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Quantifying Eocene and Miocene Extension in the Sevier Hinterland in Northeastern Nevada

Permanent Link: http://ufdc.ufl.edu/UFE0022218/00001

Material Information

Title: Quantifying Eocene and Miocene Extension in the Sevier Hinterland in Northeastern Nevada
Physical Description: 1 online resource (49 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: argon, carlin, complex, core, eocene, exhumation, hinterland, lamoille, miocene, nevada, sevier, tectonics, 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: Rocks exposed in the Ruby-East Humboldt metamorphic core-complex, NE Nevada, provide a guide for reconstructing the pre-Eocene crustal structure in the hinterland of the Sevier Orogen. These rocks occupy the footwall of a major west-dipping normal-sense shear system that may extend ~ 50 km further west under part of the mineralized Carlin trend. Previous thermochronologic studies showed mineral cooling ages from the Ruby Mountains core complex generally grow younger to the WNW, in the direction of fault slip. 40Ar/39Ar biotite and muscovite analyses from transects in the direction of slip on the Ruby detachment give apparent ages between ca. 31 and 21 Ma and are, therefore, consistent with the previous K-Ar data in showing a decrease in cooling age from east to west. These data indicate rapid cooling due to exhumation of the footwall of the detachment started at about 23 Ma. 40Ar/39Ar data from muscovite and biotite along transects across Lamoille Canyon and the East Humboldt Range give apparent ages of ca. 33?31 Ma in the eastern part of the footwall and ca. 25?20 Ma in the western part of the footwall. The ca. 33?31 Ma apparent ages may indicate an Oligocene phase of extension at a poorly-defined rate of 4.5 +6.7/-1.9 km/m.y. A change in slope of the mica age vs. slip distance relationship at ca. 23 Ma suggests that extension began at that time. The gradient in mica cooling ages to the west of the break in slope of this relationship suggests a slip rate of 4.2 to 3.8 km/m.y.
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: UFE0022218:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022218/00001

Material Information

Title: Quantifying Eocene and Miocene Extension in the Sevier Hinterland in Northeastern Nevada
Physical Description: 1 online resource (49 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: argon, carlin, complex, core, eocene, exhumation, hinterland, lamoille, miocene, nevada, sevier, tectonics, 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: Rocks exposed in the Ruby-East Humboldt metamorphic core-complex, NE Nevada, provide a guide for reconstructing the pre-Eocene crustal structure in the hinterland of the Sevier Orogen. These rocks occupy the footwall of a major west-dipping normal-sense shear system that may extend ~ 50 km further west under part of the mineralized Carlin trend. Previous thermochronologic studies showed mineral cooling ages from the Ruby Mountains core complex generally grow younger to the WNW, in the direction of fault slip. 40Ar/39Ar biotite and muscovite analyses from transects in the direction of slip on the Ruby detachment give apparent ages between ca. 31 and 21 Ma and are, therefore, consistent with the previous K-Ar data in showing a decrease in cooling age from east to west. These data indicate rapid cooling due to exhumation of the footwall of the detachment started at about 23 Ma. 40Ar/39Ar data from muscovite and biotite along transects across Lamoille Canyon and the East Humboldt Range give apparent ages of ca. 33?31 Ma in the eastern part of the footwall and ca. 25?20 Ma in the western part of the footwall. The ca. 33?31 Ma apparent ages may indicate an Oligocene phase of extension at a poorly-defined rate of 4.5 +6.7/-1.9 km/m.y. A change in slope of the mica age vs. slip distance relationship at ca. 23 Ma suggests that extension began at that time. The gradient in mica cooling ages to the west of the break in slope of this relationship suggests a slip rate of 4.2 to 3.8 km/m.y.
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: UFE0022218:00001


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357502e66f5275ead4206496963766c4
9a83edbe304e9031cd2f11de21837f43a9338dca







QUANTIFYING EOCENE AND MIOCENE EXTENSION IN THE SEVIER HINTERLAND
IN NORTHEASTERN NEVADA






















By

JENNIFER N. GIFFORD


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 Jennifer N. Gifford































To my mother, who greatly encouraged me in my pursuit of science









ACKNOWLEDGMENTS

I wish to thank my Advisor Dr. David Foster for his help and patience. I also wish to

thank my committee members for their guidance and helpful reviews of my thesis material. I

especially wish to thank Shawn J. Malone for his endless assistance. I also wish to thank Warren

Grice and Misty Stroud for their assistance in analysis procedures. Richard McKenzie for all of

his help with my figures. Finally, I wish to thank my family for their un-ending love and

support.









TABLE OF CONTENTS

page

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

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

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

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

CHAPTER

1 IN TR OD U CTION .......................................................................... .. ... .... 10

2 R E G IO N A L G E O L O G Y ............................................................................. .....................12

3 40Ar/39Ar TH ERM OCHRON OLOGY ...................................................................... ........ 16

P reviou s T herm chronology ......................................................................... ................... 16
40A r/39A r Therm ochronology Results...................... .............. ..................... ............... 18
B io tite ..........................................................................1 9
M u sco v ite ................................................................2 1

4 D ISC U S SIO N ............................................................................... 30

Eocene Exhumation of the Ruby Mountains Metamorphic Core Complex defined by
40Ar/39Ar Thermochronology ...... ................................30
Constraints on the Timing of the Onset of Extension ...................... ................ 31
Constraints on the Detachment Slip Rate ...................................................................................32

5 C O N C L U SIO N S ................................................................4 1

APPENDIX 40Ar/39Ar THERMOCHRONOLOGY METHODS..................... ............... 42

Sam ple Preparation and Irradiation .................................................................................. 42
40Ar/39Ar Analytical Instrumentation and Procedures ............................... .............43

LIST OF REFERENCES ................. .. ......... ...................... 45

BIOGRAPHICAL SKETCH .................. ............... ......... 48









LIST OF TABLES


Table page

3-1 Summary of previous 40Ar/39Ar thermochronology research .........................................26

3-2 Summary of 40Ar/39Ar thermochronology from the Ruby Mountains metamorphic
core com plex ...................................... ......................................................2 8

3-3 Summary of level of chloritization of samples from the Ruby Mountains
m etam orphic core com plex ........................................................................ ..................29









LIST OF FIGURES


Figure page

2-1 Location map of the Ruby Mountains metamorphic core complex (blue), the Elko-
Carlin gold trend (yellow), as well as the Antler and Sevier thrust belts ........................15

3-1 Biotite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples
collected from the Ruby Mountains Metamorphic Core complex along two separate
transects. All ages are weighted plateau cooling ages calculated from three or more
h e atin g step s......................................................................... 2 3

3-2 Muscovite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples
collected from the Ruby Mountains Metamorphic Core complex along two separate
transects. All ages are weighted plateau cooling ages calculated from three or more
h e atin g step s......................................................................... 2 4

4-1 Simplified geologic map showing mineral cooling ages from samples, collected
along transects across Lamoille Canyon and the East Humboldt Range, using
40A r/39A r therm chronology (2 ). .......................................................... .....................35

4-2 Plot of sample calculated ages vs. the altitude at which samples were collected .............36

4-3 All of the mica 40Ar/39Ar thermochronology ages from the Lamoille Canyon transect
were plotted versus their distance along slip direction. .......................................... 37

4-4 Mica 40Ar/39Ar thermochronology ages versus distance in slip direction from the
Lamoille Canyon transect, younger age grouping. A regression was calculated for
the sam ple set ............................................................................... 38

4-5 Mica 40Ar/39Ar thermochronology ages versus distance in slip direction from the
Lamoille Canyon transect, older age grouping. A regression was calculated for the
sa m p le se t ...................................... ..................................................... 3 9

4-6 All of the mica 40Ar/39Ar thermochronology ages from the East Humboldt Range
transect were plotted versus their distance along slip direction ............... .....................40









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

QUANTIFYING EOCENE AND MIOCENE EXTENSION IN THE SEVIER HINTERLAND
IN NORTHEASTERN NEVADA


By

Jennifer N. Gifford

May 2008

Chair: David A. Foster
Major: Geology

Rocks exposed in the Ruby-East Humboldt metamorphic core-complex, NE Nevada,

provide a guide for reconstructing the pre-Eocene crustal structure in the hinterland of the Sevier

Orogen. These rocks occupy the football of a major west-dipping normal-sense shear system

that may extend 50 km further west under part of the mineralized Carlin trend. Previous

thermochronologic studies showed mineral cooling ages from the Ruby Mountains core complex

generally grow younger to the WNW, in the direction of fault slip. 40Ar/39Ar biotite and

muscovite analyses from transects in the direction of slip on the Ruby detachment give apparent

ages between ca. 31 and 21 Ma and are, therefore, consistent with the previous K-Ar data in

showing a decrease in cooling age from east to west. These data indicate rapid cooling due to

exhumation of the football of the detachment started at about 23 Ma. 40Ar/39Ar data from

muscovite and biotite along transects across Lamoille Canyon and the East Humboldt Range

give apparent ages of ca. 33-31 Ma in the eastern part of the football and ca. 25-20 Ma in the

western part of the football. The ca. 33-31 Ma apparent ages may indicate an Oligocene phase

of extension at a poorly-defined rate of 4.5 +6.7/-1.9 km/m.y. A change in slope of the mica age

vs. slip distance relationship at ca. 23 Ma suggests that extension began at that time. The









gradient in mica cooling ages to the west of the break in slope of this relationship suggests a slip

rate of 4.2 + 3.8 km/m.y.









CHAPTER 1
INTRODUCTION

Metamorphic core complexes are important geological features in the North American

Cordillera, both for the glimpse they provide into the middle crust and the valuable tectonic

information they contain. These complexes occur in a long belt, from southern Canada to

northern Mexico in what was formerly crust thickened by Mesozoic and Cenozoic orogenic

events (Coney, 1980). Core complexes develop when low-angle detachment faults exhume

middle-crustal metamorphic rocks in the football of the fault. These complexes have been the

subject of numerous studies (e.g., Armstrong and Ward, 1991; Dallmeyer et al., 1986; Howard,

1980; Howard, 2003; McGrew and Snee, 1994; Snoke, 1980), but the timing and significance of

many remain uncertain. The metamorphic core complex exposed in the Ruby-East Humboldt

Range is of particular interest. This complex is marked by deformed and migmitized upper

amphibolite facies metamorphic rock and associated igneous bodies unroofed by slip on the main

detachment. The Ruby-East Humboldt core complex occupies a central position in the

hinterland region of the Sevier Orogen (e.g., Howard, 2003). This paper examines the timing

and rate of Cenozoic extension and thinning of the lower crust in the Ruby Mountains

metamorphic core complex via high and low temperature thermochronology (40Ar/39Ar) on rocks

within structurally controlled transects.

Metamorphic Core Complex: Core complexes form as middle-crustal metamorphic rocks

are exhumed during rapid extension, commonly in the football of a detachment fault. The rocks

in the football of the detachment fault are uplifted through a progression of metamorphic facies

and deformation mechanisms, resulting in a characteristic sequence of structures. The movement

zone is folded as the result of the bowing upwards of the lower crust to form a broad basement

culmination, and is driven by isostatic rebound due to tectonic denudation (Wernicke and Axen,









1988). Lister and Davis (1989) state that the formation of metamorphic core complexes is driven

by extensional stresses in the upper crust. Three primary models explaining the deformation

leading to the formation of metamorphic core complexes are noted by Brun et al. (1994). These

models are the simple shear model, in which low angle detachments accommodate extension

(Wemicke, 1981; Wernicke, 1985), the rotated normal fault model involving the formation of

high angle normal faults which subsequently rotate to a low dip and cease to be active slip

surfaces (Buck, 1988; Wernicke and Axen, 1988), and the domino (tilted block) model which

postulates straight faults bounding rotating crustal blocks (Angelier and Coletta, 1983; Davis,

1983). Lister and Davis (1989) address the strengths and weaknesses of the broad mechanisms,

and support a multiple detachment model. During the evolution of a metamorphic core complex,

it is necessary to have multiple generations of detachment faults all splaying from an original

master fault, or from a controlling movement zone at depth. The most recently active

detachment fault is the youngest in a succession of faults that progressively remove lower

portions of the upper plate, eating their way upwards through the overlying sequences. The

detachment faults presently observed in the metamorphic core complexes (e.g., Ruby Mountains)

are relatively young, and are only the last in a succession of low-angle normal faults that carve

through the upper crust at the upward terminations of major, shallow-dipping, ductile shear

zones in the extending Cordilleran orogen (Chery, 2001).









CHAPTER 2
REGIONAL GEOLOGY

By the Neoproterozoic (- 600 Ma), the western margin of North America had developed

into a passive continental margin (Burchfiel et al., 1992). This margin was characterized by

marine and non-marine plastic sedimentary rocks, interbedded with carbonates in a westward

thickening wedge (Burchfiel et al., 1992). These rocks were later modified by Phanerozoic

orogenic events. The first Phanerozoic orogenic event is referred to as the Antler orogeny. This

phase is characterized by late Neoproterozoic to upper Devonian shelf sequences thrust eastward

- 200 km onto the edge of the North American craton (Roberts et al, 1958; Poole et al, 1992).

