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Investigation of the History of Exhumation and Faulting in the Ruby Mountains Metamorphic Core Complex, Nevada

Permanent Link: http://ufdc.ufl.edu/UFE0022155/00001

Material Information

Title: Investigation of the History of Exhumation and Faulting in the Ruby Mountains Metamorphic Core Complex, Nevada
Physical Description: 1 online resource (61 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: apatite, basin, biotite, core, detachment, exhumation, fault, metamorphic, miocene, nevada, range, thermochronology
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: New 40Ar/39Ar and apatite fission-track data from the Ruby Mountains metamorphic core complex, in NE Nevada, helps to const ain the exhumation and cooling history of the area. Apatite fission-track data yielded a low-temperature history of exhumation along the west-rooted detachment fault flanking the range. Apatite results indicated rapid cooling in the Miocene and also provided apparent ages of samples along a transect perpendicular to the fault. The change in apatite apparent ages with respect to elevation suggest that rapid cooling started at approximately 15 Ma. Sampling perpendicular to the direction of fault slip allowed for the calculation of an average rate of slip along the fault of 9.1 to 3.1 km/my, assuming that the apparent ages reflect exhumation due to detachment slip. Biotite 40Ar/39Ar total fusion ages were determined from the same samples, and were also used to calculate a rate of slip along the fault. A rate of 1.6 to 2.2 km/my was calculated from the biotite 40Ar/39Ar data. Due to the suspicion that some of the biotite samples were in the biotite argon partial retention zone, which would cause these samples to appear older than samples closer to the fault due to the fact that the original argon was not lost, a rate of 2.4 to 0.6 km/my was recalculated using the younger biotite ages. The biotite 40 Ar/ 39 Ar data suggest that exhumation was accomplished mainly by tectonic denudation along a detachment fault rather than erosion. The vertical variation of fission-track ages alternately suggest that these data record erosional exhumation due to erosion following the uplift of the footwall block relative to the adjacent valley floors. New data from this study as well as previous data indicate that rapid tectonic exhumation started at ca. 24 Ma and continued until ca. 14 Ma.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Foster, David A.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022155:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022155/00001

Material Information

Title: Investigation of the History of Exhumation and Faulting in the Ruby Mountains Metamorphic Core Complex, Nevada
Physical Description: 1 online resource (61 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: apatite, basin, biotite, core, detachment, exhumation, fault, metamorphic, miocene, nevada, range, thermochronology
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: New 40Ar/39Ar and apatite fission-track data from the Ruby Mountains metamorphic core complex, in NE Nevada, helps to const ain the exhumation and cooling history of the area. Apatite fission-track data yielded a low-temperature history of exhumation along the west-rooted detachment fault flanking the range. Apatite results indicated rapid cooling in the Miocene and also provided apparent ages of samples along a transect perpendicular to the fault. The change in apatite apparent ages with respect to elevation suggest that rapid cooling started at approximately 15 Ma. Sampling perpendicular to the direction of fault slip allowed for the calculation of an average rate of slip along the fault of 9.1 to 3.1 km/my, assuming that the apparent ages reflect exhumation due to detachment slip. Biotite 40Ar/39Ar total fusion ages were determined from the same samples, and were also used to calculate a rate of slip along the fault. A rate of 1.6 to 2.2 km/my was calculated from the biotite 40Ar/39Ar data. Due to the suspicion that some of the biotite samples were in the biotite argon partial retention zone, which would cause these samples to appear older than samples closer to the fault due to the fact that the original argon was not lost, a rate of 2.4 to 0.6 km/my was recalculated using the younger biotite ages. The biotite 40 Ar/ 39 Ar data suggest that exhumation was accomplished mainly by tectonic denudation along a detachment fault rather than erosion. The vertical variation of fission-track ages alternately suggest that these data record erosional exhumation due to erosion following the uplift of the footwall block relative to the adjacent valley floors. New data from this study as well as previous data indicate that rapid tectonic exhumation started at ca. 24 Ma and continued until ca. 14 Ma.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Foster, David A.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022155:00001


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3cc7765563d13ce351ddb0380dccaf94c9dcef5e







INVESTIGATION OF THE HISTORY OF EXHUMATION AND FAULTING IN THE RUBY
MOUNTAINS METAMORPHIC CORE COMPLEX, NEVADA





















By

VIRGINIA NEWMAN


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2008


































2008 Virginia Newman

































To my husband, Lou and my daughters, Hannah and Rachel









ACKNOWLEDGMENTS

First and foremost, I would like to express my gratitude to Dr. David Foster, my advisor,

for his guidance and expertise during the process of this study. I also thank Dr. Keith Howard

for sharing his knowledge of the geology of Ruby Mountains as well as his assistance in the

field. I thank the many excellent professors I have had in the program, including Dr. Smith and

Dr. Randazzo, my field camp instructors. I appreciate Dr. Neuhoff and Dr. Meert, my committee

members, for their patience and assistance. I am extremely thankful to Mike Hartley, who

assisted with many aspects of the lab work, and to Jennifer Gifford, who contributed GIS

distances to this project. I would especially like to thank my husband for his financial support

and patience.

Field work for this project was funded by the USGS. USGS also funded a portion of the

fission-track analysis.









TABLE OF CONTENTS

page

A CK N O W LED G M EN T S ................................................................. ........... ............. .....

LIST O F TA BLE S ......... .... ........................................................................... 6

LIST OF FIGURES .................................. .. ..... ..... ................. .7

A B S T R A C T ......... ....................... .................. .......................... ................ .. 8

CHAPTER

1 INTRODUCTION ............... .............................. ............................. 10

2 G E O L O G IC SE T T IN G .......................................................................... ..........................16

Regional geologic background .............. ................ ......... ... ..................... ............... 16
Geologic History of the Ruby Mountains Metamorphic Core Complex............................. 17

3 M E T H O D S ............................................................................... 2 1

4 APATITE FISSION-TRACK RESULTS ........................................ ......................... 25

5 BIO TITE 40AR/39A R RE SULTS......... ........................................................ ............... 30

6 DISCU SSION ........... .... ..... ... ........... ....................... ........ 35

7 C O N C L U SIO N S ................. ......... ................................ .......... ........ ..... .... ...... .. 42

APPENDIX

A M E T H O D O L O G Y ...................................................................................... .....................4 3

B APATITE FISSION-TRACK AGE AND LENGTH DATA...............................................44

C MONTE TRAX APATITE FISSION-TRACK MODELS ............................................ 47

L IS T O F R E F E R E N C E S .................................................................................... .....................57

BIOGRAPHICAL SKETCH 61









LIST OF TABLES

Table page

3-1 Rock samples from the Ruby Mountains, Nevada.....................................................23

5-1 Biotite 40A r/39A r total fusion ages......................................................... ............... 32

B A patite fission-track age data............................................... .. .. ... ......................... 45

B-2 Apatite fission-track length data. .............................................. .............................. 46









LIST OF FIGURES


Figure page

1-1 Location map of the Ruby Mountains metamorphic core complex.............................. 14

1-2 Map of the Ruby Mountains metamorphic core complex region ................................ 15

3-1 Lamoille Valley thermochronology sample locations ....................................................24

4-1 Apatite apparent ages vs. sample elevation.. ........................................ ............... 28

4-2 Apatite apparent ages vs. distance from fault.......................... ......... .. ............ 29

5-1 B iotite apparent ages vs. elevation.......................................................... ............... 33

5-2 Biotite apparent ages vs. distance from fault........................... ...... ... ............ 34

6-1 Apatite and biotite apparent ages vs. distance from fault ..................... .........39

6-2 The younger biotite apparent ages vs. distance from fault.. ....................................40

6-3 Base of the biotite argon partial retention zone at ca. 24 Ma ...................... ........... 41

C-l Best fit M onte Trax model for H93 RUBY-4........................................ ............... 48

C-2 Best fit Monte Trax model for H93RUBY-5.................................. ...............49

C-3 Best fit M onte Trax model for H93RUBY-8.......................................... ............... 50

C-4 Best fit Monte Trax model for H97RUBY-41 ................................ .............51

C-5 Best fit M onte Trax model for H97RUBY-42....................................... ............... 52

C-6 Best fit Monte Trax model for H97RUBY-51 .............................................. ...............53

C-7 Best fit Monte Trax model for H97RUBY-53.............................................. ...............54

C-8 Best fit Monte Trax model for H97RUBY-54............................ ............... 55

C-9 Best fit M onte Trax model for H97RUBY-55............................................................. 56










Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

INVESTIGATION OF THE HISTORY OF EXHUMATION AND FAULTING IN THE RUBY
MOUNTAINS METAMORPHIC CORE COMPLEX, NEVADA


By

Virginia Newman

May 2008
Chair: David Foster
Major: Geology

New 40Ar/39Ar and apatite fission-track data from the Ruby Mountains metamorphic core

complex, in NE Nevada, helps to constrain the exhumation and cooling history of the area.

Apatite fission-track data yielded a low-temperature history of exhumation along the west-rooted

detachment fault flanking the range. Apatite results indicated rapid cooling in the Miocene and

also provided apparent ages of samples along a transect perpendicular to the fault. The change in

apatite apparent ages with respect to elevation suggest that rapid cooling started at approximately

15 Ma. Sampling perpendicular to the direction of fault slip allowed for the calculation of an

average rate of slip along the fault of 9.1 + 3.1 km/my, assuming that the apparent ages reflect

exhumation due to detachment slip. Biotite 40Ar/39Ar total fusion ages were determined from the

same samples, and were also used to calculate a rate of slip along the fault. A rate of 1.6 2.2

km/my was calculated from the biotite 40Ar/39Ar data. Due to the suspicion that some of the

biotite samples were in the biotite argon partial retention zone, which would cause these samples

to appear older than samples closer to the fault due to the fact that the original argon was not lost,

a rate of 2.4 0.6 km/my was recalculated using the younger biotite ages. The biotite 40 Ar/39 Ar

data suggest that exhumation was accomplished mainly by tectonic denudation along a









detachment fault rather than erosion. The vertical variation of fission-track ages alternately

suggest that these data record erosional exhumation due to erosion following the uplift of the

football block relative to the adjacent valley floors. New data from this study as well as

previous data indicate that rapid tectonic exhumation started at ca. 24 Ma and continued until ca.

14 Ma.









CHAPTER 1
INTRODUCTION

Metamorphic core complexes are the tectonically exhumed and highly extended footwalls

of low-angle detachment normal faults (Crittendon et al., 1978). While it is known that these

features form during continental extension, the processes that lead to core complex formation, as

opposed to more distributed extension, are not yet resolved (Buck et al., 1988). In the Basin and

Range province of the western United States, Tertiary metamorphic core complexes were

exhumed during regional extension possibly caused by hot spot movement (Best et al., 1991), the

growth of a subduction slab-gap (Sonder and Jones, 1999), a relaxation of boundary forces

(Sonder and Jones, 1999), or the orogenic collapse of overly thickened crust (Coney and Harms,

1984).

The timing of the onset of extension in many of the metamorphic core complexes in the

central and northern Basin and Range, south of the Snake River Plain, is poorly constrained.

Some place the onset of extension in the northern Basin and Range between 55 to 45 Ma (Sonder

and Jones, 1999; and McGrew et al., 1994). Others contend that most extension did not start

until the Miocene (Stockli et al., 2001; and Dokka et al., 1986). The confusion within the timing

of these events arises from the multiple episodes of extension that are overprinted in the area

(Dallmeyer et al., 1986; and Mueller et al., 1999). The issue is complicated further by the

possibility of distinctly different processes initiating and perpetuating extension at different

times. Some researchers believe there was an earlier period of extensional thinning of the ductile

middle crust followed by a later period of extension of the entire crust (Wells et al., 2000).

The Ruby Mountains, which are the focus of this study, are part of the southernmost extent

of the northern Basin and Range Province, located in northeast Nevada (Fig. 1). Previous

research in the Ruby Mountains Metamorphic Core Complex documented gradual cooling of









rocks that were at the upper middle crust in Late Cretaceous time, from 63 to 49 Ma (Mueller et

al., 1999). Data from the same study shows that deeper structural levels cooled rapidly between

36 to 29 Ma, with all rocks exposed in the core complex football being cooled at or below the

1000 C isotherm by ca. 23 Ma (Mueller et al., 1999). There is disagreement, however, about the

actual timing of rapid cooling. Dallmeyer et al. (1986) place the period of rapid cooling between

20 to 30 Ma, with all rocks cooled below 100C by ca. 20 Ma. Reese (1983) discusses two

distinct episodes of extension occurring in the Ruby Mountains football. The first occurred

between ca. 37 to 30 Ma, followed by a second between ca. 25 to 9 Ma after a tectonic hiatus.

Hodges et al. (1992) suggests that unroofing began during and immediately following Late

Cretaceous thrust burial, but most thermochronological data indicate major exhumation and

cooling in Oligocene and Miocene epochs (Mueller et al., 1999; Hurlow et al., 1991; Reese,

1983). Biotite K-Ar ages from the eastern Ruby Mountains record cooling below approximately

3000C between ca. 37.6 and 30.1 Ma, which could indicate Oligocene extension or be related to

an exhumed partial retention zone. The kinematics of detachment normal faults and timing of

cooling indicate that exhumation was derived by tectonic extension and that the football rocks in

the Ruby Mountains form a metamorphic core complex (Howard, 1980).

Rocks exposed in the Ruby Mountains were exhumed from depths of as much as 35 km

following burial by thrusting in the Late Cretaceous (McGrew et al., 2000). The metamorphic

core complex is flanked by a west-rooted, low-angle, mylonitic detachment fault that is overlain

by a more complex arrangement of normal and listric faults. The complex arrangement of high-

angle faults overlying low-angle detachment formed as the nonrigid football was exhumed from

beneath the hanging wall along a low angle detachment fault (Howard, 1980).









The current body of knowledge existing on the timing and rates of extension in the Ruby

Mountains was obtained through previous thermochronologic studies, using the K-Ar and fission

track methods (Dallmeyer et al., 1986; Dokka et al., 1986; Reese, 1983; McGrew et al., 1994).

The use of several thermochronologic systems together to study core complex footwalls allows

the reconstruction of a thermal history over a larger temperature range than one system alone

(Foster and John, 1999; Stockli, 2005). Each unique thermochronologic system can provide

information about the timing of the cooling of a rock body through the specific closing

temperature of the minerals in question and thereby the exhumation of football rocks from

different paleodepths. Additional information about the meaning of rocks cooling histories can

be obtained by integrating geologic and structural information obtained in the field.

Published K-Ar and 40Ar/39Ar apparent ages and apatite fission track apparent ages in the

Ruby Mountains core complex football young westward in the direction of extensional

unroofing for each mineral analyzed. Biotite 40Ar/39Ar apparent ages generally range from about

45 Ma to near ca. 20 Ma (Dallmeyer et al., 1986; Kistler et al., 1981; McGrew et al.,1994). If the

previous biotite K-Ar apparent ages represent rapid cooling through the argon retention isotherm

of about 300C, then the westward-younging pattern can be used to determine the rate of

unroofing and quenching of the core complex. The approximate rate of fault slip can be obtained

by inverting the rate of lateral change in cooling rate with distance (Foster et. al., 1993).

Inversion based on the previous data indicates a rate of slip on the detachment of 1 to 2 km/m.y.

