about from experimental conditions. Quality Factors Established by Coe (1978) Other quality factors established by Coe (1978) were the standard error of the slope in the Arai plot ( b), the fraction (f) of the total (ext rapolated) NRM that is used for paleointensity calculations ( Y/a), the gap factor (g) that quantifies how evenly distributed are the NRM values in the slope, and a quality factor (q) that combines the previous three factors (q = b f g / b). Like in the case of pT RM checks, the need of establishing cut-off values is common to all the quality criteria. Results and Discussion Results from data analysis are shown in Ta bles 5-1 and 5-2. Results with curved or two slope Arai plots were rejected. The quali ty criteria mentioned above were taken into consideration to analyze the data. As selection criteria, cut-off values for the q factor (that combines all the Coe (1978) quality factor s) and DRATS (that measure the success of pTRM checks) were used. For the calculation of DRATS in the set of samples No 2, the second pTRM checks at 400o and 500o C were not taken into consideration because they cover unblocking temperatures ranges accounted for in other pTRM checks. The cut-off

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74 value of the quality factor (q) used, was 1. This cut-off value was used by.Selkin and Tauxe (2000). The maximum value of DRATS considered as a successful result was 25%. This same value was used by Tauxe a nd Staudigel (2004) and even a higher value of 30% was used by Tauxe et al. (2004). Add itionally results from one sample (14-1-4) were discarded because the fraction of the NRM used for the calculation of paleointensity (f ) is very low (0.21). Results from 5 sample s from the first set of samples and 5 samples from the second set of samples met the selecti on criteria and were c onsidered successful (Figures 5-1 to 5-4). As indi cated in Tables 5-1 and 5-2 some results were discarded because they did not meet the quality crit eria or because, despite having successful quality values, they have slightly concaved curves. Most of the successful paleointensity re sults obtained are from British Columbia samples. Only one successful paleointensity results was obtained from Patagonia and one successful paleointensity result was obtained from an obsidian samples. No successful results were obtained from samples from Mexico, Australia or Hawaii. Among the successful results, there are two pairs of samples from the same paleomagnetic sites (1-2 and 25-2) which have similar paleointensity results (Tables 5-1 and 5-2, and Figures 5-1 and 5-2). Examples of unsuccessful paleointensity results are given in Figures 5-5 and 5-6. Figure 5-5 shows rejected results from sample s with slightly curved Arai plots which, nevertheless, have acceptable quality factors. Figure 5-6 shows rejected results of samples from site 25-3. Sample 25-3-8 is a good example of two slope curve that appear to have an interpretable segment of the Arai plot with successful pTRM checks. Based on the results by Dunlop et al. ( 2005), such results are consistent with both sample alteration

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75 after a certain temperature or with MD or PSD behavior. Results from sample 25-3-5 were also rejected because of unsuccessful pTRM checks. Consistent with the outcome of this st udy, great differences in success rate of paleointensity studies using the Thelli er method are observed among different study areas. For example, no success was obtained at all from samples from Iceland (Jeff Gee, personal communication), while Tauxe et al. (2004) obtained successful results from 64 out of 95 (67%) specimens analyzed from An tarctica, and Valet et al. (1998) obtained 35 out of 69 (51%) from lavas < 35 ka from Hawaii. Regardless of the apparent success from an individual Thellier experiment, it is striking to often encounter in the literature standard deviations above 20% among results from the same flow. A paper by Tarduno and Smirnov (2004) sheds a great deal of doubt about results from the Thellier experiments, arguing that very low paleointensity values (under 4 x1022 A/m2) in data compilations (e.g. Selk in and Tauxe, 2000) should not be obtained as frequently because they are typical of unstable states of the Earths magnetic field (e.g., Guyodo and Valet, 1999). Tarduno an d Smirnov (2004) state that lava flows as well as SBG could be slightly weathere d and the weathering pr oducts acquire a CRM at low temperatures during the Thellier experiment, producing misleading results. However records of relative paleointensity from sediments might not provide the real range of paleointensity varia tion of the Earths field because they record time-averaged paleointensity values. Despite the apparent simplicity of the Thellier method, achi eving good results has proven to be very difficult and a matter of c ontroversy. In principle this experiment is only applicable in the case of rocks with single domain gr ains (e.g. Dunlop et al., 2005).

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76 The extent errors caused when applying th e method in rocks with PSD has not been determined yet and in my opinion more studies are needed.

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77Table 5-1. Paleointensity results from the first set of samplesa Sample Latitude Longitude Area N T1 T2 bb/B f g q DRATS F VADM S Reason for (oC) (oC) (%) ( T) (1022Am2) Lack of Success PT10-9 -51.85 -70.52 PT --------------------------------------------Chaotic Plot 10-1-3 51.55 -126.35 BC 9 400 540 0.0411 0.0886 0.87 0.86 8.4 12.0 23.2 3.6 Concaved Plot 1-1-2 50.68 -123.48 BC --------------------------------------------Insuficient data 1-2-2 50.65 -123.44 BC 5 500 550 0.0258 0.0221 0.60 0.68 18.5 12.3 58.3 9.0 x 13-1-3 51.59 -126.43 BC --------------------------------------------Concaved Plot 14-1-5 51.59 -126.43 BC 10 400 550 0.0143 0.0279 0.53 0.87 16.6 3.4 25.6 3.9 x 15-1-3 51.59 -126.43 BC 6 500 550 0.6820 0.6727 0.48 0.76 0.5 1.5 50.7 7.8 Low q value 20-1-9 51.97 -120.13 BC --------------------------------------------Chaotic Plot 20-2-2 51.97 -120.13 BC --------------------------------------------Chaotic Plot 21-2-4 51.95 -120.08 BC --------------------------------------------Concaved Plot 22-1-2 51.93 -120.03 BC 10 350 550 0.1103 0.0569 0.64 0.88 9.9 62.2 96.8 14.8 pTRM check 2-2-2 51.62 -126.62 BC --------------------------------------------Chaotic Plot 25-10-5 51.73 -120.01 BC 13 300 550 0.0384 0.0946 0.85 0.90 8.1 32.5 20.3 3.1 pTRM check 25-1-5 51.73 -120.01 BC --------------------------------------------Concaved Plot 25-2-5 51.73 -120.01 BC 10 400 550 0.0262 0.0365 0.56 0.87 13.4 25.2 35.8 5.5 x 25-3-5 51.73 -120.01 BC 10 400 550 0.0362 0.0521 0.43 0.88 7.3 37.6 34.8 5.3 pTRM check 25-4-3 51.73 -120.01 BC --------------------------------------------Chaotic Plot 25-5-6 51.73 -120.01 BC 8 400 530 0.0208 0.0199 0.39 0.85 16.6 55.4 52.2 8.0 pTRM check 25-6-6 51.73 -120.01 BC 8 400 530 0.0717 0.0650 0.43 0.84 5.6 8.8 55.2 8.4 Concaved Plot 25-7-6 51.73 -120.01 BC 10 400 550 0.0161 0.0226 0.75 0.84 28.3 25.0 35.7 5.5 x 25-8-4 51.73 -120.01 BC 8 400 530 0.0376 0.0506 0.57 0.90 10.2 21.2 37.2 5.7 x 25-9-9 51.73 -120.01 BC 11 350 550 0.0364 0.0711 0.66 0.93 8.7 41.7 25.6 3.9 pTRM check 26-1-6 51.68 -120.05 BC --------------------------------------------Chaotic Plot 26-2-4 51.68 -120.05 BC --------------------------------------------Concaved Plot 26-3-7 51.68 -120.05 BC --------------------------------------------Chaotic Plot PT30-1 -50.55 -71.65 PT --------------------------------------------Concaved Plot PT36-5 -50.32 -71.22 PT --------------------------------------------Concaved Plot PT46-1 -49.51 -72.13 PT --------------------------------------------Concaved Plot PT49-3 -50.01 -71.87 PT --------------------------------------------Concaved Plot

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78Table 5-1. Continued Sample Latitude Longitude Area N T1 T2 b b /B f g q DRATS F VADM S Reason for (oC) (oC) (%) (T) (1022Am2) Lack of Success 5-1-7 51.62 -126.60 BC --------------------------------------------Chaotic Plot PT6-2 -51.88 -70.66 PT --------------------------------------------Concaved Plot 7-1-3 51.59 -126.45 BC --------------------------------------------Chaotic Plot 8-1-5 51.59 -126.45 BC --------------------------------------------Concaved Plot 9-1-4 51.61 -126.40 BC 10 400 550 0.0663 0.0899 0.75 0.87 7.31 5.8 36.8 5.7 Concaved Plot 9-4-4 51.61 -126.40 BC --------------------------------------------Concaved Plot 9-5-5 51.61 -126.40 BC --------------------------------------------Concaved Plot aSampling areas are British Columbia (BC) a nd Patagonia (PT). N is the number of temper ature steps used to calculate the value o f paleointensity in the interval of temperatures T1 and T2 (oC). Quality parameters defi ned by Coe et al. (1978) are: the standard error of the slope ( b), the standard error of th e slope divided by the slope ( b/B), the fraction of the NR M used for calculation of paleointensity (f), the gap fact or (g) and the quality f actor (q). DRATS is the pTRM differe nce ratio sum, F is the value of paleointensity and VADM is the virtual axial dipole moment. Cro sses under S indicate the samples considered to have successful paleointensity results.

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79Table 5-2. Paleointensity results from the second set of samplesa Sample Lat. Long. Area N T1 T2 bb/B f g q DRATS F VADM S Reason for (oC) (oC) (%) ( T) (1022 Am2) Lack of Success PT10-8 -51.9 -70.5 Patagonia -------------------------------------------Concave Plot 12-10 19.3 -99.3 Mexico 15 200 560 0.04 0.07 0.68 0.88 8.84 4.62 30.30 6.8 Concave Plot 1-2-7 50.7 -123. 4 British Columbia 14 300 560 0. 07 0.05 0.83 0.79 12.26 7.43 65.79 10.2 x PT13-2 -51.8 -70.3 Patagonia -------------------------------------------Chaotic Plot 13-2 19.3 -99.3 Mexico -------------------------------------------Concave Plot 1324.1 19.0 -99.0 Mexico* -------------------------------------------Chaotic Plot 14-1-4 51.6 -126. 4 British Columbia 12 300 540 0. 05 0.09 0.21 0.60 1.45 6.54 30.13 4.6 Low q 15-1-6 51.6 -126. 4 British Columbia 13 100 530 0. 32 0.43 0.63 0.90 1.32 15.12 37.07 5.7 x 1718.3 -------------------------------------------------------Concave Plot 1718.7 -------------------------------------------------------Concave Plot 188.1 --------New Mexico* -------------------------------------------Concave Plot 19-2 19.3 -99.4 Mexico -------------------------------------------Chaotic Plot 21.6 -37.6 144.0 Australia -------------------------------------------Concave Plot 23.1 --------Arizona* -------------------------------------------Insufficient data 23.8 -37.6 144.0 Australia -------------------------------------------Concave Plot 25-2-4 51.7 -120. 0 British Columbia 14 300 560 0. 03 0.05 0.98 0.86 18.45 8.46 33.72 x 25-3-8 51.7 -120.0 British Columbia -----------------------------------------Concave Plot 25-9-8 51.7 -120.0 British Columbia -----------------------------------------Concave Plot PA3-105 -52.0 -69.9 Patagonia -------------------------------------------Chaotic Plot 315.1* 2.5 -76.7 Colombia -------------------------------------------Unusual Plot 315.2* 1.2 -77.7 Colombia -------------------------------------------Unusual Plot 33.1* --------Arizona -------------------------------------------Concave Plot 395.1* 36.6 -106 New Mexico 4 530 560 0.15 0.21 0.92 0.62 2.71 34.76 34.72 6.2 pTRM check 53.1* --------New Mexico -------------------------------------------Chaotic Plot 6-3 22.8 -101.9 Mexico -------------------------------------------Concave Plot PT6-7 -51.9 -70.7 Patagonia 7 100 440 0.40 0.63 0.50 0.76 0.60 20.83 31.76 4.9 x 7-8 -37.6 144.0 Australia -------------------------------------------Concave Plot 718.1* 37 -107 New Mexico 11 100 560 0.01 0.035 0.96 0.50 13.83 20.28 16.38 2.9 x 823.1 --------Hawaii -------------------------------------------Chaotic Plot 823.2 --------Hawaii -------------------------------------------Concave Plot 9-5 19.3 -99.3 Mexico 5 200 420 0.14 0.105 0.35 0.70 2.34 12.17 66.21 Concave Plot aColumn headers as in Table 5-2. Sample labels with asterisks are obsidians.

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80 Figure 5-1. Successful paleointen sity results of two sample s from site 1-2 (British Columbia). Figures a and c are Arai pl ots with the corres ponding Zijderveld plots to the right (Figures b and d) Temperature steps are in centigrade degrees.

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81 Figure 5-2. Successful paleointen sity results of two sample s from site 25-2 (British Columbia). Figures a and c are Arai pl ots with the corres ponding Zijderveld plots to the right (Figures b and d) Temperature steps are in centigrade degrees.

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82 Figure 5-3. Successful paleointen ity results from samples 25-76 (a) and 25-8-4 (b). Arai plots are shown with the correspondi ng Zijderveld plots to the right. Temperature steps are in centigrade degrees.

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83 Figure 5-4. Successful paleoint enity results from samples 14-1-5 (a) and 718 (b). Arai plots are shown with the correspondi ng Zijderveld plots to the right. Temperature steps are in centigrade degrees.

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84 Figure 5-5. Examples of rejected paleointensity results that had slightly curved Arai plots but acceptable quality factors. Arai pl ots are shown with the corresponding Zijderveld plots to the right. Temperat ure steps are in centigrade degrees.

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85 Figure 5-6. Examples of rejected paleointens ity results from of samples from site 25-3. The interpretation of results from samp le 25-3-8 is ambiguous (see text). Arai plots are shown with the correspondi ng Zijderveld plots to the right. Temperature steps are in centigrade degrees.

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86 CHAPTER 6 CONCLUSIONS Several general aspects about secular va riation and time-averaged field can be concluded from the studies in British Columbia, Patagonia and Mexico presented in this dissertation: None of the studies yielded persis tent longitudinal components of the paleomagnetic field which is consistent w ith recent studies (e .g., McElhinny et al., 1997). Results from all of the studi es are consistent with m odel G (McFadden et al., 1988) of secular variation, which imply that s ecular variation was adequately sampled. The presence of a quadrupolar compontent is not clear in the resu lts obtained in this study since they are mostly at high latitude The mean directions in the areas of British Columbia and Patagonia (roughly at 50o N and 50o S latitude) coincide with the geocentric axial dipole (GAD), but a quadrupolar component of the field in these areas is difficult to discard becaus e it is expected to produce only about 1o shallower inclinations. The mean direction in the area of Mexico coincides with a GAD plus a 5% quadrupole, but the reverse polarity data coinci des both with the GAD and the GAD plus a 5% quadrupol e and it is closer to GAD. The asymmetry between the northern and southern hemisphere of the present magnetic field and particularly the 20o inclination anomaly in Patagonia, are not observed in the paleomagnetic data obta ined, implying that the present field configuration is relatively recent. The pres ent direction of the magnetic field in the area of Patagonia is at an angle from GAD greater than the angle made with the GAD by any of the successful results in th at area. This observation supports the idea that the present field could be in an anomalously noisy state favorable for occurrence of a reversal or excursion of the fiel d (Opdyke and Mejia, 2004). Recommendations for Future Studies There are volcanic areas on Earth where studies similar to those presented in this dissertation can be made to be tter characterize the paleomagne tic field in the past 5 Myr.

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87 From the experience gathered in this study, and after reviewing a substantial number of articles, I would like to provide some prag matic recommendations for future studies: A careful sampling plan must be made in which all available previous geologic studies area taken into consideration. The support of a local geologist or volcanologist is ideal. Samples must be taken in a wide area of the outcrop to avoid bias or complete lack of success due to the effects of lightning, s lightly rotated blocks, viscous remanent magnetization, etc. Orientation of the samples in the field using sun compass should be done as much as possible. The magnetic anomalies in some sites, often when affected by lightning, are large enough to substan tially deflect the magnetic compass. Sites should be located in the field as accurately as possi ble to facilitated corroboration and the use of th e data in future studies. Laboratory work should start with a pilot se t of samples. Based on the results from this initial set of samples, a plan to pr ocess the remaining samples must be made. This procedure can help determine whethe r AF or thermal demagnetization is more suitable for the particular paleomagnetic an alysis and the degree of detail (i.e. the number of demagnetization steps) as well as the ranges of unblocking temperatures or fields that are more appropriate to apply. The applic ation of numerous demagnetization steps (eg., 16 steps), that make the laboratory work a lot more time consuming, range from being unnecessary to definitive in order to obtain success. For example, results from An tarctica using only NR M (Mankienen and Cox, 1988) are not much different than th e results obtained after demagnetization of the same set of samples (Tauxe et al ., 2004) On the other hand, the application of numerous demagnetization steps in the st udy from Patagonia presented here, helped produce more precise and successful results. Good age control is important. Making e fforts to accompany the paleomagnetic results with radiometric dates will enhan ce the possibility of using the results obtained in future paleomagnetic studies (e.g. studies focusing on shorter periods on time). Attempting paleointesity measurements w ill help to improve the paleointensity dataset. The comparison of improved paleoi ntensity dataset from lava flows with paleointesity records from sediments c ould help to elucidate the validity of paleointesity results.

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88 Comparison with Recent Studies In order to venture into what recent pale omagnetic studies are telling us about timeaveraged field and secular va riation, a brief compilation of these results was made, which is presented in Table 6-1. Figure 6-1 shows th at the VGP scatter of these studies coincide with Model G of paleosecular variation (M cFadden et al., 1988). Figure 6-2 shows the mean inclination of recent studies along with the inclination expected from the GAD and the GAD plus a 5% quadrupole. As can be judge d, it is difficult to as certain if the data fits better to a GAD plus a 5% quadrupole (a common pe rcentage of quadrupole term proposed in TAF studies). In the case of the mostly unpublished results from Ecuador by Opdyke and others (Opdyke et al., 2004), the da ta strongly suggest the presence of the quadrupole term. A further analysis of the data shows that adding an axial ocupole component of 7% to the GAD plus 5 % quadrupo le curve, helps increase the fit of the data at around 30o to 40o latitude (Figure 6-3).

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89 Figure 6-1. VGP scatter of studies in Tabl e 6-1 (diamonds) compared to model G (curve). Modified from Opdyke and Mejia, 2004.

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90 Figure 6-2. Magnetic inclinati on plotted against latitude. The figures show the mean inclination obtained from paleomagnetic studies listed in Table 6-1 (filled diamonds), inclination expected from GAD (solid lines) and inclination expected from a magnetic field comp osed of GAD plus a 5% quadrupole (dashed lines). (a) Normal and revers e data. (b) Normal and reverse data combined. Modified from Opdyke and Mejia, 2004.

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91 Figure 6-3. Comparison of GAD and GAD plus 5% quadrupole, plus 7% octupole. Figure 6-3 a shows paleomagnetic inclinatio n data from Table 6-1 (diamonds) compared to the inclination expected from GAD (solid lines) and from GAD, plus a 5% quadrupole, and a 7% octupol e (dashed lines). Figure 6-3 b shows the same results (squares) plotte d as anomalies relative to GAD.

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92Table 6-1. Summary of recent secular variation studiesa Reference Location Lat. Long.F.D.Age G Dec Inc 95 N St/SbSl Su Max ND1MinND2 Disp. VGP Mankinen and Cox (1988) Mc Murdo V.F. -78.1165.439 0-5 C 4.6 -84. 7 3.5 38 21.918.9 25.96.2 0 48.71 N 358.5 -85.8 3.9 29 --------------R 194.3 81.2 8.4 9 --------------Baraldo et al. (2003) Deception Island -63.0-60.627 Brunhes N 348.8 -73.70 4.4 21 18.215.1 23.013.4 0 54.80 Mejia et al. (2004) Patagonia -51.2-70.6 46 0 4.1 C 359.0 -68.2 3.5 33 17.214.7 20.6 5 13 56.40 N 359.4 -70.6 4.3 22 --------------R 178.4 63.5 6.2 11 --------------Camps et al. (2001)1 Possession Is. -46.451.8 45 0.5 5 C 5.2 -64.4 3.7 41 18.614.6 22.416 1 453 N 359.4 -66.4 3.8 21 --------------R 190.7 61.8 6.4 20 --------------Opdyke and Musgrave (2004) Newer V. -38.0 144.038 0 4 C 356.3 -57.7 3.5 33 14.212.1 17.0 6 3 402 N 2.1 -62.3 6.3 13 --------------R 173.4 54.6 3.8 20 --------------Brown (2002) Easter Island -27.1-109.2 62 Brunhes N 357.3 -45.2 4 55 14.112.4 16.2 10 7 53.70 Yamamoto et al. (2002) Society Islands -18.0148 154 0.62 3.45C 2.1 34.7 2.6 13014.613.4 15.9 15 14 5010 N 3.8 -34.1 3.2 82 --------------R 179.7 35.5 4.5 48 --------------Opdyke et al. (in prep.) Ecuador -0.37-78. 370 0 2.5 C 359.8 -5.8 4.1 49 13.211.6 15.3 10 N 355.8 -5.8 7.1 20 14.011.5 17.8 10 R 182.5 5.7 4.9 29 12.910.9 15.7 10 Kidane et al. (2003)2 Afar Depression 11.0 42.0 35 1.4 3.3 C 359.7 16.4 5.7 26 11.69.8 17.8 15 2 --2 R 179.6 -17.9 6 21 --------------Carlut et al. (2000) Guadalupe Island 16.0 -61.7 26 1 0.05 N 0.0 29.6 4.5 23 11.39.4 14.17.7 0 65.52 Mejia et al. (submitted, 2005) Mexico 19.6 -88.0 187 0 2 C 358.8 31.6 2 18712.711.9 14.1 10 ------N 359.0 30.7 2.2 14413.012.0 14.1 10 ------R 178.0 -34.5 4.1 43 12.110.7 13.9 10 ------Laj et al. (1999)3 Hawaii 21.4 157.5105 3.22 3.11N 2.8 23.7 2.3 10312.711.6 14.113.6 1 53.41 Herrero and Valet (2002) Hawaii 21.4 157.518 0.033 0.7N 358.6 34.5 5.8 17 12.09.7 15.66.6 0 62.21

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93Table 6-1. Continued. Reference Location Lat. Long.F.D. Age G Dec Inc 95 N St/SbSl Su Max ND1MinND2 Disp. VGP Tauxe et al. (2000) La Pa lma Is. 28.8 -17.829 2 0.566 C 358.642.4 6.1 21 15.8 13.120.0 5 8 63.20 N 355.146.1 9.6 9 --------------R 180.9-39.1 8.4 12 --------------T auxe et al. (2003) San Francisco V. 35.4 -111. 847 0-5 C 351.952.6 5.5 22 15.0 11.917.9k > 10023 63.60 T auxe et al. (2003) N 349.949.1 9.4 12 --------------R 175.6-54.7 5.2 10 --------------Johnson et al. (1998) Sao Miguel Is. 37.6 -26.435 0. 78-0.88 N 357.448.5 5.3 28 17. 3 14.7 21.2k > 20 4 45.20 Mitchell et al. (1989) Washington 46.0 -121.857 Brunhes N 2.6 65.2 2.5 56 14. 9 13.2 17.97.9 1 59.70 Mejia et al. (2002) British Columbia 51.5 -122.452 Brunhes N 356.970. 2 2.8 45 17.8 15.4 20.2 5 7 52.50 Udagawa et al. (1999) Iceland 65.1 -15.0 38 1.8 0.5 N 359.073.6 4 37 22.1 19.1 26.3 7.4 0 451 aLat. and Long. are the approximate average latitude and longitude of the study areas, F.D. is the number of drilled flows, Age is given in Ma (for numerical values), G refers to the way the sites were grouped: N for normal, R for reverse and C for combined normal and reversed site s. Dec. is declination, Inc. is inclination, St/Sb is the VGP scatter (either uncorrected or corrected for within site scatter). Sl, and Su are the lower and upper 95% conf idence limits of the scatter ( Cox 1969]. Max Disp. is the maximum 95 cut-off value used to consider a site for calculations (when preceded by ") or the maximum 95 value in the data set used for calc ulations (value not preceded by "), except when the minimum k (dispersion parame ter) is stated. ND1 is the number of sites excluded because of high scatter. Min VGP is the minimum VGP cut-off value used to consider a site (if value preceded by ") or the minimum VGP value among the sites used for calculations (value not preceded by "). ND2 is the number of sites excluded because of low VGP. 1Confidence limits of scatter calculated using the method of Efron and Tibshirani (1986]. 2Only the sites presented as tectonically stable were considered. 3Scatter of the VGP was recalculated in this study using the presen t position of the hot-spot to correct for plate movement. Modified from Opdyke and M ejia, 2004.