Numerous thrust faults cut across the area affected by the Antler orogeny, leading some authors

to interpret the terrane as an accretionary complex (Oldow, 1984). The basal thrust is preserved

today as the Roberts Mountain thrust (Burchfiel et al., 1992). The next major contractional event

is referred to as the Sonoma orogeny. In central Nevada, the Sonoma orogeny was characterized

by the eastward thrusting of the Havallah assemblage over the western elements of the Antler

orogeny along the Gloconda thrust fault (Burchfiel et al., 1992). This event occurred in the latest

Paleozoic to earliest Triassic (Burchfiel et al., 1992). In the Late Jurassic a further pulse of

compression resulted in the Nevadan orogeny; however, the causes of this contractional event are

uncertain (Burchfiel et al., 1992). From the Early to Late Cretaceous, further shortening and

eastward thin-skinned thrusting define the Sevier orogeny (Burchfiel et al., 1992). Thrusting and

metamorphism associated with this event ended -70 Ma (Burchfiel et al., 1992). Some estimates

place the thickness of the crust affected by the Sevier orogeny as thick as 60 to 70 km in Nevada

(Burchfiel et al., 1992).

The tectonic character of the Cordilleran margin changed during the Cenozoic. The

Mesozoic-Cenozoic transition was marked by continued shortening and crustal thickening









related to the Laramide orogeny and shallow-angle subduction of the Farallon slab (Sonder and

Jones, 1999). During the late Eocene/early Oligocene, the Farallon slab began to roll back and

peel away from its coupling with the bottom of the North American lithosphere (Humphreys,

1995). The rollback of the subducting slab allowed hot asthenospheric material to come into

contact with metasomatized lithospheric mantle, promoting partial melting (Armstrong & Ward,

1991). Sonder and Jones (1999) suggest that this heat was transferred to the crust above the

rolling slab, leaving it mechanically weak. A second consequence of the change in plate

boundary conditions was a gradual relaxation of compressive forces from the plate boundary

(Liu, 2001). As coupling between the North American and Farallon plates diminished due to

slab rollback, so did the force driving thickening. Liu (2001) suggests that this relaxation of

stress on the thickened crust of Eastern Nevada allowed for a change from shortening to

gravitationally driven extension. Large metamorphic core complexes formed throughout

southern Idaho and eastern Nevada in the 40-25 Ma time interval (Armstrong and Ward, 1991),

and can be associated with both ignimbrite magmatism and the gravitational collapse of

thickened crust.

The Ruby Mountains Metamorphic Core Complex: Howard (2003) divides

northwestern Nevada into three broad tectonic elements: the hinterland of the Sevier fold and

thrust belt, the Ruby-East Humboldt metamorphic core complex, and the Elko-Carlin domain.

The Ruby-East Humboldt complex (Figure 2-1) exposes metamorphosed Paleozoic strata, as

well as smaller areas of Proterozoic and possibly Archean rock (Howard, 2003). These

metamorphic units were unroofed during the Tertiary via extensional normal faulting, which also

caused mylonitization and overprinted prior metamorphic features (Howard, 2003).

Metamorphic conditions in the Ruby and Humboldt ranges peaked at upper amphibolite facies,









with lower-grade facies occurring to the east and south (Howard, 2003). Snoke (1980) provides

further divisions for the lithologies present in the Ruby Mountains, dividing them between the

metamorphic core, unmetamorphosed Paleozoic sedimentary units exposed by a low angle fault

system, and Tertiary sedimentary and volcanic units. Granitic rock forms a significant volume of

the Ruby Mountains complex as well, manifested as dikes, sills, and irregular masses, commonly

of two-mica pegmatitic granite (Howard, 1980). The Sevier fold and thrust belt exposes 10 to 13

km of siliciclastic and carbonate miogeoclinal strata, Neoproterozoic to early Mesozoic in age,

east of the Ruby mountains (Howard, 2003). West of the Ruby Mountains in the Elko-Carlin

domain, deep water strata were thrust over the miogeoclinal shelf units during a series of

shortening events during the Paleozoic and Mesozoic (Howard, 2003). Backthrusting in the

Elko-Carlin domain is suggested by overturned folds in several localities (Howard, 2003). In

addition to these three tectonic elements, the region also exposes numerous Jurassic and early

Cretaceous igneous intrusions, with late Cretaceous granites emplaced into deeper crustal levels

of the metamorphic core complex (Howard, 2003).

The deep crustal rocks exposed in the Ruby-East Humboldt metamorphic core complex

provide a guide for reconstructing Eocene crustal structure in northeastern Nevada (Howard,

2003). The exposed rocks from the core complex are in the football of a west-dipping normal-

sense shear system (Howard, 2003). It has been proposed that these rocks may have underlain

the Pifion and Adobe Ranges, 50 km to the west, before Tertiary extension (Howard, 2003).

Eocene events in the Elko-Carlin region included widespread magmatic intrusions, normal

faulting, an elevated geothermal gradient from crustal thinning, and development of lacustrine

deposits in active structural basins (Howard, 2003).









Antler Belt


Sevier Belt


Figure 2-1. Location map. A) the Elko-Carlin gold trend. B) Ruby Mountains metamorphic core
complex.









CHAPTER 3
40Ar/39Ar THERMOCHRONOLOGY

Previous Thermochronology

Despite the numerous studies of Eocene extension in the Elko-Carlin region, a clear picture

of the setting and tectonics has not been formed. Evidence for Eocene extension within the high-

grade metamorphic rocks of the Ruby Mountains comes mainly from 40Ar/39Ar dating of

hornblende, which yielded discordant age spectra that suggested cooling during the late Eocene

era (McGrew and Snee, 1994). The hornblende data are not entirely satisfactory because these

data are discordant, and they can be interpreted in different ways. As shown by Howard (2003),

extensive lacustrine deposits record a large Eocene lake and wetland system, which may suggest

an early Tertiary phase of extension.

Mineral cooling ages obtained by 40Ar/39Ar thermochronology can be used to constrain the

cooling and exhumation histories of highly extended terranes such as metamorphic core

complexes (e.g., Foster and John, 1999; Stockli, 2005). Previously available

thermochronological data relevant to the exhumation and cooling history of the Ruby Mountains

Metamorphic Core Complex produced apparent ages ranging from the Oligocene to the earliest

Miocene. These data are summarized in Table 3-1. These ages are interpreted to record cooling

through the argon closure interval for biotite, which is about 350-280 at about C depending on

cooling rate (McDougall and Harrison, 1999).

Cooling ages given by mica K-Ar and apatite fission-track data from the Ruby Mountains

core complex generally grow younger in the "west-northwest direction of extensional unroofing"

(McGrew and Snee, 1994). Analyses reported by McGrew and Snee (1994), from 3 muscovites

and 5 biotites yield apparent ages between 21.9 0.2 Ma and 26.7 + 0.1 Ma. They also

determined that at ca. 20 Ma, rapid cooling occurred through the temperature interval of









approximately 3000C to below 1000C. The K-Ar biotite apparent ages indicate the time at which

the sample passed through the argon retention isotherm or partial retention interval. The closure

temperature for biotite is dependant on cooling rate, but generally varies between 280 and 350C

(McDougall and Harrison, 1999). The age progression of younger ages to the west can be

interpreted to suggest the rate of progressive westward unroofing and cooling of the core

complex, or an east-tilted oblique section through a zone of partial argon retention.

Foster and John (1999) showed that if mineral cooling ages represent quenching by

progressive unroofing of an isotherm beneath an extensional shear zone, an approximate rate of

lateral cooling and detachment slip can be obtained by inverting the rate of lateral age change.

This inversion would indicate Oligocene and early Miocene exhumation of a westward moving

hanging wall at a rate of 1 to 2 km/m.y. The westward-younging cooling age patterns, if they are

combined with older mica cooling ages from exposures on the eastern side of the core complex,

indicate prolonged extensional exhumation beginning in the early Tertiary era (McGrew and

Snee, 1994). If age-progression data were found to suggest that the metamorphic core moved

cumulatively 30-40 km east below its hanging wall over approximately 20-30 m.y., the core

would previously have been to the west, underneath the valley along the east margin of the

Adobe and Pifion ranges.

This apparent displacement rate calculated from the previously published data is several

times slower than those reported for core complexes in the Raft River and Snake ranges, and the

Colorado River extensional corridor (Miller et al., 1999; Foster and John, 1999; Wells et al.,

2000; Carter et al., 2004), but similar to those from core complexes in the northern Rocky

Mountains (Grice, 2006). Extension rates in many other areas of the Basin and Range province









accelerated markedly about 15-17 Ma, as determined by fission-track dating of apatite (Miller et

al., 1999, Carter et al., 2004).

Dallmeyer et al., (1986) obtained 40Ar/39Ar and K-Ar ages from biotite samples in the

mylonite zone located in the Northern Ruby Mountains, 40Ar/39Ar for two samples in Lamoille

Canyon, and 40Ar/39Ar for biotite samples located in the South-Eastern section of the East

Humboldt Range. In the East Humboldt Range outside of the mylonite zone, apparent ages

ranged from 32 to 33 Ma. Within the mylonite zone ranging down into the Northern Ruby

Mountains, 40Ar/39Ar ages were younger but slightly more widespread, ranging from 22 to 27

Ma. In Lamoille Canyon biotite from a non-mylonitic magmatic core rock yielded an age of

25.3 0.7 Ma. Further west towards the opening of Lamoille Canyon and nearer to the

detachment fault, a biotite from within the mylonite resulted in an age of 20.8 + 0.5 Ma.

Dallmeyer et al. (1986) hypothesized that cooling began by ~ 45 Ma and that the rocks reached

temperatures below 3000C by 20 Ma.

Dokka et al., (1986) published fission track data for apatites, sphenes, and zircons as well

as 40Ar/39Ar thermochronology on biotites across the northern Ruby Mountains, and some

samples in the lower East Humboldt Range. The fission track analyses and biotite 40Ar/39Ar

yielded concordant ages from ca. 23.4 to 25.4 Ma, indicating the timing of onset of extension and

the start of rapid cooling along the detachment fault.

4Ar/39Ar Thermochronology Results

Mineral separates biotitee and muscovite) were obtained from a total of twenty-three rock

samples (sixteen from the Lamoille Canyon transect and seven from the East Humboldt Range

transect) for the 40Ar/39Ar thermochronology in this study. These mineral separates were then

analyzed in vacuum laser total fusion, laser step-heating and furnace step-heating experiments in

the noble gas laboratory at the University of Florida. Samples were run through furnace step-









heating as well as laser step-heating due to scheduling conflicts with the laser. The methods for

rough sample preparation, analytical instrumentation and procedures, and thermochronological

data reduction are summarized in Appendix A.

A summary of calculated apparent ages from the 40Ar/39Ar thermochronology analyses are

reported in Table 3-2. These results are organized into two groups; one corresponds to rock

samples collected along the Lamoille Canyon transect and the other group to samples collected

along the East Humboldt Range transect. Individual 40Ar/39Ar cooling ages are reported in Table

3-2 according to sample name, rock type, sample latitude and longitude, elevation, and the

mineral that was dated. Total % 39Ar gas used in the weighted plateau age calculation during

step-heating and MSWD (mean standard weighted deviates) are also reported with the calculated

cooling ages in Table 3-2. MSWD values are only relevant in step-heating age calculations, and

are not reported for the laser total fusion age calculations. The higher MSWD values for some of

the step-heating age calculations is generally due to the presence of excess 40Ar in the first and

last few heating steps of the experiment. However, the plateau ages are considered robust with

these anomalous steps omitted. The J-values (from biotite GA1550 mineral flux monitors) used

in individual cooling age calculations are also included in Table 3-2. The individual mineral

cooling age spectra are shown in Figures 3-1 and 3-2.

Biotite

Laser step-heating experiments from two biotite separates across the East Humboldt

transect resulted in mostly well-defined weighted age plateaus (Figure 3-1). Biotite from granitic

gneiss sample DF02-212, yielded a flat age-plateau over 65% of the total 39Ar gas released

corresponding to an apparent age of 22.6 1.2 Ma (MSWD = 3.08). Biotite from DF02-214, a

biotite granite sampled from the same location as DF02-212, gave a plateau age of 24.3 + 1.2 Ma

(99% total 39Ar gas, MSWD = 11.38).









Furnace step-heating experiments were performed on one biotite from the East Humboldt

Range transect and three from the Lamoille Canyon transect. DF02-211, a biotite granite from

the East Humboldt Range yielded a weighted plateau age of 23.2 2.0 Ma (86% total 39Ar gas,

MSWD = 2.67). DF02-205, a leucogranite, produced a weighted average age of 24.8 2.7 Ma

using 87% of the total 39Ar gas, with a MSWD of 15.25. The age spectrum for this biotite is

internally dischordant probably because of the presence of chlorite intergrowths and 39Ar recoil.

The percentage of chloritization within the DF02 biotite samples is shown in Table 3-3. DF02-

206, a biotite granite yielded a flat weighted plateau age of 22.3 2.6 Ma, MSWD = 9.42.