A calculated rate of 1 to 2 km/m.y. indicates an onset to extension of around 30 to 25 Ma rather

than 50 to 45 Ma. This apparent slip rate is several times slower than the rates of detachment

slip in tectonically analogous areas, such as the Snake Range metamorphic core complex (Lee et

al., 1991) and the Colorado extensional corridor (Foster et al., 1993; Foster and John, 1999),









which have calculated rates of 7 to 14 km/m.y. The calculated rate of extension in the Ruby

Mountains is also an order of magnitude slower than the rates estimated from shear-strain fabrics

in the mylonitic shear zone that roofs the core complex, which suggests extension occurred at a

rate of 22 km/my (Hacker et al., 1990). The calculated rate also does not account for the peak

time of rapid quenching in the Ruby Mountains and regionally, which apparently occurred in the

middle Miocene, approximately 24 Ma (Dokka et al., 1986; Stockli et al., 2001). Furthermore,

the extension rate would have had to be many times faster to exhume the full 35 km if all the

extension occurred in one event. The resolution of this contradiction is a principle reason for the

study. The calculated rate of extension may also support or refute one or more of the previously

discussed hypotheses relating to possible causes of extension in the area.

I present new apatite fission track and 40Ar/39Ar thermochronometry that constrain the

timing of onset of extension and the rate of slip along the low angle detachment fault flanking

the Ruby Mountains metamorphic core complex. The data and conclusions generated by this

study compliment recent and ongoing thermochronologic studies in the Basin and Range

(McGrew et al., 2000; Colgin et al., 2006). These combined data have important implications for

the cause of widespread extension in the Basin and Range during Neogene times, because they

reveal the spatial distribution of the timing of extension and exhumation of a spectrum of crustal

levels.




































Figure 1-1. Location map of the Ruby Mountains metamorphic core complex, located in
northeast Nevada between the Antler orogenic belt and the Sevier orogenic belt.








I i


Iis "


)i I.







Figure 1-2. Map of the Ruby Mountains metamorphic core complex region, including the Ruby
Mountains- East Humboldt Range, Clover Hill, Wood Hills, and a portion of the
Pequop Mountains, outlined in the dashed red line. Gray areas represent ranges in the
area.









CHAPTER 2
GEOLOGIC SETTING

Regional geologic background

The Cretaceous Sevier Orogeny in NE Nevada was characterized by large scale thrusting

of miogeoclinal sediments, along with extensive ductile deformation, granitoid intrusion, and

metamorphism. By the end of the Sevier Orogeny the crust in central Nevada could have been as

thick as 60 to 70 km (Burchfiel et al., 1992). In Latest Cretaceous time the convergence rate of

the Faralon and North American plates increased and the dip of the subducting slab decreased

(Armstrong and Ward, 1991). This resulted in the migration of the magmatic arc to the east and

further crustal shortening. Rocks now exposed in the football of the Ruby Mountains core

complex represent the once deeply buried core of the Servier-Laramide Orogen. These rocks

underwent metamorphism, thrusting, nappe folding, and the intrusion of granitic rocks during

this event (Howard, 1980).

Laramide deformation continued into the early Cenozoic. At this time, the entire length

of the western edge of North America was a convergent plate boundary, with the North

American plate overriding the Faralon plate (Sonder and Jones, 1999). The dip of the subducting

slab steepened in the Nevadan part of the orogen about 40 Ma, shifting magmatism westward,

and shrinking the magmatic gap that had been caused by shallow subduction. A period of

intense magmatic activity, known as the ignimbrite flare-up, ensued in the Great Basin around 40

Ma (Armstrong and Ward, 1991). Magmatism gradually migrated west, as the dynamics of the

plate boundary changed (Sonder and Jones, 1999). The previously thickened lithosphere was

thermally weakened due to magmatism and/or asthenospheric upwelling (Armstrong and Ward,

1991). The convergent boundary was gradually replaced by right-lateral transform motion as the

Mendocino triple junction moved to the north. The switch from a convergent boundary to a









transform boundary resulted in the formation of the San Andreas fault system, the opening of the

Gulf of California, and a change in boundary forces acting on the plates. Large scale extension

in the northern Basin and Range (NBR) could have begun as early as the middle-late Eocene

(Sonder and Jones, 1999). Total extension across the Nevada segment of the Basin and Range

province has been estimated to be 120 to 150 km (Gans 1987; Wernicke 1992).

Geologic History of the Ruby Mountains Metamorphic Core Complex

The Ruby Mountains Metamorphic Core Complex is located east of the Paleozoic Antler

orogenic belt, and west of the Mesozoic Sevier belt (Howard, 1980) (Fig 1). Rocks exposed in

the East Humboldt Range, Clover Hill and Wood Hills are also part of the metamorphic core

complex football (Fig.2). The deepest structural levels exposed in the region lie in the East

Humboldt range and the Clover Hill area. Structurally shallower-level strata are exposed in

Wood Hills and the Pequop Mountains, to the east of the Ruby Mountains. The northernmost

exposure of rocks related to the core complex is located in the southern Windermere Hills

(Mueller et al, 1993).

Geologic elements in this area can be divided into four separate subsets. The deepest

structural levels are composed of metamorphic core complex rocks, including possible

Precambrian basement rocks, Late Proterozoic to Mid-Paleozoic metasedimentary rocks, and

Mesozoic and Tertiary igneous rocks. This suite of rocks comprises the migmatitic igneous and

metamorphic infrastructure of the complex. An approximately 1 km thick mylonitic shear zone

overlays and deforms the upper portion of the metamorphic and igneous infrastructure. The

mylonitic shear zone is structurally beneath a highly extended cover sequence of

unmetamorphosed to low-grade metamorphic Paleozoic through Tertiary strata (Mueller et al,

1993; MacCready, 1997). In the Northern Ruby Mountains and East Humboldt Range a low-

angle normal fault system separates the core complex from low-grade metamorphic rocks. In









more southerly portions of the complex these cover rocks are transitional into the metamorphic

rocks.

The focus of this study is on a cross-section through the center part of the range at

Lamoille Canyon, which is an excellent location to study the low-angle detachment system

because all of the structural elements are exposed. The cross-section is along a road to Lamoille

Canyon, which was carved by the Lamoille Canyon glacier. This glacier deposited moraines

approximately 12 miles long (Sharp, 1938), which are cut by fault scarps, indicating that brittle

faulting continued into the Holocene.

The mylonitic zone is exposed along the west range front. The mylonite transition zone is

approximately 1 km thick along the western flank of the mountain range. Features such as

asymmetric mica porphyroclasts and asymmetric parasitic folds within the micaceous mylonite

indicate a west-northwest sense of shear. Stratigraphic sequences in the mylonite zone have

been tectonically thinned as much as one-fifteenth to one-twentieth of the original thickness.

Metamorphic barometry studies indicate that Neoproterozoic strata were buried as much as 35

km (Hodges et. al, 1992). Deformation in the mylonite zone occurred at amphibolites and

greenschist facies conditions (Howard, 1980). The blastomylonitic fabric found in the transition

zone most likely resulted from extreme tectonic flattening and stretching. Rocks at the base of

the mylonite zone grade into the higher-grade, coarser-grained, nonmylonitic rocks of the

metamorphic infrastructure.

At deeper structural levels beneath the mylonite zone, granitic dikes, sills and irregular

bodies form more than half the exposed rocks. The granitic rocks are predominantly two-mica

granite, but granodiorite gneiss, biotite granite, and local bodies of metagabbro are also present.

Two-mica granites in the area are small discontinuous bodies so they generally are not mapped









separately from the metasediments that they intrude (MacCready et al., 1997). The exception is

the leucogranite orthogneiss of Thorpe Creek, which occurs as a sheet like body in the core of

the Lamoille Canyon Nappe. The granite in the area is predominantly Jurassic or Cretaceous in

age, and appears to be sourced from metasedimentary rocks at an intermediate depth, though

there is some Tertiary plutonic material as well. The percentage of granitic material increases in

relation to the percentage metamorphic materials toward the deeper structural levels of the

complex (Howard, 1980).

Metamorphic rocks in the football of the core complex include high-grade marble and

quartzite. The marbles and quartzites can be roughly correlated to the unmetamorphosed

Paleozoic shelf sediments in the hanging wall (Howard, 1971). The oldest stratigraphic unit is

the lower Cambrian Prospect Mountain Quartzite. The unit directly above the Prospect

Mountain Quartzite is a calc-silicate marble correlated with Cambrian limestones and shales and

the Ordovician Pogonip group. The Eureka Quartzite overlays the marble. Immediately above

the Eureka Quartzite is a massive, white, nearly pure dolomite unit. The uppermost unit is a

color-banded marble that correlates to the Guilmette Limestone.

Several large recumbent folds exist within the football of the core complex. In Lamoille

Canyon, the Lamoille nappe folds the premetamorphic Ogilvie Thrust, Prospect Mountain

Quartzite, and Verdi Peak Marble into an eastward overturned recumbent anticline that plunges

shallowly north (Howard, 1971). The recumbent folds may have been forged by diapers as the

migmatite front moved toward more shallow structural levels (Howard, 1980), or by progressive

shortening in the middle crust (Camilleri et al.1996; McCready et al., 1997).

Tertiary strata in valleys to the west of the Ruby Mountains include conglomerates with

limestone clasts that were presumably derived from limestone cover that was structurally above









the core complex (MacCready et al., 1997). Younger Tertiary strata contain metamorphic and

granitic clasts derived from the Ruby Mountains.









CHAPTER 3
METHODS

Twelve samples of amphibolite and granite were collected on a slip-orthogonal transect

representing progressively shallow to deep structural levels in the football of the low-angle

detachment fault flanking the Ruby Mountains (Table 1; Fig.4). Granitic samples were

separated for apatite, mica and potassium feldspar using standard density and magnetic methods

(Appendix A) during the fall of 2002 and spring of 2003. Amphibolite samples were processed

for mica, apatite, and amphibole.

Biotite samples were irradiated at Oregon State University and analyzed by the 40Ar/ 39Ar

method in the noble gas laboratory at the Department of Geological Sciences at the University of

Florida in the spring and summer of 2004. Biotite samples were fused in a water-cooled double-

vacuum furnace to extract argon gas from the samples. The biotite samples were fused due to

the tendency of biotite to become unstable during the step heating process and the likelihood the

fusion age would be the same as the plateau age (McDougall and Harrison, 1999).

Argon gas extracted from the samples during furnace heating was transferred through

vacuum lines and exposed to getters for ten minutes to remove reactive gases. The purified

argon gas was analyzed in a Mass Analyzer Products Model 215-50 mass spectrometer using a

Balzers electron multiplier to measure ion beams of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar. Blanks were

performed at the beginning of each day, and periodically between samples. Data were reduced

using the program ArArCALC version 2.2 by Koppers (2002). ArArCALC utilizes Excel by

Microsoft to calculate apparent ages, plot age plateaus, and isochrons. The ArArCALC program

was also used to calculate J-values from total fusion ages of the GA1550 biotite flux monitors.

These calculated J-values were then utilized in the calculation of 40Ar/39Ar total fusion age

calculations.









Fission-track analysis of apatite samples was performed by Ray Donelick at Apatite to

Zircon, Inc. during the spring and summer of 2003. Suitable grains were polished to an internal

surface that intersected fission-tracks from above and below the polished plane equally. The

fission-tracks were etched in 5.5 M HNO3 for 20 seconds at 210C. The resultant etched tracks

were then viewed using an unpolarized light microscope at 1562.5x or 2000x magnification from

grains free of large surface imperfections and possessing a minimum of inclusions and crystal

defects. The number of spontaneous fission-tracks counted over a selected area of the grain

divided by the area itself yields the spontaneous fission-track density. The external detector

method (EDM) was utilized to determine the relative uranium concentrations in the grains

selected for analysis. The external detector was a low-uranium, fission-track free muscovite

mica. The mica was placed adjacent to the apatite grain mount and both irradiated with thermal

neutrons in a nuclear reactor (Donelick, 2005). The induced fission-tracks on the mica were then

etched in 49% HF for 15 minutes at 230C. Apatite age and length data were forward modeled

using the Monte Trax program by Gallagher (1995), to estimate the thermal history of the

samples. MonteTrax was used in the genetic algorithm mode with 100 simulations of 250 runs

were used to calculate the best fit cooling lines included in Appendix C.









Table 3-1. Rock samples from the Ruby Mountains, Nevada


Sample number
H93 RBY-4
H93 RBY-5
H93 RBY-8
H97 RUBY-41
H97 RUBY-42

H97 LC-51
H97 LC-52

H97 RUBY-53
H97 RUBY-54

H97 RUBY-55


Rock type*
Tm, biotite granodiorite
Tum, amphibolite
Ocm Bio. Amphibolite
Kp, leucogranite
Kp, leucogranite
Tg bio-gar aplite,
folded
Kp,pegmatitic granite gneiss
CZp, micaceous
quartzite
Tm, biotite granite
mylonitic granite
gniess


El. (m)
3039
3017
2561
1895
2088

2981
2434

2310
2010


UTM N
4501690
4501735
4498710
4496400
4497130

4494900
4499050

4501000
4503740


UTM E
637380
637100
638150
644800
642225

638730
637900

635320
629685


1905 4505645 628630


Samples H93RBY-4, H97 LC-51, and H97 RUBY-54 are probably ca.29 intrusions. All
samples were collected on a cross-strike traverse approximately 20km long. Abbreviations
of rock units as indicated on Figure 3-1.
















625000 630000 635000 640000 645000
Sample Locations

Lithologic Units:

-4510000 Emd-Dolomite
Ezm-Quartzite
OEm-Marble
Oe-Eureka Quarizite
DOd-Dolomite
Dm-Marble
-4505000 Mzgd-Granodiorite Gneiss
Mzgm-Quartz Monzonite Gneiss
I M-Migmatite
Jg-Granite

4500000 Created by Virginia Newman
December 5, 2002
Geologic Map from
Geologic Map of
s | Ruby Mountains, Nevada
by Howard, Kistler, Snoke, Will
1975
,l -4495000

Projection and Grid:
SUTM zone 11
1983 NAD
N
625000 630000 635000 640000 645000
4 0 4 8 Miles W E


s







Figure 3-1. Lamoille Valley thermochronology sample locations









CHAPTER 4
APATITE FISSION-TRACK RESULTS

Apatite fission track data are useful for reconstructing low-temperature thermal histories of

rock masses due to the low blocking temperature (Gleadow and Brown, 1999; Gallagher et al.,

1998; Stockli, 2005). The closing temperature for fission-tracks in apatite is approximately

110C 10C for geologically rapid cooling rates (Gleadow and Duddy, 1981). In reality,

fission-tracks anneal progressively through a temperature interval called the partial annealing

zone (PAZ). The temperature range is from approximately 600C to 1100C for common

compositions of apatite, with annealing occurring more rapidly at the upper end of this range

(Gleadow, and Brown, 1999; Stockli, 2005). Fission-track dates may be combined with estimates

of the geothermal gradient to obtain information about the rate of exhumation and erosion. In

addition to obtaining the date at which a grain cooled through the closing temperature, much

information about the cooling history of a sample can be obtained by analyzing the distribution

of fission-track lengths.

Previous studies in the Ruby Mountains yield fairly consistent apatite fission-track dates.

A study by Dokka et al., (1986) showed a range in fission-track ages for titanite, zircon, and

apatite from 24 2 to 18 2 Ma from samples taken in the northern Ruby Mountains, north of

Lamoille Valley. These data suggested that rapid cooling started between ca. 25 and 23 Ma, and

that rocks had cooled from above approximately 3000C to below 700C between approximately

24 to 18 Ma. Previous apatite fission-track studies performed by the population method done in

Lamoille Valley yielded apatite ages of 12 5, 15 2, 18 2 Ma (Reese, 1983). The ages

increase from west to east along a transect through Lamoille Valley. Reese (1983) estimates

apparent rate of exhumation of 87 m/my or .087 km/my from these results. Unfortunately, the









population method is known to bias age estimates and no track length measurements were made

in this study, so the rate of cooling could not be assessed.