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102 Skewes, M. A., and Stern, C. R., Petrology and geochemistry of alkali basalts and ultramafic inclusions from the Palei-Aike volcanic field in southern Chile and the origin of the Patagonian plateau lavas, J. Volcanol. Geotherm. Res., 6, 3-25, 1979. Soler-Arechalde, A. M., and Urrutia-Fucugauchi, J., Paleomagnetism of the Acambay Graben, central Trans-Mexican volc anic belt, Tectonophysics, 318, 235-248, 2000. Steele, W.K., Paleomagnetic constraints on the volcanic history of Iztaccihuatl, Geofis. Int., 24, 159-167, 1985. Steele, W. K., Paleomagnetic directions from the Iztaccihuatl volcano, Mexico, Earth Planet. Sci. Lett., 11, 211-218, 1971. Szeremeta, N., Laj, C., Guillou, H., Kissel, C., Zamaud, A., and Carracedo, J. C., Geomagnetic paleosecular variation in th e Brunhes period, from the island of El Hierro (Canary Islands), Eart h Planet. Sci. Lett., 165, 241-253, 1999. Tanaka, H. (1999), Circular asymmetry of the paleomagnetic directions observed at low latitude volcanic sites, Earth, Planets and Space, vol.51, 1279-1286. Tauxe, L. Paleomagnetic principles and pr actice, Kluwer Academic Publishers, The Netherlands, 1998. Tauxe, L., Constable, C. Johnson, C. L., Koppers A. A. P., Miller, W. R., and Staudigel, H., Paleomagnetism of the southwestern U. S.A. recorded by 0 Ma igneous rocks, Geochem. Geophys. Geosyst., 4(4), 8802, doi:10.1029/2002GC000343, 2003. Tauxe, L., Gans, P., and Mankinen, E. A., Paleomagnetism and 40Ar/39Ar ages from volcanics extruded during the Matuyama and Brunhes Chrons near McMurdo Sound, Antarctica, Geochem. Ge ophys. Geosyst., 5, Q06H12, doi:10.1029/2003GC000656, 2004. Tauxe, L. and Kent, D. V., A simplified stat istical model for the geomagnetic field and the detection of shallow bias in pale omagnetic inclinations: was the ancient magnetic field dipolar? In Timescales of the paleomagnetic field, Geophysical Monograph Series 145, eds. Channell, J.E.T ., Kent, D.V., Lowrie, W. and Meert, J.G., 101-115, American Geophysical Union, Washington, DC, 2004. Tauxe, L., and Staudigel, H., Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine ba saltic glass of the Troodos Ophiolite, Geochem. Geophys. Geosyst., 5, Q02H06, doi:10.1029/2003GC000635, 2004. Tauxe, L., Staudigel, H., and Wijbrans, J. R., Paleomagnetism and 40Ar/39Ar ages from La Palma in the Canary Islands, Geochem. Geophys. Geosys., 1, paper 2000GC000063, 2000. Thellier, E., and Thellier, O., Sur lintensit e du champ magnetique terrestre dans le passe historique et geo logique, Ann. Geophys., 15, 285, 1959.

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PAGE 116

104 BIOGRAPHICAL SKETCH Victoria Mejia was born in Lafayette, I ndiana, while her father, a university professor from Colombia, was finishing his PhD in agricultural soils at Purdue University. She grew up in a rather conservative regi on of Colombia where coffee production is the main economic activity. Her inclination for the na tural sciences and intellectual affairs thrived in her home town, Manizales, a small city that is the home of several universities. Educati on was also stimulated at her home by her dedicated parents and six older siblings. In 1985, while she was in her first years of undergraduate studies in geology, the Nevado del Ruiz volcano, a volca no that offers an imposing view from Manizales, made a catastrophic eruption. Her re search experience star ted at the volcano observatory that was created soon after this eruption. Since then, volcanology has played a major role in all her research, which be gan with her undergradua te thesis on seismic tremors, produced by fluid flow within volcan ic vents. Later on she embraced the study of paleomagnetism using submarine basaltic glas s (for her masters thesis) and lava flows (for her PhD dissertation) as paleomagnetic recorders. Outside of academia, she has worked for the geological survey of Co lombia (Ingeominas) and for Baker Hughs.


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GEOMAGNETIC FIELD FOR THE PAST 5 MYR RECORDED IN LAVA FLOWS
FROM BRITISH COLUMBIA, PATAGONIA, AND MEXICO















By

VICTORIA MEJIA


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Victoria Mejia

































I dedicate this dissertation to my parents, Teresita and German.















ACKNOWLEDGMENTS

I am very grateful to the chair of my supervisory committee, Dr. Neil Opdyke, for

giving me the opportunity to participate in this project and for his continuous support. I

also thank the members of my supervisory committee: Dr. Dwight Adams, Dr. Jim

Channell, Dr. Mike Perfit, and Dr. David Foster, who were supportive and followed the

progress of this investigation.

I am especially grateful to all the researchers who collaborated with me in this

study, particularly those who participated during field work, and from whom I received

numerous suggestions and had important discussions: Dr. Rene Barendregt who sampled

the area of British Columbia; Dr. Harald Bohnel who did field work with Dr. Neil

Opdyke and me in Mexico; Dr. Juan Francisco Vilas who did field work with Dr. Neil

Opdyke and me in Patagonia, and Dr. Joe Stoner who collected some samples in

Patagonia. I also want to thank for complementing this study by doing radiometric dating

on the samples we collected, Dr. Brad Singer, who dated samples from Patagonia, and

Dr. Amabel Ortega-Rivera and Dr. James Lee who dated samples from Mexico. I wish to

thank many geologists who opportunely and briefly helped and shared with us their

knowledge of the areas of study during field work, like Dr. Miguel Haller, Dr. Massimo

D'Orazio and Dr. Fabrizio Innocenti, in Patagonia and Dr. Jorge Aranda, in Mexico. I

also want to express my gratitude to the numerous persons whose hospitality made field

work abroad very pleasant.









Likewise, I wish to thank important suggestions from paleomagnetists such as Dr.

Joe Meert, Dr. Lisa Tauxe, Dr. Cathy Constable and Dr Catherine Johnson.

I am greatful to Dr. Kainian Huang and Ray Thomas for their assistance in the

paleoamgentic laboratory at the Univiersity of Florida.

I am grateful to the staff, faculty and graduate students of the Department of

Geology for the team spirit lived during our every day work. I especially thank some

graduate students (some of whom have graduated): Kusali Gamage, Sergio Restrepo,

Jaime Escobar, Dr. Johan Guyodo, Helen Evans, Dr. Sharon Kanfoush, Dr. George

Kamenov, Dr. Carlos Jaramillo and Dr. John Chadwick.

I also want to thank my husband, Jorge Ivan Velez, for his moral support and for

his patience, since he remained living in Colombia, and we could not share a lot of time

together for several years.

This study was mostly funded by the National Science Foundation. Additional

funding to support myself was received from a graduate fellowship from the Florida

Georgia Alliance for Minority Participation (for one year) and the McLaughlin

Dissertation Fellowship (for one semester).
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES .................................................... ............ .............. viii

LIST OF FIGURES ......... ......................... ...... ........ ............ ix

ABSTRACT ........ .............. ............. ...... ...................... xi

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

Tim e A average Field (TAF) M odels.................................... ................... ...... ......... ..
Paleosecular V ariation (PSV ) ......................................................... .............. 3
P aleom magnetic D atasets ............................................................... ........................ 4
D ata Q u ality ...................... ............... ................................................ . 5
Content of the D issertation ........................... ......... .. ..................... ............... 6

2 PALEOSECULAR VARIATION OF BRUNHES AGE LAVA FLOWS FROM
BRITISH COLUM BIA, CANADA ........................................ ........................ 7

Sam pling and G eologic D escription....................................... .......................... 8
Laboratory Analysis................... ............ ......................... 14
D ata A analysis and Selection Criteria...................................... ........................ 16
Results from British Columbia and Discussion ............. ....................................... 18

3 PLIO-PLEISTOCENE TIME AVERAGED FIELD IN SOUTHERN PATAGONIA
RECORDED IN LAVA FLOW S.................................... .......................... .. ......... 25

Sam pling and Sam pling A rea ......................................................................... ..... 27
N northern Sam pling A rea........................................................... ............... 27
Pali-A ike V olcanic Field ......................................................... ............... 28
P aleom agn etic A n aly sis.......................................... ..............................................2 8
D ata A analysis and Selection Criteria...................................... ........................ 30
R adioisotopic A ges ................... ........ .............. .... ...... ............. 34
N northern Sam pling A rea........................................................... ............... 34
Pali-A ike V olcanic Field ........................................................ ............... 35
R results from Patagonia ........................................................................ ............... 36









4 PALEOSECULAR VARIATION AND TIME-AVERAGED FIELD RECORDED
IN LAVA S FLOW S FROM M EXICO ........................................... .....................41

G eology and Sam pling A rea............................................................ .....................4 1
Previous Paleomagnetic Studies in the TMVB ................................... ...............47
N ew D ata ............................ ....................... ........................................ ............... 4 8
Paleomagnetic Laboratory Work and Data Analysis .......................................49
R results from N ew D ata ............................................... ............................ 50
Com pilation of Paleom agnetic D ata............................................... ..................... 56
T ectonic R stations ...................................................... .. ......... .. .... 57
R results from the T M V B ..................................................................... ..................58
C conclusions from M exico................................................. .............................. 60

5 PALEO IN TEN SITY .......................................................... .... ............... 68

Laboratory W ork .................................. .. .......... .. ............70
D ata A naly sis ................................................. ........................... 7 1
Linearity of the Arai Plot............................................................ .. ....72
P TR M C hecks ................................................................ ........ ............72
Quality Factors Established by Coe (1978).............................. ...............73
R results and D discussion ..................... .. .. .......................... ..... ..... 73

6 C O N C L U SIO N S ..................... .... .......................... ........ ........ ...... ........... 86

Recom m endations for Future Studies..................................... ......... ............... 86
Com prison w ith R recent Studies........................................... ......................... 88

L IST O F R E FE R E N C E S ....................................................................... ... ................... 94

BIOGRAPHICAL SKETCH ......................................................... ................ 104
















LIST OF TABLES


Table pge

2-1 Paleomagnetic and age data of sites from British Columbia ...................................9

2-2 Statistical data among groups of sites from British Columbia.............................24

3-1 Paleomagnetic and age data of sites from Patagonia ............................................33

3-2 Statistical data among groups of sites from Patagonia..............................39

4-1 New paleomagnetic and age data of sites from Mexico.......................................54

4-2 Statistical data among new sites studied in M exico.............................................. 55

4-3 Compiled Late Pliocene Holocene paleomagnetic data from the TMVB.............61

4-4 Statistics of paleomagnetic data of late Pliocene to Recent age from studies..........67

4-5 Statistics of late Pliocene to Holocene age results from compiled data .................67

5-1 Paleointensity results from the first set of samples................. .........................77

5-2 Paleointensity results from the second set of samples ..........................................79

6-1 Sum m ary of recent secular variation studies ........................................ .............92
















LIST OF FIGURES


Figure p

2-1 Location map showing sampling sites in British Columbia..............................8..

2-2 Geology of the Silverthrone volcanic field and sampling-site locations ..............12

2-3 Geology of the Clearwater Wells Gray Park volcanic field and sampling-site .....15

2-4 Pair of Zijderveld diagrams showing AF and thermal demagnetization..................17

2-5 Equal area projection of site-mean directions from British Columbia.....................18

2-6 Equal area projection of mean directions from several groups of sites .................20

2-7 Mean virtual geomagnetic poles (VGPs) obtained from British Columbia............23

3-1 Location map of southern Patagonia showing sampling sites...............................26

3-2 Examples of Zijderveld diagrams obtained from Patagonia...................................29

3-3 Equal area projection of site mean directions from Patagonia...............................31

3-4 Paleomagnetic results from Patagonia expressed in VGPs................... ................32

3-5 Equal area projection of mean directions from several groups of sites....................39

4-1 Location of sampling areas in M exico .......................................... ............... 44

4-2 Location of compiled paleomagnetic sites (a95 < 100) from the TMVB...................46

4-3 Examples of Zijderveld diagrams from M exico ............................. ..................... 51

4-4 Remagnetization circles and directions form samples of Site 17 (Mexico).............52

4-5 Equal area projection of paleomagnetic directions obtained from Mexico .............53

4-6 Equal area projection of paleomagnetic directional data compiled from .............59

4-7 Virtual geomagnetic poles (VGPs) from the TMVB ............................................61

5-1 Successful paleointensity results of two samples from site 1-2.............................80









5-2 Successful paleointensity results of two samples from site 25-2.............................81

5-3 Successful paleointenity results from samples 25-7-6 (a) and 25-8-4 (b)...............82

5-4 Successful paleointenity results from samples 14-1-5 (a) and 718 (b). ..................83

5-5 Examples of rejected paleointensity results .......... ...............................................84

5-6 Examples of rejected paleointensity results from of samples from site 25-3...........85

6-1 VGP scatter of studies in Table 6-1 (diamonds) compared to model "G" (curve). .89

6-2 M agnetic inclination plotted against latitude. .................................. .................90

6-3 Comparison of GAD and GAD plus 5% quadrupole, plus 7% octupole. ................91















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

GEOMAGNETIC FIELD FOR THE PAST 5 MYR RECORDED IN LAVA FLOWS
FROM BRITISH COLUMBIA, PATAGONIA, AND MEXICO

By

Victoria Mejia

May 2005

Chair: Neil D. Opdyke
Major Department: Geological Sciences

Paleosecular variation (PSV) and time averaged field (TAF) results recorded in

lava flows younger than 5 million years are presented. The targeted areas of studies are

several volcanic fields from British Columbia (mainly the Silverthrone, Garibaldi, and

Wells Park volcanic fields), Southern Patagonia (the Pali-Aike volcanic field and Meseta

Viscachas lavas), and Mexico (the Trans-Mexican volcanic belt and several volcanic

areas in San Luis Potosi).

The purpose of this investigation was to obtain high quality paleomagetic data

suitable to test the presence or absence of permanent non dipolar components of the field

that have been interpreted from studies carried out with less rigor. The mean directions in

the areas of British Columbia and Patagonia (roughly at 500 N and 50 S latitude)

coincide with the expected geocentric axial dipole (GAD) at these areas. The presence of

a quadrupolar component of the field is difficult to discard because it is expected to

produce only about 1 shallower inclinations. The mean direction in the area of Mexico









coincides with a GAD plus a 5% quadrupole. The VGP scatter in the three areas of study

coincides with Model G.

The asymmetry between the northern and southern hemisphere of the present

magnetic field and particularly the 200 inclination anomaly relative to GAD in Patagonia,

are not observed in the paleomagnetic data obtained, implying that the present field

configuration is relatively recent. The results confirm that axial components prevail in the

time-averaged field.














CHAPTER 1
INTRODUCTION

The characteristics of the present Earth's magnetic field are well known from

magnetic observatories, satellite, airplane and ship data, but the history of the magnetic

field blurs as we go into the past. The implementation of the magnetic compass in Europe

by 1200 ultimately made possible historic records of the magnetic field. Compilations of

historic records dating back to about 1600 have been used to model the magnetic field

(Jackson et al., 2000). The Earth's magnetic field has also been modeled on other time

scales such as the last 3000 years using archaeomagnetic data derived from pottery

(Constable et al., 2000) and in geologic times (the past 5 Myr) using paleomagnetic data

derived from rock materials (e.g. Johnson and Constable, 1995). The analysis of these

different kinds of records of the magnetic field has helped to understand both the time

averaged field (TAF) and its secular variation, which applied to the geologic past, is

referred to as paleosecular variation (PSV).

Time Average Field (TAF) Models

The TAF can be described as an expansion of spherical harmonics, and, although a

non-unique solution, the TAF is represented in most studies as mainly (about 95%)

composed of a geocentric axial dipole term (GAD). Because of the deficiencies in quality

and spacial coverage of paleomagnetic data, it is still uncertain as to which other non-

dipolar terms best characterize the paleomagnetic field. Besides the GAD (Opdyke and

Henry, 1979), the most likely term present in the TAF is an axial quadrupole of about 5%

of the magnetic field (e.g., McElhinny et al., 1996; Johnson and Constable, 1995;









Hatakeyama and Kono, 2002). The quadrupole field is regarded as more promising

because of the consistency with which it is observed in paloemagnetic data (McElhinny

et al., 1996). The quadrupole field was first observed by Wilson (1970) who noticed that

it causes a "far sided effect". That is, the virtual geomagnetic poles (VGPs) plot away

from the pole with respect to the sampling area. The quadrupole field is observed in

paleomagnetic datasets as consistent negative inclination anomalies (observed

inclinations minus expected inclination from GAD) for the normal polarity data, which

imply shallower inclinations than expected (positive or downwards) in the northern

hemisphere and steeper inclinations than expected (negative or upwards) in the southern

hemisphere. Likewise, the quadrupole field is manifested by consistent positive

inclination anomalies in the reverse polarity data, which imply steeper inclinations than

expected (negative) in the northern hemisphere and shallower inclinations than expected

(positive) in the southern hemisphere. It is difficult to think of any source of error that

could produce such pattern of inclination anomalies, which provides certainty that the

axial quadrupole term is real and not an artifact.

Another non-dipole term, the axial octupole, is expected to produce inclinations

throughout all latitudes to be shallower (or steeper if the term has a negative sign). It is

therefore more likely to confuse the axial octupole with artifacts of the paleomagnetic

data. Despite obtaining a significant octupole term by analyzing paleomagnetic dataset,

some authors regard this term as an artifact (e.g., McElhinny et al., 1996) while others

consider it real (Hatakeyama and Kono, 2002). Likewise the existence of persistent non-

axial terms, resulting in longitudinal structures, has been suggested (e.g., Johnson and









Constable, 1995; Johnson and Constable, 1997) nonetheless with a great deal of

uncertainty due to insufficient data.

Paleosecular Variation (PSV)

PSV is most often analyzed by studying the VGP scatter relative to the axis of

rotation among a group of paleomagnetic sites in a region. Paleosecular variation studies

show that the value of VGP scatter increases with latitude. The paleosecular variation

model that best fit the observations is Model G (MacFadden et al., 1988). In Model G,

VGP scatter is analyzed separately for the dipole and quadrupole family of spherical

harmonics (i.e., spherical harmonic terms with Gauss coefficients in which degree (n)

minus order (m) are odd or even numbers, respectively). According to this model the

scatter due to the dipole family of harmonics varies with latitude and the scatter due to

the quadrupolar family of harmonics is constant though out all latitudes. The averaged

VGP scatter of the present magnetic field for the northern and southern hemispheres is

similar to Model G.

Another observable of PSV is the distribution of the directions and VGPs.

According to several studies (e.g., Tanaka, 1999; Tauxe and Kent, 2004), the distribution

of VGPs is expected to be circularly symmetric at all latitudes, while the distribution of

direction is expected go from elliptical at low latitudes to circular at high latitudes. The

Mu and Me parameter values of circularly symmetric (or fisherian) distributions should be

< 1.207, and < 1.094, respectively (Tauxe, 1998). These parameters are derived from the

comparison of the declination and inclination distributions with the correspondent

theoretical distribution of a Fisher distribution. Tauxe and Kent (2004) have model the

expected north-south elongation of the 95% confidence ellipses as a function of latitude.









Paleomagnetic Datasets

Two main archives of paleomagnetic data are available from different sources:

sediments and lavas. PSV studies in which lava flows are used as recording materials are

generally referred to as PSVL (acronym for paleosecular variation in lavas). Lava flows,

which were used for this study, have the ability to record the Earth's magnetic field at the

time they cool. The period of time in which a lava flow cools is so short (around a year)

relative to our resolution of geologic time in general, that we can consider lava flows as

archives of instant readings of the paleomagnetic field. Lee (1983) compiled a

paleomagnetic database from lavas of the past 5 Myr. This database has been

subsequently updated by Quidelleur et al. (1994), Johnson and Constable (1996) and

McElhinny and McFadden (1997). After application of different selection criteria the

number of records in the renovated databases is not much greater than the 2244 records in

Lee's original compilation. The characteristics of any of these datasets are far from being

ideal; the main deficiencies are that (a) the sample distribution over the Earth is deficient

and not uniform, (b) most studies have been undertaken using paleomagnetic procedures

that are now obsolete and (c) age control is limited.

This study emerged upon realizing the need to improve the paleomagnetic dataset

to better constrain time-averaged field (TAF) and secular variation models. The research

project that emerged from this initiative, called the time averaged field initiative (TAFI),

was undertaken with NSF funds by scientists from several institutions such as the Scripps

Institute of Oceanography, University of Florida, University of Alaska, and University of

Massachusetts.

The areas of study were strategically chosen to improve the spatial and temporal

coverage of the paleomagnetic datasets and tectonic stability was taken into









consideration. For this reason, this study comprises only the past 5 Myr, a period of time

in which significant influences of plate tectonic movements are avoided. Most of the

studies belonging to the TAFI initiative are located along the backbone of the Americas,

from the Arctic to the Antarctic. The target areas for this dissertation were several

volcanic fields from British Columbia, southern Patagonia, and Mexico.

Data Quality

The selection criteria for acceptance of paleomagnetic results are somewhat

arbitrary, despite that data quality is a key issue for obtaining accurate non-dipole

components of the TAF and secular variation estimates. Generally, for example, studies

from individual areas in which blanket or no demagnetization have been applied are

discarded and a maximum cut-off value of a95 (the 95% confidence cone around the

mean direction) is determined. With increasing attention placed on achieving very

accurate results, recent paleomagnetic results from specific areas on Earth (e.g. Carlut et

al., 2000; Johnson et al., 1998; Tauxe et al., 2000) have greatly superseded previous

selection criteria. Tauxe et al. (2000) set up new selection criteria in which the a95 of

individual sites is restricted to 5 (for comparison McElhinny and McFadden (1997) used

200). Such selection criteria are attainable not just by discarding a lot more data but by

implementing stringent laboratory and data analysis techniques that improve the accuracy

of the results. Tauxe et al. (2000) applied several strategies to obtain paleomagentic data

of improved quality. One of them emplaced in the laboratory is to increase the number or

demagnetization steps that are often performed (from around 7 to 10 steps to around 15 to

18). This strategy enables us to resolve the primary component of magnetization more

accurately and to discard spurious data more easily. Another strategy to improve the

quality of the paleomagnetic data is performed during data analysis. The principal









component of magnetization is obtained from at least 5 points of the demagnetization

curve (without anchoring to the origin) and the MAD that cannot exceed 5. The

measures of Tauxe et al. (2000) to achieve good data quality were followed in the studies

described in this dissertation. These measures increase the expectations of obtaining data

sensitive to non-dipole (5-10% of the Earth's field) components of the field.