Lastly, DF02-208, a biotite leucogranite from Lamoille Canyon yielded a weighted plateau age

of 17.0 + 1.6 Ma, but with only 42% of the 39Ar gas involved in the calculation, this sample was

excluded from further analysis because of low total K20 and a large discordance between

adjacent steps.

The higher MSWD values for samples DF02-214, DF02-205, and DF02-206 are due to the

presence of excess Ar in the first few heating steps of the experiments, and chlorite intergrowths

within the biotite grains. These samples were all collected from near the detachment interface

and were overprinted within the greenschist facies during the formation of the retrograde

mylonite. Fluid infiltration and the formation of chlorite intergrowths explains the more

discordant age spectra and lower apparent K20 values of these samples.

Laser total fusion experiments and data reduction from eight biotite separates along

Lamoille Canyon (separated by Virginia Newman, and analyzed by Mike Hartley) yielded ages

ranging from ca. 20.7 to 31 Ma. Biotite from H97RBY-42 is a leucogranite produced an age of

31.2 1.8. H97RBY-51 is a biotite garnet aplite which yielded an age of 31.3 1.5 Ma.

H93RBY-8 is a biotite amphibolite and yielded an age of 30.9 1.8 Ma. H93RBY-4 is a biotite









granodiorite which yielded an age of 23.1 + 1.0 Ma. H97RBY-53 is a biotite quartzite which

yielded an age of 24.4 + 0.8 Ma. H97RBY-54 is a biotite granite which yielded an age of 24.7 +

2.2 Ma. H97RBY-55 is a mylonitic granite gneiss which yielded an age of 20.7 + 1.0 Ma.

Muscovite

Muscovite separates from nine different samples collected from the two transects across

the Ruby Mountains underwent in-vacuo laser and furnace step-heating 40Ar/39Ar analyses.

Most of the analyses of muscovite resulted in largely well-defined, flat-shaped age spectra of

better quality than the biotite analyses (Figure 3-2).

Along the Lamoille Canyon transect, five muscovite separates were analyzed. They follow

a trend of younging from east to west. Three samples were analyzed using laser step-heating.

DF02-216 is a leucogranite. Muscovite from this sample gave a plateau age of 32.4 0.7 over

94 percent of the total 39Ar gas released (MSWD = 0.22). Muscovite of sample DF02-218

yielded a flat age plateau over 99% of the total 39Ar gas released and a cooling age of 32.4 0.6

Ma (MSWD = 0.53). Located much further to the west, muscovite from DF02-203, a quartzite

sample, yielded a plateau age of 22.6 0.8 Ma (95% 39Ar gas, MSWD = 4.32). Two samples

were analyzed using furnace step-heating. DF02-209 yielded muscovite with an age of 21.7

3.1 Ma with 82% of the 39Ar gas going into the calculation and a MSWD of 7.86. DF02-210 was

a quartzite sampled from the same location as DF02-209, which yielded an age of 27.9 + 3.8 Ma

(57% 39Ar, MSWD = 3.15) and a very discordant age spectrum.

Four muscovite samples from the East Humboldt Range transect were analyzed with laser

step-heating and furnace step-heating methods. DF02-215 is a leucogranite located furthest west

along the transect, and DF02-219 located on the furthest east side of the East Humbodlt Range

along the valley between this range and the Wood Hills. Using laser step-heating, DF02-215

yielded an age of 21.5 0.5 Ma with 100% of the 39Ar gas and an MSWD of 0.23. DF02-219 is









a muscovite quartzite which yielded an age of 27.4 0.6 (89% 39Ar, MSWD = 0.23). DF02-221

and H03WH-42 are samples both located in the Wood Hills, east of the Humboldt Range, and

were analyzed with furnace step-heating. DF02-221 was from a quartz vein and the muscovite

gave an age of 43.7 5.3 Ma (89% 39Ar, MSWD = 4.50). H03WH-42 was a marble and

produced an age of 49.3 3.6 Ma (75% 39Ar, MSWD = 5.40).


















45-

40-

35 -

30
-
25-
20-

15
IS-

10 -

5-


45-

40-

35-
30-

25-

20-

15 -

10 -

5-


50

45

40

35

30

25

20


10

5


MSWD=1525


0 10 20 30 40 50 60 70 80 90 1C
Cumulative 39Ar Released (%)


0 10 20 30 40 50 60 70 80 90
Cumulative 39Ar Released (%)


0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


0 0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%) D


5-
MSWD = 11 38

0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)F


Figure 3-1. Biotite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples

collected from the Ruby Mountains Metamorphic Core complex along two separate

transects. All ages are weighted plateau cooling ages calculated from three or more

heating steps. A) DF02-205. B) DF02-206. C) DF02-208. D) DF02-211. E) DF02-

212a. F) DF02-214a


Do *... Bi~ Ciis- g


MSWD =1 59
DFu _.,; B.R:.l.l- f, I
OFOI


MSWD = 3 08


















MSWD =4 32


0-
0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


10 20 30 40 50 60 70
Cumulative 39Ar Released (%)


Di.. zIi,.:lu-::..l- ug"-






MSWD = 0 22

S 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


25-

20- E F.., : s' ..1 ..1.i r I r

15-

10

5 -
MSWD = 0 53
0-
0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


u "i'I tl i'iu, ::,,1I- -










MSWD= 0 23

0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


25

20
o C-i.i.. 2 rC io E::L ..ii- "Q
15 1

10

5
_0 MSWD = 2 37
0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)



50

45

40

35

30

25

20 D.Fu. I i, h... .i Gr

15 ? ir,,

10

5-
MSWD = 0 67

0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


Figure 3-2. Muscovite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples

collected from the Ruby Mountains Metamorphic Core complex along two separate

transects. All ages are weighted plateau cooling ages calculated from three or more

heating steps. A) DF02-203a. B) DF02-209. C) DF02-210. D) DF02-215a. E) DF02-

216a. F) DF02-216b. G) DF02-218a. H) DF02-218b. I) DF02-219a. J) DF02-219b.

K) DF02-221. L) H03WH-42.


DFu.I.", .I (. aj


40

35

30
-
25-
--
20

15

10

5-


50

45

40

35

30

25

20

15

10

5

0
C

E


50-

45

40-

35 -

30 -



&.
























45

40

35

30

25

20

15

10

5


DFu:._ I Il i,, ,,-..: .-I






MSWD = 0 23

0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


45-

40-

35-

30-

- 25-

20-

15-

10-

5-

0-


0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)


Cumulative 39Ar Released (%)


Figure 3-2. Continued.











Table 3-1. Summary of previous 40Ar/39Ar thermochronolovg research.


Sample


Rock Type


Samples from 40Ar/39Ar transect in the East Humbolt Range
15 mylonitic orthogneiss 40"
14 protomylonitic q dioritic orthogneiss 40"
4119-25 40'
13 protomylonitic q dioritic orthogneiss 40"
4119-9 40'
880805-3 bt sill ksp pl q gar schist 40'
RM-19 hb bt q dioritic gneiss 40"
880709-2 ms gar pegmatitic gneiss 40"
880706-2A amphibolite 41
870718-1 hb bt q dioritic gneiss 41
870719-2B bt monzogranitic gneiss 41
18 nonmylonitic amphibolite 41
870614-3 ms bt schist 41
S 870623-2 ms bt sill gar schist 41
17 mylonitic q dioritic orthogneiss 41
16 mylonitic q dioritic orthogneiss 41


Latitude Longitude Mineral


49' 44"
50' 21"
50' 21"
50'31"
50'31"
56'50"
59' 43"
59' 54"
00'00"
00' 33"
00' 51"
01' 22"
02' 05"
02'38"
02' 42"
03'21"


Samples from 40Ar/39Ar transect in the Northern Ruby Mountains
5 mylonitic quartzite 40 26':


6
4
5169-19
B
8
7
G
I
H
NEV-13-80


nonmylonitic mygmatitic core
nonmylonitic mygmatitic core

monazite-bearing bt schist
mylonitic q dioritic orthogneiss
mylonitic q dioritic orthogneiss

ms gneiss porphyry


05' 50"
04' 46"
04'46"
04' 47"
04'47"
06'21"
05'26"
06'08"
05' 45"
05' 10"
05' 25"
05' 52"
05' 50"
06' 24"
00' 33"
59' 55"



16' 17"
16' 33"
17'38"
16'21"
13' 57"
16'23"
15'56"
14'23"
12'42"
14'35"
59' 55"


20"


46' 06"
46' 16"
46'39"
47'21"
49' 32"
49' 54"
51' 47"
52'04"
52'34"
03'21"


bt
bt
sphene
bt
apt
bt
bt
ms
bt
bt
bt
bt
ms
ms
bt
bt


Age
(Ma)



33.7
31.90
23.8
32.9
25.1
26.74
23.43
21.9
21.9
20.89
21.5
25.3
22.1
22.2
27.7
25.0



21.7
22.4
21.5
18.4
23.4
22.4
23.5
13.3
153.9
15.5
26.5


Errors
(+/-) 20 Method


1.1
0.8
2.5*
0.8
4.2*
0.08
0.1
0.2
0.2
0.1
0.2
0.6
0.2
0.3
0.8
0.8



0.7
0.7
0.6
2.5*
0.4
0.8
0.8
1.0
0.9
0.4
6.3*


Ar-Ar
Ar-Ar
FT
Ar-Ar
FT
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar
Ar-Ar



Ar-Ar
Ar-Ar
Ar-Ar
FT
K-Ar
Ar-Ar
Ar-Ar
K-Ar
K-Ar
K-Ar
FT


Source


Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dokka et al., 1986
Dallmeyer et al., 1986
Dokka et al., 1986
McGrew and Snee, 1994
McGrew and Snee, 1994
McGrew and Snee, 1994
McGrew and Snee, 1994
McGrew and Snee, 1994
McGrew and Snee, 1994
Dallmeyer et al., 1986
McGrew and Snee, 1994
McGrew and Snee, 1994
Dallmeyer et al., 1986
Dallmeyer et al., 1986



Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dokka et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dallmeyer et al., 1986
Dokka et al., 1986










Table 3-1. Continued


Sample Rock Type
Samples from 40Ar/39Ar transect in Lamoille Canyon
3 mylonitic
1 nonmylonitic mygmatitic core


Age
Latitude Longitude Mineral (Ma)


40 41'34" 115 28'03" bt
40 37'41" 115 21'58" bt


Errors
(+/-) 2o Method Source


20.8 0.5
25.3 0.7


Ar-Ar Dallmeyer et al., 1986
Ar-Ar Dallmeyer et al., 1986


Note: grd = granodiorite, q di = quartz diorite, gr = granite, q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, ksp = potassium-feldspar, amp
amphibolite, gar = garnet, gn = gneiss, sill = sillimanite, pi = plagioclase feldspar, sample age errors are reported at lo.











Table 3-2. Summary of 40Ar/39Ar thermochronology from the Ruby Mountains metamorphic core complex
Elevation Age Errors %
Sample Rock Classification Latitude Longitude (m) Mineral (Ma) (+/-) 39Ar, MSWD, J value Comments


Sairnles froir the 40Ar/39Ar transect


n i Lamoille Canyon


DF02-203
DF02-205
DF02-206
DF02-208
DF02-209
DF02-210
DF02-216
DF02-218
H93RBY-4
H93RBY-8
H97RBY-42
H97RBY-51
H97RBY-52
S H97RBY-53
00 H97RBY-54
H97RBY-55


2-mica quartzite
bt grd
bt gr
bt monzo-gr
2-mica q-rich grtd
ms quartzite
ms gar q syenite
ms gar syeno-gr
bt grd
bt amp
bt gr
bt gar gr
bt gn
bt quartzite
bt gr
bt gn


41.124'
40.024'
38.815'
37.628'
37.985'
37.985'
37.267'
37.539'
39.278'
37.661'
36.765'
35.596'
37.847'
38.926'
40.461'
41.500'


26.930'
24.115'
21.599'
29.436'
31.574'
31.574'
17.184'
17.818'
22.499'
21.992'
19.124'
21.631'
22.165'
23.969'
27.933'
28.658'


Samples from the 40Ar/39Ar transect in the East Humbolt Range
DF02-211 bt granodiorite 41 01.989' 115 07.087'
DF02-212 bt q-rich grtd 41 01.615' 115 05.996'
DF02-214 bt monzo-gr 41 01.615' 115 05.996'
DF02-215 ms grd 41 02.891' 115 09.392'
DF02-219 ms quartzite 41 02.278' 115 00.313'
DF02-221 ms quartzite 40 59.696' 114 51.282'
H03WH-42 ms marble 41 02.105' 114 49.893'


2667
3059
3246
3290
2793
2793
1957
2093
3039
2561
2088
2981
2654
2310
2010
1905



2674
2949
2949
2027
1968
2096
2211


4.32
15.25
9.42
1.59
7.86
3.15
0.22
0.53













2.67
3.08
11.38
0.23
0.23
4.50
5.40


0.0005128
0.0036734
0.0037164
0.0038456
0.0037380
0.0037595
0.0005353
0.0005392
0.0060690
0.0060690
0.0060690
0.0060690
0.0060690
0.0060690
0.0060140
0.0060140



0.0037810
0.0005214
0.0005300
0.0005386
0.0005432
0.0038026
0.0038672


Note: grd = granodiorite, q di = quartz diorite, gr = granite, grtd = granitoid, q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, kfs = potassium-
feldspar, amp = amphibolite, gar = garnet, gn = gneiss, % 39Arp = percent of 39Ar used in weighted plateau age calculation, and MSWDp = mean
standard weighted deviates for plateau cooling age, Ish = laser step-heating, fsh = furnace step-heating, Itf = laser total fusion. All cooling ages are
reported in 2 sigma error.