Samples collected for this study were obtained along a west to east transect through

Lamoille Valley, perpendicular to the detachment fault along the western flank of the range.

Samples were collected in this way to show the change in age as a function of distance from the

fault (Fig. 3-1). Apatite data obtained from 9 of 12 samples collected from the Lamoille Canyon

area is summarized in Appendix B. The samples yielded high quality apatite with abundant

tracks and gave reliable data. Apatite fission track ages ranged from 21.1 + 1.5 Ma to 14.4 + 1.9

Ma. All samples yielded unimodal track length distributions with mean lengths between about

14.1 to 14.9 microns. These long track lengths indicate rapid cooling rates and that the fission-

track ages reflect cooling through the partial annealing zone. The apparent ages of each sample

were plotted in ISOPLOT (Ludwig, 2004), in terms of elevation and in terms of distance from

the detachment to shed light on the possible causes and rates of exhumation (Figures4-1 and

4-2).

The graph of apparent apatite ages vs. sample elevations indicates rapid cooling at

approximately 15 Ma (Figure4-1), with the exception of samples H97 RUBY-41 and H97

RUBY-54. This relationship suggests a possible second episode of cooling at ca. 15 Ma. It

should be noted that sample H93 RBY-8 was severely weathered, showing signs of extensive

oxidation. Samples H93 RBY-4, H97 LC-51, and H97 RUBY-54 were collected from Tertiary

plutons, but this should not affect the results because the intrusion ages of the plutons are much

older than the timing of cooling and exhumation. The age of rapid cooling generally increases as

the distance from the fault increases, but the relationship is partially obscured by the large errors









(Figure4-2). This suggests that exhumation progressed from east to west due to movement along

the detachment.

Model cooling histories and track length distributions were calculated using Montetrax

(Appendix C). The mean fission-track lengths obtained from each sample ranged from 14.06 gm

to 14.86 gm. The long mean track lengths and unimodal track distributions indicate a

straightforward cooling history without the influence of any thermal anomalies (Gleadow et al.,

1986). The T-t models reveal rapid cooling through the PAZ (1100-600C) centered at the

apparent age of the samples. Calculated model track length distributions are consistent with the

observed data.



















2800


2400






2000


1600


H97Ruby-l

IH97Ruby-54


Apatite Apparent Ages (Ma)


Figure 4-1. Apatite apparent ages vs. sample elevation. This graph is a x-y plot constructed in
Isoplot. H97 RUBY-41 and H97 RUBY-54 are contained in the blue ellipse as
exceptions to the apparent cooling trend at ca. 15Ma. Apparent age errors are listed
in Appendix B, elevation errors are 20m. Error crosses are 20.


i


I


~I ~I ~I ~I ~I J ~I J ~I


I I





















0)


C.)


0l


I i I I I


Distance from Fault (km)

Figure 4-2. Apatite apparent ages vs. distance from fault. This figure was generated in Isoplot.
Data points were plotted in an x-y graph and the regression line was drawn using x-y
weighted averages. Apparent age errors are taken from Appendix B, distance errors
are .25km. The slope of the regression line is 0.11 + 0.32. Mean Squared Weighted
Deviates (MSWD) for the regression is 9.3. Error crosses are 20.









CHAPTER 5
BIOTITE 40AR/39AR RESULTS

Previous studies using the 40Ar/39Ar method on biotite in the Ruby Mountains core

complex produced predominately Oligocene and earliest Miocene ages. These results are

interpreted to indicate cooling below the argon closure in biotite or about 3000C (McDougall and

Harrison, 1999). Studies by McGrew et al. (1994) yielded 40Ar/39Ar mica ages between 21.7

0.2 Ma and 22.9 0.3 Ma. The same study found that rapid cooling from approximately 300C

to below 1000C occurred by 20 Ma. Dallmeyer et al. (1986) obtained argon ages from biotite

samples both in the mylonite zone and above it. Above the mylonite zone ages ranged from

approximately 32 to 33 Ma. Within the mylonite zone ages were younger, ranging from

approximately 22 to 24 Ma. Specifically in Lamoille Canyon, a biotite from non-mylonitic rock

produced an age of 25.3 + .7 Ma. A biotite from within the mylonite, in Lamoille Valley yielded

an age of 20.8 + .5 Ma. The authors postulate that rapid cooling began by approximately 45 Ma

and that the rocks had cooled to below 3000C by 20 Ma. Biotite data published Kistler et al.

(1981) and utilized by Reese (1983) from Lamoille Valley yielded ages from ca. 19.6 to 33 Ma,

going generally from youngest to oldest in a west to east fashion.

Samples for this study were collected along a slip orthogonal transect through Lamoille

Valley to avoid along-strike variations in cooling or exhumation history (Fig.3-1). This provides

for the determination of the change in age of samples as a function of distance from the exposed

trace of the detachment fault. Biotite was separated using standard density and magnetic

methods from the same samples used for apatite fission-track analysis (Appendix A).

40Ar/39Ar total fusion ages for the eight samples range from ca. 31.0 to 20.7 Ma (Table

5-1). The total fusion ages are in general agreement with dates obtained from previous studies

(Dallmeyer et al., 1986; McGrew et al., 1994,). The biotite ages were graphed in ISOPLOT









(Ludwig, 2004) against sample elevations and the distance of rocks sampled from the fault to aid

in determine the rate and cause of rapid extension in the area (Figs. 5-1 and 5-2).

The graph of biotite ages vs. sample elevation shows no clear relationship between biotite

apparent age and sample elevation (Fig. 5-1). The biotite apparent ages are older than the apatite

fission-track ages from the respective samples, due to the higher closure temperature of biotite,

approximately 3000 C vs. 1000 C. When biotite ages are graphed against distance from the fault

a trend does emerge (Fig. 5-2). The cooling ages generally increase with distance from the fault,

indicating that exhumation progressed from east to west along the detachment.









Table 5-1. Biotite 40Ar/39Ar total fusion ages


Sample number

H93 RBY-4
H93 RBY-8
H97 RUBY-42
H97 LC-51
H97 LC-52
H97 RUBY-53
H97 RUBY-54
H97 RUBY-55


Total fusion age
(Ma)
23.06
30.92
31.24
31.26
27.06
24.44
24.68
20.7


error+2o
(Ma)
1.04
1.75
1.75
1.53
1.17
0.83
2.16
0.98













29


27


25


<: 23


o 21


19
-2 2 6 10 14

Distance from Fault (km)
Figure 5-1. Biotite apparent ages vs. elevation. This graph was generated in Isoplot in a x-y
plot. Age errors are from Table2, elevation errors are taken to be 20m. Error crosses
are 20.












31


29


c, 27


25

ak 23


C 21


19
-2 2 6 10 14

Distance from Fault (kmn)

Figure 5-2. Biotite apparent ages vs. distance from fault. This figure was generated in Isoplot.
Data points were plotted in an x-y graph and the regression line was drawn using x-y
weighted averages. Apparent age errors are taken from Table 5-1, distance errors are
.25km. The slope of the regression line is 0.64 + 0.46. Mean Squared Weighted
Deviates (MSWD) for the regression is 16. Error crosses are 2o.









CHAPTER 6
DISCUSSION

The use of multiple isotopic systems and mineral phases for thermochronology research

allows the reconstruction of thermal histories over a greater temperature range than one system

alone (Foster and John, 1999; Stockli, 2005). Apatite fission-track data and thermal models

show that exposed rocks along the Lamoille transect in the Ruby Mountains metamorphic core

complex cooled below approximately 1000C between 21.1 + 1.5 Ma in the east and 14.4 + 1.9

Ma in the west (Appendices B and C). The early to middle Miocene cooling interval is in general

agreement with other studies (Dokka et al., 1986; Reese, 1983), but is more precise and allows

the progression of exhumation to be assessed.

Rapid cooling that progressed from east to west supports the hypothesis that the core

complex was exhumed due to slip along an asymmetric detachment fault. Previously estimated

rates of exhumation for this detachment system were generally 1 to 2 km/my (Reese, 1983;

McGrew, 1994). The rate of extension calculated from these data was more rapid than suggested

by previous studies that were based on fewer samples and more widely scattered sample

locations. Apparent apatite ages vs. distance from the fault were regressed in ISOPLOT (Fig.4-

2). The slope of the regression line generated by ISOPLOT is 0.11 0.32. By inverting the

slope and its error, the lateral rate of change, an approximate rate of 9.1 3.1 km/my is given.

This apparent slip rate is within a range of rates of detachment slip in tectonically analogous

areas, such as the Snake Range metamorphic core complex (Lee et al., 1991) and the Colorado

extensional corridor (Foster et al., 1993; Foster and John, 1999; Carter et al., 2006), where the

rates are approximately 7 to 14 km/my. The differences between the previous estimate by Reese,

1983 and the data from this study are probably due to the bias introduced by the older population

method. The slower rates may indicate different mechanisms for metamorphic core complex









development in the two areas, such as rolling hinge vs. low-angle detachment with two phases of

cooling.

A possible second phase of brittle faulting and rapid exhumation is indicated by the

relationship between apatite apparent ages and sample elevations (Fig. 4-1). H97Ruby-41 is the

easternmost sample along the transect and has been down-dropped by range-front faults,

explaining the older age of 18.1 0.9 Ma at the lower elevation of 1895 m and its plotting

outside of the rapid cooling trend. Sample H97Ruby-54 also plots outside this trend. With the

exception of these two data, samples below 2600 m elevation give concordant ages. This

suggests that rapid exhumation started at about 15 Ma due to erosion of the fault block.

Apatite fission-track and (U-Th)/He data published by Colgan et al., 2006 indicate that the

southern Ruby Mountains, in the Harrison Pass region, was rapidly exhumed between 14-15 Ma.

This data is an indication that the southern portion of the range was exhumed as an intact east-

tilted block. The fault system in the Harrison pass area merges in the north with the mylonite

zone that bounds the metamorphic core complex, indicating that rapid middle Miocene unroofing

could have occurred in the northern part of the range as well (Colgan et al., 2006)

Biotite apparent ages vs. distance from the fault were also regressed in ISOPLOT (Fig.

5-2). The slope and error of the biotite regression line, 0.64 0.46, was inverted and an apparent

slip rate of approximately 1.6 2.2 km/my was calculated. The west to east progression of

biotite ages from younger to older also supports progressive exhumation due to slip along the

detachment fault. The rate of extension calculated from these data was in agreement with

previously calculated rates from biotite data, which were in a range from 0.1 to 1.5 km/my

(Reese, 1983). The calculated rate based on the entire biotite 40Ar/39Ar data set is slower than

expected when compared to the apatite fission track data.









Biotite and apatite data were graphed together to investigate possible reasons for the

differences in the apparent slip rates calculated from each data set (Fig. 6-1). As expected,

apatite ages were younger than biotite ages due to the lower closing temperature of apatite.

However, it was expected that the biotite ages would be systematically older than the apatite

ages. Instead, the difference between apatite and biotite ages increased as the distance from the

fault increased. An explanation for the artificially high ages is that the older biotite ages, greater

than about 24 Ma, at a distance of greater than 10 km from the exposed detachment were

probably within the biotite argon partial retention interval prior to rapid extension starting at

about 24 Ma (e.g. Foster and John, 1999). New biotite and muscovite 40Ar/39Ar produced by

Jennifer Gifford at the University of Florida indicates two distinct episodes of extension and

cooling (Gifford,J., 2008). If only the younger biotite ages are regressed due to the mixed

cooling ages of the "older" samples, a rate of extension of 2.4 + 0.6 km/my is given (Fig. 6-2).

This rate is closer to the rate of slip calculated from the apatite data. These results are similar to

the results from the Chemehuevi detachment in California (John and Foster, 1993), but are still

slower than many detachment systems in the Basin and Range (Carter et al., 2004).

The significantly larger rate of slip along the detachment estimated from the apatite data

could indicate that isostatic rebound played a role in unroofing the core complex. Variable rates

of slip have been previously documented in the Harcuvar Mountains (Carter et al., 2004) and

along the Chemehuevi and Sacramento detachment faults (Carter et al., 2006). The large rate

may also indicate that some of the exhumation in this area was accomplished rapidly on brittle

faults in the middle Miocene, rather than recording only the rate of slip along the low-angle

detachment fault (Colgan et al., 2006).









The thermochronological data also reveal the time that rapid exhumation began and the

duration of extension. All of the apatite fission-track ages have long track lengths with unimodal

distributions, so they all reveal rapid cooling that must have started before ca. 20 Ma, the

apparent age of the oldest sample. The biotite 40Ar/39Ar dates for samples greater than 10 km

from the fault give apparent ages substantially older than the apatite fission-track ages, therefore,

these record slower cooling. The biotite 40Ar/39Ar ages at approximately 10 km or less from the

exposed fault are centered within error or a few m.y. older than the apatite fission-track ages.

The oldest biotite age from the samples taken less than 10 km from the fault is ca. 24 Ma. A

time of approximately 24 Ma is also at the transition in slope of the line between the ca. 30 Ma

and the ca. 24 Ma biotites (Fig.6-3), which would record the 3000 C isotherm prior to rapid

extension (Foster and John, 1999). This also suggests that cooling initiated at ca. 24 Ma. This

conclusion is consistent with previous studies (Dallmeyer et al., 1986; Dokka et al., 1986,

McGrew, 1994), as well as new unpublished data obtained in the area (Gifford, J., 2008).













30


26


a 22


o0 18



S-14
4-4
10

-6 -2 2 6 10 14 18 22

Distance from Fault (km)

Figure 6-1. Apatite and biotite apparent ages vs. distance from fault. This graph was generated
in ISOPLOT using the same methods as the individual data sets. Apatite data is
graphed as red error crosses, biotite is graphed as green error crosses. The slope of
the regression line on the younger four biotite apparent ages is 0.4 1.7. The slope of
the regression line on the apatite apparent ages is 0.11 0.32. Error crosses are 20.



















(U
bo
1<


C




.l..


Distance from Fault (km)

Figure 6-2. The younger biotite apparent ages vs. distance from fault. The graph was generated
in ISOPLOT using the same methods as for the entire biotite data set. The slope of
the regression line is 0.4 + 1.7. The Mean Squared Weighted Deviates (MSWD) of
the regression is 12. Error crosses are 20.
















0 30






26















-6 -2 2 6 10 14

Distance from Fault (km)


Figure 6-3. Base of the biotite argon partial retention zone at ca. 24 Ma. This graph was
generated in ISOPLOT by the same method as the other graphs. The trend lines were
also generated in ISOPLOT. The transition line between the two trend lines was
drawn in Illustrator to clarify where the base of the biotite argon partial retention zone
is located. The ca. 24 Ma line, black, was created to show the change in slope
between the Illustrator line and the ISOPLOT trend line. Younger biotite apparent
ages are represented by red error crosses, older biotite apparent ages are represented
by green error crosses, and the teal error cross represents the biotite that plotted
within the biotite argon partial retention zone. Error crosses are 20.









CHAPTER 7
CONCLUSIONS

The data obtained in this study are consistent with the hypotheses that rocks exposed in

the Ruby Mountains metamorphic core complex were rapidly exhumed by a detachment fault. A

possible second phase of brittle faulting and erosional exhumation at approximately 15 Ma is

suggested by the apatite fission-track data. These thermochronologic data indicate that rapid

exhumation took place during Miocene time, with an onset of approximately 24Ma. The apatite

fission-track data suggest a rate of slip on the detachment of approximately 9.1 3.1 km/my,

which may record low angle faulting and subsequent brittle faulting. An apparently slower rate

of 2.4 0.6 km/my was calculated from the quenched biotite cooling ages.