Content of the Dissertation

Chapters 2, 3, and 4 contain the paleomagnetic studies from the three areas of study

chosen for this dissertation: British Columbia, Patagonia and Mexico respectively.

Results from the studies in British Columbia (chapter 2) and Patagonia (chapter 3) have

been published (Mejia et al., 2002, and Mejia et al., 2004, respectively) and results from

the study of Mexico (chapter 4) are in the process of being published (Mejia et al., 2004,

submitted). Many segments, tables, and figures from these articles are found throughout

the dissertation in the same or somewhat modified way that they appear in the journals. In

chapter 5 some results of paleointensity analysis are presented. Finally, chapter 6 presents

the conclusions and inspection of recent paleomagnetic studies of the same kind

presented here. Part of the work of analyzing recent studies has been already published

(Opdyke and Mejia, 2004).














CHAPTER 2
PALEOSECULAR VARIATION OF BRUNHES AGE LAVA FLOWS FROM
BRITISH COLUMBIA, CANADA

Paleomagnetic results from 53 sites from southern British Columbia (latitude 50-

51.5 N) are presented. Samples were taken from the Silverthrone, Mount Meager, and

Garibaldi Lake volcanic fields of the Garibaldi Volcanic Belt, as well as from the Wells

Gray Clearwater volcanic fields of the Anahim Volcanic Belt and the Kelowna area

field of the Chilcotin Plateau Basalts (Figure 2-1). These volcanic areas rest on

metamorphic rocks and were produced in a variety of tectonic settings: arc volcanism

(Garibaldi Volcanic Belt), hot spot volcanism (Anahim Volcanic Belt), and back arc

volcanism (Chilcotin Plateau Basalts). Volcanism in this area both predates and postdates

Pleistocene glaciations. During the glacial and interglacial periods, sediments of glacial,

fluvial, and lacustrine environments were deposited along with volcanic products,

including the lava flows that are the focus of this study. A rich variety of geomorphic

shapes such as tuya volcanoes, horns, glacial valleys, and moraines are a legacy of the

glacial age. Many of the lava flows studied here are valley-filling basalts and display a

wide variety of sub-aqueous and sub aerial textures. Age control was achieved by: (a)

paleomagnetic sampling of previously studied lava flows, and (b) accepting the age of the

volcanic unit given in the literature. Based on these ages, the sampled lava flows

represent mostly the last 550 ka, although flows ranging from 550 to 760 ka were also

sampled.










-1300 -1250 -1200 -115


VNince British Columbia Alberta
.. *Ruren



::'- \Cleawater-Wells \
Gray Park V. F.
~ilverthrone V. F.

Mount Meager V. F. Kamloops
< .....'U- ,*. j a .-
Garibaldi Lake V, F,
Kefowna V. F. 50
g* Kelowna


Vii|tor Washington ILnln




Figure 2-1. Location map showing sampling sites in British Columbia (black dots) and
their correspondent volcanic fields.

Sampling and Geologic Description

A total of 53 paleomagnetic sites were sampled by Barendregt during the summers

of 1998 and 1999. Samples were taken using a hand held drill and oriented using

magnetic compass and, when possible, sun compass (30% of the samples). From 8 to 10

cores were collected from all sites, each site corresponding to a single lava flow. Site

location was determined using GPS. Most sites were accessed by road and short hikes

were necessary. A helicopter was used to access two flows. Most of the sampled lava

flows fill valleys and are exposed as a result of subsequent erosion. Other lava flows were

sampled closer to volcanoes and are at a higher altitude. Several sites are from a series of

overlapping lava flows, generally from 3 to 9 in number. The lava sequences and their

stratigraphic relationship are given in Table 2-1.


















Table 2-1. Paleomagnetic and age data of sites from British Columbiaa


Site U/ DG V.F. Site Site
ID L Lat. Long.

1-1 MM 50.68 -123.48

1-2* MM 50.65 -123.44
1-3 GL 50.07 -123.04

1-4 U GL 50.07 -123.09

1-5 L GL 50.04 -123.11
2-1 U ST 51.62 -126.62
2-2 ST 51.62 -126.62
2-3 L ST 51.62 -126.62

5-1 ST 51.62 -126.60
7-1 ST 51.59 -126.45
8-1 ST 51.59 -126.45
9-1 L a ST 51.61 -126.40
9-2 ST 51.61 -126.40
9-3 ST 51.61 -126.40
9-4 a ST 51.61 -126.40
9-5 U ST 51.61 -126.40
10-1 ST 51.55 -126.35
13-1 ST 51.59 -126.43
14-1 ST 51.59 -126.43
15-1 ST 51.59 -126.43
16-1 U ST 51.40 -126.31
16-2 L ST 51.40 -126.31
18-1 CW 52.13 -120.23
19-1 CW 52.17 -120.22


D I SC n/N K c95
Dir. Dir.

23.2 53.2 10 6/10 642 2.6

23.1 56.2 10 10/10 110 4.6
41.3 77.5 12 11/12 350 2.4

342.1 68.7 10 10/10 401 2.4

323.1 77.0 10 10/10 502 2.2
343.4 68.1 0 6/7 146 5.6
46.0 56.4 0 7/7 327 3.3
311.5 74.0 0 6/7 21 14.8

358.5 67.3 0 5/7 488 3.5
6.3 61.3 0 7/7 68 7.4
5.1 56.1 0 7/7 523 2.6
1.6 67.9 0 7/7 696 2.3
356.2 72.8 0 6/6 102 6.7
357.9 63.6 0 7/7 93 6.3
8.9 72.2 0 6/7 480 3.1
1.3 68.3 0 6/7 409 3.3
12.6 68.9 0 8/8 206 3.9
23.1 67.9 0 5/7 352 4.1
13.0 68.0 0 7/8 254 3.8
14.7 68.7 0 7/7 391 3.1
348.3 58.3 0 5/10 476 3.5
357.1 51.3 0 10/10 96 4.9
5.7 69.2 1 8/8 151 4.5
23.6 71.6 0 6/10 576 2.8


VGP
Long.

2.5

357.1
273.7

164.1

199.5
148.4
329.3
180.2

87.2
22.2
37.7
356.7
217.5
65.2
272.1
326.9
311.8
316.9
320.7
313.6
91.7
60.2
309.3
303.3


K AF/ Age
VGP Th (ka)

410 Th 2.34

73 Th 2.34
109 Th 50 to 150

157 Th <34.2

153 Th <34.2
60 Th <150
241 Th <150
7 Th <150

234 Th 12
41 Th <150
322 Th <150
301 Th <150
35 Th <150
40 Th <150
180 Th <150
152 Th <150
80 Th <150
137 Th <150
107 Th <150
177 Th <150
310 Th 400
65 Th 400
59 AF 300
231 AF 300


U (+/-)
(ka)

0.05

0.05
































100
100
300
300


R Meth.


1 14C

1 14C
2 ST
3 14C
3 C

3 14C
2 K/Ar
2 K/Ar
2 K/Ar

4 14C
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
2 K/Ar
5 K/Ar
5 K/Ar


















Table 2-1. Continued

U/ DG V.F. Site Site D I SC n/N K cs95
ID L Lat. Long. Dir. Dir.
21-1 U b CW 51.95 -120.08 15.4 85.8 0 9/12 898 1.7
21-2 L b CW 51.95 -120.08 78.9 88.0 0 8/11 155 4.5
22-1 U c CW 51.93 -120.03 26.2 73.5 0 7/10 610 2.4
22-2 c CW 51.93 -120.03 20.4 76.2 11 9/11 459 2.4
22-3 L c CW 51.93 -120.03 9.7 74.6 10 8/10 157 4.4
23-1 CW 51.79 -120.01 312.0 66.4 9 3/12 291 7.2
24-1 CW 51.77 -120.01 330.8 62.0 0 4/8 166 7.1
25-1 CW 51.73 -120.01 348.3 66.7 0 11/12 323 2.5
25-2 U CW 51.73 -120.01 336.3 69.3 0 10/10 596 2
25-3 CW 51.73 -120.01 329.7 64.0 0 9/10 665 2
25-4 CW 51.73 -120.01 359.6 68.4 0 10/10 290 2.8
25-5 CW 51.73 -120.01 341.9 70.0 0 8/10 814 1.9
25-6 d CW 51.73 -120.01 327.6 70.9 8 10/10 398 2.4
25-7 d CW 51.73 -120.01 324.8 66.9 0 10/10 239 3.1
25-8 e CW 51.73 -120.01 351.9 66.5 3 7/10 220 4.1
25-9 e CW 51.73 -120.01 350.3 66.5 0 10/10 237 3.1
25-10 L e CW 51.73 -120.01 350.6 65.1 0 11/11 134 4
26-1 L f CW 51.68 -120.05 350.5 68.7 10 8/10 401 2.8
26-2 f CW 51.68 -120.05 344.1 66.7 10 7/10 226 4
26-3 U f CW 51.68 -120.05 351.8 61.8 10 5/10 333 4.2
27-1 L g CW 51.67 -120.04 327.8 71.1 0 8/10 202 3.9
27-2 g CW 51.67 -120.04 314.2 66.0 10 10/10 101 4.8
27-3 U g CW 51.67 -120.04 311.1 69.2 0 10/10 169 3.7
28-1 CW 51.63 -120.13 332.1 69.3 9 9/10 200 3.6


VGP
Long.
244.4
246.2
292.3
273.7
265.2
164.0
138.4
138.1
164.5
145.3
141.8
168.9
173.7
158.6
126.9
133.5
120.3
157.6
143.5
96.4
175.4
162.9
172.0
165.5


K AF/ Age U (+/-)
VGP Th (ka) (ka)
232 Th 200/280 110 150
40 Th 200/280 110/150


203 Th 561
145 Th 561
55 Th 561
145 Th 547
96 AF 547
140 th 547
222 th 547
329 th 547
112 th 547
338 th 547
148 th 547
98 th 547
102 th 547
122 th 547
65 th 547
152 th 350
93 th 350
224 th 350
81 th 350
45 th 350
65 th 350
74 th 350/500


106
106
106
102
102
102
102
102
102
102
102
102
102
102
102
90
90
90
90
90
90
90/50


R Meth.


6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
6 K/Ar
7 K/Ar
7 K/Ar
7 K/Ar
7 K/Ar
7 K/Ar
7 K/Ar
6 K/Ar













Table 2-1. Continued
Site U/ DG V.F. Site Site D I SC n/N K cs95 VGP VGP cs95 K AF/ Age U (+/-) R Meth.
ID L Lat. Long. Dir. Dir. Lat. Long. VGP VGP Th (ka) (ka)
29-1 L KA 49.83 -119.75 3.3 81.9 0 11/11 355 2.4 65.6 242.3 4.7 96 th 760 110 8 K/Ar
29-2 h KA 49.83 -119.75 345.9 77.6 0 10/11 821 1.7 72.0 221.8 3.1 241 th 760 110 8 K/Ar
29-3 U h KA 49.83 -119.75 343.3 76.4 0 9/10 744 1.9 73.1 214.8 3.4 226 th 760 110 8 K/Ar
aSite identification number (ID) is composed of two numbers separated by a dash. Sites with the same number before the dash usually
belong to stratigraphic sequences. Asterisk is indicated in a site that was not considered for paleomagnetic calculations. The number
after the dash indicates the position of the lava flow in the sequence (upwards or downwards). U/L indicate the uppermost and
lowermost lava flows of these sequences. DG refers to directional group (see text for explanation). Volcanic field abbreviations are:
MM for Mount Meager; GL for Garibaldi Lake; ST for Silverthrone; CW for Clearwater Wells Gray Park and KA for Kelowna
Area. D and I are the mean site declination and inclination; SC indicates the number of samples oriented with sun compass in the site;
n/N are the number of samples used for calculation of site mean direction per number of processed samples. K is the dispersion
parameter of directions (dir) or VGPs; c095 is the 95% confidence cone about the mean direction (Dir.) or mean VGP; AF/Th represent
mean direction results after AF or thermal demagnetization. Age is the age(s) obtained in previous studies from the flow or volcanic
unit (see text). U is the uncertainty range of the age. R is the reference source on which a given age is based: (1) is Read, 1990 and
Hickson et al, 1999; (2) Green et al., 1988; (3) Green, 1977; (4) Roddick, 1996; (5) Hickson et al., 1995; (6) Hickson and Souther,
1984; (7) Hickson, 1986; (8) Church, 1980. Meth. is the method used to obtain the age (Str. = stratigraphic relationship).







































Figure 2-2. Geology of the Silverthrone volcanic field and sampling-site locations
(triangles) (modified from Green et al., 1988). The sites that fall outside the
mapped area belong to the unit of basaltic andesite flows (crosses).

The Silverthrone volcanic field (figure 2) was studied by Green et al. (1988) who

defined three units: (a) breccias overlying metamorphic rocks in angular discontinuity;

(b) ryolitic, dacitic, and andesitic flows overlying the breccia, and (c) unconsolidated

fluvial and volcano-sedimentary deposits resting on deeply eroded flow surfaces. The

main focus of sampling in this volcanic field were the basaltic andesite flows which

originated from numerous centers and filled the Machmell and Pashleth river valleys and

are locally interstratified with sediments (unit c). Sites 16-1 and 16-2 were sampled from

older andesitic flows near Kingcome Glacier (unit b). An age of 0.4 + 0.1 Ma (K-Ar) was









obtained by Green et al. (1988) at a nearby site. The remaining 15 paleomagnetic sites

from this volcanic field were obtained from the valley filling basaltic andesitic flows of

unit c. The radiometric dates obtained for some of these basaltic flows place them in the

last 150 Ka (Green et al., 1988). Site 5-1 was taken from a lava flow associated with a

boulder covered in part by barnacles which yielded a 12 Ka 14C radiometric date

(Roddick, 1996). Sites from this volcanic field are indicated in table 1 by the abbreviation

ST.

The Garibaldi Lake Volcanic Field was studied by Green et al. (1988) and

comprises nine volcanic centers that have been active through the Quaternary. Three

flows were sampled from the volcanic center that occupies the Cheakamus river valley.

One of these flows (site 1-3) corresponds to a unit that locally contains pebbles striated in

two directions suggesting that the unit predates the Fraser Glaciation ice sheet, which has

an age of 50 Ka. The other two sites (sites 1-4 and 1-5) correspond to lava flows that

overlie the previously described unit and underlie glacio-lacustrine sediments with a 14C

radiometric date of 34.2 Ka (Green, 1977). Sites from this volcanic field are indicated in

Table 2-1 by the abbreviation GL.

The Mount Meager Volcanic Field has been active since the Pliocene (Green,

1988) and has several volcanic centers. The only lava flow that was sampled here was a

dacitic lava flow, erupted from Mount Meager along with other volcanic products

(volcanic ash and pyroclastic flows) and has a 14C radiometric date of 2350 yr BP (Read,

1990 and Hickson et al. 1999). This lava flow was sampled twice (site 1-1 and site 1-2),

and although the range of uncertainty of the directions at both sites overlap, the result of









site 1-2 was discarded because of field observations and site 1-1 was preferred. Sites

from this volcanic field are indicated in table 1 by the abbreviation MM.

Lava flows from the Clearwater Wells Gray Park Volcanic Field (Figure 3) are

exposed along the Clearwater and Murtle river basins. Hickson and Souther (1984)

differentiated four volcanic assemblages that began forming 3 Ma ago: (a) an early

glacial assemblage; (b) a deeply dissected, valley-filling assemblage composed of

basaltic lava flows originating from different eruptive centers, and locally interstratified

with sediments, which forms the main focus of sampling in this area; (c) a late

intraglacial assemblage that includes ice-contact deposits, and (d) post-glacial pyroclastic

cones and lava flows. Twenty-eight basaltic lava flows (sites 20-1 to 28-1) were sampled

from assemblage (b). We assigned ages to these flows, ranging between 0.2 and 0.5 Ma,

based on K/Ar ages obtained from nearby outcrops which were studied previously (table

1). Sites 18-1 and 19-1 were obtained from eroded lava conduits near Ray Mountain

Volcano dated at 0.03 + 0.03 Ma (K-Ar) (Hickson et al., 1995). Sites from this volcanic

field are indicated in table 1 by the abbreviation CW.

Lavas from the Kelowna area (Table 1) were sampled from eroded Pleistocene

volcanic outcrops. Three overlapping lava flows at Lambly Creek, near Kelowna, were

sampled. A radiometric date of 0.76 0.11 Ma (K/Ar) was given by Church (1980) to

these flows. Sites from this volcanic field are indicated in table 1 by the abbreviation KA.

Laboratory Analysis

Most of the directions derived from sun and magnetic compass readings coincided

within few degrees (<3). Directions from sun compass readings were used at four sites

(sites 1-1, 1-2, 1-5 and 26-3) in which some of the directions derived from sun compass

and magnetic compass differed 5 or more.






























S Post-Glacial assemblage
S Late-Glacial assemblage 24-1
S ] \ il. Fiill'ir .j.,-.:n. 25-1 -25-10 5145'
v Early-Glacial assemblage 261 26-
Piteoii.ni 'eli Site 27-1 -"-: Clearwater
28- 9, 5 *
/i Blackpool i.. L .i ,r
Figure 2-3. Geology of the Clearwater Wells Gray Park volcanic field and sampling-site
locations (dots) (modified from Hickson and Souther, 1984).

Stepwise AF demagnetization was performed on one sample per site. AF

demagnetization was done using a Schonstedt AF demagnetizer and later a Dtech D-200

AF demagnetizer. All samples were thermally demagnetized in 14 to 21 steps utilizing a

Schonstedt oven. The measurements were made in a 2G Cryogenic magnetometer in a

shielded room at the University of Florida. Generally good agreement between AF and

thermal demagnetization was observed (Figures 2-4a to 2-4c) and the results from

thermal demagnetization were used to calculate the primary component of magnetization

of most sites. However, AF demagnetization gave better results (Figure 2-4d) at sites 23-

1, 18-1, 19-1, and 24-1 and was used for analysis of the latter three sites. Magnetic

susceptibility was monitored during thermal demagnetization, using a Sapphire

susceptibility meter (except for samples from Clearwater-Wells Gray Park and Lambly


Clearwater L.

19-1


Blde River

52'00'









Creek volcanic fields). The behavior of the basaltic rock material during laboratory

analysis was in general typical for subaerial basalts. The NRM intensity varied between

about 1 to 10 A/m, but was more commonly around 4 A/m. The principal component of

magnetization was easily defined by removing the viscous remanent magnetization

(VRM) in the first few demagnetization steps. In most cases a small (<5%) or no fraction

of the NRM remained after the 570C heating step, suggesting titanomagnetite as the

magnetic carrier; however few sites still had >5% of the NRM remaining at 6000C (the

highest temperature step applied) which can indicate the presence of small amounts of

hematite.

Data Analysis and Selection Criteria

In order to obtain suitable measurements of the paleomagnetic field and avoid

spurious data that could blur real non-dipole contributions to magnetic field models, it is

necessary to establish stringent procedures and selection criteria for statistical parameters.

The data analysis procedures and selection criteria used in this study closely follow those

proposed by Tauxe et al. (2000) and supersede those of McElhinny and McFadden

(1997). The direction of the primary component of magnetization was obtained through

principal component analysis (Kirschvink, 1980) using a segment of at least 5 points of

the demagnetization curve directed toward the origin with a maximum angular deviation

(MAD) < 5. Site-mean directions (Figure 2-5, Table 2-1) were obtained applying Fisher

(1953) statistics from at least 5 cores and selected for further analysis if c95 values were

<5 (rounded to the nearest integer). Based on these criteria, we selected 45 of the 52

flows sampled; that is, 7 sites were rejected (Figure 2-5).

















Samples 16-2-1


. n1t


-o" ...*
*


-1


-.1' \ 400


S: 00


-4


OUe c u


Samples 19-1-6



- ~woo


Figure 2-4. Pair of Zijderveld diagrams showing AF and thermal demagnetization (left
and right side plots, respectively) on replicate samples from British Columbia.
Good agreement between the two methods is shown in a, b and c. Difficult to
interpret thermal demagnetization curves relative to AF demagnetization, with
both methods still indicating the same general direction, is shown in d.
Approximate intensity values in A/m.


Secular variation analysis were made by calculating the angular standard deviation


(Sb) of the virtual geomagnetic poles among the selected group of sites with respect to


the Earth's axis of rotation using the method described by Johnson and Constable (1996)


in which the effect of within-site scatter is subtracted from the total scatter. The 95%


confidence limits of the resultant scatter (angular standard deviation) were calculated


using the method described by Cox (1969).


Ix l
- r.


0.5


Samples 25-3-1





550


-15- -" -I.N


Samples 1-5-2


-1.


,:2W
a-(k










N








0
S0 \
/n o 0 0






Figure 2-5. Equal area projection of site-mean directions from British Columbia. Squares
represent directions of lava flows <150 Ka and circles represent directions of
lava flows 200 to 760 Ka old. Rejected sites are shown with dots inside their
respective symbols (squares or circles). All inclination point downwards.

Results from British Columbia and Discussion

The directions of sites within volcanic fields tend to cluster together indicating that

their ages are closer in time compared to the sites from other volcanic fields. More

precisely, it can be observed that younger sites (1-1 to 15-1) that are < 150 Ka and older

sites (16-1 to 29-3) that are 200 to 760 Ka (table 1) show a distinct distribution of the

paleomagnetic directions in time (Figure 2-5). The mean directions among the younger

and older lava flows have circles of confidence that barely overlap and only the mean

direction from the group of younger lavas coincides with the GAD (Figure 2-6, Table 2-

2). Given the age uncertainty of the lava flows and the episodic nature of volcanism, it is

uncertain to what extent has the paleomagnetic field been sampled in time. That is, it is

not known which fractions, corresponding to periods of enhanced volcanism, of the total

period of time being studied (mainly the last 560 Kyr) have been sampled.









The mean direction of magnetization and the mean VGP of the 45 selected lava

flows with a95 < 5 (Table 1) coincide at the 95% confidence level, with the expected

direction of the GAD (Inc = 68.3) in the area (mean latitude = 51.50 N) and the

geographic north respectively (Figures 2-6 and 2-7). The directional distribution is

fisherian while the VGP distribution is not. Similar results are observed when considering

all the lava flows (including results from the 7 lava flows that were rejected), but in this

case, at the 95% confidence level, the mean direction almost falls away from the GAD

and the mean VGP does not coincide with the Earth's axis of rotation. The shift in the

direction obtained by taking into consideration the previously discarded sites produces a

shift of the mean direction away from the GAD despite the fact that 5 of the rejected sites

belong to the group of younger flows that tend to plot closer to the GAD. With a

relatively small number of observations, it would be premature to argue that the

application of selection criteria yields better results. Studies that explain the possible

underlying reasons supporting the use of selection criteria are necessary.

The mean direction of the selected sites lies somewhat closer to the group of older

flows, because they are represented by more than twice as many flows as the group of

younger lavas, although that does not imply an uneven sampling in time since the group

of older flows represents about twice the period of time represented by the younger group

(only the three flows from the Kelowna area are older than 560 Ka). This observation

highlights the need for uniform sampling through time.