Samnles fro the 40ArP9Ar transectin Lamoille Canvon









Table 3-3. Summary of level of chloritization of samples from the Ruby Mountains
metamorphic core complex


Rock Classification


% chloritization of
Mineral biotite


Samples from the 40Ar/39Ar transect in Lamoille Canyon


DF02-203
DF02-205
DF02-206
DF02-208
DF02-209
DF02-210
DF02-216
DF02-218
H93RBY-4
H93RBY-8
H97RBY-42
H97RBY-51
H97RBY-52
H97RBY-53
H97RBY-54
H97RBY-55


2-mica quartzite
bt granodiorite
bt granite
bt monzo-granite
2-mica q-rich granitoid
ms quartzite
ms gar q syenite
ms gar syeno-granite
bt granodiorite
bt amphibolite
bt granite
bt gar granite
biotite gneiss
bt quartzite
bt granite
bt gneiss


0-5
5-10
15-20


Samples from the 40Ar/39Ar transect in the East Humbolt Range
DF02-211 bt granodiorite bt 10 15
DF02-212 bt q-rich granitoid bt 15 20
DF02-214 bt monzo-granite bt 0 5
DF02-215 ms granodiorite ms
DF02-219 ms quartzite ms
DF02-221 ms quartzite ms
H03WH-42 ms marble ms
Note: q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, kfs
potassium-feldspar, gar = garnet


Sample









CHAPTER 4
DISCUSSION

Eocene Exhumation of the Ruby Mountains Metamorphic Core Complex defined by
40Ar/9Ar Thermochronology

The 40Ar /39Ar thermochronological data obtained from rock samples collected across the

Ruby Mountains define a lateral cooling age gradient, in which cooling ages young progressively

to the west. Individual mineral cooling ages and transect sample localities are shown in Figure

4-1, a simplified geologic map.

Thermochronological data obtained by 40Ar/39Ar thermochronology in this study are

discussed below in the context of the tectonic exhumation of the Ruby Mountains metamorphic

core complex. In particular, the new thermochronologic data, combined with previous

thermochronology, provide constraints on (1) the onset and duration of extension in the Ruby

Mountains metamorphic core complex, and (2) the slip rate of the bounding normal detachment

fault.

Samples were graphed age vs. altitude to see if there was any relationship. Based on

Figure 4-2, there is no visible relationship between altitude and calculated age.

One complication for any discussion of this data is the location of the detachment fault

plane. In many other metamorphic core complexes like the Harcuvar core complex in Arizona

(Foster et al., 1993), physical evidence of the surface is preserved within the mountain range.

This is not true for the Ruby-East Humboldt metamorphic core complex. The fault plane could

have been located anywhere above the current exposure. If the fault plane was directly above the

current mountain top exposures, we would have expected to see a clear pattern between the

altitude of the high elevation samples and the low elevation samples compared to their calculated

ages. If this were the case, the higher elevation samples would show older apparent ages, and the

lower elevation samples would have passed through the closure isotherms for muscovite and









biotite at younger ages as exhumation progressed. As Figure 4-2 illustrates, there is no trend

consistent with this pattern. This leads to the conclusion that the fault plane was located high

enough above the current exposure that the high altitude samples had not yet passed through the

biotite and muscovite Ar retention isotherms.

Constraints on the Timing of the Onset of Extension

The method outlined by Foster and John (1999) was used to evaluate the onset of extension

along the Ruby Mountains metamorphic core complex detachment fault. Figure 4-3 shows an

'age vs. distance in slip direction' graph using muscovite and biotite cooling ages (40Ar/39Ar

thermochronology) obtained from lower plate rock samples. Figure 4-2 also shows a simplified

geologic map of the Lamoille Canyon area with sample locations along a projection line oriented

at 2850, which is the slip direction for the detachment. The samples collected along Lamoille

Canyon were input into ArcGIS, onto a geologic map, and projected orthogonally back to the

slip line, defining their distances in slip direction, which are shown along the x-axis of the graph

in Figure 4-3. The accuracy of this projection was estimated, and error values of 10 meters

were assigned to the samples. The same method was used for the transect in the East Humboldt

Range. The vertical error bars seen in Figures 4-3, 4-4, 4-5, and 4-6 represent the errors in the

individual mica cooling ages, all calculated at 95% confidence (2 sigma).

Muscovite and biotite data were plotted together because at a cooling rate of> 250C/m.y.

the closure temperature difference is less than about 50C, and the error from using both types of

data is, therefore, relatively small. Both muscovite and biotite cooling ages from Lamoille

Canyon young in a progressive pattern towards the west across the Ruby Mountains following

the direction of slip on the detachment fault system. At a distance in slip direction of- 12 km

along the transect, mica cooling ages drop from the late Eocene (> 33 Ma) to the early Oligocene

(< 25 Ma). Further west along the transect, the mica cooling ages decrease more gradually to ~









20 Ma. The change in slope of the mica cooling age curve defines the position of the mica partial

argon retention zone at the onset of exhumation and the start of a period of rapid slip along the

detachment fault. A regression plotted through the collection of younger samples (Figure 4-4)

shows an change in slope at the 12 km mark to intersect the age line at 25 Ma, indicating the

age of the onset of extension in the Ruby Mountains metamorphic core complex.

The mica ages from the East Humboldt Range transect, while more sparse, are consistent

with those of the Lamoille Canyon transect. There is a drop in age at 27 Ma to -24 Ma, and

then a more gradual drop to 21 Ma to the east along the transect. When a regression is plotted

for only the younger samples (Figure 4-7), the regression line gives an age of 25.5 Ma, clearly

coinciding with the onset of extension in the Lamoille Canyon transect samples.

Constraints on the Detachment Slip Rate

Thermochronological data from rock samples collected along lower plate transects in

metamorphic core complexes parallel to the direction of tectonic unroofing have also been

successfully used to estimate previous slip rates on bounding detachment faults (e.g., Foster et

al., 1993; Foster and John, 1999; Stockli, 2005; Wells et al., 2000). The mica ages obtained in

this study likewise may be used to estimate the slip rate along the detachment fault. Slip rate

estimates were made using a combination of both muscovite and biotite cooling age and distance

data. To conduct these regressions, a computer program called Isoplot v. 3.09a (Ludwig, 1991)

was used. Straight lines were fit to the thermochronological data using least-squares regressions.

Note, samples falling into an intermediate age range (i.e., H97RBY-52, DF02-219) were

excluded from both the older and younger regressions for both the Lamoille Canyon transect as

well as the younger age grouping from the East Humboldt Range transect.

Figures 4-4, 4-5, and 4-7 show the results from the Isoplot least-squares regressions. For

Lamoille Canyon, there were two distinct age groupings. The younger group includes eight









samples (from left to right on Figure 4-4) DF02-206, H93RBY-4, H97RBY-53, DF02-205,

DF02-203, H97RBY-54, H97RBY-55, and DF02-209. The older group includes five samples

(from left to right on Figure 4-5) DF02-216, DF02-218, H97RBY-42, H97RBY-51, and

H93RBY-8. H97RBY-52 fell into the middle of the apparent age range and was excluded from

both the older and the younger groups. The older group of samples (Figure 4-5) yielded a

regression slope (m) = -0.24 0.22 (slope errors are 2c). Slip rate estimates for the detachment

were made from this regression by taking the inverse of the absolute values of the regression

slopes and their errors (e.g., see Foster and John, 1999). The slip rate for the older group of

samples was calculated to be 4.2 +4.5 km/m.y. (or cm/yr). The younger group of samples from

Lamoille Canyon (Figure 4-4) yielded a regression slope (m) = -0.24 0.26 (slope errors again at

20). The slip rate for the younger group of samples from the Lamoille Canyon transect was

calculated to be 4.2 3.8 km/m.y. (or cm/yr).

It is important to note that the slip rate calculations are averaged over the time interval

from 33-30 Ma for the older Lamoille Canyon samples and 26-20 Ma for the younger

Lamoille Canyon samples. These ranges are the range of cooling ages used in the slip rate

estimates. The slip rate calculations do not account for increases or decreases in slip along the

detachment within the time interval of- 33-30 Ma and -26-20 Ma (Foster and John, 1999;

Stockli, 2005).

The muscovite and biotite 40Ar/39Ar data from the slightly older group of Lamoille Canyon

transect samples gave apparent ages of ca. 33-31 Ma in the eastern part of the football. The ca.

33-31 Ma apparent age could indicate an Oligocene phase of extension at a poorly defined rate

of- 4.2 + 4.5 km/m.y. However, the apparent ages of these samples are all within error of each









other and could simply be due to slow cooling of the shallower part of the football through the

biotite partial retention interval, or a crustal thermal event at about 30 Ma.

Figures 4-6 shows the mica 40Ar/39Ar ages (Ma) from the East Humboldt Range transect

plotted versus their distance along slip direction (km). The group includes five samples DF02-

219, DF02-212, DF02-214, DF02-211, and DF02-215, as well as 2 samples from Dallmeyer et

al. (1986) and 6 samples from McGrew and Snee (1994).

It appears that the East Humboldt Range might have older and younger grouping similar to

the transect in Lamoille Canyon, but the structural complexity of the region and the geographical

scattering of the samples doesn't allow a regression for the data.












cR.R2a D-B L-21 DFlm-211
iT2: .- 91 1 ,, 1,l..e Br3 2I .-2. DF 1212 OFrni
ST 2 1i 1 l .n 1" DF G-,n
MOF, -.I .


o 2 M, k 115


IHrtbddl
Entsj
Wrht--J
pt-v.


-410


Hkod H HI


S Palagen


S^Pae----P-e-


SMe Boic

- Fault Mesoaaic
S bLoaw-angle Pae
rwneal f aull
-A- Thrustfault Lower P
Proteraa
,' P Mylonite
- Fd Archean
?:S K-Ar biotie Dcding agechrontours


a srata &


e intruaeis


intrudias

& upper
0 strata

SWetela


HSnEY- HOMBSWW
HO-l &in B3T 311O -w m 1.6 BT a1.2 %'. 1.8
BFTr .a T .1. 1.
eT sin. ,a-..


ME FaI DF- L M
MSif m-flb U s M 1 *f-l.A


Figure 4-1. Simplified geologic map showing mineral cooling ages from samples, collected
along transects across Lamoille Canyon and the East Humboldt Range, using
40Ar/39Ar thermochronology (2c).


sar 2 .. it


1M ". 1.. "



ufD2eD-
WYzi.;e~ i


Mcunlairls


HlIV H-4a
as 4ta 4 -ae


..F


Bszn 12


BT .I ,-..










34
data-point error crosses are 2o
M M
32 -


30 -


28 1


S26
o I
0) *B IB B
S24 -


22 M B


20 -


18
1800 2000 2200 2400 2600 2800 3000 3200

Altitude (m)


Figure 4-2. Plot of sample calculated ages vs. the altitude at which samples were collected.













34
data-point error crosses are 2o

32


30

Onset of extension
28


26


24


22


20


18
4 8 12 16 20 24

Distance in Slip Direction (km)


Figure 4-3. All of the mica 40Ar/39Ar thermochronology ages from the Lamoille Canyon transect
were plotted versus their distance along slip direction.

















26



s 24

0)

22



20



18


14 16 18 20 22 24

Distance in Slip Direction (km)


Figure 4-4. Mica 40Ar/39Ar thermochronology ages versus distance in slip direction from the
Lamoille Canyon transect, younger age grouping. A regression was calculated for the
sample set.

















ls \M


Slope = -0.240.22 (2a)
Inter = 33.761.5
Xbar = 6.77129, Ybar =32.1386
MSWD = 0.24, Probability = 0.87


I[e]


data-point error crosses are 20











l~jB


Distance in Slip Direction (km)


Figure 4-5. Mica 40Ar/39Ar thermochronology ages versus distance in slip direction from the
Lamoille Canyon transect, older age grouping. A regression was calculated for the
sample set.


33




32




31




30




29









data-point error crosses are 2o


25 27 29 31
Distance in Slip Direction (km)


All of the mica 40Ar/39Ar thermochronology ages from the East Humboldt Range
transect were plotted versus their distance along slip direction.