APPENDIX A
METHODOLOGY

The samples utilized in this study were crushed at the U.S. Geological Survey. They

were then washed to float the dust off and sieved in 60 to 120 mesh sieves. The 60 and 120

mesh fraction was used for thermochronology preparation. The samples were passed through

pure tetrabromoethylene (TBE), specific gravity 2.96, to separate quartz, feldspars and other

light minerals from heavier mafic minerals. The sinks were sent through the Frantz

magnetometer to separate the magnetic minerals from the nonmagnetic. The magnetic fractions

contained predominantly amphibole and mica. The nonmagnetic fractions primarily contained

apatite and zircon. The nonmagnetic fractions were then passed through pure methyleneiodide

(MI), specific gravity 3.32, to separate apatite from zircon. Following inspection under the

binocular microscope the apatite separates were sent to Apatite to Zircon, Inc. for fission-track

analysis. The magnetic fractions were inspected under a binocular microscope and picked when

necessary for biotite, muscovite in one case, and amphibole. The original TBE floats were sent

through a dilute solution of TBE in order to separate potassium feldspars from plagioclase.

Additional field work was done in September, 2004.

Apatite fission-track data and 40Ar/39Ar thermochronology data were utilized to constrain

the timing of the onset of slippage along the detachment fault as well as the time averaged rate of

slippage along the fault.









APPENDIX B
APATITE FISSION-TRACK AGE AND LENGTH DATA

Apatite fission-track age and fission-track length data were obtained from Ray Donelick at

Apatite to Zircon, Inc.









Table B-1. Apatite fission-track age data
Pooled Median
ps N pi Ni pd Fission- Mean Fission- Fission-

(10^6 (tracks) (10A6 (tracks) (10A6 Track Age Track Age Track Age

tracks tracks tracks (Ma) (Ma) (Ma)
cm^- cm^- cm^-
2) 2) 2)
20.4 0.7-
0.478 212 1.725 2060 3.795 20.41.6 21.11.5 1.6+
17.3 1.2-
0.088 104 1.101 1298 3.818 16.01.7 19.52.3 3.4+
13.8 2.0-
0.036 43 0.521 627 3.842 13.82.2 15.92.6 2.7+
18.5 0.7-
0.508 607 5.698 6803 3.889 18.1+0.9 18.51.2 0.7+
15.7 2.3-
0.084 98 1.081 1254 3.913 16.01.7 14.41.9 1.1+
17.1 1.4-
0.082 98 0.086 1173 3.936 17.21.9 18.61.8 1.0+
14.1 1.1-
0.501 588 7.155 8390 3.96 14.50.7 15.11.2 1.3+
16.0 1.4-
0.172 198 2.034 2340 3.984 17.61.4 18.11.7 1.8+
14.4 2.0-
0.063 71 0.935 1056 4.007 14.1+1.8 14.71.6 1.7+
ps is the density of spontaneous fission-tracks, N is the number of fission-tracks, pi is the density
of induced fission-tracks, Ni is the number of induced fission-tracks, and pd is the density of
induced fission-tracks in the dosemeter.









Table B-2. Apatite fission-track length data.
9- 10- 11- 12- 13- 14- 15- 16- 17-
Sample Tracks Mean Standard 5-6 6-7 7-8 8-9 10 11 12 13 14 15 16 17 18

Numbers Standard Deviation

Error (pm) (pm)
H93Ruby-
4 131 14.730.11 1.25 0 0 0 0 0 2 1 9 16 49 34 16 4

H93Ruby-5 63 14.600.13 1.05 0 0 0 0 0 0 1 4 10 23 20 5 0

H93Ruby-8 82 14.540.10 0.9 0 0 0 0 0 0 1 4 15 36 22 4 0
H97Ruby-41
129 14.570.11 1.19 0 0 0 1 0 0 0 12 24 41 39 11 1

H97Ruby-42 25 14.860.22 1.07 0 0 0 0 0 0 0 1 4 9 8 2 1

H97Ruby-51 97 14.650.10 1 0 0 0 0 0 0 2 4 13 46 23 7 2

H97Ruby-53 142 14.58 0.09 1.02 0 0 0 0 0 0 0 10 35 47 41 7 2

H97Ruby-54 137 14.06 0.13 1.55 0 0 0 0 1 4 3 17 28 48 28 5 2

H97Ruby-55 89 14.210.14 1.33 1 0 0 0 1 3 2 4 24 31 21 2 1









APPENDIX C
MONTE TRAX APATITE FISSION-TRACK MODELS

Figures 1 through 9 are individual runs showing best-fit lines generated by the Monte

Trax program (Gallagher, 1995). In order to generate these figures, broad time and temperature

boundaries were input, as well as the observed mean fission-track ages. A genetic algorithm was

then used to select time-temperature points from within the initial time-temperature boundaries

and construct a thermal history. Built-in statistical tests determine which is the best-fit model for

each sample. The track length histograms were created by inputting individual grain counts and

track length measurements into the program. Data used to create these models is listed in

Appendix B.











O

20

40

6D

00

1 DO

120


55 44 33 22 11 n

Time (Ma)






I I
I-

I I

-- I I I
I I I I -


I I I I I I I I I I I I I


1 2 3 4 5 6 7


I I I I I I


9 10 11 12 13 14 15 17 17 1 1~ 20


Track Length (microns)

Figure C-i. Best fit Monte Trax model for H93 RUBY-4. Observed age is 21.10 Ma, predicted
age is 21.05 Ma. Observed mean track length is 14.730 gm, predicted mean track
length is 14.735 Lm. Observed standard deviation is 1.250, predicted standard
deviation is 1.271. This model run indicates rocks were at 1500C at 23 Ma, 490C at
19 Ma, and 290C at 0 Ma.











0

20 -









100 -

120 -


44
44


IIlI I I I
33 22 11 0
Time (ia)


301



20-

I
I I
I I
10- I


I Z I I


1 2 9 4 5 6 7 9 9 10 11 12 13 14 15 16 17 18 19 20

Track Lengths (Microns)

Figure C-2. Best fit Monte Trax model for H93RUBY-5. Observed age is 19.50 Ma, predicted
age is 19.10 Ma. Observed mean track length is 14.600 pim, predicted mean track
length is 14.603 rm. Observed standard deviation is 1.050, predicted standard
deviation is 1.056. This model run indicates that rocks were at 1960C at 20 Ma, 69C
at 20 Ma, and 300C at 0 Ma.


-- - - - - - - -












20





80-
3





100 -

120


44


33 22
Time (Ma)


N N
I I
20- 5


10-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 16 19 20
Track Lengths (Microns)

Figure C-3. Best fit Monte Trax model for H93RUBY-8. Observed age is 15.90 Ma, predicted
age is 16.13 Ma. Observed mean track length is 14.540 pm, predicted mean track
length is 14.565 rm. observed standard deviation is 0.900, predicted standard
deviation is 0.935. This model run indicates rocks were at 1630C at 17 Ma, 460C at
17 Ma, and 470C at 0 Ma.


0
0











20
20

40



n on

100

120


55 44 33 22 11 D
Time (Ma)







-1


.. I I II
r I I

I~ I I


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 13 19 20

Track Length (microns)

Figure C-4. Best fit Monte Trax model for H97RUBY-41. Observed age is 18.50 Ma, predicted
age is 19.01 Ma. Observed mean track length is 14.570 pmr, predicted mean track
length is 14.534 rm. Observed standard deviation is 1.190, predicted standard
deviation is 1.186. This model run indicates rocks were at 1020C at 20 Ma, 520C at
15 Ma, and 330C at 0 Ma.










0










100

120

55 44 33 22 11 0
Time (Ma)
20-






I L




1 2 3 d 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Trark Length (microns)

Figure C-5. Best fit Monte Trax model for H97RUBY-42. Observed age is 14.40 Ma, predicted
age is 14.35 Ma. Observed mean track length is 14.860 pm, predicted mean track
length is 14.862. Observed standard deviation is 1.070, predicted standard deviation
is 1.067. This model run indicates that rocks were at 1520C at 15 Ma, 500C at 14 Ma,
and 320C at 0 Ma.











0


20


i40


0o




100DO


120


~1


.JU


40-


N 30-


2D-


10-


I
44


I I I I
33 22
Time (Ma)


I I
11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Track Lengths (microns)


Figure C-6. Best fit Monte Trax model for H97RUBY-51. Observed age is 18.60 Ma, predicted
age is 18.61 Ma. Observed mean track length is 14.650 gm, predicted mean track
length is 14.662 gm. Observed standard deviation is 1.000, predicted standard
deviation is 1.027. This model run indicates that rocks were 1430C at 19 Ma, 610C at
19 Ma. and 330C at 0 Ma.


I I
I
I I


I I
I I
I I I
I I
I l l
I l
I I I
II











D


2D -


440 -





150





120 -


1


44


Jn .


Vw


50-

40-

30-

20-

10-


33 22 11 0

Time (MNa)


1 2 3 4 5 6 7 3 9 10 11 12 13 14 15 16 17 18 19 20

Track Lengths (microns)

Figure C-7. Best fit Monte Trax model for H97RUBY-53. Observed age is 15.10 Ma, predicted
age is 15.04 Ma. Observed mean track length is 14.580 pm, predicted mena track
length is 14.569 pmr. Observed standard deviation is 1.020, predicted standard
deviation is 1.016. This model run indicates that rocks were at 1950C at 16 Ma, 65C
at 15 Ma, and 360C at 0 Ma.


~1
I I
I I I
I I I

I I I
Il
II I I I
I I I
I I
I I I
I I I


- - - - - - - ;- - -











0

20

S40

a60

p 80

100

120


55 44 33 22 11 0

Time (Ma)


3U


40-
Na-
N 30-


20-


10-


I I I I I I I I I I I I I I I I I I I I
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Track Length (microns)


Figure C-8. Best fit Monte Trax model for H97RUBY-54. Observed age is 18.10 Ma, predicted
age is 18.10 Ma. Observed mean track length is 14.060 pm, predicted mean track
length is 14.072 im. Observed standard deviation is 1.550, predicted standard
deviation is 1.517. This model run indicates that rocks were 2370C at 28 Ma, 75C at
16 Ma, and 330C at 0 Ma.


I I

I I

I I
I I I
I I
1-11 1-1
I I I I ,. _
I I I
r I I I
I I I I
I I
I I I I I..
I I I I













20

s40

60

80

100

120


55 44 33 22 11 0

Time (Ma)
40-


30-


20- T"



10- I
4J I I I L..


I I I I I I I I I I I I


I I I I I I I


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Track Length (microns)


Figure C-9. Best fit Monte Trax model for H97RUBY-55. Observed age is 14.70 Ma, predicted
age is 14.80 Ma. Observed mean track length is 14.210 gm, predicted mean track
length is 14.198 im. Observed standard deviation is 1.330, predicted standard
deviation is 1.320. This model run indicates that rocks were 1740C at 17 Ma, 800C at
15 Ma, and 340C at 0 Ma.









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the North American cordillera: The temporal and spatial association of magmatism and
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Best, M. G., and Christiansen, E. H., 1991, Limited extension during peak Tertiary volcanism,
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12,528.

Buck, W., 1988, Flexural rotation of normal faults: Tectonics, vol.7, i. 5, p. 959-973.

Burchfiel, B. C., Cowan, D. S., Davis, G. A., 1992, Tectonic overview of the cordilleran orogen
in the western United States: The geology of North America: The Geological Society of
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Mountains metamorphic core complex became cool: Evidence from apatite (U-Th)/He
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Carter, T. J., Kohn, B. P., Foster, D. A., Gleadow, A. J. W., and Woodhead, J. D., 2006, Late-
stage evolution of the Chemehuevi and Sacramento detachment faults from apatite (U-
Th)/He thermochronometry: Evidence for mid-Miocene accelerated slip: Geological
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Colgan, P. A., Metcalf, J. R., 2006, Rapid middle Miocene unroofing of the southern
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Colgan, P. A., Dumitru, T. A., McWilliams, M, and Miller, E. L., 2006, Timing of Cenozoic
volcanism and Basin and Range extension in northeastern, Nevada: New constrainsts
from the northern Pine Forest Range: Geological Society of America Bulletin, v. 118,
i. 1, p. 126-139.

Coney, P. J., and Harms, T. A., 1984, Cordilleran metamorphic core complexes: Cenozoic
extensional relics of Mesozoic compression: Geology, v. 12, p.550-554.

Crittendon, M., Jr., Coney, P. J., and Davis, G., 1977, Tectonic significance of metamorphic core
complexes in the North American cordillera: Geology, v.6, p. 79-80

Dallmeyer, R. D., Snoke, a. W., and McKee, E. H., 1986, The Mesozoic-Cenozoic
tectonothermal evolution of the Ruby Mountains-East Humboldt Range, Nevada: a
cordilleran metamorphic core complex: Tectonic, v. 5, p. 931-954.









Dokka, R. K., and Mahaffie, M. J., 1986, Thermochronologic evidence of major tectonic
denudation associated with detachment faulting, northern Ruby Mountians-East
Humboldt Range, Nevada: Tectonics, v.5 p. 995-1006.

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metamorphic core complexes and the reconstruction og the transition zone, west central
Arizona: Constraints from apatite fission-track thermochronology: Journal of
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detachment faulting in the southern Basin and Range: The Chemehuevi Mountains case
study: Geological Society of America, v. 105, p. 1091-1108.

Foster, D. A., and John, B. E., 1999, Quantifying tectonic exhumation in an extensional orogen
witt thermochronology: Examples from the southern Basin and Range Province:
Geological Society, London, Special Publications, v. 154, p. 343-364.

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geological problems: Annual Review of Earth and Planetary Sciences, v. 26, p. 519-572.

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Gifford, J., 2008, Quantifying Eocene and Miocene extension in the Sevier hinterland of the
Ruby-East Humboldt metamorphic core complex in northeastern Nevada. Gainesville,
Florida, The University of Florida, M. S. thesis, 90??p.

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apatite: Nuclear Tracks and Radiation Measurements, v. 5, p. 169-174.

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denudational response to tectonics, In (Ed.), Summerfield, M. A., Geomorphology and
Global Tectonics, John Wiley and Sons LTD., Chichester, p. 57-75.

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of mylonization in the Ruby Mountains, Nevada: Journal of Geophysical Researcg, v.
95, p. 8569-8580.









Hodges, K. V., Snoke, A. W., and Hurlow, H. A., 1992, Thermal evolution of a portion of the
Sevier hinterland: The Ruby Mountain-East Humboldt Range and Wood Hills,
northeastern Nevada: Tectonics, v. 11, p. 154-164.

Howard, K. A., 1971, Paleozoic metasediments in the northern Ruby Mountains, Nevada:
Geological Society of America Bulletin, v. 82, p. 259-264.

Howard, K. A., 1980, Metamorphic infrastructure in the northern Ruby Mountains, Nevada:
Geological Society of America, Memoir 153, p. 335-347.

Hurlow, H. A., Snoke, A. W., and Hodges, K. V., 1991, Temperature and pressure mylonization
in a Tertiary extensional shear zone, Ruby Mountain-East Humboldt Range, Nevada:
Tectonic implications: Geology, v. 19, p. 82-86.

Kistler, R. W., Ghent, E. D., and O'Neil, J. R., 1981, Petrogenesis of garnet two-mica granites in
the Ruby Mountains, Nevada: Journal of Geophysical Research, v. 86, p. 10,597-10,606.

Koppers, A. A. P., 2002, ArArCALC, software for 40 Ar/39 Ar age calculations: Computers and
Geosciences, v. 5, p. 605-619.

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the northern Snake Range, Nevada: Tectonic, v. 10, p. 77-100.

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3.09a).