Results of sites obtained from lava sequences deserve a closer examination because

they are also likely to incorporate a bias by over-sampling a particular period of time. In

order to investigate this problem, mean directions from contiguous lava flows having









a) N
















b) _











600 900 600

Figure 2-6. Equal area projection of mean directions from several groups of sites from
British Columbia. Comparisons of the mean direction from the selected group
of sites (gray-filled circle), GAD (triangle pointing down) and IGRF (triangle
pointing up) are shown in plot (a) with the mean direction among the younger
group of lava flows (< 150 Ka) (gray-filled diamond) and the older group of
lava flows (200 to 760 Ka) (striped diamond); and in plot (b) with the mean
direction of all lava flows (square) and filtered data (empty diamond).

directions with overlapping circles of confidence were obtained, and were used to

calculate an overall mean direction along with the remaining flows that pass the selection

criteria. Mankinen et al. (1985) applied this method and identified directional groups









among the flows having overlapping circles of confidence. The same terminology is used

here, and flows conforming to directional groups are shown in Table 2-1. This procedure

produces a mean direction and mean VGP (Table 2-2 and Figure 2-7) that is very similar

to the unfiltered result, although a95 increases and the mean is slightly displaced toward

the GAD.

It is uncertain that the process of filtering the data benefits the quality of the results.

The inclusion or exclusion of sites from the group of older or younger flows slightly

shifts the balance between the two groups, driving the mean in one way or another. In this

case more data points were taken out from the older group (13) than from the younger

group (3), driving the mean (of filtered sites) toward the cluster of younger sites and

toward the GAD (Figure 2-6). Instead, it is uncertain if contiguous lava flows with

overlapping 95% circles of confidence record a relatively stationary direction of the field

rather than that those flows erupted in a very short period of time (Love, 2000). The

validity of this concern is observed in the overlapping directions between flows 9-4 and

9-5 which are separated by a soil layer, thus indicating a substantial period of time

between the two flows (for this reason these two flows were not included in the same

directional group, see Table 2-1). Several cases in which filtering has been applied in

lava sequences have also produced mean directional results that do not differ

substantially from those obtained from the unfiltered data (McElhinny et al., 1996b;

Szeremeta et al., 1999; Laj et al., 1999). In general, it can be said that the relative low

accumulation rate in the Garibaldi volcanic belt (Sherrod and Smith, 1990) and the

presence of multiple volcanic centers in the Silverthrone and Wells-Grey Park volcanic

fields helped to evenly sample the paleomagnetic field in time.









The fact that the mean direction and mean VGP among the selected flows and

filtered dataset coincide with the expected GAD and geographic pole, respectively, does

not suggest a significant persistent contribution of the non-dipole components of the

field. Particularly the "far-sided" effect (Wilson, 1970), although expected to be

relatively small at this latitude, that would cause the VGP to plot further from the North

Pole with respect to the sampling area, is not observed. Likewise, the small negative

inclination anomaly (-2 to -40) for the area obtained as a result of modeling Brunhes-age

paleomagnetic records from lava flows (Johnson and Constable, 1995) is not present in

the dataset.

The corresponding angular standard deviation of the VGPs relative to the Earth's

axis of rotation (Sb) for the selected sites is 17.50 with lower (Sl) and upper (Su) 95%

confidence limits (Cox, 1969) of 20.5 and 15.2 respectively (Table 2-2). These values

are within the range expected from Model G (McFadden et al., 1988) of secular variation

for that latitude (17.4 with upper and lower confidence limits of 19.3 and 15.60

respectively). Similar scatter results are obtained when considering all the flows and

after filtering (Table 2-2). The agreement of the VGP dispersion of this study with the

dispersion value predicted in Model G suggests that the secular variation has been

adequately sampled.

The mean VGP reported from a similar study (in terms of age, rock type and

quality) further south in the Indian Heaven Volcanic Field of Washington State (Mitchell

et al., 1989) plots, as in this study, in a near-sided position although coinciding with the

rotational axis at the 95% confidence level (Figure 2-7). The VGP scatter of 15.00 (with

upper and lower limits of 17.9 and 13.2) obtained in that study coincides less precisely









than in this study with the scatter expected from Model G at 460 N (16.60 with lower and

upper confidence limits of 15.00 and 18.3).


Figure 2-7. Mean virtual geomagnetic poles (VGPs) obtained from British Columbia. The
mean VGPs among selected sites (circle), all lava flows (empty diamond),
filtered data (gray-filled diamond), is shown along with results (Mitchell et al.
1989) from Indian Heaven volcanic field (square).


















Table 2-2. Statistical data among groups of sites from British Columbiaa
D I N K Dir U95 Dir VGP Long VGP Lat K VGP U95 VGP St Sb Su SI O.G O.A
Selected Sites 356.9 70.2 45 57 2.8 215.8 85.5 23 4.5 17.7 17.5 20.5 15.2 yes yes
All Sites 354.7 70.0 52 57 2.6 199.6 85.1 23 4.2 17.8 17.5 20.2 15.4 yes no
Filtered Sites 0.0 69.4 33 55 3.4 237.3 87.2 23 5.4 16.9 16.7 20.0 14.3 yes yes
Older Flows 349.5 70.7 31 65 3.2 191.0 82.5 25 5.3 18.0 17.8 21.6 15.2 no no
Younger Flows 11.7 68.1 14 61 5.1 311.4 83.0 26 7.9 17.4 17.3 23.1 13.8 yes yes

aAbbreviations for columns D to U95 VGP (first to ninth) are as in table 1. Data of VGP scatter relative to the Earth's axis of rotation is given in columns: St (total
scatter), Sb (corrected scatter), Su (upper 95% confidence limit of the scatter) and SL (lower 95% confidence limit of the scatter). O.G./O.A indicates whether the
95% confidence limits (U95) of the mean direction/mean VGP overlap the GAD/Earth's rotation axis.














CHAPTER 3
PLIO-PLEISTOCENE TIME AVERAGED FIELD IN SOUTHERN PATAGONIA
RECORDED IN LAVA FLOWS

A paleosecular variation study from two areas of southern Patagonia (latitudes

51.5 to 52.50 S and latitudes 49.50 to 50.50 S) is presented in this chapter (Figure 3-1).

The data will help to fill a gap of paleomagnetic data from high southern latitudes. The

results that were obtained were compared with those predicted by existing TAF and

PSVL models. The inclination anomalies depicted by some TAF models for this area

(JC95) as well as the expected scatter of the virtual geomagnetic poles (VGPs) relative to

the Earth's axis of rotation according to Model G of secular variation (McFadden et al.,

1988) are tested.

Samples were obtained from and around Meseta Viscachas (northern sampling

area) and from the Pali-Aike volcanic field (Figure 3-1), the southernmost among a series

of Late Cretaceous Holocene alkali basaltic plateaus (Skewes and Stern, 1979) that

extend east of the Patagonian Andes. Ramos and Kay (1992) have proposed that back-arc

volcanism in southern Patagonia is the result of slab window formation in the mantle

produced by the collision of the Chile ridge with the South American plate. The tectonic

environment in southernmost Patagonia is complicated by sinestral motion of the Scotia

plate along the southwestern tip of Tierra del Fuego. Skewes and Ster (1979) suggest the

presence of thermal or mechanical perturbations of the mantle related to the trench-

transform triple junction between South American, Antarctic, and Scotia plates, based on

findings of ultramafic inclusions and chemical characteristics of the Pali-Aike basalts,









that are indicative of a mantle origin and are not observed in other Patagonian plateau

basalts.


Figure 3-1. Location map of southern Patagonia showing sampling sites and radiometric
dates obtained in this and previous studies. The general location map to the
left (modified from Skews and Stern, 1979) shows Upper Cretaceous to
Quaternary alkali basaltic plateaus of Patagonia (red). The locations of Meseta
Viscachas and the Pali-Aike Volcanic field (red-fill areas) are designated as
MV and PAVF, respectively. The map to the right contains the location of
radiometric (40Ar/39Ar) and/or paleomagnetic sampling sites (red-filled
triangles pointing up). The sampling sites of previously obtained 40Ar/39Ar
(Meglioli, 1992) and K/Ar (Mercer, 1976) radiometric dates in the area are
shown by squares and triangles pointing down, respectively. Results of
radioisotopic dates (in Ma) are within ellipses.









The Meseta Viscachas and Pali-Aike basaltic flows are locally interbedded with

tills. K-Ar and 40Ar/39Ar radioisotopic dates of these flows (Mercer, 1976; Meglioli,

1992; Singer et al., 2004) obtained to help depict the glacial history in Patagonia indicate

primarily Pliocene Pleistocene ages. 40Ar/39Ar radioisotopic dates from 17 of the

paleomagnetic sites were obtained by Dr. Brad Singer (Mejia et al., 2004) which support

and complement previous results.

Sampling and Sampling Area

Most of the samples (49 sites) were collected in Argentina during February of

2000. Four sites were collected by Joe Stoner during February of 1998 in the Chilean part

of the Pali-Aike volcanic field. Each site represents an individual lava flow. Access to the

outcrops was achieved by road and tracks. Short hikes were occasionally necessary.

Normally 10 samples were collected at each site and oriented using a magnetic compass

and sun compass, when possible. Sun compass declinations were obtained for 75% of the

collected samples and used for further calculations when they differed by 5 or more from

the magnetic compass declinations. Two lava flows from the Chilean side of Patagonia

were sampled twice and their data combined. This way, the pairs of sites PA3 and PA6

(PA3-6) and sites PA4 and PA5 (PA4-5) were combined.

Northern Sampling Area

This region is adjacent to the Andes and consequently the relief is high. Lava flows

east of the Andes are now often exposed in cliffs that have resulted from scarp retreat.

Lava flows along these scarps outcrop in stratigraphic sequences usually composed of

several (more than 3) flows.









Pali-Aike Volcanic Field

The area sampled in the Pali-Aike volcanic field is predominantly flat and covered

with Patagonian gravel. Some topography is created by lava flows and volcanic centers.

We sampled lava flows, usually < 10 m thick, exposed in areas around and roughly along

the eastern parts of the Rio Gallegos and Rio Chico valleys. In the area around Rio

Gallegos, individual flows can be traced for a few kilometers, and up to three lava flows

were sampled in stratigraphic order. Volcanic cones and eruptive centers are more eroded

in the Rio Gallegos than in the Rio Chico area. In the Rio Chico area cinder cones (often

aligned indicating fissure volcanism) are very well preserved. Examples of these cinder

cones are Cerro de los Frailes, Cerro Conventos, and Cerro Tres Hermanos. The

geomorphologic differences between these two areas of the Pali-Aike volcanic field

suggest that the lava flows that outcrop along Rio Gallegos are generally older than those

that outcrop along Rio Chico, which is in agreement with results obtained from

radioisotopic dating.

Paleomagnetic Analysis

Laboratory analysis was carried out in the paleomagnetic laboratory at the

University of Florida. AF demagnetization was done using a Dtech D-200 AF

demagnetizer and thermal demagnetization using a Schonstedt oven. Magnetic

measurements were made in a 2G Cryogenic magnetometer in a shielded room. Pilot sets

of samples composed of one sample per site and three samples per site were run using

stepwise (around 17 steps) AF and thermal demagnetization, respectively, to choose

which method of demagnetization was more appropriate for each site. The general

agreement between the directions obtained from AF and thermal demagnetization

(Figures 3-2a and 3-2b) indicates that any CRM acquired during heating does not alter













significantly the primary direction of magnetization, possibly due to low field conditions


within the oven.


Samples PT26.5


25 Oe



I W$

3.3.
aoo


200

4 4~
400


Samples PT46.2


S i ii)~~~~~~~~~~~c


1000
Zo


5XC.

A


I-'D f


Sample PT5.6


1 2 3

"0 ..... --
xI
'l V


, N


Sample PT29.1




W/U













I3 i
W iA
i .^'


Figure 3-2. Examples of Zijderveld diagrams obtained from Patagonia. Figures 3-2a and
3-2b are AF and thermal demagnetization (left and right respectively) on
replicate samples. Figure 3-2a shows a case in which the thermal
demagnetization curve is difficult to interpret and the AF demagnetization
curve is not, while both methods still indicate the same general direction.
Figure 3-2b is an example of both AF and thermal demagnetization curves
showing similar results and Figures 3-2c and 3-2d show AF demagnetization
curves of sites affected by lightning. Approximate intensity values are in A/m.


Thermal demagnetization was the preferred procedure for processing all the


samples from each site, except when the orthogonal projections from thermal









demagnetization were more difficult to interpret than those from AF demagnetization

(Figure 3-2a) or when the site was affected by lightning (Figures 3-2c and 3-2d). The

most distinctive symptoms of sites being affected by lightning are scatter in directions

and high intensity of the NRM (reaching around 20 A/m in some of the samples whereas

the intensity of magnetization of the sites not affected by lightning is around 4 A/m).

Twelve of our sites showed signs of being affected by lightning; therefore we applied AF

demagnetization, which is the most successful method to remove the overprint caused by

lightning-induced IRM. The remaining 20 sites that were treated using AF

demagnetization were those in which the thermal treatment produced a noisy orthogonal

projection or rapid loss of most of the magnetization during the first few temperature

steps, while results from AF demagnetization were clear. Other researchers have also

documented more successful AF versus thermal demagnetization treatments on basaltic

flows (e.g., Camps et al., 2001, and Szeremeta et al., 1999). Despite the preference to

apply thermal demagnetization, AF demagnetization was used more times (in 32 sites)

than thermal demagnetization (in 21 sites).

Data Analysis and Selection Criteria

The procedures to obtain and process paleomagnetic data were aimed at obtaining

high quality results. We followed the procedures and selection criteria for data analysis

used by Tauxe et al. (2000). That is, the primary component of magnetization from

individual samples was obtained using principal component analysis (Kirschvink, 1980)

from a segment of at least 5 points of the orthogonal projection directed toward the origin

and with maximum angular deviation < 5. Site mean directions were calculated from at

least 3 samples per site using Fisher (1953) statistics and selected as successful when a95

values were < 5 (rounded to the nearest integer).











































Figure 3-3. Equal area projection of site mean directions from Patagonia. The figure
shows site mean directions of sites thought to be Plio-Pleistocene in age
(circles) and of Miocene or unconstrained ages (squares). Magnetic vectors
pointing up and down are represented by shaded and unshaded areas
respectively; crossed sites are rejected sites because of the quality of the
paleomagnetic data. The IGRF (blue triangle) is plotted as a reference point.

A summary of the paleomagnetic results of all the sites is contained in Table 3-1

and the data plotted in Figures 3-3 and 3-4a. We obtained successful paleomagnetic

results from most of the sites (except site PAl) that were treated using thermal

demagnetization. AF demagnetization was applied to 32 sites. Among the 12 sites treated

using AF demagnetization, as an alternative for treating samples affected by lightning, 4









sites had random directions (no results), 6 sites had a95 values > 5, and only 2 sites had

a95 < 5, complying with our selection criteria. Among the remaining 20 sites that were

treated using AF demagnetization, only 4 sites did not pass the selection criteria.

a) 900 b) 900


















2700 2700

Figure 3-4. Paleomagnetic results from Patagonia expressed in VGPs. Normal and
reverse sites are represented by shaded and unshaded areas respectively. (3-
4a) Site-mean VGPs and (3-4b) Mean VGP from selected group of sites
(square), normal sites (filled diamond) and reversed sites (empty diamond).
Triangle indicates the position of the geomagnetic North Pole.

Many of the sites that were rejected occupy the periphery of the overall distribution

of paleomagnetic directions (Figure 3-3), which suggests that the applied selection

criteria are successfully filtering out noise in the paleomagnetic data. Likewise the

application of detailed stepwise demagnetization seems to improve the quality of the

data. Previous paleomagnetic analysis by Fleck et al. (1972) in a sequence of lava flows

interbedded with tills from Cerro El Fraile (South of Lago Argentino), that used mild or

no demagnetization techniques, rendered results with a95 values higher than 10o, that do

not pass our selection criteria.













Table 3-1. Paleomagnetic and age data of sites from Patagoniaa
Site U/L Site Site Dec Inc SC N cU95 K Th/ L VGP VGP cU95 K R.D. U(+/-) R


Dir Dir AF Lat Long VGP VGP (Ma)


-51.74 -70.15 9
-51.74 -70.17 140.8 64.7 0
-51.78 -70.23 357.1 -65.7 9
-51.94 -70.42 142 60.5 0
-51.68 -70.19 196.7 66.0 0
-51.88 -70.66 217.7 59.0 0
-51.89 -70.72 186.4 60.4 9
-51.87 -70.59 185.4 60.7 8
-51.84 -70.51 183.4 71 10
-51.85 -70.52 172.9 76.5 10
-51.86 -70.52 4.5 -57.1 11
-51.79 -70.28 184.6 60.3 10
-51.78 -70.27 355.8 -71.7 10
L -51.72 -70.15 200.5 68.4 11
-51.72 -70.15 170.4 54.4 1
U -51.71 -70.15 150.2 72.6 7
-51.79 -70.00 10
-51.83 -70.03 0
-51.90 -70.05 356 -76.5 0
-51.87 -69.42 30.3 -37.6 6
-51.94 -69.57 352.2 -73 10
-51.94 -69.60 342 -65.7 10
-51.91 -69.64 295.8 -81.8 0
-51.98 -69.73 351.6 -57.8 9
-51.87 -69.39 43.2 -51.7 0
-51.88 -69.19 18.1 -73.3 10
-51.87 -69.17 10.4 -72 11
-51.84 -69.40 10
-51.99 -69.85 350.7 -63.6 10
-50.55 -71.65 317.8 -75.8 9
-50.52 -71.70 10
-50.52 -71.70 180.4 63.1 10
-50.52 -71.70 49.2 49.1 10
-50.53 -71.61 297.9 -77.6 10
* -50.32 -71.22 44.2 -70.6 10
U -50.32 -71.22 24.8 -63.7 9
L -50.32 -71.21 11.7 -66.5 10
-50.34 -71.23 34.5 -65.4 5
L -50.33 -71.07 8.2 -74.2 10
U -50.34 -71.07 6.3 -79.2 10
U -50.29 -71.13 38.4 -72.5 0
-50.29 -71.13 27.2 -67.8 6
L -50.29 -71.12 1
L -49.51 -72.13 179.3 52.8 10
-49.51 -72.13 180 58.3 10
U -49.51 -72.13 184.3 56.9 10
L -50.01 -71.87 345.3 -66.3 8
-50.01 -71.87 352.4 -74.1 11
U -50.01 -71.87 342.2 -67.4 10


0/9
6/10 14.4 22
8/9 3.7 231
7/10 3.2 368
5/10 4.7 264
10/10 1.4 1202
8/9 2.3 605
7/8 4 229
4/10 7.1 167
10/10 2.4 391
9/11 3.4 235
3/10 12 107
10/10 2.4 400
9/11 4.3 147
5/11 3.3 537
6/10 5.5 149
0/10 -
0/10 -
9/10 3.5 222
5/10 8.7 79
7/10 2.7 513
7/9 3.9 244
7/10 4.5 184
9/10 4.4 140
3/10 8.2 226
10/10 3.7 174
11/11 3.6 163
0/10 -
6/10 5 179
8/10 2.8 386
0/10 -
10/10 2.1 512
10/10 4.6 113
6/10 9.7 49
6/10 4.5 221
8/10 2.9 366
6/10 5.9 131
6/9 6.9 95
7/10 3.4 312
9/10 4.6 126
6/11 2.2 912
5/9 3.6 462
0/9 -
10/10 1.8 694
9/10 2.9 308
9/10 2.3 495
7/10 3 408
10/11 4.2 130
9/10 1.5 1129


AF X
Th
Th
AF X
Th
AF
AF
AF X
Th
AF
AF X
Th
AF
AF
AF
AF X
AF X
AF
AF X
AF
AF
AF
AF
AF X
AF
AF
AF X
AF X
Th
AF
Th
Th
AF
Th
Th
AF
AF X
AF
AF
Th
AF
AF X
Th
Th
Th
Th
Th
Th


9.9
263
26.4
189.9
189.4
134.4
132.2
264.4
302.8
303.9
130.2
134.8
209.2
83.8
346.3



117.2
339.9
140.7
211.3
136.7
263.4
5.7
63.5
66.2


15 1.3
103
216
107
721 8.67
302 1.14
108
73 0.857
129
158 9.16
55
142 0.486
63 1.79
326
63


0.03 a


0.032 a


0.08 a


0.096 a
0.12 a


69 0.69 0.05
155
175 0.32 0.02
116 0.165 0.046
53 0.23 0.02
102
201
58 0.31 0.03
62 0.31 0.03


80.9 242.7 7.2 87 0.665
64.4 152.5 4.7 142


-73.9
-79.6
-77.6
80.4
78.1
79


111
329.3
144.6
46.3
12.1
15.2
25.2
88.3
102.7
52.5
31.4


105.8
108.4
123.7
199.6
126.9
190.2


15.42
253 15.41
69
20
87
179
58 3.02
40
107
39
336 2.98
174


554 4.08
181
254
199 3.02
49
536 3


0.168 a


0.17 a
0.16 a






0.04 a





0.03 a



0.12 a



0.03 a


0.04 a


Lat Long










Table 3-1. Continued
Site U/L Site Site Dec Inc SC N C95 K Th L VGP VGP s95 K R.D. U(+/-) R
Lat Long Dir Dir AF Lat Long VGP VGP (Ma)
PA1 -52.05 -70.02 31.9 -61.9 0 4/4 17.3 29 Th 67 10.4 26.1 13
PA2 -52.05 -70.02 26.2 -51.9 6 5/8 4 358 Th 62.9 344.3 4.7 267
PA3-6 L -52.02 -69.93 323.7 -62.0 17 13/17 2.5 271 Th 64.4 205.2 3.5 139 1.12 0.01 a
PA4-5 U -52.02 -69.93 315 -65.1 19 17/19 1.8 404 Th 61.4 192 2.5 204
a U/L indicates the uppermost and lowermost lava flows of lava sequences; the asterisk in site 35 indicates
that it is in-between the following two sites. Dec and Inc are the mean site declination and inclination; SC
is the number of sun compass declinations obtained in each site; n/N is the number of samples used for
calculation of site-mean direction per number of processed samples. K is the dispersion parameter of
directions (Dir) or VGPs; c95 is the 95% confidence cone about the mean direction (Dir) or mean VGP;
Th/AF represent mean direction results after thermal or AF demagnetization; R.D. is the 40Ar/39Ar
radioisotopic date obtained for some sites; U is the uncertainty range of the radioisotopic date. R is the
reference source on which a given radioisotopic date is based: (a) is this study and (b) is Meglioli, 1992.
Sites PA1, PA2, PA3, PA4, PA5 and PA6 were labeled on the samples as BN1, BN2, SDL1, SDU1, SDU2
and SDL2 respectively.


Radioisotopic Ages

The radioisotopic dates that we obtained from 17 of the 53 paleomagnetic sites

sampled indicate mostly Pliocene Pleistocene ages. Mercer (1976), Meglioli (1992) and

Singer et al. (2004) have obtained similar results (Figure 3-1 and Table 3-1).