22 I


Figure 4-6.


to1

*@









CHAPTER 5
CONCLUSIONS

The thermochronological data set obtained from the Ruby Mountains metamorphic core

complex transect across Lamoille Canyon shown in this study provide several constraints on the

exhumation onset and slip rate during the Eocene and Miocene: (1) The age of the onset of

extension in the Ruby Mountains is indicated to be -25 Ma by the marked break in the slope of

the cooling age curve on the age vs. distance diagram constructed from the mica cooling ages

(Fig. 4-3). This thermochronology-based age constraint is in good agreement with the previous

thermochronology done in the area (e.g., Dallmeyer et al., 1986; Dokka et al., 1986; Howard,

2003; McGrew and Snee, 1994). Thus, the onset of extension in the Ruby Mountains is now

well constrained and confirmed to be at 25 Ma. Furthermore, (2) the cooling ages from micas

show that extension in the Ruby Mountains across Lamoille Canyon continued until at least 20

Ma. (3) Muscovite and biotite 40Ar/39Ar thermochronological data obtained during this study

were also used to constrain the slip rate on the detachment fault. These data show that between -

26-18 Ma, the averaged slip rate on the detachment was 4.2 + 3.8 km/m.y. (4) 40Ar/39Ar data

from the eastern part of the football may indicate an Oligocene phase of extension at a rate of-

4.2 + 4.5 km/m.y. (or cm/yr) from 33-31 Ma, or record the graduate cooling before the onset

of extension.









APPENDIX
40Ar/39Ar THERMOCHRONOLOGY METHODS

Sample Preparation and Irradiation

20 samples were collected along two transects in 2002. Billets were cut from the samples

and were sent to Texas Petrographic Inc. to be cut and polished into thin sections. Examination

of the thin sections provided information on the extent of alteration to various mineral phases in

the rock samples to be used in 40Ar/39Ar analyses. Samples that showed un-altered micas, K-

feldspars, and hornblende were selected for mineral separation and 40Ar/39Ar analyses.

The selected samples were crushed and milled into a sand sized fraction using a Sturtevant

rock Jaw Crusher and Bico Pulverizer type UA disk mill. The pulverized sample was sieved.

Each sample was then run through the water table to separate minerals according to density. The

IV and III (lightest) water table fractions were used further to process for K-feldspar and

muscovite, and the water table II and I fractions were processed for biotite and hornblende.

Following water table processing, tetrabromoethane (TBE) and methylene iodine (MI)

heavy liquids (densities of 2.96 and 3.33 g/cm3, respectively) were used to separate the micas,

K-feldspar, and hornblende from less dense minerals. These separates were rinsed with ethanol

(for TBE) and acetone (for MI) 2-3 times following separation. Special dilute TBE mixes were

used to further separate the TBE floats (containing quartz, K-feldspar, plagioclase feldspar,

muscovite, etc.) to first sink the quartz and feldspars and allow the muscovite to float, and then to

sink the quartz and plagioclase feldspar and allow the K-feldspar to float.

A Frantz magnetic separator Model L-1 was used to separate the biotite and hornblende

from non-magnetic phases. Only the most magnetic biotite and hornblende Frantz separates

were kept to avoid inclusion-rich minerals. All mineral separates were then hand picked under a

binocular microscope for better refinement using standard picking tools (i.e., nylon brushes,









Pyrex glass dishes, and wax weighing paper). Some separates were given an ultrasonic bath in

de-ionized water for approximately fifteen minutes to remove any altered materials.

Nine muscovite separates and seven biotite separates along with GA1550 biotite flux

monitors (98.79 + 0.5 Ma, see Reene et al., 1998) were individually packaged in aluminum foil

(- 5 mg for mineral separate, 1 mg for flux monitors) and sealed in a pure quartz glass. The

mineral separates and flux monitors were irradiated in 2 different batches in a 1.1 MW TRIGA

MARK II research nuclear reactor at the Oregon State University Radiation Center. For a more

detailed description of these facilities and irradiation methods see

http://ne.oregonstate.edu/facilities/radiation_center/. The first batch, OS10, was irradiated for 2

hours. This included samples: DF02-203 (muscovite), DF02-212 biotitee), DF02-214 biotitee),

DF02-215 (muscovite), DF02-216 (muscovite), DF02-218 (muscovite), and DF02-219

(muscovite). The second batch, OS11, was irradiated for 7 hours and included samples DF02-

204 biotitee), DF02-205 biotitee), DF02-206 biotitee), DF02-208 biotitee), DF02-209

(muscovite), DF02-210 (muscovite), DF02-211 biotitee), DF02-221 (muscovite), and H03WH-

42 (muscovite).

40Ar/9Ar Analytical Instrumentation and Procedures

The 40Ar/39Ar analyses were carried out in the noble gas laboratory at the Department of

Geological Sciences, University of Florida. A combination of both laser ablation step-heating

and furnace step-heating techniques were utilized to extract Ar gas from mineral separates.

Laser ablation step-heating of mica separates was done by a water-cooled New Wave Research

model MIR10 30W CO2 laser. During laser step-heating the New Wave laser was manually

controlled using LAS (laser ablation software) version 1.3.0.1 by New Wave Research. Mica

separates were ablated a total of 5-15 steps under a 1750 [tm continuous wavelength focused

laser beam at 2-5.5% power. A final laser fusing-step was done at 10-12% power. The step-









heating schedule used for laser ablation varied and was adjusted accordingly to maximize Ar gas

output for the mineral separates.

A water-cooled, double vacuum, resistively heated furnace was used to step-heat the

remaining mica separates. The furnace step-heating analyses were controlled automatically. The

heating schedule was programmed into the computer to maximize the Ar gas output for each

heating step ranging from a de-gassing step at 400 degree to a fusing step at 1450 degrees.

Ar gas extracted from mineral samples by laser and furnace step-heating was transferred

by vacuumed lines to a getters trap for 10 minutes to remove reactive gasses. The purified Ar

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

equipped with a filament for gas ionization and a magnetic sector mass discriminator followed

by a Balzers Electron Multiplier collector to measure the isotopic abundances of 36Ar, 37Ar, 38Ar,

39Ar, and 40Ar. Cold laser blanks were analyzed at the beginning of each analytical session and

every five steps after. The laser blanks were obtained by closing the laser chamber for two

minutes and then passing the blank gas to the mass spectrometer following the same steps as a

regular sample step. Heated furnace blanks were made by closing and heating the empty furnace

to 4000C, 600C, 800C, 1000C, 1200C and 1450C prior to sample step-heating.

Data files produced from the mass spectrometer sample and blank analyses were imported

into the program ArArCALC version 2.4 by Koppers (2002) for data reduction and 40Ar/39Ar

cooling age calculations. ArArCALC uses Microsoft Excel to plot data tables, age plateaus

and isochrons. ArArCALC was also used to calculate J-values from laser total fusion analyses of

the GA1550 biotite flux monitors. These J-values were then applied to their individual samples

to better constrain their 40Ar/39Ar cooling age calculations.









LIST OF REFERENCES


Angelier, J., and Coletta, B., 1983, Tension fractures and extensional tectonics: Nature, v. 301, p.
203-222.

Armstrong, R.L., and Ward, P., 1991, Evolving geographic patterns of Cenozoic magmatism in
the North American cordillera: The termoral and spatial association of magmatism and
metamorphic core complexes: Journal of Geophysical Research, v. 96, p. 13,201-13,224.

Brun, J., Sokoutis, D., and Driessche, J. V. D., 1994, Analogue modeling of detachment fault
systems and core complexes: Geology, v. 22, p. 319-322.

Buck, W.R., 1988, Flexural rotation of normal faults: Tectonics, v. 7, 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.

Carter, T.J., Kohn, B.P., Foster, D.A., and Gleadow, A.J.W., 2004, How the Harcuvar Mountains
metamorphic core complex became cool: Evidence from apatite and (U-Th)/He
thermochronometry: Geology, v. 32, p. 985-988.

Chery, J., 2001, Core complex mechanics: From the Gulf of Corinth to the Snake Range:
Geology, v. 21, p. 439-442.

Coney, P.J., 1980, Cordilleran metamorphic core complexes: An overview: Geological Society
of America, Memoir 153, p. 7-31.

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: Tectonics, v. 5, p. 931-954.

Davis, G.H., 1983, Shear-zone model for the origin of metamorphic core complexes: Geology, v.
11, p. 342-347.

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

Foster, D.A., Gleadow, A.J.W., Reynolds, S.J., Fitzgerald, P.G., 1993, Denudation of
metamorphic core complexes and the reconstruction of the transition zone, West Central
Arizona: Constraints from apatite fission track thermochronology: Journal of
Geophysical Research, v. 98, p. 2167-2185.









Foster, D.A., and John, B.E., 1999, Quantifying tectonic exhumation in an extensional orogen
with thermochronology: examples from the southern Basin and Range Province: In Ring,
U., Brandon, M., Lister, G.S., and Willett, S.D. (eds), Exhumation Processes: normal
faulting, ductile flow, and erosion: Geological Society (London) Special Publication, 154,
p. 356-378.

Grice, W., 2006, Exhumation and cooling history of the middle Eocene Anaconda metamorphic
core complex, western Montana: Master of Science Thesis Defense, University of
Florida.

Henry, C.D., and Boden, D.R., 1998, Eocene Magmatism: The heat source for Carlin-type gold
deposits of northern Nevada: Geology, v. 26, p. 1067-1070.

Hofstra, A.H., and Cline, J.S., 2000, Characteristics and models for Carlin-type gold deposits:
Reviews in Economic Geology, v. 13, p. 163-220.

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

Howard, K.A., 2003, Crustal structure in the Elko-Carlin region, Nevada, during Eocene gold
mineralization: Ruby-East Humboldt metamorphic core complex as a guide to the deep
crust: Economic Geology, v. 98, p. 249-268.

Kistler, R.W., Ghent, E.D., O'Neil, J.R., 1981, Petrogenesis of garnet two-mica granites in the
Ruby Mountains, Nevada: Journal of Geophysical Research, v. 86, p. 10,591-10,606.

Koppers, A.A.P., 2002, ArArCALC software for Ar-40/Ar-39 age calculations: Computers and
Geosciences, v. 28, p. 605-619.

Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic core complexes and detachment
faults formed during Tertiary continental extension in the northern Colorado River
region, USA: Journal of Structural Geology, v. 11, p. 65-94.

McDougall, I., and Harrison, T.M., 1999, Geochronology and Thermochronology by the
40Ar/39Ar method, 2nd edition. Oxford University Press, N.Y.

McGrew, A.J., and Snee, L.W., 1994, 40Ar/39Ar thermochronologic constraints on the
tectonothermal evolution of the northern East Humboldt Range metamorphic core
complex, Nevada: Tectonophysics, v. 238, p. 425-450.

Miller, E.L., Dumitru, T.A., Brown, R.W., and Gans, P.B., 1999, Rapid Miocene slip on the
Snake Range-Deep Creek Range fault system, east-central Nevada: Geological Society of
America Bulletin, v. 111, p. 886-905.

Oldow, J.S., 1984, Evolution of a late Mesozoic back-arc fold and thrust belt, northwestern Great
Basin, U.S.A.: Tectonophysics, v. 102, n. 1-4, p. 245-274.









Poole, F.G., 1992, Distribution and thickness of late Proterozoic through middle Ordovician
rocks: The Geology of North America, The Geological Society of America, vol. G-3, p.
407-479.

Roberts, R.J., Ferguson, H.G., Gilluly, J., Hotz, P.E., 1958, Paleozoic rocks of north-central
Nevada: Bulletin of the American Association of Petroleum Geologists, v. 42, n. 12, p.
2813-2857.

Snoke, A.W., 1980, Transition from infrastructure to suprastructure in the northern Ruby
Mountains, Nevada: Geological Society of America, Memoir 153, p. 287-333.

Stockli, D.F., 2005, Application of low-temperature thermochronometry of extensional tectonic
settings: Reviews in Mineralogy and Geochemistry, v. 58, p. 411-448.

Wells, M.L., Snee, L.W., and Blythe, A.L., 2000, Dating of major normal fault systems using
thermochronology: An example from the Raft River Mountains, Basin and Range,
western United States: Journal of Geophysical Research, v. 105, p. 16,303-16,327.

Wernicke, B., 1981, Low-angle faults in the Basin and Range province: Nappe tectonics in an
extending orogen: Nature, v. 291, p. 645-648.

Wernicke, B., 1985, Uniform-sense normal simple shear of the continental lithosphere: Canadian
Journal of Earth Sciences, v. 22, p. 108-126.

Wernicke, B., and Axen, G. J., 1988, On the role of isostasy in the evolution of normal fault
systems: Geology, v. 16, p. 848-851.









BIOGRAPHICAL SKETCH

Jennifer was born in Hartford, CT, in 1983. She is the youngest of three sisters. She

graduated from RHAM high school (Hebron, CT) in 2001. She received a Bachelor of Science

degree in geology from Syracuse University (Syracuse, NY) in 2005. While attending graduate

school at the University of Florida (Gainesville, FL), Jennifer served as a teaching assistant for

several courses in the Department of Geological Sciences and as a research assistant for Dr.