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Tertiary extension in the Ruby Mountains core complex, Nevad: Geological Society of
America Bulletin, v. 109, p. 1576+1594.

McDougall, I., and Harrison, T. M., 1999, Geochronology and Thermochronology by the 40 Ar/
39 Ar Method: New York, New York, Oxford University Press, Oxford, UK.

McGrew, A. J., and Snee, L. W., 1994, Ar/Ar thermochronologic constraints on the
tectonothermal evolution of the northern East Humboldt Range metamorphic core
complex, Nevada: Tectonophysics, v. 238, p. 425-450.

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tectonothermal evolution of the East Humboldt Range metamorphic core complex,
Nevada: Geological Society of America Bulletin, v. 112, p. 45-60.

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extension in the Windermere Hills, northeast Nevada: Geological Society of America
Bulletin, v. 111, p. 11-27.

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their role in Tertiary exhumation of the East Humboldt-Wood Hills metamorphic
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Reese, N. M., 1983, Cenozoic tectonic history of the Ruby Mountains and adjacent areas,
northeastern Nevada: Constraints from radiometric dating and seismic reflection profiles:
Dallas, Texas, Southern Methodist University, M. S. thesis, 88p.

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Bulletin, v. 50, p. 881-917.

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Wright, J. E., 1997, The grand tour of the Ruby-East Humboldt metamorphic core
complex, northeaster Nevada: Part I-Introduction and road log: BYU Geology Studies, v.
42, part 1, p. 225-269.

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widened: Annual Review of Earth and Planetary Sciences, v. 27, p. 417-462.

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Canyon Range during extension along the Sevier Desert Detachment, west central Utah:
Tectonics, v. 20, p. 289-307.

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settings: Reviews in Mineralogy and Geochemistry, v. 58, p. 411-448.

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BIOGRAPHICAL SKETCH

Virginia Newman was born in Melbourne, FL in 1975. She graduated from Deltona High

School (with honors) in 1993. She earned a Bachelor of Arts degree in linguistics from the

University of Florida-Gainesville in 1997. Virginia took several years off from education in

order to marry and start a family. Once admitted to graduate school at the University of Florida,

she spent a year as a teaching assistant. Virginia received a Master of Science degree from the

University of Florida in May of 2008. Virginia will continue to pursue her interests in the

geological sciences in the private sector.





PAGE 1

1 INVESTIGATION OF THE HISTORY OF E XHUMATION AND FAULTING IN THE RUBY MOUNTAINS METAMORPHIC CORE COMPLEX, NEVADA By VIRGINIA NEWMAN A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008

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2 2008 Virginia Newman

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3 To my husband, Lou and my daughters, Hannah and Rachel

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4 ACKNOWLEDGMENTS First and foremo st, I would like to express my gratitude to Dr. David Foster, my advisor, for his guidance and expertise during the process of this study. I also thank Dr. Keith Howard for sharing his knowledge of the geology of Ruby Mountains as well as his assistance in the field. I thank the many excellent professors I have had in the program, including Dr. Smith and Dr. Randazzo, my field camp instructors. I appr eciate Dr. Neuhoff and Dr. Meert, my committee members, for their patience and assistance. I am extremely thankful to Mike Hartley, who assisted with many aspects of the lab work, and to Jennifer Giffor d, who contributed GIS distances to this project. I would especially like to thank my husband for his financial support and patience. Field work for this project was funded by th e USGS. USGS also funded a portion of the fission-track analysis.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4LIST OF TABLES................................................................................................................. ..........6LIST OF FIGURES.........................................................................................................................7ABSTRACT.....................................................................................................................................8CHAPTER 1 INTRODUCTION..................................................................................................................102 GEOLOGIC SETTING..........................................................................................................16Regional geologic background...............................................................................................16Geologic History of the Ruby Mountai ns Metamorphic Core Complex................................ 173 METHODS.............................................................................................................................214 APATITE FISSION-TRACK RESULTS.............................................................................. 255 BIOTITE 40AR/39AR RESULTS............................................................................................ 306 DISCUSSION.........................................................................................................................357 CONCLUSIONS.................................................................................................................... 42APPENDIX A METHODOLOGY................................................................................................................. 43B APATITE FISSION-TRACK AGE AND LE NGTH DATA................................................. 44C MONTE TRAX APATITE FISSION-TRACK MODELS.................................................... 47LIST OF REFERENCES...............................................................................................................57BIOGRAPHICAL SKETCH.........................................................................................................61

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6 LIST OF TABLES Table page 3-1 Rock samples from the Ruby Mountains, Nevada............................................................. 235-1 Biotite 40Ar/39Ar total fusion ages................................................................................... 32B-1 Apatite fission-track age data.............................................................................................45B-2 Apatite fission-track length data........................................................................................ 46

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7 LIST OF FIGURES Figure page 1-1 Location map of the Ruby Mountains metamorphic core complex................................... 141-2 Map of the Ruby Mountains metamorphic core complex region...................................... 153-1 Lamoille Valley thermochronology sample locations.......................................................244-1 Apatite apparent ages vs. sample elevation....................................................................... 284-2 Apatite apparent ages vs. distance from fault.................................................................... 295-1 Biotite apparent ages vs. elevation.....................................................................................335-2 Biotite apparent ages vs. distance from fault.....................................................................346-1 Apatite and biotite apparent ages vs. distance from fault.................................................. 396-2 The younger biotite apparent ag es vs. distance from fault................................................ 406-3 Base of the biotite argon part ial retention zone at ca. 24 Ma............................................ 41C-1 Best fit Monte Trax model for H93 RUBY-4.................................................................... 48C-2 Best fit Monte Trax model for H93RUBY-5..................................................................... 49C-3 Best fit Monte Trax model for H93RUBY-8..................................................................... 50C-4 Best fit Monte Trax model for H97RUBY-41................................................................... 51C-5 Best fit Monte Trax model for H97RUBY-42................................................................... 52C-6 Best fit Monte Trax model for H97RUBY-51................................................................... 53C-7 Best fit Monte Trax model for H97RUBY-53................................................................... 54C-8 Best fit Monte Trax model for H97RUBY-54................................................................... 55C-9 Best fit Monte Trax model for H97RUBY-55................................................................... 56

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8 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INVESTIGATION OF THE HISTORY OF EX HUMATION AND FAULTING IN THE RUBY MOUNTAINS METAMORPHIC CORE COMPLEX, NEVADA By Virginia Newman May 2008 Chair: David Foster Major: Geology New 40Ar/39Ar and apatite fission-track data from the Ruby Mountains metamorphic core complex, in NE Nevada, helps to constrain the exhumation and cooling history of the area. Apatite fission-track data yielded a low-temper ature history of exhumation along the west-rooted detachment fault flanking the range. Apatite re sults indicated rapid cooling in the Miocene and also provided apparent ages of samples along a transect perpendicular to the fault. The change in apatite apparent ages with respect to elevation su ggest that rapid cooling started at approximately 15 Ma. Sampling perpendicular to the direction of fault slip allowed for the calculation of an average rate of slip along the fault of 9.1 3.1 km/my, assuming that the apparent ages reflect exhumation due to detach ment slip. Biotite 40Ar/39Ar total fusion ages were determined from the same samples, and were also used to calculate a rate of slip along the fault. A rate of 1.6 2.2 km/my was calculated from the biotite 40Ar/39Ar data. Due to the suspicion that some of the biotite samples were in the biotite argon partial retention zone, which would cause these samples to appear older than samples closer to the fault du e to the fact that the or iginal argon was not lost, a rate of 2.4 0.6 km/my was recalculated using the younger biotite ages. The biotite 40 Ar/ 39 Ar data suggest that exhumation was accomplis hed mainly by tectonic denudation along a

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9 detachment fault rather than erosion. The vert ical variation of fission-track ages alternately suggest that these data record erosional exhumation due to er osion following the uplift of the footwall block relative to the adjacent valley fl oors. New data from this study as well as previous data indicate that ra pid tectonic exhumation started at ca. 24 Ma and continued until ca. 14 Ma.

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10 CHAPTER 1 INTRODUCTION Metamo rphic core complexes are the tectonic ally exhumed and highly extended footwalls of low-angle detachment normal faults (Crittendon et al., 1978). While it is known that these features form during continental extension, the processes that lead to core complex formation, as opposed to more distributed extension, are not yet resolved (Buck et al., 1988). In the Basin and Range province of the western United States, Tertiary metamorphic core complexes were exhumed during regional extension possibly caused by hot spot movement (Best et al., 1991), the growth of a subduction slab-gap (Sonder and Jones, 1999), a relaxation of boundary forces (Sonder and Jones, 1999), or the orogenic collapse of overly thickened crust (Coney and Harms, 1984). The timing of the onset of extension in many of the metamorphic core complexes in the central and northern Basin and Range, south of the Snake River Plain, is poorly constrained. Some place the onset of extensi on in the northern Basin and Ra nge between 55 to 45 Ma (Sonder and Jones, 1999; and McGrew et al., 1994). Others contend that most extension did not start until the Miocene (Stockli et al., 2001; and Dokka et al., 1986). The confusion within the timing of these events arises from the multiple episodes of extension that are overprinted in the area (Dallmeyer et al., 1986; and Muel ler et al., 1999). The issue is complicated further by the possibility of distinctly different processes in itiating and perpetuating extension at different times. Some researchers believe there was an earl ier period of extensional thinning of the ductile middle crust followed by a later period of extens ion of the entire crus t (Wells et al., 2000). The Ruby Mountains, which are the focus of this study, are part of the southernmost extent of the northern Basin and Range Province, locat ed in northeast Nevada (Fig. 1). Previous research in the Ruby Mountains Metamorphic Core Complex documented gradual cooling of

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11 rocks that were at the upper middle crust in Late Cretaceous time, from 63 to 49 Ma (Mueller et al., 1999). Data from the same study shows that d eeper structural levels cooled rapidly between 36 to 29 Ma, with all rocks exposed in the core complex footwall being c ooled at or below the 100 C isotherm by ca. 23 Ma (Mueller et al., 199 9). There is disagree ment, however, about the actual timing of rapid cooling. Dallmeyer et al. (1986) place the period of rapid cooling between 20 to 30 Ma, with all rocks cooled below 100C by ca. 20 Ma. Reese (1983) discusses two distinct episodes of ex tension occurring in the Ruby Mountai ns footwall. The first occurred between ca. 37 to 30 Ma, followed by a second between ca. 25 to 9 Ma afte r a tectonic hiatus. Hodges et al. (1992) suggests that unroofing began during and immediately following Late Cretaceous thrust burial, but most thermoch ronological data indicate major exhumation and cooling in Oligocene and Miocen e epochs (Mueller et al., 1999; Hurlow et al., 1991; Reese, 1983). Biotite K-Ar ages from the eastern Ruby Mountains record cooli ng below approximately 300C between ca. 37.6 and 30.1 Ma, which could indica te Oligocene extension or be related to an exhumed partial retention zone. The kinema tics of detachment normal faults and timing of cooling indicate that exhumation was derived by tectonic extension and that the footwall rocks in the Ruby Mountains form a metamor phic core complex (Howard, 1980). Rocks exposed in the Ruby Mountains were ex humed from depths of as much as 35 km following burial by thrusting in the Late Cretace ous (McGrew et al., 2000). The metamorphic core complex is flanked by a west-rooted, low-angl e, mylonitic detachment fault that is overlain by a more complex arrangement of normal and lis tric faults. The comple x arrangement of highangle faults overlying lo w-angle detachment formed as the nonrigid footwall was exhumed from beneath the hanging wall along a low a ngle detachment fault (Howard, 1980).

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12 The current body of knowledge existing on the timing and rates of extension in the Ruby Mountains was obtained through previous thermoch ronologic studies, usi ng the K-Ar and fission track methods (Dallmeyer et al., 1986; Dokka et al., 1986; Reese, 1983; McGrew et al., 1994). The use of several thermochronologic systems toge ther to study core complex footwalls allows the reconstruction of a thermal history over a la rger temperature range than one system alone (Foster and John, 1999; Stockli, 2005). Each unique thermochronologic system can provide information about the timing of the cooli ng of a rock body through the specific closing temperature of the minerals in question and thereby the exhumation of footwall rocks from different paleodepths. Additional information about the meaning of rocks cooling histories can be obtained by integrating geologic and structural inform ation obtained in the field. Published K-Ar and 40Ar/39Ar apparent ages and apatite fiss ion track apparent ages in the Ruby Mountains core complex footwall young west ward in the direction of extensional unroofing for each mineral analyzed. Biotite 40Ar/39Ar apparent ages generally range from about 45 Ma to near ca. 20 Ma (Dallmey er et al., 1986; Kistler et al., 1981; McGrew et al.,1994). If the previous biotite K-Ar apparent ages represent rapid cooling th rough the argon retention isotherm of about 300 C, then the westward-younging pattern can be used to determine the rate of unroofing and quenching of the core complex. The approximate rate of fault slip can be obtained by inverting the rate of lateral change in cooling rate with distance (Foster et. al., 1993). Inversion based on the previous data indicates a rate of slip on th e detachment of 1 to 2 km/m.y. A calculated rate of 1 to 2 km/m.y. indicates an onset to extension of around 30 to 25 Ma rather than 50 to 45 Ma. This apparent slip rate is several times slower than the rates of detachment slip in tectonically analogous areas, such as th e Snake Range metamorphic core complex (Lee et al., 1991) and the Colorado extensional corridor (Foster et al., 1993; Foster and John, 1999),

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13 which have calculated rates of 7 to 14 km/m.y. The calculated rate of extension in the Ruby Mountains is also an order of ma gnitude slower than the rates estimated from shear-strain fabrics in the mylonitic shear zone that roofs the core complex, which suggests extension occurred at a rate of 22 km/my (Hacker et al., 1990). The calculated rate also does not account for the peak time of rapid quenching in the R uby Mountains and regionally, wh ich apparently occurred in the middle Miocene, approximately 24 Ma (Dokka et al., 1986; Stockli et al., 2001). Furthermore, the extension rate would have had to be many ti mes faster to exhume the full 35 km if all the extension occurred in one event. The resolution of this contradiction is a pr inciple reason for the study. The calculated rate of extension may also s upport or refute one or mo re of the previously discussed hypotheses relating to possibl e causes of extension in the area. I present new apatite fission track and 40Ar/39Ar thermochronometry that constrain the timing of onset of extension and the rate of slip along the low angle detachment fault flanking the Ruby Mountains metamorphic core complex. The data and conclusions generated by this study compliment recent and ongoing thermochronologic studies in the Basin and Range (McGrew et al., 2000; Colgin et al., 2006). These combined data have important implications for the cause of widespread extension in the Basin and Range during Neogene times, because they reveal the spatial distribution of the timing of extension and exhu mation of a spectrum of crustal levels.

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14 \002 btnfrntfrnt t Figure 1-1. Location map of the Ruby Mountai ns metamorphic core complex, located in northeast Nevada between the Antler orogenic belt and the Sevier orogenic belt.

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15 Figure 1-2. Map of the Ruby Mountains metamo rphic core complex re gion, including the Ruby MountainsEast Humboldt Range, Clover Hill, Wood Hills, and a portion of the Pequop Mountains, outlined in the dashed red line. Gray areas represent ranges in the area.