Northern Sampling Area

7 of the 20 sites in this area were dated. In all cases the magnetic polarities

coincided with the ones expected from the magnetic polarity time scale (Cande and Kent,

1995). Flows PT31 and PT32 with reverse polarity were dated at 15.4 Ma. These two

flows were not considered for the calculation of mean directions or mean VGPs, because

their ages are out of the scope of this study. The nearby flows PT33 (a dyke with

intermediate direction), PT34 and PT30 located close to the previously mentioned sites

were not considered either, because of lack of age control. The isochron ages of the

remaining dated flows in this area indicated either Gauss or Gilbert magnetic chrons

(spanning from 2.98 to 4.08 Ma) in agreement with previously dated nearby sites

(Mercer, 1976). Most of the sites that were not dated crop out in stratigraphic sequences









in which at least one of the flows was dated. In these cases we assumed for the undated

flows the magnetic chron obtained for the dated lava flow in that sequence, provided that

the same polarity was observed among all lava sequences. The only stratigraphic

sequence in which no lava flow was radioisotopically dated was that composed of flows

PT39 and PT40. Because of their normal polarity and closeness to flows PT41

(determined as Gauss) they probably correspond to the Gauss polarity chron. The nearby

site PT38 is not discussed because paleomagnetic results were considered unsuccessful

for this site.

Pali-Aike Volcanic Field

Ten of the 33 sites of this area were dated. The radioisotopic dates that we obtained

range from 9.15 Ma to 0.165 Ma. This age range is similar to that indicated by previous

radiometric dates (Mercer, 1976; Meglioli, 1992 and Singer et al., 2004) in this volcanic

field (Figure 1). The radioisotopic age obtained for site PT11 was checked and confirmed

by a second measurement using whole rock material (Table 3-1). The initial result was

preferred for being more precise and the product of a measurement of ground mass

material. Site PT24 corresponds to a lava flow that has not been covered by soil that

according to Skewes and Stern (1979) represents the most recent volcanic activity in the

Pali-Aike volcanic field that took place 5000 to 10000 yr B.P., based on anthropologic

studies (Bird, 1938). In all but one case the magnetic polarities that we obtained for the

dated lava flows were in agreement with those expected from their ages, according to the

magnetic polarity time scale (Cande and Kent, 1995). Despite its lack of coincidence with

the magnetic polarity time scale, the normal polarity of site PA3-6 (isochron age of 1.12

0.01 Ma) does coincide with the Punaruu Event (Singer, 1999) with a recalculated age

of 1.12 0.01 Ma (Singer et al., 2002).









The northern part of the Pali-Aike volcanic field, along Gallegos river has two

flows with radioisotopic dates > 8.67 Ma (PT6 and PT11) that occupy relatively high

elevations and are remnants of old lava flows. Similar ages were obtained by Meglioli

(1992) around 80 km west of these sites (Figure 3-1). These two flows were excluded for

the calculation of mean directions or mean VGPs in order to focus the analysis on

Pliocene-Pleistocene lava flows. All the other sites that were dated in the Pali-Aike

volcanic field are less than 1.79 Ma. The remaining sites that were not dated in this

volcanic field were considered for paleomagnetic analysis. A Pliocene-Pleistocene age

was assumed for some of the undated flows by: (a) inferring a similar age of that of a

dated flow within the same lava sequence (such is the case of flows PT15 and PT16 that

are assumed to have a similar age of flow PT14); (b) inferring a young age when the

sampled lava correspond to a volcanic structure (such is the case of flow PT3, PT10,

PT20 and PT25) and (c) inferring a similar age of a previously obtained nearby

radioisotopic date (Figure 3-1) like in the case of the flows related to cinder cones that

crop out roughly along Rio Chico (sites PT21, PT23, PT26 and PT27). A relationship of

sites PT5 and PT19 with previous nearby radiometric dates of 2.1 Ma and 0.69 Ma,

respectively (Meglioli, 1992) is more difficult to determine due to the relative greater

distance and the complex stratigraphic relations of the lava flows in the area. The

previously mentioned two flows along with the undated flows PT4, PT8 and PT12 were

considered for calculation of mean directions and mean VGPs, despite that there is some

possibility that these flows are older than Pliocene in age.

Results from Patagonia

No attempt was made to filter the data of serial correlation in lava sequences,

considering the contention of Love (2000) that this procedure can be inadequate due to









the possibility of slow secular variation of the magnetic field rather than fast

accumulation rate of lava flows. However, the only flows in lava sequences that have

overlapping circles of confidence are flows PT45-PT46, PT39-PT40 and PA3-6 PA4-5.

Table 3-1 summarizes mean directions and mean VGPs among groups of sites

calculated using the statistical methods of Fisher (1953). According to the selection

criteria previously described we obtained 38 successful paleomagnetic results out of the

53 sites that were studied. Excluding the sites that had Miocene ages or were likely to

have that age, we calculated mean directions among 33 sites. The mean direction (D =

358.70, I = -68.2, a95 3.5) and mean VGP (Lat = 88.5, Long = 141.30, a95 = 5.4)

among the selected group of sites coincide at the 95% confidence level with the expected

direction of the GAD (+ 68.1) and axis of rotation respectively (Figures 3-4b and 3-6a

and Table 3-2). Likewise the mean direction and mean VGPs among the normal, reverse

and all the results (without consideration of selection criteria, N = 41) coincide at the

95% confidence level with the GAD and axis of rotation respectively (Figures 3-4b and

3-6a and Table 3-3). The normal and reversed groups of sites pass the reversal test

(McFadden and McElhinny, 1990) with a "B" classification. The VGP scatter with

respect to the Earth's axis of rotation (traditionally used as indicative of secular variation,

Table 3-2) that we obtained from the selected group of results is 17.1 (within-site scatter

considered, e.g. Johnson and Constable, 1996) with upper and lower 95% confidence

limits of 20.6 and 14.60 (Cox, 1969). This value is in close agreement with that predicted

by Model G (McFadden et al., 1988) of 17.4 (with upper and lower confidence limits of

19.3 and 15.60) for that latitude. Scatter values from high latitudes compatible with those

expected from Model G have been also recently obtained from lavas younger than 5 Ma









of British Columbia (Mejia et al., 2002), Deception Island (Baraldo et al., 2003), and

Possession Island (Camps et al., 2001). However this has not been the case in the recent

study from Patagonia (around 47 S) by Brown et al. (2004), in which scatter is

substantially higher than that expected from Model G.

The IGRF of the year 2000 for the studied area is Dec = 13.5 and Inc = -48.1 and

the GAD for this same area is Dec = 0, Inc = -68.1. Therefore the present inclination

anomaly in the area is 200. Such anomaly reflects the pronounced non-dipole structure of

the present field in South America. The present inclination anomaly in the area of study

is greater than the inclination anomaly of any of our sites that passed the selection criteria

(Figure 3-3). This observation suggests that the present inclination anomaly is among the

greatest that has occurred in the area at least during non-transitional states of the

magnetic field.

The mean directions of the normal and reverse data of this study were compared to

ranges of directions corresponding to the declination and inclination anomalies (obtained

mean values minus those expected from GAD) resulting from the TAF models (Figure 3-

6b) obtained by JC95 for the past 5 Ma, that are based on normal (LN1 model) and

reverse (LR1 model) data sets derived from lava flows (Johnson and Constable, 1996).

The agreement between the values modeled by JC95 for the Patagonia area and our

results (Figure 3-6b) is facilitated by large 95% confidence ranges among our normal and

reverse mean directions. However, the departure from the more ubiquitous negative and

positive inclination anomalies (for normal and reverse data respectively) depicted in the

TAF models of JC95 is not clearly observed in our data set. Only the inclination anomaly

of -4.7 (63.4 68.1) that we obtained from the reverse sites closely agrees with the -4










a) N b) N










600 600 600














Figure 3-5. Equal area projection of mean directions from several groups of sites. (Figure
3-5a) Comparison of the mean direction of the selected group of sites (shaded
square) and the selected plus rejected group of sites (empty square) with the
GAD (triangle pointing down). (Figure 3-5b) comparison of the mean
direction from normal (gray-filled diamond) and reverse (empty diamond)
data with the GAD (triangle pointing down) and ranges of inclination and
declination in the area of study (shaded quadrilaterals) as modeled by Johnson
and Constable (1995).

Table 3-2. Statistical data among groups of sites from Patagoniaa
Group Dec Inc N a95 K Fish VGP VGP a95 K Fish O.G. O.A. St Sb Su Si
Dir Dir Dir Long Lat VGP VGP VGP
Sel 358.7 -68.2 33 3.5 51 yes 141.3 88.5 5.4 22 yes yes yes 17.3 17.1 20.6 14.6
Sel + Rej 3.3 -67.3 41 3.6 40 yes 24.5 88.2 5.3 19 yes yes yes 18.9 18.6 21.9 16.6
Normal 359.3 -70.6 22 4.3 53 yes 113.8 85.5 6.8 22 yes yes yes 18.2 18.0 22.6 14.9
Reverse 177.7 63.4 11 6.1 57 yes 91.3 -84.6 8.7 28 yes yes yes 16.3 16.1 22.4 12.6
aSel and rej are selected and rejected groups of sites. Abbreviations for columns Dec, Inc, c95 Dir, K Dir,
VGP Long, VGP Lat, a95 VGP, K VGP are as in table 3-1. Fish Dir/Fish VGP indicate whether the
distribution of the directional/VGP data is fisherian. O.G./O.A indicate whether the 95% confidence limits
(aU9) of the mean direction/mean VGP overlap the GAD/Earth's rotation axis respectively. Data of VGP
scatter relative to the Earth's axis of rotation is given in columns: St (total scatter), Sb (scatter corrected for
within-site scatter), S, (upper 95% confidence limit of the scatter) and S1 (lower 95% confidence limit of
the scatter).

to -6 inclination anomaly range obtained for the Patagonia area by JC95. But at the same

time, the declination anomaly of this same group of sites of AD = 2.30 (177.70 1800) is






40


quite distant from the range of -10 to -12o of the model. Detecting true departures from

negative and positive inclination anomalies (for normal and reverse data respectively)

would be important, because they would represent a contribution opposite to the axial

quadrupole term (that causes the so called far-sided effect seen in VGP plots (e.g.

Wilson, 1970)) that is nevertheless expected to be small close to + 550 latitudes such as in

this study. Such axial quadrupole term (g20) contribution becomes zero at these latitudes

because of the shape of the wave of the Legendre polynomials of this spherical harmonic

term.














CHAPTER 4
PALEOSECULAR VARIATION AND TIME-AVERAGED FIELD RECORDED IN
LAVAS FLOWS FROM MEXICO

The Trans-Mexican Volcanic Belt (TMVB) was targeted for investigation in order

to increase the number of paleomagnetic sites with reverse directions since less than 25%

of the sites from previous paleomagnetic studies in Mexico showed reverse polarity. This

chapter presents results from 13 paleomagnetic sites from an area west of Mexico City

and 7 sites from an area of dispersed monogenetic volcanism in the state of San Luis

Potosi, accompanied by seven 40Ar/39Ar radiometric dates.

The time averaged field is usually compiled from lavas that are 0 to 5 Myr in age.

This time range is used in order to avoid the data being compromised by plate tectonics

or true polar wander. This assumed safe period of time is invalid where fast moving

plates carry the lavas across lines of latitude (Yamamoto et al., 2004), or where rotations

of continental blocks occur over a short time (e.g., Luyendyk, 1990). Tectonic rotations

appear to be a particular problem in the TMVB (Ruiz-Martinez et al., 2000) which lead

us to restrict the analysis of the paleomagnetic field in this area to the past 2 Myr. In this

study our results from the TMVB were analyzed along with data compiled from previous

studies, thus updating the compilation by Bohnel et al. (1990).

Geology and Sampling Area

The Trans-Mexican Volcanic Belt (TMVB) crosses Mexico in east-west direction

between around 190 to 21 north latitude. This volcanic belt has developed since the

Miocene and both polygenetic and monogenetic volcanoes have contributed to its









formation. The tectonic setting most often proposed to explain the volcanism in the

TMVB is subduction of the Rivera and Cocos plates under the North America plate

(Pardo and Suarez, 1995), however others have interpreted rifting unrelated to

subduction, based on several characteristics such as the oblique position of the TMVB

relative to the subduction plate boundary and the presence of lavas with composition

similar to oceanic island basalts (OIB) (Marquez et al., 1999; Verma, 2002).

The oblique subduction beneath the TMVB is accommodated by a transtensive

tectonic regime, shear and rifting that produces two fault systems of E-W and N-NW

directions (Ferrari et al., 1994; Alaniz-Alvarez et al., 1998). Volcanism in the TMVB,

closely related to faulting (Johnson and Harrison, 1990), is concentrated in several areas

along its length. The paleomagnetic sites obtained by us (Figure 4-1) or compiled from

the literature (Figure 4-2) that were analyzed in this study are from the following areas:

* In the western TMVB, the paleomagnetic sites compiled are mostly of Pleistocene
to Holocene age and closely related to major fault systems. Most of the sampled
areas are the Jalisco Block, the Chapala region, Valle de Santiago Volcanics, Rio
Grande de Santiago Volcanics and the Michoacan-Guanajuato Volcanic Field
(MGVF). The MGFV has numerous monogenetic volcanoes and is located at the
intersection of a tectonic triple junction (Johnson and Harrison, 1990).

* In the central TMVB, surrounding the basin of Mexico, we obtained data from the
volcanic ranges Sierra de las Cruces, Sierra Santa Catalina, Sierra Nevada, Sierra
de Rio Frio and the Sierra Chichinautzin. All the ranges around the Mexico basin,
except Sierra de las Cruces, were considered of Pleistocene to Holocene age (Nixon
et al., 1987). Sierra de las Cruces, as described by Osete et al. (2000), gets younger
toward the south with ages ranging from 3.7 to 0.29 Ma (late Pliocene -
Pleistocene). When radiometric dates were not available, we assumed the age for
some sites from Sierra de las Cruces, as expected from the areas of Gauss to
Brunhes age, mapped by Osete et al. (2000). The volcanic ranges around the basin
of Mexico are mostly composed of stratovolcanoes. However the Sierra
Chichinautzin is a volcanic field that consists mainly of numerous monogenetic
volcanoes. The exposed lavas from this volcanic field are very young, probably less
than 100 Ka old (Urrutia-Fucuchauchi and Martin del Pozzo, 1993).

* In the eastern TMVB, many of the paleomagnetic sites compiled are from the
Altiplano Area. In the northern part lavas are mostly Miocene to Pliocene in age






43


(Ruiz-Martinez et al., 2000; Cantagrel and Robin, 1979). In the southeastern part of
the Altiplano, the shield volcano Cofre de Perote and the stratovolcano Citlaltepetl
(Orizaba Peak), are surrounded mainly by monogenetic volcanoes that reach lower
elevation. These monogenetic volcanoes are of Brunhes age (Negendank et al.,
1985). Other volcanic areas in the eastern TMVB where paleomagnetic studies
have been made are Los Tuxtlas (Plio-Pleistocene) and Palma Sola Massif
(Miocene Pleistocene). These two areas are often not considered to belong to the
TMVB (e.g., Cantagrel and Robin, 1979).







44




a)
S Km I I SW
0 10 20




:: : -- Toluca : 13. 12
190 15 --' -

j-:-IuscoV 1. 4


Goro0 V
S Nevado de Toluca V.



*8
S .- Cuernavoca
Pa!iFh Villa Guerrero
Ocean
-100" -990 45' -99 30' -990 15'

b)

Villa de Ramos 0 Km 40 2
6^- *:- A2
22 45' ,
.--* 3

220 30'

S---- 5 San Nicolas de
220 1' Tolentino
220 4 415_

Pacific -San Luis Potosi
OL'tn I I
-101 -100


Figure 4-1. Location of sampling areas in Mexico using geologic base maps (Carta
geologica 1:250000 Estados Unidos Mexicanos, 1983). Figure 4-la shows the
sampling area in the central part of the TMVB. Sampling sites (black dots),
volcanoes (filled triangles), and areas of upper Tertiary andesites (crosses) are
represented. Figure 4-lb shows the sampling area in the state of San Luis
Potosi. Sampling sites (filled triangles) and areas of Quaternary basalts (dots)
are represented












Figure 4-2. Location of compiled paleomagnetic sites (a95 < 100) from the TMVB plotted on neotectonic map (modified from Ego and
Ansan, 2002). MB is Mexico Basin, Abbreviated for some volcanic areas are: Jalisco Block (JB), Tepic-Chapala Rift
(TCHR), Rio Grande de Santiago Volcanics (RGS), Chapala Region (CHR), Michoacan-Guanajuato Volcanic Field (MG),
Valle de Santiago Volcanics (VS), Acambay Graben (AG), Sierra de las Cruces (SC), Sierra Chichinautzin (SCH), Sierra
Santa Catalina (SSC), Sierra Rio Frio (SRF), Sierra Nevada (SN), Altiplano Area (AA), Los Tuxtlas Volcanic Field (LT)
and Palma Sola Massif (PSM). Abbreviations for some volcanoes are: Toluca Volcano (Tol), Iztaccihuatl Volcano (IZ),
Cofre de Perote Volcano (CP), and Orizaba Peak, i.e. Citlaltepetl Volcano (PO). Site locations have different symbols
depending of their reference number (as they appear in Tables 5 and 6). Empty squares represent site-locations of ref 8
(dark green), ref 1 l(light green), ref 5 (light blue), Watkins et al. (1971) (black), ref 17 (magenta), ref 16 (grey), ref 2
(dark blue). Filled squares represent site-locations of ref 14 (light brown), ref 3 (magenta), ref 4 (yellow). Circles represent
site-location of this study (dark blue), ref 12 (light green), ref 13 (dark green), ref 9 (gray), ref 15 (light blue), ref 10
(brown) and ref 7 (magenta).














-1060


-1040


- 02-


-1000









Previous Paleomagnetic Studies in the TMVB

One of the earliest most important paleomagnetic studies from the TMVB is by

Mooser et al. (1974) in the central TMVB, surrounding the basin of Mexico. That study

was made primarily for magnetostratigraphic purposes, and the standard paleomagnetic

techniques of the time were applied. That is, stepwise AF demagnetization was applied to

one sample per site (out of 5 to 10 field-oriented cores). The remaining samples of the

site were then blanket demagnetized at a field judged to be most appropriate based on the

data from the only sample of the site that had several demagnetization steps. The data

reported include 187 sites of igneous rocks, both volcanic and plutonic, from several

stratigraphic groups (Miocene-Pleistocene) from the TMVB.

Several subsequent paleomagnetic studies in the TMVB were made during the late

70s and 80s. Although paleomagenetic techniques similar to those applied by Mooser et

al. (1974) were used, secular variation was also analyzed. Most of those studies were

made in the Sierra Chichinautzin (e.g. Herrero-Bervera and Pal, 1977; Herrero-Bervera et

al., 1985) and the nearby Iztaccihuatl Volcano (Steele, 1971; Steele, 1985), both of

Brunhes age. The only available paleomagntic study from the 80s on mostly Plio-

Pleistocene lava flows from the eastern part of the TMVB (Altiplano area and Palma Sola

Massif) is by Bohnel and Negendank (1981) who report results from 53 paleomagnetic

sites. These rocks were demagnetized in more detail (at least 3 steps), but no principal

component analysis (PCA) was applied to determine remanent directions. H. Bohnel

(unpublished data, 2005) has enhanced the results from this preliminary study by

applying PCA and constraining the age of the lava flows based on new data. These data

are reported in this paper.









During the 90s up to the present, paleomagnetic studies on lavas increased,

extending from east to west along the TMVB. In these studies modem procedures

expected to produce data of improved quality have been applied. In particular, all the

samples have been step-wise demagnetized using AF or thermal demagnetization and

principal component analysis has been used to determine remanent directions. Most of

the recent studies have been dedicated to studying specific volcanic areas and age

interpretations have been made based on radiometric dates and magnetic polarity.

Recent paleomagnetic data has been obtained on very young lava flows (< 40 Ka)

along the TMVB. These lava flows belong mostly to the Chichinautzin Volcanic field

(e.g., Morales et al., 2001; Gonzales et al., 1997) and to the MGVF (Gonzales et al.,

1997). Bohnel and Molina-Garza (2002) have compiled paleomagnetic data for secular

variation analysis for the past 40 Ka. In their study results of the same flows from

different studies have been taken into consideration and averaged.

New Data

Our sampling within the TMVB took place in the southern part of the Sierra de las

Cruces, that limits the basin of Mexico City to the west as well as around the Nevado de

Toluca Volcano (Figure 4-la). We also sampled lava flows away from the TMVB in an

area of about 20000 km2 of the state of San Luis Potosi (Figure 4-1b) where dispersed

monogenetic basaltic volcanoes occur (Aranda-Gomez et al., 1993). This study reports

for the first time paleomagnetic data from this area.

Because of the scarcity of paleomagnetic data from reversely magnetized flows,

one of our goals was to focus our sampling on reverse flows. We attempted to determine

in the field which flows were reverse by taking rock-blocks from the outcrops while

roughly keeping track of their orientation and measuring with a portable flux-gate vector









magnetometer the sign of the variation in magnetic moment, parallel to the north, that

occurred as the rock-blocks were positioned closer or away from the magnetometer's

sensor; when the intensity of magnetization increased, the lavas were interpreted to have

normal polarity and vice-versa. In order to avoid sampling in areas that had been

tectonically affected (e.g., Ruiz-Martinez et al., 2000; Osete et al., 2000), we sampled in

areas of the TMVB thought to be of Pleistocene age, as well as off the TMVB in an

extensive area in the state of San Luis Potosi (Figure 4-1b). Sampling was carried out

during May of 1999. Sampling sites were chosen to be representative of a single lava

flow and the location established using GPS. At least 10 cores a few cm long were drilled

in each site using a portable gasoline-powered drill. Samples were oriented using

magnetic compass and sun compass when possible.

Paleomagnetic Laboratory Work and Data Analysis

Laboratory paleomagnetic analysis was conducted at the University of Florida.

Magnetic measurements were made in a 2G cryogenic magnetometer in a shielded room.

Thermal demagnetization was done using a Schonstedt oven and AF demagnetized using

a Dtech D-200 demagnetizer. The minimum number of demagnetization steps used

during thermal demagnetization was 14 and, during AF demagnetization, 13. High scatter

and strong intensity of the natural remanent magnetization (NRM) was observed on sites

1 through 5, indicating that these sites were affected by lightning. Samples from these

sites were then AF demagnetized since this is the preferred method to remove the

isothermal remanent magnetization (IRM) acquired by lightning (Schmidt, 1993). All the

remaining samples were subjected to stepwise thermal demagnetization on all the

samples and AF demagnetization on one replicate sample. Subsequently replicate

samples of 9 sites were subjected to stepwise AF demagnetization after obtaining









relatively high within-site scatter and realizing that many of those sites were also affected

by lightning (Figure 4-3). The effects of lightning on those sites were not as obvious as in

sites 1 to 5 since fewer samples per site had very high NRM intensities and the scatter of

NRM directions was smaller. The use of AF demagnetization successfully decreased the

values of the 95% confidence angle around the site mean direction (t95) obtained from

thermal demagnetization but the changes in site-mean directions were rather small, and

did not exceed 4.8.

The demagnetization curves of individual specimens sampled in our study were

analyzed by determining directions trending to the origin (lines) as well as

remagnetization circles (planes, Figure 4-4) when direct lines could not be obtained

(Kirschvink, 1980). The quality criteria with which the components of magnetization

were calculated were those suggested by Tauxe et al. (2000): at least 5 points of the

demagnetization curve and a maximum angular deviation (MAD) < 5. Site-mean

directions (Table 4-1, Figure 4-5a) were determined from at least 3 samples and

calculated using Fisher (1953) statistics or, when it was applicable, the method for

combined analysis of remagnetization circles and direct observations by McFadden and

McElhinny (1988a). As quality control for the analysis of the results, the cut-off values of

a95 < 100 (used by McElhinny and McFadden, 1997) and a95 < 5 (used by Tauxe et al.,

2000) were considered.