David A. Foster. She received a master's degree in geology from the University of Florida in

May of 2008. Jennifer continues to pursue her interests in the PhD program in geology at the

University of Florida (Gainesville, FL).









QUANTIFYING EOCENE AND MIOCENE EXTENSION IN THE SEVIER HINTERLAND
IN NORTHEASTERN NEVADA

Jennifer N. Gifford
Geological Sciences
David A. Foster
Master of Science
May 2008

This project provides a comprehensive low-temperature thermochronologic history of the

Ruby Mountain metamorphic core complex in northeast Nevada, and helps to define the Tertiary

extensional history of the region. Samples from two transects across the range were analyzed

using furnace and laser 40Ar/39Ar methods. Data generated by this project was used to: a)

establish the timing of Paleogene and Neogene exhumation and rate of westward slip on the fault

system that separates the upper crustal block that include the Carlin gold trend mineralizations

from the Ruby Mountains metamorphic core complex, and b) better understand the geologic

setting and origin of the Carlin-type deposits that form the Carlin gold trend.





PAGE 1

1 QUANTIFYING EOCENE AND MIOCENE EX TENSION IN THE SEVIER HINTERLAND IN NORTHEASTERN NEVADA By JENNIFER N. GIFFORD 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

PAGE 2

2 2008 Jennifer N. Gifford

PAGE 3

3 To my mother, who greatly encourag ed me in my pursuit of science

PAGE 4

4 ACKNOWLEDGMENTS I wish to thank my Advi sor Dr. David Foster for his help and patience. I also wish to thank my committee members for their guidance a nd helpful reviews of my thesis material. I especially wish to thank Shawn J. Malone for his endless assistance. I al so wish to thank Warren Grice and Misty Stroud for their assistance in anal ysis procedures. Richard McKenzie for all of his help with my figures. Fi nally, I wish to thank my fam ily for their un-ending love and support.

PAGE 5

5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ............................................................................................................... 4 LIST OF TABLES ...........................................................................................................................6 LIST OF FIGURES .........................................................................................................................7 ABSTRACT ...................................................................................................................... ...............8 CHAPTER 1 INTRODUCTION .................................................................................................................. 10 2 REGIONAL GEOLOGY ........................................................................................................12 3 40Ar/39Ar THERMOCHRONOLOGY ....................................................................................16 Previous Thermochronology .................................................................................................. 16 40Ar/39Ar Thermochronology Results .....................................................................................18 Biotite ....................................................................................................................... .......19 Muscovite ..................................................................................................................... ...21 4 DISCUSSION .................................................................................................................... .....30 Eocene Exhumation of the Ruby Mountains Metamorphic Core Complex defined by 40Ar/39Ar Thermochronology ..............................................................................................30 Constraints on the Timing of the Onset of Extension ............................................................. 31 Constraints on the Deta chme nt Slip Rate ............................................................................... 32 5 CONCLUSIONS ................................................................................................................... .41 APPENDIX 40Ar/39Ar THERMOCHRONOLOGY METHODS .................................................. 42 Sample Preparation and Irradiation ........................................................................................ 42 40Ar/39Ar Analytical Instrument ation and Procedures ............................................................ 43 LIST OF REFERENCES ...............................................................................................................45 BIOGRAPHICAL SKETCH .........................................................................................................48

PAGE 6

6 LIST OF TABLES Table page 3-1 Summary of previous 40Ar/39Ar thermochronology research. ........................................... 263-2 Summary of 40Ar/39Ar thermochronology from the Ruby Mountains metamorphic core complex .................................................................................................................. ....283-3 Summary of level of chloritization of samples from the Ruby Mountains metamorphic core complex ................................................................................................ 29

PAGE 7

7 LIST OF FIGURES Figure page 2-1 Location map of the Ruby Mountains me tamo rphic core complex (blue), the ElkoCarlin gold trend (yellow), as well as the Antler and Sevi er thrust belts. ......................... 153-1 Biotite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples collected from the Ruby Mountains Metamo rphic Core complex along two separate transects. All ages are weighted plateau cooling ages calculated from three or more heating steps. ................................................................................................................ ......233-2 Muscovite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples collected from the Ruby Mountains Metamo rphic Core complex along two separate transects. All ages are weighted plateau cooling ages calculated from three or more heating steps. ................................................................................................................ ......244-1 Simplified geologic map showing mineral cooling ages from samples, collected along transects across Lamoille Canyon a nd the East Humboldt Range, using 40Ar/39Ar thermochronology (2 ). ..................................................................................... 354-2 Plot of sample calculated ages vs. th e altitude at which sa mples were collected. .............364-3 All of the mica 40Ar/39Ar thermochronology ages from the Lamoille Canyon transect were plotted versus their dist ance along slip direction. ..................................................... 374-4 Mica 40Ar/39Ar thermochronology ages versus dist ance in slip direction from the Lamoille Canyon transect, younger age grouping. A regression was calculated for the sample set. ............................................................................................................... .....384-5 Mica 40Ar/39Ar thermochronology ages versus dist ance in slip direction from the Lamoille Canyon transect, older age grouping. A regression was calculated for the sample set. ................................................................................................................... .......394-6 All of the mica 40Ar/39Ar thermochronology ages from the East Humboldt Range transect were plotted versus thei r distance along slip direction. ........................................40

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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 QUANTIFYING EOCENE AND MIOCENE EX TENSION IN THE SEVIER HINTERLAND IN NORTHEASTERN NEVADA By Jennifer N. Gifford May 2008 Chair: David A. Foster Major: Geology Rocks exposed in the Ruby-East Humboldt metamorphic core-complex, NE Nevada, provide a guide for reconstructing the pre-Eocene crustal structure in the hinterland of the Sevier Orogen. These rocks occupy the footwall of a major west-dipping normal-sense shear system that may extend ~ 50 km further west under part of the mineralized Carlin trend. Previous thermochronologic studies showed mineral cooli ng ages from the Ruby Mountains core complex generally grow younger to the WNW, in the direction of fault slip. 40Ar/39Ar biotite and muscovite analyses from transects in the directi on of slip on the Ruby detachment give apparent ages between ca. 31 and 21 Ma and are, therefore, consistent with the pr evious K-Ar data in showing a decrease in cooling age from east to we st. These data indicate rapid cooling due to exhumation of the footwall of the de tachment started at about 23 Ma. 40Ar/39Ar data from muscovite and biotite along transects across La moille Canyon and the East Humboldt Range give apparent ages of ca. 33 Ma in the easte rn part of the footwall and ca. 25 Ma in the western part of the footwall. The ca. 33 Ma apparent ages may indicate an Oligocene phase of extension at a poorly-defined rate of 4.5 +6.7/ -1.9 km/m.y. A change in slope of the mica age vs. slip distance relationship at ca. 23 Ma sugge sts that extension began at that time. The

PAGE 9

9 gradient in mica cooling ages to the west of the break in slope of this re lationship suggests a slip rate of 4.2 3.8 km/m.y.

PAGE 10

10 CHAPTER 1 INTRODUCTION Metamorphic core com plexes are important ge ological features in the North American Cordillera, both for the glimpse they provide into the middle crust and the valuable tectonic information they contain. These complexes occur in a long belt, from southern Canada to northern Mexico in what was formerly crust thickened by Mesozoic and Cenozoic orogenic events (Coney, 1980). Core complexes develo p when low-angle detachment faults exhume middle-crustal metamorphic rocks in the footwall of the fault. These complexes have been the subject of numerous studies (e.g., Armstrong and Ward, 1991; Dallmeyer et al., 1986; Howard, 1980; Howard, 2003; McGrew and Snee, 1994; S noke, 1980), but the timing and significance of many remain uncertain. The metamorphic core complex exposed in th e Ruby-East Humboldt Range is of particular interest. This comp lex is marked by deformed and migmitized upper amphibolite facies metamorphic rock and associat ed igneous bodies unroofed by slip on the main detachment. The Ruby-East Humboldt core complex occupies a central position in the hinterland region of the Sevier Orogen (e.g., Howard, 2003). This paper examines the timing and rate of Cenozoic extension and thinning of the lower crust in the Ruby Mountains metamorphic core complex via high a nd low temperature thermochronology (40Ar/39Ar) on rocks within structurally controlled transects. Metamorphic Core Complex: Core complexes form as middle-crustal metamorphic rocks are exhumed during rapid extension, commonly in th e footwall of a detachment fault. The rocks in the footwall of the detachment fault are upl ifted through a progression of metamorphic facies and deformation mechanisms, resulting in a charact eristic sequence of structures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, and is driven by isostatic rebound due to tectonic denudation (Wernicke and Axen,

PAGE 11

11 1988). Lister and Davis (1989) stat e that the formation of metamor phic core complexes is driven by extensional stresse s in the upper crust. Three primary models explaining the deformation leading to the formation of meta morphic core complexes are noted by Brun et al. (1994). These models are the simple shear model, in whic h low angle detachments accommodate extension (Wernicke, 1981; Wernicke, 1985), the rotated normal fault model involving the formation of high angle normal faults which subsequently ro tate to a low dip and cease to be active slip surfaces (Buck, 1988; Wernicke and Axen, 1988), and the domino (tilted block) model which postulates straight faults boundi ng rotating crustal blocks (Ange lier and Coletta, 1983; Davis, 1983). Lister and Davis (1989) a ddress the strengths and weaknesses of the broad mechanisms, and support a multiple detachment model. During the evolution of a metamorphic core complex, it is necessary to have multiple generations of detachment faults all splaying from an original master fault, or from a controlling movement zone at depth. The most recently active detachment fault is the youngest in a succession of faults that progressively remove lower portions of the upper plate, eating their way upw ards through the overlying sequences. The detachment faults presently observed in the metamorphic core complexes (e.g., Ruby Mountains) are relatively young, and are only the last in a succe ssion of low-angle normal faults that carve through the upper crust at the upw ard terminations of major, shallow-dipping, ductile shear zones in the extending Cord illeran orogen (Chery, 2001).

PAGE 12

12 CHAPTER 2 REGIONAL GEOLOGY By the Neoproterozoic (~ 600 Ma), the wester n ma rgin of North America had developed into a passive continental margin (Burchfiel et al., 1992). This marg in was characterized by marine and non-marine clastic sedimentary rocks, interbedded with car bonates in a westward thickening wedge (Burchfiel et al., 1992). These rocks were later modified by Phanerozoic orogenic events. The first Phaner ozoic orogenic event is referred to as the Antler orogeny. This phase is characterized by late Neoproterozoic to upper Devonian shelf sequences thrust eastward ~ 200 km onto the edge of the North American craton (Roberts et al, 1958 ; Poole et al, 1992). Numerous thrust faults cut across the area aff ected by the Antler orogen y, leading some authors to interpret the terrane as an accretionary comp lex (Oldow, 1984). The basal thrust is preserved today as the Roberts Mountain thru st (Burchfiel et al., 1992). Th e next major contractional event is referred to as the Sonoma orogeny. In cen tral Nevada, the Sonoma orogeny was characterized by the eastward thrusting of the Havallah assemblage over the western elements of the Antler orogeny along the Gloconda thrust fa ult (Burchfiel et al., 1992). Th is event occurred in the latest Paleozoic to earliest Triassic (Burchfiel et al., 1992). In the Late Jurassic a further pulse of compression resulted in the Nevada n orogeny; however, the causes of this contractional event are uncertain (Burchfiel et al., 1992). From the Early to Late Cretaceous, further shortening and eastward thin-skinned thrusting defi ne the Sevier orogeny (Burchfiel et al., 1992). Thrusting and metamorphism associated with this event ended ~ 70 Ma (Burchfiel et al., 1992). Some estimates place the thickness of the crust affected by the Sevi er orogeny as thick as 60 to 70 km in Nevada (Burchfiel et al., 1992). The tectonic character of the Cordilleran margin changed during the Cenozoic. The Mesozoic-Cenozoic transition was marked by continued shortening a nd crustal thickening

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13 related to the Laramide orogeny and shallow-an gle subduction of the Farallon slab (Sonder and Jones, 1999). During the late Eocene/early Oligo cene, the Farallon slab began to roll back and peel away from its coupling with the bottom of the North Amer ican lithosphere (Humphreys, 1995). The rollback of the subducting slab allowe d hot asthenospheric material to come into contact with metasomatized lithospheric man tle, promoting partial melting (Armstrong & Ward, 1991). Sonder and Jones (1999) suggest that this heat was transferred to the crust above the rolling slab, leaving it mechanic ally weak. A second consequence of the change in plate boundary conditions was a gradual relaxation of compressive forces from the plate boundary (Liu, 2001). As coupling between the North Amer ican and Farallon plates diminished due to slab rollback, so did the force driving thickening Liu (2001) suggests th at this relaxation of stress on the thickened crust of Eastern Neva da allowed for a change from shortening to gravitationally driven extension. Large me tamorphic core complexes formed throughout southern Idaho and eastern Nevada in the 40 Ma time interval (Armstrong and Ward, 1991), and can be associated with both ignimbrite magmatism and the grav itational collapse of thickened crust. The Ruby Mountains Metamorphic Core Complex: Howard (2003) divides northwestern Nevada into three broad tectonic el ements: the hinterland of the Sevier fold and thrust belt, the Ruby-East Humboldt metamorphi c core complex, and the Elko-Carlin domain. The Ruby-East Humboldt complex (Figure 2-1) exposes metamorphosed Paleozoic strata, as well as smaller areas of Proterozoic and possibly Archean rock (Howard, 2003). These metamorphic units were unroofed during the Tertia ry via extensional normal faulting, which also caused mylonitization and overprinted prio r metamorphic features (Howard, 2003). Metamorphic conditions in the Ruby and Humboldt ranges peaked at upper amphibolite facies,