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16 CHAPTER 2 GEOLOGIC SETTING Regional geologic background The Cretaceous Sevier Orogeny in NE Nevada was characteri zed by large scale thrusting of m iogeoclinal sediments, along with extensiv e ductile deformation, granitoid intrusion, and metamorphism. By the end of the Sevier Orogeny the crust in central Nevada could have been as thick as 60 to 70 km (Burchfiel et al., 1992). In Latest Cretaceous time the convergence rate of the Faralon and North American plates increased and the dip of the subducting slab decreased (Armstrong and Ward, 1991). This resulted in the migration of the magmatic arc to the east and further crustal shortening. Rocks now exposed in the footwall of the Ruby Mountains core complex represent the once deeply buried core of the Servier-Laramide Orogen. These rocks underwent metamorphism, thrusting, nappe foldi ng, and the intrusion of granitic rocks during this event (Howard, 1980). Laramide deformation continued into the earl y Cenozoic. At this time, the entire length of the western edge of North America was a convergent plate boundary, with the North American plate overriding the Fa ralon plate (Sonder a nd Jones, 1999). The dip of the subducting slab steepened in the Nevadan part of the orogen about 40 Ma, shifting magmatism westward, and shrinking the magmatic gap that had been caused by shallow subduction. A period of intense magmatic activity, known as the ignimbr ite flare-up, ensued in the Great Basin around 40 Ma (Armstrong and Ward, 1991). Magmatism gradually migrated west, as the dynamics of the plate boundary changed (Sonder and Jones, 1999). The previously thickened lithosphere was thermally weakened due to magmatism and/or asthenospheric upwelling (Armstrong and Ward, 1991). The convergent boundary was gradually repla ced by right-lateral transform motion as the Mendocino triple junction moved to the nort h. The switch from a convergent boundary to a

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17 transform boundary resulted in the formation of the San Andreas fault system, the opening of the Gulf of California, and a change in boundary for ces acting on the plates. Large scale extension in the northern Basin and Range (NBR) could have begun as early as the middle-late Eocene (Sonder and Jones, 1999). Total extension across the Nevada segment of the Basin and Range province has been estimated to be 120 to 150 km (Gans 1987; Wernicke 1992). Geologic History of the Ruby Mo untains Metam orphic Core Complex The Ruby Mountains Metamorphic Core Comple x is located east of the Paleozoic Antler orogenic belt, and west of the Mesozoic Sevier belt (Howard, 1980) (Fig 1). Rocks exposed in the East Humboldt Range, Clover Hill and Wood H ills are also part of the metamorphic core complex footwall (Fig.2). The deepest structural levels exposed in the region lie in the East Humboldt range and the Clover Hill area. Structurally shallowe r-level strata are exposed in Wood Hills and the Pequop Mountains, to the ea st of the Ruby Mountains. The northernmost exposure of rocks related to th e core complex is located in the southern Windermere Hills (Mueller et al, 1993). Geologic elements in this area can be divide d into four separate subsets. The deepest structural levels are composed of metamor phic core complex rocks, including possible Precambrian basement rocks, Late Proterozoic to Mid-Paleozoic metasedimentary rocks, and Mesozoic and Tertiary igneous rocks. This suit e of rocks comprises the migmatitic igneous and metamorphic infrastructure of the complex. An approximately 1 km thick mylonitic shear zone overlays and deforms the upper portion of the me tamorphic and igneous infrastructure. The mylonitic shear zone is structurally beneath a highly extended cover sequence of unmetamorphosed to low-grade metamorphic Paleoz oic through Tertiary strata (Mueller et al, 1993; MacCready, 1997). In the Northern Ruby Mountains and East Hu mboldt Range a lowangle normal fault system separates the core co mplex from low-grade metamorphic rocks. In

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18 more southerly portions of the complex these cover rocks are transitional into the metamorphic rocks. The focus of this study is on a cross-secti on through the center part of the range at Lamoille Canyon, which is an excellent location to study the low-angle detachment system because all of the structural elements are expos ed. The cross-section is along a road to Lamoille Canyon, which was carved by the Lamoille Canyon glacier. This glacier deposited moraines approximately 12 miles long (Sharp, 1938), which ar e cut by fault scarps, i ndicating that brittle faulting continued into the Holocene. The mylonitic zone is exposed along the west range front. The mylonite transition zone is approximately 1 km thick along the western flank of the mountain range. Features such as asymmetric mica porphyroclasts and asymmetric parasitic folds within the micaceous mylonite indicate a west-northwest sense of shear. Stra tigraphic sequences in the mylonite zone have been tectonically thinned as much as one-fifteenth to one-twentie th of the original thickness. Metamorphic barometry studies indi cate that Neoproterozoic strata were buried as much as 35 km (Hodges et. al, 1992). Deformation in the mylonite zone occurred at amphibolites and greenschist facies conditions (H oward, 1980). The blastomylonitic fabric found in the transition zone most likely resulted from extreme tectonic flattening and stretching. Rocks at the base of the mylonite zone grade into the higher-grade, coarser-grai ned, nonmylonitic rocks of the metamorphic infrastructure. At deeper structural levels beneath the myl onite zone, granitic dikes, sills and irregular bodies form more than half the exposed rocks. The granitic rocks ar e predominantly two-mica granite, but granodiorite gneiss, biotite granite, and local bodies of metagabbro are also present. Two-mica granites in the area are small discont inuous bodies so they generally are not mapped

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19 separately from the metasediments that they intrude (MacCready et al., 1997). The exception is the leucogranite orthogneiss of Thorpe Creek, whic h occurs as a sheet like body in the core of the Lamoille Canyon Nappe. The granite in the area is predominantly Jurassic or Cretaceous in age, and appears to be sourced from metasedi mentary rocks at an in termediate depth, though there is some Tertiary plutonic material as well. The percentage of granitic material increases in relation to the percentage metamorphic materials toward the deeper structural levels of the complex (Howard, 1980). Metamorphic rocks in the footwall of the core complex include high-grade marble and quartzite. The marbles and quartzites can be roughly correlated to the unmetamorphosed Paleozoic shelf sediments in the hanging wall (Howard, 1971). The oldest stratigraphic unit is the lower Cambrian Prospect Mountain Quartzite. The unit directly above the Prospect Mountain Quartzite is a calc-silicate marble corre lated with Cambrian limestones and shales and the Ordovician Pogonip group. The Eureka Quartz ite overlays the marble. Immediately above the Eureka Quartzite is a massive, white, near ly pure dolomite unit. The uppermost unit is a color-banded marble that correlates to the Guilmette Limestone. Several large recumbent folds exist within the footwall of the core complex. In Lamoille Canyon, the Lamoille nappe folds the premetamorphic Ogilvie Thrust, Prospect Mountain Quartzite, and Verdi Peak Marble into an eastward overturned recumben t anticline that plunges shallowly north (Howard, 1971). The recumbent folds may have been forged by diapers as the migmatite front moved toward more shallow stru ctural levels (Howard, 1980), or by progressive shortening in the middle crust (Camille ri et al.1996; McCready et al., 1997). Tertiary strata in valleys to the west of the Ruby Mountains include conglomerates with limestone clasts that were presumably derived from limestone cover that was structurally above

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20 the core complex (MacCready et al., 1997). Y ounger Tertiary strata contain metamorphic and granitic clasts derived from the Ruby Mountains.

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21 CHAPTER 3 METHODS Twelve samples of am phibolite and granite were collected on a slip-orthogonal transect representing progressively shallow to deep stru ctural levels in the footwall of the low-angle detachment fault flanking the Ruby Mountains (Table 1; Fig.4). Granitic samples were separated for apatite, mica and potassium feldspar using standard density and magnetic methods (Appendix A) during the fall of 2002 and spring of 2003. Amphibolite samples were processed for mica, apatite, and amphibole. Biotite samples were irradiated at Or egon State University and analyzed by the 40Ar/ 39Ar method in the noble gas laboratory at the Department of Geological Sc iences at the University of Florida in the spring and summer of 2004. Biotit e samples were fused in a water-cooled doublevacuum furnace to extract argon gas from the samples. The biotite samples were fused due to the tendency of biotite to become unstable duri ng the step heating process and the likelihood the fusion age would be the same as the plateau age (McDougall and Harrison, 1999). Argon gas extracted from the samples during furnace heating was transferred through vacuum lines and exposed to getters for ten minutes to remove reactive gases. The purified argon gas was analyzed in a Mass Analyzer Pr oducts Model 215-50 mass spectrometer using a Balzers electron multiplier to measure ion beams of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar. Blanks were performed at the beginning of each day, and periodi cally between samples. Data were reduced using the program ArArCALC version 2.2 by Koppers (2002). ArArCALC utilizes Excel by Microsoft to calculate apparent ages, plot age plateaus, and isochrons. The ArArCALC program was also used to calculate J-values from total fusion ages of the GA1550 biotite flux monitors. These calculated J-values were then utilized in the calculation of 40Ar/39Ar total fusion age calculations.

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22 Fission-track analysis of apatite samples was performed by Ray Donelick at Apatite to Zircon, Inc. during the spring and summer of 2003. Suitable grains were polished to an internal surface that intersected fission-t racks from above and below the polished plane equally. The fission-tracks were etched in 5.5 M HNO3 for 20 seconds at 21C. The resultant etched tracks were then viewed using an unpolarized light microscope at 1562.5x or 2000x magnification from grains free of large surface imperfections a nd possessing a minimum of inclusions and crystal defects. The number of spontan eous fission-tracks counted over a selected area of the grain divided by the area itself yields the spontaneous fission-track dens ity. The external detector method (EDM) was utilized to determine the re lative uranium concentrations in the grains selected for analysis. The external detector was a low-uranium, fission-track free muscovite mica. The mica was placed adjacent to the apatit e grain mount and both irradiated with thermal neutrons in a nuclear reactor (Donelick, 2005). The induced fissi on-tracks on the mica were then etched in 49% HF for 15 minutes at 23C. Ap atite age and length data were forward modeled using the Monte Trax program by Gallagher (19 95), to estimate the th ermal history of the samples. MonteTrax was used in the genetic al gorithm mode with 100 simulations of 250 runs were used to calculate the best fit cooling lines included in Appendix C.

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23 Table 3-1. Rock samples from the Ruby Mountains, Nevada Sample number Rock type* El. (m) UTM_N UTM_E H93 RBY-4 Tm, biotite granodiorite 30394501690 637380 H93 RBY-5 Tum, amphibolite 30174501735 637100 H93 RBY-8 Ocm Bio. Amphibolite 25614498710 638150 H97 RUBY-41 Kp, leucogranite 18954496400 644800 H97 RUBY-42 Kp, leucogranite 20884497130 642225 H97 LC-51 Tg bio-gar aplite, folded 29814494900 638730 H97 LC-52 Kp,pegmatitic granite gneiss 24344499050 637900 H97 RUBY-53 CZp, micaceous quartzite 23104501000 635320 H97 RUBY-54 Tm, biotite granite 20104503740 629685 H97 RUBY-55 mylonitic granite gniess 19054505645 628630 Samples H93RBY-4, H97 LC-51, and H97 RUBY54 are probably ca.29 intrusions. All samples were collected on a cross-strike tr averse approximately 20k m long. Abbreviations of rock units as indicated on Figure 3-1.

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24 ## # # # # # # # #\002btnbfr r )Tj-3.054 -3.138 Td( b tnfrrnn rrn n !" # $ n !rrn # "% rnr n%n "rrn%n ""nn &% n #()n nn\033* # +,, %rrn"-. r %rrn"-r. /#("rn+\0330 #(1r* +2n +r$+n b 4 r5nr% n "rbb b:, :8:8:: :,:8:8:: b Figure 3-1. Lamoille Valley thermochronology sample locations

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25 CHAPTER 4 APATITE FISSION-TRACK RESULTS Apatite fi ssion track data are useful for reconstructing low-temperature thermal histories of rock masses due to the low blocking temperature (Gleadow and Brown, 1999; Gallagher et al., 1998; Stockli, 2005). The closing temperature fo r fission-tracks in apat ite is approximately 110C 10C for geologically rapid cooling rates (Gleadow and Duddy, 1981). In reality, fission-tracks anneal progressively through a temperature interval called the partial annealing zone (PAZ). The temperature range is fr om approximately 60C to 110C for common compositions of apatite, with a nnealing occurring more rapidly at the upper end of this range (Gleadow, and Brown, 1999; Stockl i, 2005). Fission-track dates may be combined with estimates of the geothermal gradient to obtain informati on about the rate of exhumation and erosion. In addition to obtaining the date at which a grain cooled through the clos ing temperature, much information about the cooling history of a sample can be obtained by analyzing the distribution of fission-track lengths. Previous studies in the Ruby Mountains yield fairly consistent apatite fission-track dates. A study by Dokka et al., (1986) showed a range in fission-track ages for titanite, zircon, and apatite from 24 2 to 18 2 Ma from samples taken in the northern Ruby Mountains, north of Lamoille Valley. These data suggested that rapid cooling started between ca. 25 and 23 Ma, and that rocks had cooled from above approximate ly 300C to below 70C between approximately 24 to 18 Ma. Previous apatite fission-track studies performe d by the population method done in Lamoille Valley yielded apatite ages of 12 5, 15 2, 18 2 Ma (Reese, 1983). The ages increase from west to east along a transect through Lamoille Valley. Reese (1983) estimates apparent rate of exhumation of 87 m/my or .087 km/my from these results. Unfortunately, the

PAGE 26

26 population method is known to bias age estimates and no track length measurements were made in this study, so the rate of cooling could not be assessed. Samples collected for this study were obtained along a west to east transect through Lamoille Valley, perpendicular to the detachment fault along the western flank of the range. Samples were collected in this way to show the ch ange in age as a function of distance from the fault (Fig. 3-1). Apatite data obtained from 9 of 12 samples collected from the Lamoille Canyon area is summarized in Appendix B. The samp les yielded high quality apatite with abundant tracks and gave reliable data. Apatite fission track ages ranged from 21.1 1.5 Ma to 14.4 1.9 Ma. All samples yielded unimodal track length di stributions with mean lengths between about 14.1 to 14.9 microns. These long track lengths indi cate rapid cooling rate s and that the fissiontrack ages reflect cooling through the partial annealing zone. The apparent ages of each sample were plotted in ISOPLOT (Ludwig, 2004), in term s of elevation and in terms of distance from the detachment to shed light on the possible causes and rates of exhumation (Figures4-1 and 4-2). The graph of apparent apatite ages vs. sa mple elevations indicates rapid cooling at approximately 15 Ma (Figure4-1), with th e exception of samples H97 RUBY-41 and H97 RUBY-54. This relationship suggests a possible second episode of cooling at ca. 15 Ma. It should be noted that sample H93 RBY-8 was se verely weathered, showing signs of extensive oxidation. Samples H93 RBY-4, H97 LC-51, and H 97 RUBY-54 were collected from Tertiary plutons, but this should not affect the results because the intrusi on ages of the plutons are much older than the timing of cooling and exhumation. The age of rapid cooling generally increases as the distance from the fault increa ses, but the relationship is par tially obscured by the large errors

PAGE 27

27 (Figure4-2). This suggests that exhumation progressed from east to west due to movement along the detachment. Model cooling histories and track length dist ributions were calcul ated using Montetrax (Appendix C). The mean fission-track lengths obta ined from each sample ranged from 14.06 m to 14.86 m. The long mean track lengths a nd unimodal track distributions indicate a straightforward cooling history wi thout the influence of any ther mal anomalies (Gleadow et al., 1986). The T-t models reveal rapid cooling th rough the PAZ (110-60C) centered at the apparent age of the samples. Calculated model tr ack length distributions are consistent with the observed data.