Results from New Data

As shown in Table 4-1, 15 out of 20 sampled sites yielded coherent paleomagnetic

results (Figure 4-5a); no mean direction of magnetization could be interpreted from the

remaining 5 sites. The sites collected in the area of San Luis Potosi are so affected by

lightning that only 4 out of the 7 sites from that area have interpretable results. All,







51


a) Sample 14.4 W/Up c) Sample 15.3
2
W/Up




300 0.5 4M


.... 0,2
... ... -- -- ---- --- -,.... ------- ---- I


_-0.2
-3 -2

b) Sample 14.4 W/Up d) Sample 15.3

2 W/Up

------




He O3
...... 0 .5 -0 .2
.............-3 ....................... 2 ............ ............... .. ^ -i I -0 2
-3 -2 .1


Figure 4-3. Examples of Zijderveld diagrams from Mexico. The pairs of zijderveld plots
in Figures 4-3a and 4-3b, and Figures 4-3c and 4-3d represent, respectively,
thermal and AF demagnetization curves on replicate samples, showing a
better definition of the primary component of magnetization on the AF than
on the thermal demagnetization curves. Approximate intensity values are in
A/m.

except three sites from the San Luis Potosi area, meet the conditions of having a9.5 < 100

(the same sites meet the condition of having a9.5 < 5 and a dispersion parameter k > 100,

the last being a selection criterion introduced by Tauxe et al., 2003). The homogeneity of

the data obtained is disrupted by the presence of two sites with normal polarity and one

site with intermediate direction (thus our aim of collecting samples with reverse polarity

was unsuccessful in these cases), as well as the considerable latitudinal differences of the

two areas of study that imply significant differences of the GAD values with which we

compare our results (GAD inclination is around 34.90 in the TMVB versus 40.00 in the









state of San Luis Potosi). Therefore we grouped our results as presented in Table 4. The

group of reverse sites from the TMVB is the most homogeneous. The statistical results

among all the groups of sites are very similar (Table 4-2), for this reason only the results

corresponding to the selected group of sites were plotted (Figure 4-5b). All the mean

directions overlap both with GAD and GAD plus a 5% quadrupole (GAD + Q5), and the

inclination anomalies are very low, indicating a closer approach to GAD than to the GAD

+ Q5. Likewise all the mean VGPs coincide within the errors limits with the axis of

rotation. The VGP scatter of the results we obtained from the TMVB is around 10 which

is low compared to the value expected from Model G (13.5) but in all cases it coincides

within the uncertainty range (Table 4-2). Tectonic rotations are not observed in this

dataset.

a) N b) N
















Figure 4-4. Remagnetization circles and directions form samples of Site 17 (Mexico).
Figure 4-4a is an equal area projection showing an example of
demagnetization steps (circles) obtained from sample 17.5. Filled circles
represent the steps used to calculate the remagnetization circle (arc).
Embedded is the respective zijderveld plot for the sample. Figure 4-4b shows
the remagnetization circles (arcs) and directions (circles) used to calculate the
site-mean direction of magnetization (triangle). Arcs represent portions of
planes pointing down.












a) b)














o0




Figure 4-5. Equal area projection of paleomagnetic directions obtained from Mexico.
Figure 4-5a represents the site-mean directions of sites collected in the TMVB
(circles) and SLP (squares) areas. Crossed out sites are sites that were
rejected. Figure 4-5b represents the mean directions among the selected sites
(diamond) surrounded by the 95% confidence angle (a95). Black and blue
triangles are the expected GAD and GAD plus a 5% quadrupole expected in
the area.

New 40Ar/39Ar dates obtained by Dr. Amabel Ortega-Rivera and Dr. James K. W.

Lee (Mejia et al., unpublished data, 2005) for seven of the sampled paleomagnetic sites

(Table 4-1) range from 1.28 0.54 Ma to 4.14 0.37 Ma. Results from 5 sites from

Sierra de las Cruces (9, 12, 13, 19 and 20) suggest late Pliocene Pleistocene ages, that

are consistent with previous radiometric dates by Osete et al. (2000) and Mora-Alvarez et

al. (1991) in that area. A result from a site in the Toluca volcano region (site 18) suggests

a Pleistocene age that is consistent with radiometric dates obtained by Cantagrel et al.

(1981). The oldest age obtained is for site 6, from the San Luis Potosi area (4.14 0.37

Ma), which correspond to the Pliocene epoch.















Table 4-1. New paleomagnetic and age data of sites from Mexicoa
Site Site Site Dec Inc SC n/N Dir Dir Th L VGP VGP VGP VGP Age U (+-)
Lat Long cU95 k AF Long Lat cU95 k (Ma) (Ma)
1 21.270 -100.520 ----- ----- 7/11 0/12 ----- ----- AF x
2 22.837 -99.878 346.7 23.5 5/9 5/10 37 5 AF x 136.2 75.5 25.7 10
3 22.660 -99.903 46.8 25.2 10/10 4/10 13 51 AF x 353.5 44.8 11.2 68
4 22.260 -100.564 351.4 24.1 8/10 5-5/10 11.8 ----- AF x 121.2 77.3 ---
5 22.306 -100.610 ----- ----- 9/9 0/10 ----- ----- AF x
6 22.820 -101.914 189 -37.1 8/10 8/10 2.8 396 Th 180.3 -81.3 2.8 407 4.14 0.37
7 22.834 -101.889 ----- ----- 7/7 0/10 ----- ----- AF x ---
8 18.976 -99.648 187.5 -36.4 4/11 8/11 4.8 134 Th x 158.1 -82.9 4.6 148
9 19.309 -99.305 182.8 -33.7 0/7 7/10 4.3 196 Th 187.3 -87.2 4.6 172 1.28 0.54
10 19.322 99.301 184.5 -37.7 0/0 5/10 4.5 291 AF x 146.8 -85.3 4.2 333
11 19.291 -99.261 9.9 29.4 2/9 8/9 4.6 148 Th 9.6 80 4 195
12 19.279 -99.278 161.2 -43.1 0/3 10/10 3.9 153 Th 13 -71.8 4.2 134 1.43 0.17
13 19.267 -99.292 166.9 -47.8 1/10 8/10 3.7 230 Th 32.2 -74.6 4.3 170 2.29 0.58
14 19.199 -99.250 185.7 -42.1 3/10 6/10 4.1 270 AF x 125.1 -82.6 3.9 297
15 19.230 99.272 182.2 -47.7 0/0 3/10 2.6 2308 AF x 92.4 -80.2 2.9 1759
16 19.197 -99.260 ----- ----- 2/10 0/10 ----- ----- AF x ---
17 19.156 99.806 181.4 -16.1 0/0 4-4/11 4.8 ----- AF x 252.9 -79.0
18 19.168 -99.805 177.7 -16.7 3/9 5/9 2.5 968 Th x 272.6 -79.1 2.3 1129 1.49 0.51
19 19.301 -99.375 186.1 -24.4 0/0 6/10 3 503 AF 217.5 -81.2 2.7 638 1.69 0.21
20 19.339 -99.362 182.6 -41.3 0/0 7/10 3.4 307 AF x 108.6 -84.9 2.9 428 2.61 0.52
aDec and Inc are the site-mean declination and inclination; SC is the number of sun compass declinations obtained in each site; n/N is
the number of samples used to calculate the site-mean direction (when two numbers separated by a dash they indicate the number of
lines and planes used in calculation of great circles) per the number of processed samples. K is the dispersion paramenter of directions
(Dir) or VGPs; 095 is the 95% confidence cone about the mean direction (Dir) or mean VGP; Th/AF represent whether the
paleomagnetic technique applied for the result reported was Thermal or AF demagnetization; the sites affected by lighting are points
out with an "x" under L; R.D. is the 40Ar/39Ar radiometric date obtained for some sites; U is the uncertainty range of the radiometric
date reported as 2c (Mejia et al., unpublished data, 2005).















Table 4-2. Statistical data among new sites studied in Mexicoa
Dir Dir VGP VGP VGP K
Group of Sites Dec Inc N 095 k Long Lat 095 VGP O.G. O.Q O.A. A Inc A Dec St Sb Sl Su
All sites 180.4 -34.3 14 6.2 42 178.3 -89.2 5.2 59 yes Yes Yes 1.32 0.40 10.7 10.5 8.4 14.0
Selected (095 < 100) 181.7 -35.2 13 6.4 43 149.5 -88.3 5.3 63 yes Yes Yes 0.13 1.70 10.5 10.3 8.2 13.9
Selected (from TMVB) 181.1 -35.0 12 6.9 40 132.0 -88.8 5.6 60 yes Yes Yes -0.09 1.10 10.6 10.4 8.2 14.3
Selected ( TMVB and Reverse) 180.2 -35.5 11 7.5 39 90.1 -88.8 6.0 60 yes Yes Yes -0.60 0.20 10.7 10.5 8.2 14.6
aAbbreviations for columns Dec, Inc, N, Dir c95, Dir k, VGP Long, VGP Lat, VGP c95, VGP k are as in Table 4-1. O.G./O.A indicate whether the 95%
confidence limits (as9) of the mean direction/mean VGP overlap the GAD/Earth's rotation axis respectively. O.Q. indicates whether the 95% confidence limits
(aU9) of the mean direction overlaps the GAD plus a 5% quadrupole. Data of VGP scatter relative to the Earth's axis of rotation is given in columns: St (total
scatter), Sb (scatter corrected for within-site 'catter), Su (upper 95% confidence limit of the scatter) and Si (lower 95% confidence limit of the scatter).











Compilation of Paleomagnetic Data

The data compilation includes paleomagnetic sites from lava flows ranging from

Pliocene to Recent age. We excluded sites with a 95% confidence angle around the site

mean direction (095) > 100 (rounded to the nearest integer) and results from studies in

which no demagnetization techniques were applied. Table 4-3 lists the sites that were

used for TAF and secular variation analysis, after excluding the sites that appear to be

affected by rotations.

The compiled dataset was obtained from the published literature and the

unpublished reanalysis of data initially presented by Bohnel and Negendank (1981) (H.

B6hnel, unpublished data, 2005). Included in the dataset are the compiled paleomagnetic

sites from the secular variation study by Bohnel and Molina-Garza (2002) for the past 40

Ka. Efforts were made to bring the dataset into a homogeneous state. When not reported,

VGPs were calculated. Some of the compiled paleomagnetic studies don't report specific

site coordinates, and they had to be determined from maps. Site coordinates were used to

show the spatial distribution of sampling sites (Figure 4-2), and help determine which

lava flows might have been sampled in several studies. The coordinates of the

paleomagnetic sites studied by Mooser et al. (1974) were measured by relocating them on

geologic maps (Carta geologica de la Republica Mexicana, Scale 1:50000) using the

location descriptions and location maps provided in the paper. Site coordinates were

measured from the location maps contained in the studies of Alva-Valdivia et al. (2001),

Urrutia-Fucugauchi and Rosas-Elguera (1994), Soler-Arechalde, Urrutia-Fucugauchi

(2000), Uribe-Cifuentes and Urrutia-Fucugauchi (1999), Steele (1971) and Watkins et al.









(1971). The precise locations of the sites studied by Herrero-Bervera (1986) in Sierra

Chichinautzin could not be obtained because insufficient data was provided in the paper.

The possibility of compiling paleomagnetic results from a lava flow that has been

studied several times was taken into consideration, and when noticed only one of the

results was included in the dataset. This is the case of the well studied Xitle lava flow

(e.g. Urrutia-Fucugauchi, 1996). In this particular case, only the paleomagnetic result

reported by Bohnel and Molina-Garza (2002) was used.

Tectonic Rotations

Previous paleomagnetic studies from the western TMVB where tectonic rotations

have been interpreted were examined. The reanalysis of these results (Table 4-4) was

done using our selection criteria and excluding sites of Pliocene age (only 7) that are

more likely to be affected by tectonic rotations. As can be deduced from Table 4-4,

evidence of tectonic rotations in each of the studies is limited. The mean direction often

coincides with the GAD because the number of sites (N) in each of these studies is low

(usually < 10), which leads to relatively high a95 values. There is however a general

tendency toward negative declination anomalies (AD). The nature of the observed

rotations is difficult to interpret and beyond of the scope of this study. Most of the

discarded data are located in areas of intense faulting and local tectonic rotations are

likely. Tectonic rotations would also be consistent with the observation by Johnson and

Harrison (1990) that the western TMVB is more intensely affected by recent faulting than

the central TMVB. However, because of the small number of paleomagnetic sites and the

overall low quality of the existing results (40% of the sites with a95 <150 have a95 > 7.50),

we think that more studies area needed in this area to study tectonic rotations. The studies









that show obvious declination anomalies were not considered for further analysis. The

only sites from the western TMVB that were used for TAF and secular variation analysis

are the ones studied by Maillol et al. (1997) and by Bohnel and Molina-Garza (2002).

Ruiz-Martinez et al. (2000) interpreted about -10o counterclock rotations in the

central and eastern TMVB among 28 sites of Pliocene age. Included in this analysis were

sites from the Altiplano, Palma Sola Massif and from Sierra de las Cruces. Similarly

negative declination anomalies are obtained among these sites and/or eight additional

sites of similar age from the Sierra de las Cruces, Los Tuxtlas volcanic field and Palma

Sola Massif. Therefore all these sites were excluded from the final analysis.

Results from the TMVB

The mean direction of magnetization was calculated for the normal, reverse and

combined data (Figure 4-6a). Sites with very low VGP latitude, usually interpreted to be

representative of a transitional or a reversing field, were excluded from these

calculations. The cut-off value of VGP latitude was calculated using the method by

Vandamme (1994), in which the VGP cut-off value is a function of the VGP scatter as

calculated from Model G (McFadden et al., 1988b). The VGP cut-off value calculated

that way depends on the sampling latitude. VGP cut-off values were calculated for each

site and ranged from 60.40 to 60.80.

The mean direction among the sites was compared both to the GAD and to the

GAD plus a 5% quadrupole (GAD+Q5). A small quadrupolar component of this order

has been proposed in several models (e.g., Johnson and Constable, 1995; Hatakeyama

and Kono, 2002) and, as argued by Merrill (2003), is the only non-dipole term

significantly different from zero in the TAF. The mean direction among the normal

polarity sites (within the 95% confidence level) is close but significantly different from









GAD and coincides with the GAD+Q5. The mean direction among the reverse polarity

sites is not significantly different from GAD or GAD+ Q5 (Table 4-5, Figure 4-6b). The

normal and reverse mean directions pass the reversal test with an A classification

(McFadden and McElhinny, 1990). The mean direction of the overall results doesn't

overlap the GAD but coincides with GAD+Q5 (Table 4-5). The coincidence with

GAD+Q5 of the overall mean direction originated from the sites of normal polarity that

outnumber the reverse polarity sites by 101 (N is 144 / 43 for normal/reverse polarity

sites).

a) N b) N









I i + i i +




Cbo o
00

00o



Figure 4-6. Equal area projection of paleomagnetic directional data compiled from the
TMVB using a selection criterion of C95 < 100, after excluding sites with
transitional directions or that appear to be affected by rotations. Filled (or
crosses)/open symbols indicate downward/upward directions. Figure 4-6a
shows site mean directions. Figure 4-6b shows the mean directions, for
normal and reverse data, of the groups of sites in Figure 4-6a with the 95%
confidence ellipse (Tauxe, 1998). Blue triangles represent the expected GAD
and orange squares represent the expected GAD plus a 5% quadrupole.









The distribution of site-mean directions is not fisherian, but elongated in a north-

south direction. This is consistent with previous studies (e.g., Tanaka, 1999; Tauxe and

Kent, 2004) in which the north-south elongation is expected to decrease with latitude. As

modeled by Tauxe and Kent (2004) the elongation (zeta/eta) at the area of study should

be = 2.5. The elongation of the 95% confidence ellipses (Tauxe, 1998) of the normal and

reverse mean directions in this study (Figure 4-7) are 1.4 and 2.3, respectively. Therefore,

only the elongation of the confidence ellipse of the reverse mean direction is close to the

value expected from the secular variation model by Tauxe and Kent (2004).

The values of VGP scatter among the normal and reverse polarity sites (Table 4-5)

coincide with the value expected from Model G (13.50). The mean VGP of the sites plots

slightly toward the far side of the pole relative to the sampling area (Figure 4-7). This is

the so called far-sided effect (Wilson, 1970) that results from the quadrupole portion of

the field.

When the criterion of a9.5 < 5 is applied to the dataset the mean statistical values

are similar to those using the selection criterion of a9.5 < 10.

Conclusions from Mexico

The data obtained in this study increase the number of reverse direction

paleomagnetic data from the Trans-Mexican Volcanic Belt (TMVB) by about 25%. After

avoiding the troublesome influence of rotated sites, the mean direction among selected

compiled sites of late Pliocene to Holocene age (< 2 Myr) reveals a clear coincidence

with the GAD plus a 5% quadrupole. Such small quadrupole contribution would be

increasingly less detectable at higher latitudes. The VGP scatter is best described by

Model "G" of McFadden et al. (1988).


























180


2700 2700


Figure 4-7. Virtual geomagnetic poles (VGPs) from the TMVB using a selection criterion
of C95 < 100 after excluding sites with transitional directions or that appear to
be affected by rotations. Figure 7a shows site VGPs. Filled/open circles
represent normal/reverse polarities. Figure 7b shows the mean VGP, for the
combined normal and reverse data, surrounded by the 95% confidence ellipse
(Tauxe, 1998).


Table 4-3. Compiled Late Pliocene Holocene paleomagnetic data from the TMVBa
TMVB Site Lat Long Dec. Inc. N 095 k P VGP VGP R
Area Long Lat
AA ALJ (25) 19.08 -97.52 350.4 33.4 5 10.3 56.1 N 168.8 80.9 2
AA ALM (1) 19.59 -96.78 351.9 26 7 3.6 283.9 N 137.3 80.3 2
AA ATZ (3) 19.00 -97.30 357.4 44.8 6 5.5 149.2 N 245.2 82.2 2
AA BAN (4) 19.61 -96.94 358.4 20.9 8 2.4 516.4 N 93.3 81.1 2


19.65 -96.92 4.3 24.9
19.60 -96.88 331 49
19.43 -96.90 1.3 33.4
19.50 -97.16 344.5 19.8
18.90 -97.40 351.6 22.5
18.90 -97.40 9.5 -3
19.46 -96.75 4.1 22
19.30 -97.51 336.9 39.5
19.53 -96.91 354.5 26
19.61 -97.09 8.5 31.7
19.67 -96.87 353.1 25


2.6 389.5 N
5.7 136.1 N
2.6 401.3 N
3.6 206.7 N
7.8 139.9 N
8.5 63.4 N
5.9 167 N
3.5 361.3 N
8.7 60 N
4 195.5 N
6 85.5 N


COA (5)
CON (6)
COZ (7)
CPC (8)
CSB (11)
CSC (12)
FAL (29)
GIL (34)
MAC (15)
MIC (31)
NAO (16)


50.2 82.3
200.2 61.8
36.0 88.3
143.2 72.4
132.2 79.2
57.0 67.6
56.7 81.0
184.7 68.2
126.2 82.2
8.4 81.6
129.3 80.7













Table 4-3 (continued)


TMVB


Site


NED (17)
OLL (18)
RIO (30)
SDA (13)
SLF (21)
SSS (28)
TEO (22)
VIG (23)
GV+LF (21&26)
CPA+CPB(9&10)
P+PN (27)
ACT (33)
MIO (32)
PES (54)
TAT (45)
SGV (20)
Toxtlacuaya
Jalapa
La Joya
La Primavera
Colima
MAS-1
MAS-5
MAS-6
MAS-7
MAS-8
MAS-9
MAS-10
MAS-13
MAS-15
MAS-20
MAS-21
MAS-2
MAS-4
MAS-12
MAS-16
TS
TP
SRG6
SRG8
Paracutin
El Jabali
La Mina
El Pueblito
El Metate


Lat Long Dec. Inc.


19.29 -97.41 15.4 57.5
19.62 -97.03 1.3 21.6
19.49 -96.80 4.2 20.3
18.90 -97.40 350 38.8
19.30 -97.31 16.2 51.1
19.13 -97.61 15 15.7
19.40 -96.97 358.3 44.2
19.63 -97.08 348.2 33.5
19.28 97.30 11.9 48.4
19.50 -97.16 350.1 29.8
19.01 -97.27 0.5 31.2
19.50 -96.59 179.8 -19.5
19.60 -97.09 178.8 -33.4
19.35 -96.80 160.6 -39.1
19.66 -97.14 182.5 -27.9
19.25 -97.37 338.6 -10.8
19.40 -96.90 345 34.4
19.65 -96.96 7.8 27.8
19.59 -96.99 356.6 23.2
20.66 -103.46 4.7 24.8
19.25 -103.53 1.4 33.7
20.55 -104.87 1.9 -3.6
20.51 -104.76 340.7 20.1
20.57 -104.76 356.8 38.2
20.58 -104.88 3.3 35.3
20.53 -104.85 359.5 47.4
20.50 -104.79 344.6 55
20.54 -104.72 345.7 18.8
20.46 -104.86 14 18.8
20.47 -104.76 347.9 65
20.82 -104.97 2.6 34.6
20.81 -104.93 352.8 31.5
20.51 -104.89 180.9 -50
20.49 -104.88 173.3 -20.5
20.42 -104.85 179.8 -49
20.45 -104.76 168 26.4
18.46 -95.16 1.6 18.2
18.44 -95.08 358.9 37.1
18.23 -94.86 173.9 -30.4
18.38 -94.96 172.2 -42.6
19.47 -102.25 10.7 37.8
19.45 -102.11 12.5 34.3
19.71 -101.42 339.7 58.2
19.82 -101.92 3.6 39.9
19.54 -101.99 82 41.5


N s95 K P VGP VGP R
Long Lat
8 9.1 37.6 N 294.8 66.9 2
7 6.9 76.3 N 74.3 81.5 2
10 3.1 233.2 N 58.4 80.1 2
7 6.7 79.8 N 192.1 80.2 2
7 4.8 156.6 N 26.9 70.8 2
7 7 76 N 27.7 71.7 2
10 2.4 406.9 N 249.8 83.3 2
8 3.4 271.7 N 168.3 78.8 2
11 4.3 111.8 N 142.0 75.2 2
15 1.6 641.6 N 135.9 79.9 2
15 2.4 248.6 N 70.2 87.8 2
6 4.8 193.2 R 264.6 -80.5 2
7 3 398 R 303.1 -88.2 2
9 2.3 491.3 R 5.1 -71.7 2
11 3.2 208.6 R 236.4 -84.6 2
8 8.1 47.4 T 125.1 57.2 2
26 1.8 248 N 173.6 75.8 4
8 3.8 218 N 25.1 81.1 4
3 5.8 450 N 107.1 81.8 4
7 7.7 63 N 45.4 81.1 4
23 9 12 N 17.6 88.4 4
8 3.7 221.2 N 70.2 67.6 8
9 4.7 122.5 N 139.5 68.8 8
8 2.7 422.4 N 182.8 86.9 8
5 5.7 181.2 N 3.8 86.7 8
9 2.6 397 N 252.0 82.0 8
9 7.1 54 N 216.5 69.8 8
9 8 42.4 N 129.1 72.4 8
9 1.7 878.3 N 21.8 72.7 8
8 2.4 529.1 N 237.7 61.7 8
10 3.4 198 N 20.8 87.0 8
10 7.8 39.3 N 137.3 82.2 8
9 3.1 280 R 79.4 -79.7 8
9 3.1 284.9 R 289.2 -78.2 8
10 2.3 442.5 R 74.1 -80.5 8
8 2.2 635.9 T 275.1 -53.6 8
10 5.2 86 N 1.4 74.9 3
7 6.3 79 N 242.8 87.3 3
6 7 72 R 338.3 -83.9 3
9 4.3 166 R 38.2 -80.3 3
6 4.4 238 N 336.1 79.8 4
6 2.7 597 N 348.8 78.2 4
6 4.6 213 N 220.5 64.0 4
5 5.1 227 N 306.9 85.6 4
5 4.4 301 T 327.5 14.8 4


JB
JB
JB
JB
LT
LT
LT
LT
MGVF
MGVF
MGVF
MGVF
MGVF













Table 4-3 (continued)


TMVB
Area
MGVF
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SC
SCH
SCH
SCH
SCH


Site


El Huanillo
11
8
9
10
12
13
14
15
19
20
13
26
28
6
8
11
14
16
20
27
18
CR
T02
T04
TO1
T03
ST2
Xitle
Tres Cruces
Cima
Maninal
Cuautl
Texontle
XA
P 1
P 2
P 3
P 4
TEU-1
TEU 2
0Z
ACO
JU
CHI 11
CHI 1


Lat Long Dec. Inc.