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14 with lower-grade facies occurr ing to the east and south (How ard, 2003). Snoke (1980) provides further divisions for the lithol ogies present in the Ruby Mountai ns, dividing them between the metamorphic core, unmetamorphosed Paleozoic sedimentary units exposed by a low angle fault system, and Tertiary sedimentary and volcanic units Granitic rock forms a significant volume of the Ruby Mountains complex as well, manifested as dikes, sills, and irregular masses, commonly of two-mica pegmatitic granite (Howard, 1980). The Sevier fold and thrust belt exposes 10 to 13 km of siliciclastic and carbonate miogeoclinal strata, Neoproterozoic to early Mesozoic in age, east of the Ruby mountains (Howard, 2003). West of the Ruby Mountains in the Elko-Carlin domain, deep water strata were thrust over th e miogeoclinal shelf units during a series of shortening events during the Paleozoic and Mesozoic (Howard, 2003). Backthrusting in the Elko-Carlin domain is suggested by overturned folds in several locali ties (Howard, 2003). In addition to these three tectonic elements, the re gion also exposes numerous Jurassic and early Cretaceous igneous intrusions, with late Cretaceous granites emplaced into deeper crustal levels of the metamorphic core complex (Howard, 2003). The deep crustal rocks exposed in the Ruby-East Humboldt metamorphic core complex provide a guide for reconstructing Eocene crusta l structure in northeas tern Nevada (Howard, 2003). The exposed rocks from the core complex are in the footwall of a west-dipping normalsense shear system (Howard, 2003). It has been proposed that these rocks may have underlain the Pion and Adobe Ranges, ~ 50 km to the west before Tertiary extension (Howard, 2003). Eocene events in the Elko-Carlin region incl uded widespread magmatic intrusions, normal faulting, an elevated geothermal gradient from crustal thinning, and development of lacustrine deposits in active structural basins (Howard, 2003).

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15 Figure 2-1. Location map. A) the Elko-Carlin gold trend. B) Ruby Mountains metamorphic core complex.

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16 CHAPTER 3 40Ar/39Ar THERMOCHRONOLOGY Previous Thermochronology Despite the nume rous studies of Eocene extens ion in the Elko-Carlin region, a clear picture of the setting and tectonics has not been formed. Evidence for Eocene extension within the highgrade metamorphic rocks of the Ruby Mountains comes mainly from 40Ar/39Ar dating of hornblende, which yielded discorda nt age spectra that suggested cooling during the late Eocene era (McGrew and Snee, 1994). The hornblende data are not entirely satisfactory because these data are discordant, and they can be interprete d in different ways. As shown by Howard (2003), extensive lacustrine deposits r ecord a large Eocene lake and wetland system, which may suggest an early Tertiary phase of extension. Mineral cooling ages obtained by 40Ar/39Ar thermochronology can be used to constrain the cooling and exhumation historie s of highly extended terranes such as metamorphic core complexes (e.g., Foster and John, 1999; St ockli, 2005). Previously available thermochronological data relevant to the exhumation and cooling history of the Ruby Mountains Metamorphic Core Complex produced apparent ages ranging from the Oligocene to the earliest Miocene. These data are summarized in Table 31. These ages are interpre ted to record cooling through the argon closure interval for biotite, which is about 350-280 at about C depending on cooling rate (McDougall and Harrison, 1999). Cooling ages given by mica K-Ar and apatite fission-track data from the Ruby Mountains core complex generally grow younger in the westnorthwest direction of extensional unroofing (McGrew and Snee, 1994). Analyses reported by McGrew and Snee (1994), from 3 muscovites and 5 biotites yield apparent ages between 21.9 0.2 Ma and 26.7 0.1 Ma. They also determined that at ca. 20 Ma, rapid cooling occurred through the temp erature interval of

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17 approximately 300C to below 100C. The K-Ar biot ite apparent ages indi cate the time at which the sample passed through the argon retention isotherm or partial re tention interval The closure temperature for biotite is dependant on cooli ng rate, but generally va ries between 280 and 350C (McDougall and Harrison, 1999). The age progre ssion of younger ages to the west can be interpreted to suggest the rate of progressive westward unroofing and cooling of the core complex, or an east-tilted oblique section th rough a zone of part ial argon retention. Foster and John (1999) showed that if mi neral cooling ages re present quenching by progressive unroofing of an isotherm beneath an extensional shear zone, an approximate rate of lateral cooling and detachment s lip can be obtained by inverting th e rate of lateral age change. This inversion would indicate Oligocene and early Miocene e xhumation of a westward moving hanging wall at a rate of 1 to 2 km/m.y. The westward-younging cooling age patterns, if they are combined with older mica cooling ages from expo sures on the eastern side of the core complex, indicate prolonged extensional exhumation begi nning in the early Tert iary era (McGrew and Snee, 1994). If age-progression data were found to suggest that the metamorphic core moved cumulatively 30 km east below its hanging wall over approximately 20 m.y., the core would previously have been to the west, undern eath the valley along the east margin of the Adobe and Pion ranges. This apparent displacement rate calculated fr om the previously published data is several times slower than those reported for core comple xes in the Raft River and Snake ranges, and the Colorado River extensional corrid or (Miller et al., 1999; Fost er and John, 1999; Wells et al., 2000; Carter et al., 2004), but similar to thos e from core complexes in the northern Rocky Mountains (Grice, 2006). Extension rates in ma ny other areas of the Basin and Range province

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18 accelerated markedly about 15 Ma, as determined by fission-track dating of apatite (Miller et al., 1999, Carter et al., 2004). Dallmeyer et al., (1986) obtained 40Ar/39Ar and K-Ar ages from biotite samples in the mylonite zone located in the Northern Ruby Mountains, 40Ar/39Ar for two samples in Lamoille Canyon, and 40Ar/39Ar for biotite samples located in th e South-Eastern section of the East Humboldt Range. In the East Humboldt Range outside of the mylonite zone, apparent ages ranged from ~ 32 to 33 Ma. Within the mylonite zone ranging down into the Northern Ruby Mountains, 40Ar/39Ar ages were younger but slightly more widespread, ranging from ~ 22 to 27 Ma. In Lamoille Canyon biotite from a non-mylon itic magmatic core rock yielded an age of 25.3 0.7 Ma. Further west towards the opening of Lamoille Canyon and nearer to the detachment fault, a biotite from within the mylonite resulted in an age of 20.8 0.5 Ma. Dallmeyer et al. (1986) hypothesized that coolin g began by ~ 45 Ma and that the rocks reached temperatures below 300C by 20 Ma. Dokka et al., (1986) published fission track data for apatites, sphenes, and zircons as well as 40Ar/39Ar thermochronology on biotites across the northern Ruby Mountains, and some samples in the lower East Humboldt Range The fission track analyses and biotite 40Ar/39Ar yielded concordant ages from ca. 23.4 to 25.4 Ma, indicating the tim ing of onset of extension and the start of rapid cooling along the detachment fault. 40Ar/39Ar Thermochronology Results Mineral separates (biotite and muscovite) were obtained from a total of twenty-three rock samples (sixteen from the Lamoille Canyon transect and seven from the East Humboldt Range transect) for the 40Ar/39Ar thermochronology in this study. These mineral separates were then analyzed in vacuum laser total fusion, laser st ep-heating and furnace step-heating experiments in the noble gas laboratory at the University of Fl orida. Samples were run through furnace step-

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19 heating as well as laser step-hea ting due to scheduling conflicts w ith the laser. The methods for rough sample preparation, analytical instrumentation and procedures, and thermochronological data reduction are summarized in Appendix A. A summary of calculated a pparent ages from the 40Ar/39Ar thermochronology analyses are reported in Table 3-2. These re sults are organized into two groups; one corresponds to rock samples collected along the Lamoille Canyon trans ect and the other group to samples collected along the East Humboldt Range transect. Individual 40Ar/39Ar cooling ages are reported in Table 3-2 according to sample name, rock type, sample latitude and longitude, elevation, and the mineral that was dated. Total % 39Ar gas used in the weighted plateau age calculation during step-heating and MSWD (mean standard weighted deviates) are also reported with the calculated cooling ages in Table 3-2. MSWD values are onl y relevant in step-heating age calculations, and are not reported for the laser tota l fusion age calculations. The higher MSWD values for some of the step-heating age calculations is ge nerally due to the presence of excess 40Ar in the first and last few heating steps of the e xperiment. However, the plateau ages are considered robust with these anomalous steps omitted. The J-values (from biotite GA1550 mineral flux monitors) used in individual cooling age calcula tions are also included in Table 3-2. The individual mineral cooling age spectra are shown in Figures 3-1 and 3-2. Biotite Laser step-heating experiments from two bi otite separates across the East Humboldt transect resulted in mostly well-defined weighted age plateaus (Figure 3-1). Biotite from granitic gneiss sample DF02-212, yielded a flat age-plateau over 65% of the total 39Ar gas released corresponding to an apparent age of 22.6 1.2 Ma (MSWD = 3.08). Biotite from DF02-214, a biotite granite sampled from the same locati on as DF02-212, gave a plateau age of 24.3 1.2 Ma (99% total 39Ar gas, MSWD = 11.38).

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20 Furnace step-heating experiments were performed on one biotite from the East Humboldt Range transect and three from the Lamoille Ca nyon transect. DF02-211, a biotite granite from the East Humboldt Range yielded a weighted plateau ag e of 23.2 2.0 Ma (86% total 39Ar gas, MSWD = 2.67). DF02-205, a leucogranite, produ ced a weighted average age of 24.8 2.7 Ma using 87% of the total 39Ar gas, with a MSWD of 15.25. The age spectrum for this biotite is internally dischordant probably because of the presence of chlori te intergrowths and 39Ar recoil. The percentage of chloritization within the DF 02 biotite samples is shown in Table 3-3. DF02206, a biotite granite yielded a flat weight ed plateau age of 22.3 2.6 Ma, MSWD = 9.42. Lastly, DF02-208, a biotite leucogranite from Lamoille Canyon yielded a weighted plateau age of 17.0 1.6 Ma, but with only 42% of the 39Ar gas involved in the calc ulation, this sample was excluded from further analysis because of low total K2O and a large discordance between adjacent steps. The higher MSWD values for samples DF02-214, DF02-205, and DF02-206 are due to the presence of excess Ar in the first few heating step s of the experiments, a nd chlorite intergrowths within the biotite grains. Thes e samples were all collected from near the detachment interface and were overprinted within the greenschist facies during the formation of the retrograde mylonite. Fluid infiltration and the formation of chlorite intergrowths explains the more discordant age spectra and lower apparent K2O values of these samples. Laser total fusion experiments and data re duction from eight bi otite separates along Lamoille Canyon (separated by Virginia Newman, and analyzed by Mike Hartley) yielded ages ranging from ca. 20.7 to 31 Ma. Biotite from H97 RBY-42 is a leucogranite produced an age of 31.2 1.8. H97RBY-51 is a biotite garnet ap lite which yielded an age of 31.3 1.5 Ma. H93RBY-8 is a biotite amphibolite and yielded an age of 30.9 1.8 Ma. H93RBY-4 is a biotite

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21 granodiorite which yielded an age of 23.1 1.0 Ma. H97RBY-53 is a biotite quartzite which yielded an age of 24.4 0.8 Ma. H97RBY-54 is a biotite granite which yielded an age of 24.7 2.2 Ma. H97RBY-55 is a mylonitic granite gne iss which yielded an age of 20.7 1.0 Ma. Muscovite Muscovite separates from nine different sa mples collected from the two transects across the Ruby Mountains underwent in-vacuo laser and furnace step-heating 40Ar/39Ar analyses. Most of the analyses of muscovite resulted in largely well-defined, flat-shaped age spectra of better quality than the biotite analyses (Figure 3-2). Along the Lamoille Canyon transect, five muscovite separates were analyzed. They follow a trend of younging from east to west. Three sa mples were analyzed using laser step-heating. DF02-216 is a leucogranite. Muscovite from th is sample gave a plateau age of 32.4 0.7 over 94 percent of the total 39Ar gas released (MSWD = 0.22). Muscovite of sample DF02-218 yielded a flat age plateau over 99% of the total 39Ar gas released and a cooling age of 32.4 0.6 Ma (MSWD = 0.53). Located much further to the west, muscovite from DF02-203, a quartzite sample, yielded a plateau age of 22.6 0.8 Ma (95% 39Ar gas, MSWD = 4.32). Two samples were analyzed using furnace step-heating. DF 02-209 yielded muscovite with an age of 21.7 3.1 Ma with 82% of the 39Ar gas going into the calculati on and a MSWD of 7.86. DF02-210 was a quartzite sampled from the same location as DF02-209, which yielded an age of 27.9 3.8 Ma (57% 39Ar, MSWD = 3.15) and a very discordant age spectrum. Four muscovite samples from the East Humboldt Range transect were analyzed with laser step-heating and furnace step-heating methods. DF02-215 is a leucogranite located furthest west along the transect, and DF02-219 located on the furt hest east side of the East Humbodlt Range along the valley between this range and the Wo od Hills. Using laser step-heating, DF02-215 yielded an age of 21.5 0.5 Ma with 100% of the 39Ar gas and an MSWD of 0.23. DF02-219 is

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22 a muscovite quartzite which yielded an age of 27.4 0.6 (89% 39Ar, MSWD = 0.23). DF02-221 and H03WH-42 are samples both located in the Wood Hills, east of the Humboldt Range, and were analyzed with furnace step-heating. DF02221 was from a quartz vein and the muscovite gave an age of 43.7 5.3 Ma (89% 39Ar, MSWD = 4.50). H03WH-42 was a marble and produced an age of 49.3 3.6 Ma (75% 39Ar, MSWD = 5.40).