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28 Figure 4-1. Apatite apparent ages vs. sample elev ation. This graph is a x-y plot constructed in Isoplot. H97 RUBY-41 and H97 RUBY-54 ar e contained in the blue ellipse as exceptions to the apparent cooling trend at ca. 15Ma. Apparent ag e errors are listed in Appendix B, elevation errors are 20m. Error crosses are 2

PAGE 29

29 Figure 4-2. Apatite apparent ages vs. distance from fault. This figure was generated in Isoplot. Data points were plotted in an x-y graph and the regression line was drawn using x-y weighted averages. Apparent age errors are taken from Appendix B, distance errors are .25km. The slope of the regression lin e is 0.11 0.32. Mean Squared Weighted Deviates (MSWD) for the regre ssion is 9.3. Error crosses are 2

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30 CHAPTER 5 BIOTITE 40AR/39AR RESULTS Previous studies using the 40Ar/39Ar method on biotite in the Ruby Mountains core complex produced predominately Oligocene and earliest Miocene ages. These results are interpreted to indicate cooli ng below the argon closure in biot ite or about 300C (McDougall and Harrison, 1999). Studies by McGr ew et al. (1994) yielded 40Ar/39Ar mica ages between 21.7 0.2 Ma and 22.9 0.3 Ma. The same study found th at rapid cooling from approximately 300C to below 100C occurred by 20 Ma. Dallmeyer et al. (1986) obtained arg on ages from biotite samples both in the mylonite zone and above it. Above the mylonite zone ages ranged from approximately 32 to 33 Ma. Within the myl onite zone ages were younger, ranging from approximately 22 to 24 Ma. Specifically in La moille Canyon, a biotite from non-mylonitic rock produced an age of 25.3 .7 Ma. A biotite from w ithin the mylonite, in Lamoille Valley yielded an age of 20.8 .5 Ma. The authors postulate th at rapid cooling began by approximately 45 Ma and that the rocks had cooled to below 300C by 20 Ma. Biotite data published Kistler et al. (1981) and utilized by Reese (1983) from Lamoille Valley yielded ages from ca. 19.6 to 33 Ma, going generally from youngest to oldest in a west to east fashion. Samples for this study were collected along a slip orthogonal transect through Lamoille Valley to avoid along-strike variat ions in cooling or exhumation hi story (Fig.3-1). This provides for the determination of the change in age of samples as a function of distance from the exposed trace of the detachment fault. Biotite was separated using standard density and magnetic methods from the same samples used for apatite fission-track an alysis (Appendix A). 40Ar/39Ar total fusion ages for the eight samples range from ca. 31.0 to 20.7 Ma (Table 5-1). The total fusion ages are in general agreement with dates obtained from previous studies (Dallmeyer et al., 1986; McGrew et al., 1994,). The biotite ag es were graphed in ISOPLOT

PAGE 31

31 (Ludwig, 2004) against sample elevat ions and the distance of rocks sampled from the fault to aid in determine the rate and cause of rapid ex tension in the area (Figs. 5-1 and 5-2). The graph of biotite ages vs. sample elevation shows no clear relationship between biotite apparent age and sample elevation (Fig. 5-1). The biotite apparent ages are older than the apatite fission-track ages from the respective samples, due to the higher closure temperature of biotite, approximately 300 C vs. 100 C. When biotite ag es are graphed against di stance from the fault a trend does emerge (Fig. 5-2). The cooling ages generally increase with distance from the fault, indicating that exhumation progressed from east to west along the detachment.

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32 Table 5-1. Biotite 40Ar/39Ar total fusion ages Sample number Total fusion age error (Ma) (Ma) H93 RBY-4 23.06 1.04 H93 RBY-8 30.92 1.75 H97 RUBY-42 31.24 1.75 H97 LC-51 31.26 1.53 H97 LC-52 27.06 1.17 H97 RUBY-53 24.44 0.83 H97 RUBY-54 24.68 2.16 H97 RUBY-55 20.7 0.98

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33 Figure 5-1. Biotite apparent ages vs. elevation. This graph was generated in Isoplot in a x-y plot. Age errors are from Table2, elevation errors are taken to be 20m. Error crosses are 2

PAGE 34

34 Figure 5-2. Biotite apparent ages vs. distance from fault. This figure was generated in Isoplot. Data points were plotted in an x-y graph and the regression line was drawn using x-y weighted averages. Apparent age errors are taken from Table 5-1, distance errors are .25km. The slope of the regression line is 0.64 0.46. Mean Squared Weighted Deviates (MSWD) for the regre ssion is 16. Error crosses are 2

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35 CHAPTER 6 DISCUSSION The use of multiple isotopic systems and m ineral phases for thermochronology research allows the reconstruction of thermal histories ov er a greater temperature range than one system alone (Foster and John, 1999; Stockli, 2005). Apatite fission-track data and thermal models show that exposed rocks along the Lamoille tran sect in the Ruby Mountains metamorphic core complex cooled below approximately 100C between 21.1 1.5 Ma in the east and 14.4 1.9 Ma in the west (Appendices B a nd C). The early to middle Miocene cooling interval is in general agreement with other studies (Dokk a et al., 1986; Reese, 1983), but is more precise and allows the progression of exhumation to be assessed. Rapid cooling that progressed from east to west supports the hypothesis that the core complex was exhumed due to slip along an asymmetric detachment fault. Previously estimated rates of exhumation for this detachment syst em were generally 1 to 2 km/my (Reese, 1983; McGrew, 1994). The rate of extens ion calculated from these data was more rapid than suggested by previous studies that were based on fewer samples and more widely scattered sample locations. Apparent apatite ages vs. distance from the fault we re regressed in ISOPLOT (Fig.42). The slope of the regression line genera ted by ISOPLOT is 0.11 0.32. By inverting the slope and its error, the lateral rate of change, an approxima te rate of 9.1 3.1 km/my is given. This apparent slip rate is with in a range of rates of detachme nt slip in tectonically analogous areas, such as the Snake Range metamorphic core complex (Lee et al., 1991) and the Colorado extensional corridor (Foster et al., 1993; Foster and John, 1999; Carter et al., 2006), where the rates are approximately 7 to 14 km/my. The differe nces between the previous estimate by Reese, 1983 and the data from this study are probably due to the bias introduced by the older population method. The slower rates may indicate different mechanisms for metamorphic core complex

PAGE 36

36 development in the two areas, such as rolling hinge vs. low-angle detachment with two phases of cooling. A possible second phase of brittle faulting and rapid exhumation is indicated by the relationship between apatite appare nt ages and sample elevations (Fig. 4-1). H97Ruby-41 is the easternmost sample along the transect and ha s been down-dropped by range-front faults, explaining the older age of 18.1 0.9 Ma at th e lower elevation of 1895 m and its plotting outside of the rapid cooling tre nd. Sample H97Ruby-54 also plots outside this trend. With the exception of these two data, samples below 2600 m elevation give concordant ages. This suggests that rapid exhumation started at about 15 Ma due to erosion of the fault block. Apatite fission-track and (U-Th)/He data publis hed by Colgan et al., 2006 indicate that the southern Ruby Mountains, in the Harrison Pass region, was rapidly exhumed between 14-15 Ma. This data is an indication that the southern portion of the rang e was exhumed as an intact easttilted block. The fault system in the Harrison pa ss area merges in the north with the mylonite zone that bounds the metamorphic core comple x, indicating that rapid middle Miocene unroofing could have occurred in the northern part of the range as well (Colgan et al., 2006) Biotite apparent ages vs. distance from the fault were also regre ssed in ISOPLOT (Fig. 5-2). The slope and error of the biotite regres sion line, 0.64 0.46, was inve rted and an apparent slip rate of approximately 1.6 2.2 km/my was calculated. The west to east progression of biotite ages from younger to ol der also supports progressive e xhumation due to slip along the detachment fault. The rate of extension calcu lated from these data was in agreement with previously calculated rates from biotite data which were in a range from 0.1 to 1.5 km/my (Reese, 1983). The calculated ra te based on the entire biotite 40Ar/39Ar data set is slower than expected when compared to the apatite fission track data.

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37 Biotite and apatite data were graphed toge ther to investigate possible reasons for the differences in the apparent slip rates calculated from each data set (Fig. 6-1). As expected, apatite ages were younger than biot ite ages due to the lower clos ing temperature of apatite. However, it was expected that the biotite ages would be systematically older than the apatite ages. Instead, the difference between apatite and biotite ages increased as the distance from the fault increased. An explanation fo r the artificially high ages is th at the older biotite ages, greater than about 24 Ma, at a distance of greater th an 10 km from the exposed detachment were probably within the biotite argon partial retention interval prior to rapid extension starting at about 24 Ma (e.g. Foster and John, 1999). New biotite and muscovite 40Ar/39Ar produced by Jennifer Gifford at the University of Florida i ndicates two distinct epis odes of extension and cooling (Gifford,J., 2008). If only the younger bi otite ages are regressed due to the mixed cooling ages of the older samples, a rate of extension of 2.4 0.6 km/my is given (Fig. 6-2). This rate is closer to the rate of slip calculate d from the apatite data. These results are similar to the results from the Chemehuevi detachment in California (John and Foster, 1993), but are still slower than many detachment systems in the Basin and Range (Carter et al., 2004). The significantly larger rate of slip along the detachment es timated from the apatite data could indicate that isostatic re bound played a role in unroofing the core complex. Variable rates of slip have been previously documented in the Harcuvar Mountains (C arter et al., 2004) and along the Chemehuevi and Sacramento detachment faults (Carter et al., 2006). The large rate may also indicate that some of the exhumation in this area was accomplished rapidly on brittle faults in the middle Miocene, rather than recording only the ra te of slip along the low-angle detachment fault (Colgan et al., 2006).

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38 The thermochronological data also reveal the time that rapid exhumation began and the duration of extension. All of the apatite fissiontrack ages have long track lengths with unimodal distributions, so they all reveal rapid cooling that must have started before ca. 20 Ma, the apparent age of the oldest sample. The biotite 40Ar/39Ar dates for samples greater than 10 km from the fault give apparent ages substantially ol der than the apatite fission-track ages, therefore, these record slower cooling. The biotite 40Ar/39Ar ages at approximately 10 km or less from the exposed fault are centered within error or a few m.y. older than the apatite fission-track ages. The oldest biotite age from the samples taken le ss than 10 km from the fault is ca. 24 Ma. A time of approximately 24 Ma is also at the tran sition in slope of the line between the ca. 30 Ma and the ca. 24 Ma biotites (Fig.6-3), which woul d record the 300 C isot herm prior to rapid extension (Foster and John, 1999). Th is also suggests that cooling initiated at ca. 24 Ma. This conclusion is consistent with previous studi es (Dallmeyer et al., 1986; Dokka et al., 1986, McGrew, 1994), as well as new unpublished data obtained in the area (Gifford, J., 2008).

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39 Figure 6-1. Apatite and biotite a pparent ages vs. distance from fault. This graph was generated in ISOPLOT using the same methods as the individual data sets. Apatite data is graphed as red error crosses, biotite is graphed as green e rror crosses. The slope of the regression line on the younger four biotite apparent ages is 0.4 1.7. The slope of the regression line on the apatite apparent ages is 0.11 0.32. Error crosses are 2

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40 Figure 6-2. The younger biotite appa rent ages vs. distance from fa ult. The graph was generated in ISOPLOT using the same methods as for th e entire biotite data set. The slope of the regression line is 0.4 1.7. The Mean Squared Weighted Deviates (MSWD) of the regression is 12. Error crosses are 2

PAGE 41

41 Figure 6-3. Base of the biotit e argon partial retention zone at ca. 24 Ma. This graph was generated in ISOPLOT by the same method as the other graphs. The trend lines were also generated in ISOPLOT. The trans ition line between the two trend lines was drawn in Illustrator to clarify where the base of the biotite argon partial retention zone is located. The ca. 24 Ma line, black, wa s created to show the change in slope between the Illustrator line and the ISOP LOT trend line. Younger biotite apparent ages are represented by red error crosses, older biotite apparent ages are represented by green error crosses, and the teal error cross represents the biotite that plotted within the biotite argon partial retention zone. Error crosses are 2

PAGE 42

42 CHAPTER 7 CONCLUSIONS The data obtained in this st udy are consistent with the hypot heses that rocks exposed in the Ruby Mountains m etamorphic core complex were rapidly exhumed by a detachment fault. A possible second phase of brittle faulting and erosional exhumation at approximately 15 Ma is suggested by the apatite fission-track data. Th ese thermochronologic data indicate that rapid exhumation took place during Miocene time, with an onset of approximately 24Ma. The apatite fission-track data suggest a rate of slip on the detachment of approximately 9.1 3.1 km/my, which may record low angle faulting and subsequent brittle faulting. An apparently slower rate of 2.4 0.6 km/my was calculated from the quenched biotite cooling ages.

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43 APPENDIX A METHODOLOGY The sam ples utilized in this study were cr ushed at the U.S. Geological Survey. They were then washed to float the dust off and sieved in 60 to 120 mesh sieves. The 60 and 120 mesh fraction was used for thermochronology pr eparation. The samples were passed through pure tetrabromoethylene (TBE), sp ecific gravity 2.96, to separate quartz, feldspars and other light minerals from heavier mafic minerals. The sinks were sent through the Frantz magnetometer to separate the magnetic minerals from the nonmagnetic. The magnetic fractions contained predominantly amphibole and mica. The nonmagnetic fractions primarily contained apatite and zircon. The nonmagnetic fractions we re then passed through pure methyleneiodide (MI), specific gravity 3.32, to separate apatite from zircon. Following inspection under the binocular microscope the apatite separates were sent to Apatite to Zircon, Inc. for fission-track analysis. The magnetic fractions were inspecte d under a binocular microscope and picked when necessary for biotite, muscovite in one case, and amphibole. The original TBE floats were sent through a dilute solution of TBE in order to se parate potassium feldspars from plagioclase. Additional field work was done in September, 2004. Apatite fission-track data and 40Ar/39Ar thermochronology data were utilized to constrain the timing of the onset of slippage along the detach ment fault as well as the time averaged rate of slippage along the fault.

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44 APPENDIX B APATITE FISSION-TRACK AGE AND LENGTH DATA Apatite f ission-track age and fission-track lengt h data were obtained from Ray Donelick at Apatite to Zircon, Inc.

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45 Table B-1. Apatite fission-track age data s N i Ni d Pooled FissionMean FissionMedian Fission(10^6 (tracks) (10^6 (tracks) (10^6 Track Age Track Age Track Age tracks tracks tracks (Ma) (Ma) (Ma) cm^2) cm^2) cm^2) 0.478 212 1.725 2060 3.795 20.4.6 21.1.5 20.4 0.71.6+ 0.088 104 1.101 1298 3.818 16.0.7 19.5.3 17.3 1.23.4+ 0.036 43 0.521 627 3.842 13.8.2 15.9.6 13.8 2.02.7+ 0.508 607 5.698 6803 3.889 18.1.9 18.5.2 18.5 0.70.7+ 0.084 98 1.081 1254 3.913 16.0.7 14.4.9 15.7 2.31.1+ 0.082 98 0.086 1173 3.936 17.2.9 18.6.8 17.1 1.41.0+ 0.501 588 7.155 8390 3.96 14.5.7 15.1.2 14.1 1.11.3+ 0.172 198 2.034 2340 3.984 17.6.4 18.1.7 16.0 1.41.8+ 0.063 71 0.935 1056 4.007 14.1.8 14.7.6 14.4 2.01.7+ s is the density of spontaneous fission-tracks, N is the number of fission-tracks, i is the density of induced fission-tracks, Ni is the number of induced fission-tracks, and d is the density of induced fission-tracks in the dosemeter.