19.67 -101.98 42.2 10.7
19.29 -99.26 9.9 29.4
18.98 -99.65 187.5 -36.4
19.31 -99.30 182.8 -33.7
19.32 -99.30 184.5 -37.7
18.98 -99.65 161.2 -43.1
19.31 -99.30 166.9 -47.8
19.20 -99.25 185.7 -42.1
19.23 -99.27 182.2 -47.7
19.30 -99.38 186.1 -24.4
19.34 -99.36 182.6 -41.3
19.29 -99.25 5.1 36.1
19.31 -99.20 11.5 53.8
19.22 -99.27 9.1 10.3
19.48 -99.26 179.5 -18.1
19.35 -99.38 173.2 -53.7
19.32 -99.32 174.2 -61.2
19.27 -99.29 164.8 -39.5
19.23 -99.28 194.3 -41
19.19 -99.23 173.5 -53.8
19.30 -99.26 192.3 -46.7
19.17 -99.27 148.2 -64.8
19.52 -99.42 27.6 41
19.32 -99.32 362.2 32.2
19.32 -99.33 352.1 45.6
19.28 -99.40 167.3 -26.7
19.27 -99.30 177 -53.9
19.51 -99.48 145.3 -34.6
19.36 -99.17 0.6 34.4
19.10 -99.50 338.5 53
19.10 -99.18 354.6 40.8
19.22 -99.21 359.1 33.7
19.17 -99.42 342.6 16.6
19.22 -99.47 353.3 64.4
19.20 -99.20 343.8 22.1
19.20 -99.20 5 23.7
19.20 -99.20 3 25.7
19.20 -99.20 355.9 29.9
19.20 -99.20 357.6 28.4
19.20 -99.20 338.7 17.1
19.20 -99.20 358.2 22.4
19.20 -99.20 352.8 23.1
19.20 -99.20 357.6 32.6
19.20 -99.20 323.2 22.4
19.30 -99.20 16.1 36.1
19.10 -99.10 357.6 26.7


140.6 70.0
250.5 62.4
146.9 72.6
-296.3 81.6
-287.5 83.6
132.5 84.9
111.0 85.3
147.2 66.9
93.8 82.3
125.9 80.1
138.5 87.3
164.5 53.8
-14.6 74.8
-253.9 84.5


N s95 K P VGP VGP
Long Lat
3 5.3 540 T 1.4 46.6
8 4.6 148 N 9.6 80.0
8 4.8 134 R 158.1 -82.9
7 4.3 196 R 187.3 -87.2
5 4.5 291 R 146.8 -85.3
10 3.9 153 R 13.0 -71.8
8 3.7 230 R 32.2 -74.6
6 4.1 270 R 125.1 -82.6
3 2.6 2308 R 92.4 -80.2
6 3 503 R 217.5 -81.2
7 3.4 307 R 108.6 -84.9
7 4.8 162 N 341.1 85.1
7 5.3 129 N 292.7 71.8
7 5.5 120 N 47.3 73.4
6 9.4 52 R 263.5 -79.8
6 8.4 64 R 59.9 -73.9
7 4.8 157 R 69.9 -66.5
8 5.8 94 R 5.8 -75.5
6 6.3 113 R 150.5 -76.0
6 6 128 R 61.2 -73.8
6 6.8 98 R 130.9 -75.8
8 8.6 42 T 44.7 -52.1
9 3.1 275.9 N 336.8 64.1
9 4 170.4 N 31.8 87.2
8 3 337.7 N 218.9 79.4
10 4.2 130.5 R 329.5 -76.8
10 5.4 80.1 R 71.3 -74.6
5 7 121.4 T 355.6 -57.3
113 0.8 263 N 29.9 89.3
15 2.6 216 N 211.8 66.0
7 5.1 139 N 211.9 83.4
5 5.8 175 N 128.5 88.8


4.2 255 N
3.4 318 N
4.9 242 N
7.9 72.4 N
7.1 62.2 N
6.8 67.3 N
8.7 41.2 N
4.2 199.8 N
7.8 95.4 N
4.5 411.8 N
6.7 81.9 N
3.5 289 T
2.9 229.7 N
6.5 72.7 N













Table 4-3 (continued)


TMVB
Area
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SCH
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN


Site


CHI 2
CHI 3
CHI 4
CHI 5
CHI -12
16
17
21
24
34
35
10
34
5
8
36
1
4
7
9
23
25
27
31
37
28
28
38
JB
JD
JE
JH
JJ
JL
Tetimpa
16
19
20
21
29
47
53
24
55
56
14


Lat Long Dec. Inc.


19.00 -98.80 6.1 12.9
19.10 -98.80 330.3 46.6
19.03 -99.03 342.7 11.6
19.20 -99.02 345.4 18.6
19.10 -99.20 6.4 15.9
19.27 -99.16 3.7 35.4
19.26 -99.17 4.1 33.8
19.22 -99.16 1.1 36.1
19.20 -99.14 0.3 35.9
19.03 -99.20 352.8 41.8
19.02 -99.17 8.3 9.8
19.25 -99.07 356.9 34.9
19.03 -99.20 352.8 41.8
19.21 -98.92 346.5 38.3
19.25 -99.03 352 23.9
19.01 -99.13 346.6 36.1
19.19 -98.80 353.3 24.1
19.13 -98.82 23.8 21.5
19.25 -99.01 353.3 23.7
19.20 -99.01 10 17.3
19.23 -99.15 1 38.5
19.17 -99.15 352.6 15.4
19.14 -99.16 1.9 17.6
19.06 -99.24 16.2 25.3
19.03 -99.26 338.4 47.8
19.11 -99.18 358.5 33.4
19.16 -98.76 178.6 -43.3
19.03 -99.29 46.4 12.2
19.19 -99.17 10.4 17
19.03 -99.27 13.8 10.8
19.04 -99.31 4 23.1
19.22 -99.27 342.7 21.5
19.10 -99.18 352.8 33
19.20 -99.25 358.8 45
19.05 -98.45 352.6 38.6
19.21 -98.78 359.4 13
19.26 -98.66 356.6 31.9
19.26 -98.64 3.8 35.9
19.23 -98.67 7.4 38.1
19.17 -98.63 355.5 -8
19.13 -98.66 13.8 31.2
19.10 -98.60 356.1 39.8
19.20 -98.64 7.8 21.4
19.07 -98.68 352.7 18
19.07 -98.68 358.3 35.5
19.25 -98.72 177.6 -28.5


163.7 83.1
152.9 82.4
194.1 82.6
84.0 77.4
140.5 86.2
340.6 86.4
332.6 82.7
92.6 66.4
359.0 76.7
215.0 84.9
36.8 78.9
117.1 77.8
186.0 88.3
291.1 -85.3


N o95 K P VGP VGP
Long Lat
14 2.8 205 N -304.9 76.2
12 1.8 576.7 N -165.4 61.5
9 4.1 160.2 N -224.9 68.6
11 4.2 118 N -221.4 72.9
12 9.2 21 N -309.6 77.4
3 10.1 150 N 345.4 86.5
6 8.65 61 N 1.2 86.0
8 6.8 68 N 312.5 88.7
6 8.7 60 N 282.9 89.2
7 2.7 489 N 209.1 81.6
8 7.4 50 N 49.9 73.8
6 7 92 N 172.0 87.1
7 2.7 489 N 208.3 81.7
8 5.2 114 N 183.8 77.1
6 4.5 219 N 131.0 79.8
7 6 101 N 176.9 77.4
8 5.1 118 N 125.3 80.6
5 9.4 67 N 7.1 65.6
9 8.3 39 N 124.3 80.5
7 10.3 36 N 36.9 75.7
8 3.5 259 N 282.9 87.5
5 7.9 95 N 113.7 76.4
10 9.6 26 N 70.5 79.5
8 9.2 37 N 9.8 73.3
8 10.1 31 N 200.7 68.2
7 3.8 253 N 137.3 88.3
7 3.7 274 R 69.4 -83.8
6 8.8 59 T 0.8 43.0
8 3.9 198 N 35.2 75.6
8 3 353 N 34.4 70.9
8 3.3 277 N 51.7 82.0
8 2.9 371 N 148.3 71.6


498 N
131 N
201 N
178 N
79 N
64 N
73 N
71 N
96 N
178 N
192 N
76 N
153 N
209 R













Table 4-3 (continued)


TMVB
Area
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SN
SRF
SRF
SRF


Site


17
27
SM7
SM8
SM11
SM13
SM14
SM15
SM17
SM16
IZT -82
IZT -29
IZT -30
IZT 31
IZT 10
IZT 13
IZT -78
IZT 11
IZT 18
IZT 21
IZT -24
IZT -133
IZT -27
IZT -32
IZT -23
IZT -79
IZT -84
IZT -20
IZT -25
2
3
4


Lat Long Dec. Inc.


19.21 -98.74 183.5 -24.5
19.18 -98.77 170.2 -30.1
19.29 -98.68 356.5 38.2
19.28 -98.72 6.2 34.6
19.06 -98.38 5.8 16.7
19.09 -98.59 2.1 32.9
19.09 -98.62 0.9 19.7
19.14 -98.65 4.4 32.4
19.19 -98.80 355.2 24.4
19.21 -98.74 180.8 -16.5
19.11 -98.64 356.1 39.8
19.21 -98.66 7.4 38.1
19.22 -98.65 356.6 31.9
19.22 -98.64 3.8 35.9
19.14 -98.65 7.4 28.5
19.13 -98.65 355.9 31.5
19.14 -98.66 1.3 28.3
19.14 -98.65 3.6 23.5
19.13 -98.64 17.3 52.9
19.14 -98.63 352.4 32.1
19.14 -98.63 355.3 21.3
19.19 -98.66 10.2 25.5
19.19 -98.66 354 30
19.22 -98.65 356.7 30
19.14 -98.63 0.3 24.6
19.13 -98.66 13.8 31.8
19.15 -98.65 355 -8
19.13 -98.64 22.7 60.8
19.14 -98.63 27 61.5
19.50 -98.81 177.1 -32.8
19.50 -98.82 180.3 -62.3
19.39 -98.87 176.7 -27.9
19.32 -98.87 129.2 70.5
19.32 -98.79 144.5 78.6
19.33 -98.78 358.3 33.8
19.34 -98.69 343.7 34
19.32 -98.73 191 -32
19.34 -98.71 324.6 36.3
19.35 -99.09 348.9 32.8
19.33 -98.97 9.7 35.1
19.40 -98.97 168.7 -7.7
19.32 -98.91 183.2 -10.6
19.24 -98.87 176.8 -24.5
19.37 -99.03 175.4 -12.6


287.8 -4.3
273.8 1.1
144.4 88.2
171.4 74.6
180.2 -79.4
178.8 56.7
164.3 79.4
349.4 80.9
297.8 -71.0
248.1 -75.7
287.2 -82.9
280.6 -76.3


8 8.2 47 R 296.8 -84.5 9


N a95 K P VGP VGP
Long Lat
7 4.5 178 R 232.9 -82.8
7 5.2 135 R 334.9 -80.2
13 3.9 112.7 N 205.6 86.1
10 5.3 84.4 N 352.7 84.1
10 7 48.4 N 52.7 78.1
13 3.9 116 N 21.4 87.7
9 3.9 117.5 N 75.7 81.0
11 5 84.1 N 10.8 85.6
11 4.6 98.6 N 117.6 82.1
11 2.4 357.2 R 257.0 -79.2
8 4.2 178 N 215.4 85.0
7 7.1 72.6 N 332.3 82.7
7 6.8 79.4 N 141.6 86.3
7 7.6 64.3 N 339.4 86.4
6 4.8 199.3 N 19.7 81.9
7 2.5 569.1 N 143.2 85.6
7 3.9 243.2 N 64.4 85.7
7 4.7 168.4 N 54.3 82.3
7 7.5 65.6 N 305.1 69.0
7 5.3 131.7 N 158.9 82.6
7 6.7 82.4 N 111.2 80.7
6 4.7 207.5 N 20.4 78.6
6 7.6 79.6 N 144.0 83.5
7 9.3 43.2 N 127.2 85.6
7 9.2 43.8 N 78.6 83.7
4 9.4 96.1 N 357.7 76.7
7 7.2 70.5 N 92.6 66.4
7 4.8 158.4 T 296.9 60.3
6 3.7 327.6 T 299.6 57.3
8 8.6 43 R 320.8 -86.8
6 7.9 74 R 81.7 -65.9
8 8.2 47 R 296.3 -84.5


4.8 162 T
6.8 98 T
4.7 108.8 N
7.8 35.1 N
2.5 389.1 R
4.8 93 T
7.8 61 N
6.4 77 N
9.5 51 R
7.1 117 R
4.9 131 R
3.4 316 R


SSC


53 19.32 -98.74 176.7 -27.9







66


Table 4-3 (continued)
TMVB Site Lat Long Dec. Inc. N a95 K P VGP VGP R
Area Long Lat
TCHR Ceboruco 21.14 -104.50 360 36.6 7 3.2 361 N 75.5 89.2 4
TOL 17 19.16 -99.81 181.4 -16.1 8 4.8 0 R 252.9 -79.0 1
TOL 18 19.17 -99.81 177.7 -16.7 5 2.5 968 R 272.6 -79.1 1

aAreas in the TMVB are: Altiplano Area (AA), Jalisco Block (JB), Los Tuxtlas Volcanic Field (LT),
Michuacan-Guanajuato Volcanic Field (MGVF), Sierra de las Cruces (SC), Sierra Chichinautiz (SCH),
Sierra Nevada (NS), Sierra Rio Frio (SRF), Sierra Santa Catalina (SSC), Tepic-Chapala Rift (TCHR) and
Nevado de Toluca (TOL). Site is the nomenclature of the site as it appears in the compiled study; Dec, Inc,
are the site-mean declination and inclination; N is the number of samples used to calculate the site-mean
direction, c95 is the 95% confidence cone about the mean direction, K is the dispersion paramenter; P is the
polarity: normal (N), reverse (R) or transitional (T); R is the reference used. References are: (1) This study,
(2) Bohnel (unpublished data, 2005); numbers in parenthesis indicate the correspondent site-identifications
as they appear in the preliminary study by Bohnel and Negendank (1981), (3) Alva-Valdivia et al. (2001),
(4) Bohnel and Molina-Garza (2002), (6) Herrero-Bervera et al. (1986), (7) Herrero-Bervera and Pal,
(1977), (8) Maillol et al. (1997), (9) Mooser et al. (1974), (10) Morales et al. (2001), (12) Osete et al.
(2000), (13) Ruiz-Martinez et al. (2000), (15) Steele (1985) and Steele (1971).
















Table 4-4. Statistics of paleomagnetic data of late Pliocene to Recent age from studies in the western TMVBa
Group of Sites Ref Dec Inc N Dir s195 Dir k Fish O.G. O.Q AD St
Bohnel and Molina-Garza (2002) 4 3 38.2 7 9.4 42 yes yes yes 3 13.0
Delgado-Granados etal. (1995) 5 348.6 28.3 15 9.4 17 yes no no -11.4 20.8
Maillol et al. (1997) 8 356.3 37.4 14 9.9 17 no yes yes -3.7 15.8
Nieto-Obregon et al. (1992) 11 354.4 42.6 9 8.6 37 yes yes no -5.6 15.1
Soler-Arechalde and Urrutia-Fucugauchi (2000) 14 349 36.4 4 19.7 23 yes yes yes -11 19.1
Uribe and Urrutia (1999) 16 354.5 39.6 8 8.9 9 yes yes yes -5.5 15.1
Urrutia-Fucugauchi and Rosas-Elguera (1994) 17 344.9 30.6 5 6.6 137 yes no no -15.1 17.6

All Sites with AD > 50 (N + R) 350.4 34.8 41 4.6 25 yes no no -9.6 17.2

All Sites with AD > 50(N) 351.7 35.2 30 5.5 24 no no no -8.3 16.9
All Sites with AD > 50(R) 167 -33.8 11 9.3 25 yes no no -13 18.6
a Ref. is a reference number used in Figure 4-2. The meaning of the remaining columns is as indicated in Table 4-2.



Table 4-5. Statistics of late Pliocene to Holocene age results from compiled dataa
Dir Dir Dir VGP VGP VGP K VGP A A
Group of Sites Dec Inc N s95 k Fish Long Lat 095 VGP Fish O.G. O.Q O.A. Inc Dec St Sl Su
Selected Data (N + R) 358.8 31.6 187 2 29 no 119.7 87.8 1.6 42 no no yes no -3.5 -1.2 12.73 11.9 14.1
Selected Data (N) 359 30.7 144 2.2 29 no 107.6 87.3 1.8 41 no no yes no -4.4 -1.0 12.95 12.0 14.1
Selected Data (R) 178 -34.5 43 4.1 29 no 11.8 -88 3.2 46 no yes yes yes 0.6 -2.0 12.11 10.7 13.9
aAbbreviations of most columns as in Table 4-2. Fish indicates whether the site directions or VGPs have a fisherian distribution.














CHAPTER 5
PALEOINTENSITY

Previous chapters have dealt with paleomagnetic directional data (i.e. declination

and inclination) from lava flows. In this chapter paleomagnetic intensity results obtained

from some samples of the same lava flows plus other samples described below are

presented. In addition to directions, paleointensity values are necessary to fully define the

magnetic vector at a paleomagnetic site. Obtaining paleointensity data from lava flows

has been rather unsuccessful in most studies and the success rate is usually around 20%.

Several laboratory procedures have been designed to obtain paleointensity data.

The Thellier method (Thellier and Thellier, 1959) as modified by Coe (1967) has been

the most commonly applied. These methods rely on the assumption that NRM acquired

during cooling of the igneous rock is proportional to the intensity of the Earth's magnetic

field in which they cool. In principle, these methods are essentially simple: they consist in

comparing the NRM lost through thermal demagnetization with a TRM acquired at the

same temperature in a known magnetic field applied in the laboratory. The results of

these experiments are therefore regarded as "absolute". However other assumptions and

conditions on which these methods rely are difficult to meet which result in a rather low

rate of success of the experiment.

One of the conditions of the Thellier method (and its modifications) is that the

magnetic characteristics of the rock don't change significantly after the NRM acquisition.

It is often the case that mineralogic changes take place in-situ through the geologic time

as well as during heating of the samples in the laboratory, and therefore the rocks no









longer magnetize proportionally to the field in which they were initially magnetized in

nature. When heating in air, oxidation of the magnetic carriers can occur, leading to an

increase in the capacity of the sample to magnetize. To get around the problem of

alteration during heating of the samples, one of the primary controls devised in the

Thellier method, is to measure the NRM lost and the TRM acquired in the laboratory

field (pTRM) at several increasing temperature steps. This allows using the portion of

lower unblocking temperatures that are free of mineralogic changes (for example titano-

magnetites, depending on their composition, can oxidize at temperatures as low as 3000

C). If no mineralogic changes of the magnetic carriers occur during heating the relation

between NRM lost and pTRM gained at each temperature step is expected to be linear.

When chemical alteration occurs during heating this relationship is no longer linear.

Flattening of the corresponding plot (Arai plot) is often observed when oxidation takes

place. One method to detect chemical alteration is to perform the so called pTRM checks

(Thellier and Thellier, 1959) which consists on performing additional measurement of

pTRM after having taken the sample to a higher temperature. If chemical alteration

occurs at higher temperatures the pTRM from the pTRM check should be significantly

different from the pTRM initially obtained. Additionally, as found by Dunlop et al.

(2005), pTRM check can also be unsuccessful when using multidomain (MD) materials,

even if no chemical alteration takes place. When MD materials are studied, interpreting

the steep portion of a curved Arai Plot with successful pTRM checks, that is followed by

a shallow portion of the curve with unsuccessful pTRM checks, as a linear segment

suitable for paleointensity calculations leads to misleading results. Therefore, in practice,

two slope or curved Arai plots (due to chemical alteration or MD behavior, respectively)









are difficult to distinguish from each other and paleointensity determination should not be

made in these cases.

Because of the need of several temperature steps in the Thellier experiment and its

modifications, a second condition for the reliability of the results is that the blocking

temperature spectrum coincides with the unblocking temperature spectrum, this condition

is called reciprocity. This condition is met in the case of rocks with single domain (SD)

magnetic carriers, but it is not met in the case of pseudo single-domain (PSD) and multi-

domain (MD) minerals and the resulting Arai plots are usually concaved down. Ideally a

paleointensity result could be derived from the NRM pTRM of the highest and lowest

temperature steps, however chemical alteration often occurs at high temperatures.

The lava flows studied here have been formed under sub-aerial conditions. Sub-

aerial lava flows are relatively suitable for paleointensity studies. The process of deuteric

oxidation usually occurs when cooling during the formation of the lava flows is a key

process by which titano-magnetites become more stable magnetic carriers. Submarine

basalts that are formed virtually without deuteric oxidation, are rather unsuitable for

paleointensity studies. An exception to this statement is the glassy portion of submarine

basalts (SBG) which as a result of disequilibrium during quenching have very low Ti

titanomagnetites (Zhou et al., 2000) and are therefore excellent materials for

paleointensity studies. In addition to lava flows, paleointensity results obtained from

obsidians are presented. Such material to my knowledge has not been used previously for

paleointensity analysis.

Laboratory Work

Paleointensity experiments were run in the paleomagnetic laboratory at the

University of Florida in a shielded room, using a MMTD80 thermal demagnetizer special









for paleointensity experiments. Measurements were made in a 2G Cryogenic

magnetometer. Two sets of 36 and 31 samples, respectively, were chosen from the set of

samples from British Columbia, Patagonia and Mexico. In addition to those, other

samples run were: some belonging to the set of samples from Australia collected and

studied by Opdyke and Musgrave (2004). Other samples were not oriented obsidians

most of them from New Mexico and Arizona donated by Steve Shackley, lava flows

donated by Jeff Gee for inter-laboratory comparison of results, and recently extruded

basaltic flows from Hawaii collected by Dr. Neil Opdyke. Replicates of samples

previously run for directional studies, were selected based on their stability of remanence

in an attempt to improve the success rate of the experiment.

One set of samples was run using the Thellier's method as modified by Coe (1967),

and the second set of samples was run using an additional modification to the Coe

method proposed by Aitken et al. (1988). The Coe method consists of a series of stepwise

double heating and cooling under 0 field and known field conditions (in this case 50 [tT).

The method also involves performing pTRM checks. The modification to the Coe method

by Aitken et al. (1988) consists in running the inn-field step prior to the off-field step. In

theory the Aitken method has the advantage over the Coe method of allowing the

detection of mineralogic alteration. The temperature steps used to apply the Thellier

experiment were determined based on the behavior observed during thermal

demagnetization for initial directional analysis.

Data Analysis

Arai plots and Zijderveld plots from the off-field steps of the Thellier experiment

were made in order to interpret the results. The analysis of the Arai plots in conjunction

with the zijderveld plots is important to ensure that the segment of unblocking









temperatures used to calculate a paleiontensity value trends to the origin. The analysis of

Zijderveld plots is also important to observe any changes in the direction caused by

magnetomineralogic changes during heating.

Paleointensity data was interpreted from some samples that displayed linearity in a

portion of the Arai plot and had good pTRM checks. However no interpretation was

made in the cases of curved Arai plots (suggesting MD magnetic particles) or scattered

data. There is, however, a lack of consensus about the standards of acceptance of

paleointensity results. The following paragraphs explain quality criteria that have been

used by other authors and that were taken into consideration to analyze the data presented

in this chapter.

Linearity of the Arai Plot

A way of assessing linearity of the NRM-TRM segment chosen for paleointensity

calculation established by Coe (1978), is that the slope of the segment chosen for

paloeintensity calculations can not differ by more than 20% with the slope of any

subsegment that is at least half of the segment chosen for paleointensity calculations.

PTRM Checks

One of the main quality criteria is the coincidence of pTRM checks (Coe, 1967)

with the initial measure of pTRM. Coe (1967) did not set up any percentage of difference

between the two values that could be "significant", thus leaving this matter to a judgment

that one could call "by eye". Pick and Tauxe (1993) using SBG established that pTRM

checks should coincide within 5% of the initially measured pTRM. However this

standard is rarely met when using materials such as lava flows. A variety of alternatives

have been implemented to analyze pTRM checks (Juarez and Tauxe (2000), Selkin and

Tauxe (2000) and Tauxe and Staudigel (2004)). In this study the approach by Tauxe and









Staudigel (2004) was used, in part because it appears to be more practical. These authors

defined a parameter called "DRATS" (pTRM difference ratio sum) which is the sum of

the differences between the initially measured pTRM and the pTRM checks, normalized

by the maximum pTRM used for paleointensity calculations. By doing this, the negative

and positive differences between initial pTRM and pTRM checks tend canceled out,

which according to Tauxe et al. (2004) is advantageous because it gives relevance to

trends indicating mineralogic changes rather than to scatter brought about from

experimental conditions.

Quality Factors Established by Coe (1978)

Other quality factors established by Coe (1978) were the standard error of the slope

in the Arai plot (ob), the fraction (f) of the total (extrapolated) NRM that is used for

paleointensity calculations (AY/a), the gap factor (g) that quantifies how evenly

distributed are the NRM values in the slope, and a quality factor (q) that combines the

previous three factors (q = b f g / Ob). Like in the case of pTRM checks, the need of

establishing cut-off values is common to all the quality criteria.

Results and Discussion

Results from data analysis are shown in Tables 5-1 and 5-2. Results with curved or

two slope Arai plots were rejected. The quality criteria mentioned above were taken into

consideration to analyze the data. As selection criteria, cut-off values for the q factor (that

combines all the Coe (1978) quality factors) and DRATS (that measure the success of

pTRM checks) were used. For the calculation of DRATS in the set of samples No 2, the

second pTRM checks at 4000 and 500 C were not taken into consideration because they

cover unblocking temperatures ranges accounted for in other pTRM checks. The cut-off









value of the quality factor (q) used, was > 1. This cut-off value was used by.Selkin and

Tauxe (2000). The maximum value of"DRATS" considered as a successful result was

25%. This same value was used by Tauxe and Staudigel (2004) and even a higher value

of 30% was used by Tauxe et al. (2004). Additionally results from one sample (14-1-4)

were discarded because the fraction of the NRM used for the calculation of paleointensity

(f) is very low (0.21). Results from 5 samples from the first set of samples and 5 samples

from the second set of samples met the selection criteria and were considered successful

(Figures 5-1 to 5-4). As indicated in Tables 5-1 and 5-2 some results were discarded

because they did not meet the quality criteria or because, despite having successful

quality values, they have slightly concaved curves.

Most of the successful paleointensity results obtained are from British Columbia

samples. Only one successful paleointensity results was obtained from Patagonia and one

successful paleointensity result was obtained from an obsidian samples. No successful

results were obtained from samples from Mexico, Australia or Hawaii. Among the

successful results, there are two pairs of samples from the same paleomagnetic sites (1-2

and 25-2) which have similar paleointensity results (Tables 5-1 and 5-2, and Figures 5-1

and 5-2).

Examples of unsuccessful paleointensity results are given in Figures 5-5 and 5-6.

Figure 5-5 shows rejected results from samples with slightly curved Arai plots which,

nevertheless, have acceptable quality factors. Figure 5-6 shows rejected results of

samples from site 25-3. Sample 25-3-8 is a good example of two slope curve that appear

to have an interpretable segment of the Arai plot with successful pTRM checks. Based on

the results by Dunlop et al. (2005), such results are consistent with both sample alteration









after a certain temperature or with MD or PSD behavior. Results from sample 25-3-5

were also rejected because of unsuccessful pTRM checks.

Consistent with the outcome of this study, great differences in success rate of

paleointensity studies using the Thellier method are observed among different study

areas. For example, no success was obtained at all from samples from Iceland (Jeff Gee,

personal communication), while Tauxe et al. (2004) obtained successful results from 64

out of 95 (67%) specimens analyzed from Antarctica, and Valet et al. (1998) obtained 35

out of 69 (51%) from lavas < 35 ka from Hawaii.

Regardless of the apparent success from an individual Thellier experiment, it is

striking to often encounter in the literature standard deviations above 20% among results

from the same flow. A paper by Tarduno and Smimov (2004) sheds a great deal of doubt

about results from the Thellier experiments, arguing that very low paleointensity values

(under 4 x1022 A/m2) in data compilations (e.g. Selkin and Tauxe, 2000) should not be

obtained as frequently because they are typical of unstable states of the Earth's magnetic

field (e.g., Guyodo and Valet, 1999). Tarduno and Smirnov (2004) state that lava flows

as well as SBG could be slightly weathered and the weathering products acquire a CRM

at low temperatures during the Thellier experiment, producing misleading results.

However records of relative paleointensity from sediments might not provide the real

range of paleointensity variation of the Earth's field because they record time-averaged

paleointensity values.

Despite the apparent simplicity of the Thellier method, achieving good results has

proven to be very difficult and a matter of controversy. In principle this experiment is

only applicable in the case of rocks with single domain grains (e.g. Dunlop et al., 2005).






76


The extent errors caused when applying the method in rocks with PSD has not been

determined yet and in my opinion more studies are needed.

















Table 5-1. Paleointensity results from the first set of samples


Sample Latitude Longitude Area N


PT10-9
10-1-3
1-1-2
1-2-2
13-1-3
14-1-5
15-1-3
20-1-9
20-2-2
21-2-4
22-1-2
2-2-2
25-10-5
25-1-5
25-2-5
25-3-5
25-4-3
25-5-6
25-6-6
25-7-6
25-8-4
25-9-9
26-1-6
26-2-4
26-3-7
PT30-1
PT36-5
PT46-1
PT49-3


-51.85
51.55
50.68
50.65
51.59
51.59
51.59
51.97
51.97
51.95
51.93
51.62
51.73
51.73
51.73
51.73
51.73
51.73
51.73
51.73
51.73
51.73
51.68
51.68
51.68
-50.55
-50.32
-49.51
-50.01


T1 T2
(C) (C)


ob Ob/B f g q DRATS F VADM S Reason for


(%) (pT) (1022Am2)


-70.52 PT -- -- -- -- -- -- -
-126.35 BC 9 400 540 0.0411 0.0886 0.87 0.86 8.4 12.0
-123.48 BC ----- ----- ----- ----
-123.44 BC 5 500 550 0.0258 0.0221 0.60 0.68 18.5 12.3
-126.43 BC --- ---- ---- ---- ---- ---- --- --- ---
-126.43 BC 10 400 550 0.0143 0.0279 0.53 0.87 16.6 3.4
-126.43 BC 6 500 550 0.6820 0.6727 0.48 0.76 0.5 1.5
-120.13 BC ----- ----- ----- -----
-120.13 BC --- ---- ---- ---- ---- --- ---- -- --
-120.08 BC ----- ----- ----- -----
-120.03 BC 10 350 550 0.1103 0.0569 0.64 0.88 9.9 62.2
-126.62 BC - - -
-120.01 BC 13 300 550 0.0384 0.0946 0.85 0.90 8.1 32.5
-120.01 BC - - -
-120.01 BC 10 400 550 0.0262 0.0365 0.56 0.87 13.4 25.2
-120.01 BC 10 400 550 0.0362 0.0521 0.43 0.88 7.3 37.6
-120.01 BC - - -
-120.01 BC 8 400 530 0.0208 0.0199 0.39 0.85 16.6 55.4
-120.01 BC 8 400 530 0.0717 0.0650 0.43 0.84 5.6 8.8
-120.01 BC 10 400 550 0.0161 0.0226 0.75 0.84 28.3 25.0
-120.01 BC 8 400 530 0.0376 0.0506 0.57 0.90 10.2 21.2
-120.01 BC 11 350 550 0.0364 0.0711 0.66 0.93 8.7 41.7
-120.05 BC ----- ----- ----- ----
-120.05 BC ----- ----- ----- ----
-120.05 BC - - -
-71.65 PT -- -- -- -- -- -- -
-71.22 PT -- -- -- -- -- -- -
-72.13 PT -- -- -- -- -- -- -
-71.87 PT -- -- -- -- -- -- -


9.0 x


Lack of Success
Chaotic Plot
Concaved Plot
Insuficient data


Concaved Plot

Low q value
Chaotic Plot
Chaotic Plot
Concaved Plot
pTRM check
Chaotic Plot
pTRM check
Concaved Plot

pTRM check
Chaotic Plot
pTRM check
Concaved Plot



pTRM check
Chaotic Plot
Concaved Plot
Chaotic Plot
Concaved Plot
Concaved Plot
Concaved Plot
Concaved Plot














Table 5-1. Continued
Sample Latitude Longitude Area N T1 T2 ob b /B f g q DRATS F VADM S Reason for
(oC) (oC) (%) (pT) (1022Am2) Lack of Success
5-1-7 51.62 -126.60 BC -- ---- --- --- --- ---- --- ----- -- ----- Chaotic Plot
PT6-2 -51.88 -70.66 PT ---- --- --- --- ---- --- --- --- ---- --- -Concaved Plot
7-1-3 51.59 -126.45 BC --- --- --- ---- --- --- --- --- ---- --- --- Chaotic Plot
8-1-5 51.59 -126.45 BC ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- ----- Concaved Plot
9-1-4 51.61 -126.40 BC 10 400 550 0.0663 0.0899 0.75 0.87 7.31 5.8 36.8 5.7 Concaved Plot
9-4-4 51.61 -126.40 BC --- ---- ---- ---- ---- ---- --- --- --- -- --- Concaved Plot
9-5-5 51.61 -126.40 BC --- --- --- --- --- --- --- --- ----- ----- ----- Concaved Plot
aSampling areas are British Columbia (BC) and Patagonia (PT). N is the number of temperature steps used to calculate the value of
paleointensity in the interval of temperatures T1 and T2 (C). Quality parameters defined by Coe et al. (1978) are: the standard error
of the slope (ob), the standard error of the slope divided by the slope (Gb/B), the fraction of the NRM used for calculation of
paleointensity (f), the gap factor (g) and the quality factor (q). DRATS is the pTRM difference ratio sum, F is the value of
paleointensity and VADM is the virtual axial dipole moment. Crosses under S indicate the samples considered to have successful
paleointensity results.

00


















Table 5-2. Paleointensity results from the second set of samples


Sample Lat. Long.


N T1 T2 Ob Cb/B
(C) (C)


f g q DRATS F


PT10-8 -51.9 -70.5 Patagonia ---- --- --- --- --- ----
12-10 19.3 -99.3 Mexico 15 200 560 0.04 0.07 0.68 0.88
1-2-7 50.7 -123.4 British Columbia 14 300 560 0.07 0.05 0.83 0.79
PT13-2 -51.8 -70.3 Patagonia ---- ---- ---- ---- ---- ---
13-2 19.3 -99.3 Mexico -- - -
1324.1 19.0 -99.0 Mexico* ---- --- ---- --- --- ---
14-1-4 51.6 -126.4 British Columbia 12 300 540 0.05 0.09 0.21 0.60
15-1-6 51.6 -126.4 British Columbia 13 100 530 0.32 0.43 0.63 0.90
1718.3 - - -
1718.7 - - -
188.1 ----- ----- New Mexico* ---- ---- ---- ---- --- ---
19-2 19.3 -99.4 Mexico -- - -
21.6 -37.6 144.0 Australia ---- --- ---- --- --- ---
23.1 ----- ----- Arizona* --- --- ---- --- --- --- --
23.8 -37.6 144.0 Australia ---- --- --- --- --- ----
25-2-4 51.7 -120.0 British Columbia 14 300 560 0.03 0.05 0.98 0.86
25-3-8 51.7 -120.0 British Columbia ---- ---- --- ---- --- ---
25-9-8 51.7 -120.0 British Columbia --- --- --- ---- --- --- --
PA3-105 -52.0 -69.9 Patagonia --- --- ---- --- --- --- --
315.1* 2.5 -76.7 Colombia ---- --- --- --- --- ----
315.2* 1.2 -77.7 Colombia -- - -
33.1* ----- ----- Arizona ---- --- --- --- --- ----
395.1* 36.6 -106 New Mexico 4 530 560 0.15 0.21 0.92 0.62
53.1* ----- ----- New Mexico --- --- ---- --- --- --- --
6-3 22.8 -101.9 Mexico --- --- --- ---- --- --- --
PT6-7 -51.9 -70.7 Patagonia 7 100 440 0.40 0.63 0.50 0.76
7-8 -37.6 144.0 Australia ---- ---- ---- --- --- ----
718.1* 37 -107 New Mexico 11 100 560 0.01 0.035 0.96 0.50
823.1 ----- ----- Hawaii --- --- --- --- ---- --- --
823.2 ----- ----- Hawaii --- --- --- ---- --- --- --
9-5 19.3 -99.3 Mexico 5 200 420 0.14 0.105 0.35 0.70
aColumn headers as in Table 5-2. Sample labels with asterisks are obsidians.


VADM S Reason for


(%) (pT) (1022 Am2)
----- ----- ----- -----
8.84 4.62 30.30 6.8
12.26 7.43 65.79 10.2


----- -----
6.54 30.13
15.12 37.07


18.45 8.46 33.72
18.45 8.46 33.72


2.71 34.76 34.72
2.71 34.76 34.72


----- -----
20.83 31.76

20.28 16.38
20.28 16.38


2.34 12.17 66.21
2.34 12.17 66.21


Lack of Success
Concave Plot
Concave Plot


Chaotic Plot
Concave Plot
Chaotic Plot
Low q


Concave Plot
Concave Plot
Concave Plot
Chaotic Plot
Concave Plot
Insufficient data
Concave Plot


Concave Plot
Concave Plot
Chaotic Plot
Unusual Plot
Unusual Plot
Concave Plot
pTRM check
Chaotic Plot
Concave Plot


Concave Plot


Chaotic Plot
Concave Plot
Concave Plot


















Sample 1-2-2 Brirtsh Columbia)


m= -1.17
Pl= 583 .T


Sample 1-2-7 (British Columbia)


m= -1.32
PI= 65,8 AT







\ 340


0 I 2 3 4
pTRM (A/m)


5 6 7


Figure 5-1. Successful paleointensity results of two samples from site 1-2 (British
Columbia). Figures a and c are Arai plots with the corresponding Zijderveld
plots to the right (Figures b and d). Temperature steps are in centigrade
degrees.


510
&b 520


4
pTRM A/mn)


N/Up









550








81



a) b)
Sample 2525.2

12

400 m=-0.72
5 PI= 35.8 p 550
10 450
475
500

8 L__ 510
520
400
530
0
6 540 E/Dn


550
4








0
0 2 4 8 10 12
pTRM (A/m)



c) Sample 25-2-4 (British Columbia)


1 100
12 a 300 m=-0.67 W/Up
a 400 PI= 33.7 pT

10 4460 N
480 560
5500
5410





4| S40
500
4 550


2 560


0
0 2 4 6 8 10 12 14 16
pTRM (A/m)



Figure 5-2. Successful paleointensity results of two samples from site 25-2 (British
Columbia). Figures a and c are Arai plots with the corresponding Zijderveld
plots to the right (Figures b and d). Temperature steps are in centigrade
degrees.













Sample 25-7-6 (British Columbia)


425
450
. 475


m= -0,71
Pl1 35.7 pT


W/Up














400

E/Dn


15
pTRM (A/m)


Sample 28-8-4 (British Columbia)


W/Up


m= -0.74
Pl= 37.2 T[


pTRM (A/m)


Figure 5-3. Successful paleointenity results from samples 25-7-6 (a) and 25-8-4 (b). Arai
plots are shown with the corresponding Zijderveld plots to the right.
Temperature steps are in centigrade degrees.









83




Sample 14-1-5 (British Columbia)


m= -0.51
PI = 25.6 iT


3-




2.5








2









10
0.5




0


0 0.5 I 1.5


2.5 3 3.5 4 4.5


pTRM (A/m)




Sample 718.1 (Grand Ridge, NM)


m = -0.33
Pl= 16.4 il














560


0.000 0.005 0.010 0.015 0.020 01.05 0.030
NRM (A/)



Figure 5-4. Successful paleointenity results from samples 14-1-5 (a) and 718 (b). Arai

plots are shown with the corresponding Zijderveld plots to the right.

Temperature steps are in centigrade degrees.


400
425
tL 450


550 E/Dn


O00
*, 530


0.00- 4


0.007

0,006



0.004-

0.003

0.002

0.001

0.000 -


()I













Sample 9-1-4 (British Columbia)

m= -0.74
PI= 36.8 pT


2 3 4 5
pTRAM (A/m)


6 7 8 9


Sample 12-1 (Mexico)


1.8 10


1.6

1.4 -

" 1.2 -

S 1.o-
t.-


300
350


m= -0.61
PI= 30.3 pT


E

560


540
550


pTRM (A/m)


Figure 5-5. Examples of rejected paleointensity results that had slightly curved Arai plots
but acceptable quality factors. Arai plots are shown with the corresponding
Zijderveld plots to the right. Temperature steps are in centigrade degrees.


400

- 425


475


N

550








400


0
E/Dn













Sample 25-3-8 (British Columbia)


100
200
300
350
400
420
440
460
480


m =-0.9
PI = 44.8 pT


0 540


0.5 1.0 1.5 2.0 2550
pTRM (A/m)




Sample 25-3-5 (British Columbia)


2.5



2 -



1.5



i 1-



0.5



0 i-
0 0.5


m = -0.7
PI= 34.8 pT


530


1.5 2 2.5


pT RM (A/m)



Figure 5-6. Examples of rejected paleointensity results from of samples from site 25-3.
The interpretation of results from sample 25-3-8 is ambiguous (see text). Arai
plots are shown with the corresponding Zijderveld plots to the right.
Temperature steps are in centigrade degrees.


N

550








E/Dn 0


560





3.0


N


550




400

0
E/Dn














CHAPTER 6
CONCLUSIONS

Several general aspects about secular variation and time-averaged field can be

concluded from the studies in British Columbia, Patagonia and Mexico presented in this

dissertation:

* None of the studies yielded persistent longitudinal components of the
paleomagnetic field which is consistent with recent studies (e.g., McElhinny et al.,
1997).

* Results from all of the studies are consistent with model G (McFadden et al., 1988)
of secular variation, which imply that secular variation was adequately sampled.

* The presence of a quadrupolar compontent is not clear in the results obtained in this
study since they are mostly at high latitude. The mean directions in the areas of
British Columbia and Patagonia (roughly at 50 N and 50 S latitude) coincide with
the geocentric axial dipole (GAD), but a quadrupolar component of the field in
these areas is difficult to discard because it is expected to produce only about 1
shallower inclinations. The mean direction in the area of Mexico coincides with a
GAD plus a 5% quadrupole, but the reverse polarity data coincides both with the
GAD and the GAD plus a 5% quadrupole and it is closer to GAD.

* The asymmetry between the northern and southern hemisphere of the present
magnetic field and particularly the 200 inclination anomaly in Patagonia, are not
observed in the paleomagnetic data obtained, implying that the present field
configuration is relatively recent. The present direction of the magnetic field in the
area of Patagonia is at an angle from GAD greater than the angle made with the
GAD by any of the successful results in that area. This observation supports the
idea that the present field could be in an anomalously noisy state favorable for
occurrence of a reversal or excursion of the field (Opdyke and Mejia, 2004).

Recommendations for Future Studies

There are volcanic areas on Earth where studies similar to those presented in this

dissertation can be made to better characterize the paleomagnetic field in the past 5 Myr.









From the experience gathered in this study, and after reviewing a substantial number of

articles, I would like to provide some pragmatic recommendations for future studies:

* A careful sampling plan must be made in which all available previous geologic
studies area taken into consideration. The support of a local geologist or
volcanologist is ideal.

* Samples must be taken in a wide area of the outcrop to avoid bias or complete lack
of success due to the effects of lightning, slightly rotated blocks, viscous remanent
magnetization, etc.

* Orientation of the samples in the field using sun compass should be done as much
as possible. The magnetic anomalies in some sites, often when affected by
lightning, are large enough to substantially deflect the magnetic compass.

* Sites should be located in the field as accurately as possible to facilitated
corroboration and the use of the data in future studies.

* Laboratory work should start with a pilot set of samples. Based on the results from
this initial set of samples, a plan to process the remaining samples must be made.
This procedure can help determine whether AF or thermal demagnetization is more
suitable for the particular paleomagnetic analysis and the degree of detail (i.e. the
number of demagnetization steps) as well as the ranges of unblocking temperatures
or fields that are more appropriate to apply. The application of numerous
demagnetization steps (eg., 16 steps), that make the laboratory work a lot more
time consuming, range from being unnecessary to definitive in order to obtain
success. For example, results from Antarctica using only NRM (Mankienen and
Cox, 1988) are not much different than the results obtained after demagnetization
of the same set of samples (Tauxe et al., 2004) On the other hand, the application of
numerous demagnetization steps in the study from Patagonia presented here, helped
produce more precise and successful results.

* Good age control is important. Making efforts to accompany the paleomagnetic
results with radiometric dates will enhance the possibility of using the results
obtained in future paleomagnetic studies (e.g. studies focusing on shorter periods
on time).

* Attempting paleointesity measurements will help to improve the paleointensity
dataset. The comparison of improved paleointensity dataset from lava flows with
paleointesity records from sediments could help to elucidate the validity of
paleointesity results.









Comparison with Recent Studies

In order to venture into what recent paleomagnetic studies are telling us about time-

averaged field and secular variation, a brief compilation of these results was made, which

is presented in Table 6-1. Figure 6-1 shows that the VGP scatter of these studies coincide

with Model G of paleosecular variation (McFadden et al., 1988). Figure 6-2 shows the

mean inclination of recent studies along with the inclination expected from the GAD and

the GAD plus a 5% quadrupole. As can be judged, it is difficult to ascertain if the data

fits better to a GAD plus a 5% quadrupole (a common percentage of quadrupole term

proposed in TAF studies). In the case of the mostly unpublished results from Ecuador by

Opdyke and others (Opdyke et al., 2004), the data strongly suggest the presence of the

quadrupole term. A further analysis of the data shows that adding an axial ocupole

component of 7% to the GAD plus 5 % quadrupole curve, helps increase the fit of the

data at around 300 to 400 latitude (Figure 6-3).