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Table B-2. Apatite fission-track length data. Sample Tracks Mean Standard 5-6 6-7 7-8 8-9 910 1011 1112 1213 1314 1415 1516 1617 1718 Numbers Standard Deviation Error (m) (m) H93Ruby4 131 14.73 0.11 1.25 00000 219164934164 H93Ruby-5 63 14.60 0.13 1.05 00000 01410232050 H93Ruby-8 82 14.54 0.10 0.9 00000 01415362240 H97Ruby-41 129 14.57 0.11 1.19 00010 0012244139111 H97Ruby-42 25 14.86 0.22 1.07 00000 00149821 H97Ruby-51 97 14.65 0.10 1 00000 02413462372 H97Ruby-53 142 14.58 0.09 1.02 00000 001035474172 H97Ruby-54 137 14.06 0.13 1.55 00001 431728482852 H97Ruby-55 89 14.21 0.14 1.33 10001 32424312121 46

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47 APPENDIX C MONTE TRAX APATITE FISSION-TRACK MODELS Figures 1 through 9 are individua l runs showing best-fit lin es generated by the Monte Trax program (Gallagher, 1995). In order to ge nerate these figures, broad time and temperature boundaries were input, as well as the observed m ean fissi on-track ages. A genetic algorithm was then used to select time-tempe rature points from within the in itial time-temperature boundaries and construct a thermal history. Built-in statistica l tests determine which is the best-fit model for each sample. The track length histograms were created by inputting individual grain counts and track length measurements into th e program. Data used to create these models is listed in Appendix B.

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48 Figure C-1. Best fit Monte Trax model for H 93 RUBY-4. Observed age is 21.10 Ma, predicted age is 21.05 Ma. Observed mean track length is 14.730 m, predicted mean track length is 14.735 m. Observed standard deviation is 1.250, predicted standard deviation is 1.271. This model run indicates rocks were at 150C at 23 Ma, 49C at 19 Ma, and 29C at 0 Ma.

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49 Figure C-2. Best fit Monte Trax model for H 93RUBY-5. Observed age is 19.50 Ma, predicted age is 19.10 Ma. Observed mean track length is 14.600 m, predicted mean track length is 14.603 m. Observed standard deviation is 1.050, predicted standard deviation is 1.056. This model run indicates that rocks were at 196C at 20 Ma, 69C at 20 Ma, and 30C at 0 Ma.

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50 Figure C-3. Best fit Monte Trax model for H 93RUBY-8. Observed age is 15.90 Ma, predicted age is 16.13 Ma. Observed mean track length is 14.540 m, predicted mean track length is 14.565 m. observed standard deviation is 0.900, predicted standard deviation is 0.935. This model run indicates rocks were at 163C at 17 Ma, 46C at 17 Ma, and 47C at 0 Ma.

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51 Figure C-4. Best fit Monte Trax model for H 97RUBY-41. Observed age is 18.50 Ma, predicted age is 19.01 Ma. Observed mean track length is 14.570 m, predicted mean track length is 14.534 m. Observed standard deviation is 1.190, predicted standard deviation is 1.186. This model run indicates rocks were at 102C at 20 Ma, 52C at 15 Ma, and 33C at 0 Ma.

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52 Figure C-5. Best fit Monte Trax model for H 97RUBY-42. Observed age is 14.40 Ma, predicted age is 14.35 Ma. Observed mean track length is 14.860 m, predicted mean track length is 14.862. Observed standard deviat ion is 1.070, predicted standard deviation is 1.067. This model run indicates that ro cks were at 152C at 15 Ma, 50C at 14 Ma, and 32C at 0 Ma.

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53 Figure C-6. Best fit Monte Trax model for H 97RUBY-51. Observed age is 18.60 Ma predicted age is 18.61 Ma. Observed mean track length is 14.650 m, predicted mean track length is 14.662 m. Observed standard deviation is 1.000, predicted standard deviation is 1.027. This model run indicates that rocks were 143C at 19 Ma, 61C at 19 Ma, and 33C at 0 Ma.

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54 Figure C-7. Best fit Monte Trax model for H 97RUBY-53. Observed age is 15.10 Ma, predicted age is 15.04 Ma. Observed mean track length is 14.580 m, predicted mena track length is 14.569 m. Observed standard deviation is 1.020, predicted standard deviation is 1.016. This model run indicates that rocks were at 195C at 16 Ma, 65C at 15 Ma, and 36C at 0 Ma.

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55 Figure C-8. Best fit Monte Trax model for H 97RUBY-54. Observed age is 18.10 Ma, predicted age is 18.10 Ma. Observed mean track length is 14.060 m, predicted mean track length is 14.072 m. Observed standard deviation is 1.550, predicted standard deviation is 1.517. This model run indicates that rocks were 237C at 28 Ma, 75C at 16 Ma, and 33C at 0 Ma.

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56 Figure C-9. Best fit Monte Trax model for H 97RUBY-55. Observed age is 14.70 Ma, predicted age is 14.80 Ma. Observed mean track length is 14.210 m, predicted mean track length is 14.198 m. Observed standard deviation is 1.330, predicted standard deviation is 1.320. This model run indicates that rocks were 174C at 17 Ma, 80C at 15 Ma, and 34C at 0 Ma.

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57 LIST OF REFERENCES Arm strong, R. L., and Ward, P., 1991, Evolving ge ographic patterns of Cenozoic magmatism in the North American cordillera: The temporal and spatial association of magmatism and metamorphic core complexes: Journa l of Geophysical Research, v. 96, p. 13,201-13,224. Best, M. G., and Christiansen, E. H., 1991, Lim ited extension during peak Tertiary volcanism, Great Basin of Nevada and Utah: Journal of Geophysical Research, v. 96, p. 13,50912,528. Buck, W., 1988, Flexural rota tion of normal faults: Tectonics, vol.7, i. 5, p. 959-973. Burchfiel, B. C., Cowan, D. S., Davis, G. A., 1992, Tectonic overview of the cordilleran orogen in the western United States: The geology of North America: The Geological Society of America, v. G-3, p. 407-479. Camilleri, P. A., and Chamberlain, K. R., 1997, Me sozoic tectonics and metamorphism in the Pequop Mountains and Wood Hills region, nor theast Nevada: Implications for the architecture and evolution of the Sevier orogen: Geologica l Society of America Bulletin, v. 109, i. 1, p. 74-94. Carter, T. J., Kohn, B. P., Fo ster, D. A., and Gleadow, A. J. W., 2004,How the Harcuvar Mountains metamorphic core complex became cool: Evidence from apatite (U-Th)/He thermochronometry: Geology, v. 32, n. 11, p. 985-988. Carter, T. J., Kohn, B. P., Fost er, D. A., Gleadow, A. J. W., and Woodhead, J. D., 2006, Latestage evolution of the Chemehuevi and Sacramento detachment faults from apatite (UTh)/He thermochronometry: Evidence for mi d-Miocene accelerated slip: Geological Society of America Bulletin, v.118, n. 5/6, p. 698-709. Colgan, P. A., Metcalf, J. R., 2006, Rapid mi ddle Miocene unroofing of the southern Ruby Mountains, Nevada: Geological Society of Am erica Abstracts with Programs, v. 38, n. 7, p. 417. Colgan, P. A., Dumitru, T. A., McWilliams, M, and Miller, E. L., 2006, Timing of Cenozoic volcanism and Basin and Range extension in northeastern, Nevada: New constrainsts from the northern Pine Forest Range: Geologi cal Society of America Bulletin, v. 118, i. 1, p. 126-139. Coney, P. J., and Harms, T. A., 1984, Cordiller an metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression: Geology, v. 12, p.550-554. Crittendon, M., Jr., Coney, P. J., and Davis, G., 1977, Tectonic significance of metamorphic core complexes in the North American cordillera: Geology, v.6, p. 79-80 Dallmeyer, R. D., Snoke, a. W., and McKee, E. H., 1986, The Mesozoic-Cenozoic tectonothermal evolution of the Ruby Mount ains-East Humboldt Range, Nevada: a cordilleran metamorphic core complex: Tectonic, v. 5, p. 931-954.

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58 Dokka, R. K., and Mahaffie, M. J., 1986, Ther mochronologic evidence of major tectonic denudation associated with detachment faulting, northern Ruby Mountians-East Humboldt Range, Nevada: Tectonics, v.5 p. 995-1006. Donelick, R. A., OSullivan, B. P., and Ketcham, R. A., 2005, Apatite fission-track analysis: Reviews in Mineralogy and Ge ochemistry, v. 51; 1, p. 49-94. Faure, G., 1986, Principles of Isotope Geology: New York, New Yor k, John Wiley and Sons, Inc. Foster, D. A., Gleadow, A. J. W., Reynolds, S. J., and Fitzgerald, P. G., 1993, Denudation of metamorphic core complexes and the reconstr uction og the transition zone, west central Arizona: Constraints from apatite fissi on-track thermochronology: Journal of Geophysical Research, v. 98, p. 2167-2185. Foster, D. A., and John, B. E., 1993, Structural and thermal constraints on the initiation angle of detachment faulting in the southern Basin and Range: The Chemehuevi Mountains case study: Geological Society of America, v. 105, p. 1091-1108. Foster, D. A., and John, B. E., 1999, Quantifying t ectonic exhumation in an extensional orogen witt thermochronology: Examples from th e southern Basin and Range Province: Geological Society, London, Speci al Publications, v. 154, p. 343-364. Gallagher, K., 1995, Monte Trax: On Track: The Newsletter of the International Fission-Track community, v. 5, n. 1, i. 10 Gallagher, K., Brown, R., and Johnson, C., 1998, Fissi on-track analysis and its application to geological problems: Annual Review of Ea rth and Planetary Sciences, v. 26, p. 519-572. Gans, P., 1987, An open-system, two-layer crustal st retching model for the eastern Great Basin: Tectonics, v. 6, i. 1, p. 1-12. Gifford, J., 2008, Quantifying Eocene and Miocene ex tension in the Sevier hinterland of the Ruby-East Humboldt metamorphic core comple x in northeastern Nevada. Gainesville, Florida, The University of Fl orida, M. S. thesis, 90??p. Gleadow, A. J. W., and Duddy, I. R., 1981, A na tural long-term annealing experiment for apatite: Nuclear Tracks and Radiation Measurements, v. 5, p. 169-174. Gleadow, A. J. W., and Brown, R. W., 1999, Fi ssion-track thermochronology and the long-term denudational response to tect onics, In (Ed.), Summerfield, M. A., Geomorphology and Global Tectonics, John Wiley and Sons LTD., Chichester, p. 57-75. Hacker, B. R.,Yin, A., and Christie, J. M., 1990, Di fferential stress, strain rate, and temperatures of mylonization in the Ruby Mountains, Neva da: Journal of Geophysical Researcg, v. 95, p. 8569-8580.

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59 Hodges, K. V., Snoke, A. W., and Hurlow, H. A., 1992, Thermal evolution of a portion of the Sevier hinterland: The Ruby MountainEast Humboldt Range and Wood Hills, northeastern Nevada: Tectonics, v. 11, p. 154-164. Howard, K. A., 1971, Paleozoic metasediment s in the northern Ruby Mountains, Nevada: Geological Society of America Bulletin, v. 82, p. 259-264. Howard, K. A., 1980, Metamorphic infrastructure in the northern Ruby Mountains, Nevada: Geological Society of Am erica, Memoir 153, p. 335-347. Hurlow, H. A., Snoke, A. W., and Hodges, K. V., 1991, Temperature and pr essure mylonization in a Tertiary extensional shear zone, Ruby Mountain-East Humboldt Range, Nevada: Tectonic implications: Geology, v. 19, p. 82-86. Kistler, R. W., Ghent, E. D., and ONeil, J. R ., 1981, Petrogenesis of garn et two-mica granites in the Ruby Mountains, Nevada: Journal of Geophysical Research, v. 86, p. 10,597-10,606. Koppers, A. A. P., 2002, ArArCALC, software for 40 Ar/39 Ar age calculations: Computers and Geosciences, v. 5, p. 605-619. Lee, J., and Sutter, J. F., 1991, Incremental 40Ar/39 Ar thermochronology of mylonitic rocks from the northern Snake Range, Neva da: Tectonic, v. 10, p. 77-100. Ludwig, K. R., 2004, Users manual for ISOPLOT, a geochemical toolkit for Microsoft Excel (v. 3.09a). MacCready, T., Snoke, A. W., Wright, J. E., and Howard, K. A., 1997, Mi d-crustal flow during Tertiary extension in the R uby Mountains core complex, Neva d: Geological Society of America Bulletin, v. 109, p. 1576+1594. McDougall, I., and Harrison, T. M., 1999, Geochronology and Thermochronology by the 40 Ar/ 39 Ar Method: New York, New York, Oxford University Press, Oxford, UK. McGrew, A. J., and Snee, L. W., 1994, Ar /Ar thermochronologic constraints on the tectonothermal evolution of the northern East Humboldt Range metamorphic core complex, Nevada: Tectonophysics, v. 238, p. 425-450. McGrew, A. J., Peters, M. T., and Wright, J. E., 2000, Thermobarometric constraints on the tectonothermal evolution of the East Hu mboldt Range metamorphic core complex, Nevada: Geological Society of America Bulletin, v. 112, p. 45-60. Mueller, K. J., Cerveny, P. K., Perkins, M. E., and Snee, L. W., 1999, Chronology of polyphase extension in the Windermere Hills, northeast Nevada: Geological Society of America Bulletin, v. 111, p. 11-27. Mueller, K. J., and Snoke, A. W., 1993, Progres sive overprinting of normal fault systems and their role in Tertiary exhumation of th e East Humboldt-Wood Hills metamorphic complex northeast Nevada: Tectonics, v. 12, p. 361-371.

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60 Reese, N. M., 1983, Cenozoic tectonic history of the Ruby Mountains and adjacent areas, northeastern Nevada: Constraints from radiomet ric dating and seismic reflection profiles: Dallas, Texas, Southern Methodist University, M. S. thesis, 88p. Sharp, R. P., 1938, Structure of the Ruby-East Hu mboldt Range: Geological Society of America Bulletin, v. 50, p. 881-917. Snoke, A. W., Howard, K. A., McGrew, A. J., Bu rton, B. R., Barnes, C. G., Peters, M. T., and Wright, J. E., 1997, The grand tour of th e Ruby-East Humboldt metamorphic core complex, northeaster Nevada: Part I-Introduc tion and road log: BYU Geology Studies, v. 42, part 1, p. 225-269. Sonder, L. J., and Jones, C. H., 1999, Western United States extension: How the west was widened: Annual Review of Earth and Planetary Sciences, v. 27, p. 417-462. Stockli, D. F., Linn, J. K., Dumitru, T. A., and Miller, E. L., 2001, Miocene unroofing of the Canyon Range durin extension along the Sevier Desert Detachment, west central Utah: Tectonics, v. 20, p. 289-307. Stockli, D. F., 2005, Application of low-temperat ure thermochronometry of extensional tectonic settings: Reviews in Mineral ogy and Geochemistry, v. 58, p. 411-448. Wells, M., Snee, L., and Blythe, A., 2000, Dating of major normal fault systems using thermochronology: an example from the Ra ft River detachment: Basin and Range, western United Staes: Journal of Geophysical Research, v. 105(B7), p. 16303-16327. Wernicke, B., 1992, Cenozoic extens ional tectonics of the U.S. cordillera: The Geology of North America: The Geological So ciety of America, v. G-3, p. 553-579. Wernicke, B., and Axen, G. J., 1988, On the role of isostacy in the evolution of normal fault systems: Geology, v. 16, p. 848-851.

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BIOGRAPHICAL SKETCH Virginia Newman was born in Melbourne, FL in 1975. She graduated from Deltona High School (with honors) in 1993. She earned a Bachel or of Arts degree in linguistics from the University of FloridaGainesville in 1997. Virginia took several years off from education in order to marry and start a family. Once admitted to graduate school at the University of Florida, she spent a year as a teaching assistant. Virginia received a Master of Science degree from the University of Florida in May of 2008. Virginia will continue to pursue her interests in the geological sciences in the private sector.