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23 A 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100 Cumulative 39Ar Released (%)Age (Ma) DF02-205 Biotite Age 24.79 2.67 MaMSWD = 15.25 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100 Cumulative 39Ar Released (%)Age (Ma) DF02-206 Biotite Age 22.25 2.63 Ma MSWD = 9.42B C 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-208 Biotite Age 16.95 1.63 MaMSWD = 1.59 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-211 Biotite Age 23.16 1.96 MaMSWD = 2.67D E 22.59 1.22 Ma0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100 Cumulative 39Ar Released (%)Age (Ma) DF02-212a Biotite Age 22.59 1.22 MaMSWD = 3.08 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-214a Biotite Age 24.28 1.21 MaMSWD = 11.38F Figure 3-1. Biotite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples collected from the Ruby Mountains Metamo rphic Core com plex along two separate transects. All ages are weighted plateau cooling ages calculated from three or more heating steps. A) DF02-205. B) DF02 -206. C) DF02-208. D) DF02-211. E) DF02212a. F) DF02-214a

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24 A 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100 Cumulative 39Ar Released (%)Age (Ma) DF02-203a Muscovite Age 22.59 0.79 Ma MSWD = 4.32 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-209 Muscovite Age 21.66 3.13 MaMSWD = 7.86B C 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-210 Muscovite Age 27.91 3.79 MaMSWD = 3.15 21.53 0.48 Ma0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100 Cumulative 39Ar Released (%)Age (Ma) DF02-215a Muscovite Age 21.53 0.48 MaMSWD = 0.23D E 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-216a Muscovite Age 32.14 0.69 MaMSWD = 0.22 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-216b Muscovite Age 32.73 0.79 MaMSWD = 2.37F G 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-218a Muscovite Age 32.02 0.56 MaMSWD = 0.53 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-218b Muscovite Age 32.68 0.71 MaMSWD = 0.67H Figure 3-2. Muscovite 40Ar/39Ar thermochronology step-heating mineral age ranges for samples collected from the Ruby Mountains Metamo rphic Core com plex along two separate transects. All ages are weighted plateau cooling ages calculated from three or more heating steps. A) DF02-203a. B) DF02-209. C) DF02-210. D) DF02-215a. E) DF02216a. F) DF02-216b. G) DF02-218a. H) DF02-218b. I) DF02-219a. J) DF02-219b. K) DF02-221. L) H03WH-42.

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25 I 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-219a Muscovite Age 27.15 0.54 MaMSWD = 0.23 0 5 10 15 20 25 30 35 40 45 50 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-219b Muscovite Age 27.58 0.67 MaMSWD = 2.08J K 0 10 20 30 40 50 60 70 80 90 100 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) DF02-221 Muscovite Age 43.68 5.31 MaM S WD = 4. 50 0 10 20 30 40 50 60 70 80 90 100 0102030405060708090100Cumulative 39Ar Released (%)Age (Ma) H03WH-42 Muscovite Age 49.28 3.62 MaMSWD = 5.40L Figure 3-2. Continued.

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26 26 Table 3-1. Summary of previous 40Ar/39Ar thermochronology research. Sample Rock Type Latitude Longitude Mineral Age (Ma) Errors (+/-) 2 Method Source Samples from 40Ar/39Ar transect in the East Humbolt Range 15 mylonitic orthogneiss 40 49' 44"115 05' 50"bt 33.7 1.1 Ar-Ar Dallmeyer et al., 1986 14 protomylonitic q dioritic orthogneiss 40 50' 21"115 04' 46"bt 31.90 0.8 Ar-Ar Dallmeyer et al., 1986 4119-25 40 50' 21"115 04' 46"sphene 23.8 2.5* FT Dokka et al., 1986 13 protomylonitic q dioritic orthogneiss 40 50' 31"115 04' 47"bt 32.9 0.8 Ar-Ar Dallmeyer et al., 1986 4119-9 40 50' 31"115 04' 47"apt 25.1 4.2* FT Dokka et al., 1986 880805-3 bt sill ksp pl q gar schist 40 56' 50"115 06' 21"bt 26.74 0.08 Ar-Ar McGrew and Snee, 1994 RM-19 hb bt q dioritic gneiss 40 59' 43"115 05' 26"bt 23.43 0.1 Ar-Ar McGrew and Snee, 1994 880709-2 ms gar pegmatitic gneiss 40 59' 54"115 06' 08"ms 21.9 0.2 Ar-Ar McGrew and Snee, 1994 880706-2A amphibolite 41 00' 00"115 05' 45"bt 21.9 0.2 Ar-Ar McGrew and Snee, 1994 870718-1 hb bt q dioritic gneiss 41 00' 33"115 05' 10"bt 20.89 0.1 Ar-Ar McGrew and Snee, 1994 870719-2B bt monzogranitic gneiss 41 00' 51"115 05' 25"bt 21.5 0.2 Ar-Ar McGrew and Snee, 1994 18 nonmylonitic amphibolite 41 01' 22"115 05' 52"bt 25.3 0.6 Ar-Ar Dallmeyer et al., 1986 870614-3 ms bt schist 41 02' 05"115 05' 50"ms 22.1 0.2 Ar-Ar McGrew and Snee, 1994 870623-2 ms bt sill gar schist 41 02' 38"115 06' 24"ms 22.2 0.3 Ar-Ar McGrew and Snee, 1994 17 mylonitic q dioritic orthogneiss 41 02' 42"115 00' 33"bt 27.7 0.8 Ar-Ar Dallmeyer et al., 1986 16 mylonitic q dioritic orthogneiss 41 03' 21"115 59' 55"bt 25.0 0.8 Ar-Ar Dallmeyer et al., 1986 Samples from 40Ar/39Ar transect in the Northern Ruby Mountains 5 mylonitic quartzite 40 26' 20"115 16' 17"bt 21.7 0.7 Ar-Ar Dallmeyer et al., 1986 6 nonmylonitic mygmatitic core 40 46' 06"115 16' 33"bt 22.4 0.7 Ar-Ar Dallmeyer et al., 1986 4 nonmylonitic mygmatitic core 40 46' 16"115 17' 38"bt 21.5 0.6 Ar-Ar Dallmeyer et al., 1986 5169-19 40 46' 39"115 16' 21"apt 18.4 2.5* FT Dokka et al., 1986 B monazite-bearing bt schist 40 47' 21"115 13' 57"bt 23.4 0.4 K-Ar Dallmeyer et al., 1986 8 mylonitic q dioritic orthogneiss 40 49' 32"115 16' 23"bt 22.4 0.8 Ar-Ar Dallmeyer et al., 1986 7 mylonitic q dioritic orthogneiss 40 49' 54"115 15' 56"bt 23.5 0.8 Ar-Ar Dallmeyer et al., 1986 G 40 51' 47"115 14' 23"bt 13.3 1.0 K-Ar Dallmeyer et al., 1986 I ms gneiss porphyry 40 52' 04"115 12' 42"ms 153.9 0.9 K-Ar Dallmeyer et al., 1986 H 40 52' 34"115 14' 35"bt 15.5 0.4 K-Ar Dallmeyer et al., 1986 NEV-13-80 41 03' 21"114 59' 55"apt 26.5 6.3* FT Dokka et al., 1986

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27 27 Table 3-1. Continued Sample Rock Type Latitude Longitude Mineral Age (Ma) Errors (+/-) 2 Method Source Samples from 40Ar/39Ar transect in Lamoille Canyon 3 mylonitic 40 41' 34"115 28' 03"bt 20.8 0.5 Ar-Ar Dallmeyer et al., 1986 1 nonmylonitic mygmatitic core 40 37' 41"115 21' 58"bt 25.3 0.7 Ar-Ar Dallmeyer et al., 1986 Note: grd = granodiorite, q di = quartz di orite, gr = granite, q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, ksp = potassium-feldspar, amp = amphibolite, gar = garnet, gn = gneiss, sill = sillimanite, pl = plagioclase feldspar, sample age errors are reported at 1

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28 28 Table 3-2. Summary of 40Ar/39Ar thermochronology from the Ruby Mountains metamorphic core complex Sample Rock Classification Latitude Longitude Elevation (m) Mineral Age (Ma) Errors (+/-) % 39Ar p MSWD p J value Comments Samples from the 40Ar/39Ar transect in Lamoille Canyon DF02-203 2-mica quartzite 40 41.124' 115 26.930' 2667 ms 22.6 0.8 95 4.32 0.0005128 lsh DF02-205 bt grd 40 40.024' 115 24.115' 3059 bt 24.8 2.7 87 15.25 0.0036734 fsh DF02-206 bt gr 40 38.815' 115 21.599' 3246 bt 22.3 2.6 86 9.42 0.0037164 fsh DF02-208 bt monzo-gr 40 37.628' 115 29.436' 3290 bt 17.0 1.6 42 1.59 0.0038456 fsh DF02-209 2-mica q-rich grtd 40 37.985' 115 31.574' 2793 ms 21.7 3.1 82 7.86 0.0037380 fsh DF02-210 ms quartzite 40 37.985' 115 31.574' 2793 ms 27.9 3.8 57 3.15 0.0037595 fsh DF02-216 ms gar q syenite 40 37.267' 115 17.184' 1957 ms 32.4 0.7 94 0.22 0.0005353 lsh DF02-218 ms gar syeno-gr 40 37.539' 115 17.818' 2093 ms 32.4 0.6 99 0.53 0.0005392 lsh H93RBY-4 bt grd 40 39.278' 115 22.499' 3039 bt 23.1 1.0 100 0.0060690 ltf H93RBY-8 bt amp 40 37.661' 115 21.992' 2561 bt 30.9 1.8 100 0.0060690 ltf H97RBY-42 bt gr 40 36.765' 115 19.124' 2088 bt 31.2 1.8 100 0.0060690 ltf H97RBY-51 bt gar gr 40 35.596' 115 21.631' 2981 bt 31.3 1.5 100 0.0060690 ltf H97RBY-52 bt gn 40 37.847' 115 22.165' 2654 bt 27.1 1.2 100 0.0060690 ltf H97RBY-53 bt quartzite 40 38.926' 115 23.969' 2310 bt 24.4 0.8 100 0.0060690 ltf H97RBY-54 bt gr 40 40.461' 115 27.933' 2010 bt 24.7 2.2 100 0.0060140 ltf H97RBY-55 bt gn 40 41.500' 115 28.658' 1905 bt 20.7 1.0 100 0.0060140 ltf Samples from the 40Ar/39Ar transect in the East Humbolt Range DF02-211 bt granodiorite 41 01.989' 115 07.087' 2674 bt 23.2 2.0 86 2.67 0.0037810 fsh DF02-212 bt q-rich grtd 41 01.615' 115 05.996' 2949 bt 22.6 1.2 65 3.08 0.0005214 lsh DF02-214 bt monzo-gr 41 01.615' 115 05.996' 2949 bt 24.3 1.2 99 11.38 0.0005300 lsh DF02-215 ms grd 41 02.891' 115 09.392' 2027 ms 21.5 0.5 100 0.23 0.0005386 lsh DF02-219 ms quartzite 41 02.278' 115 00.313' 1968 ms 27.4 0.6 89 0.23 0.0005432 lsh DF02-221 ms quartzite 40 59.696' 114 51.282' 2096 ms 43.7 5.3 89 4.50 0.0038026 fsh H03WH-42 ms marble 41 02.105' 114 49.893' 2211 ms 49.3 3.6 75 5.40 0.0038672 fsh Note: grd = granodiorite, q di = quartz diorite, gr = granite, grtd = granitoid, q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, kfs = potassiumfeldspar, amp = amphibolite, gar = garnet, gn = gneiss, % 39Arp = percent of 39Ar used in weighted plateau age calculation, and MSWDp = mean standard weighted deviates for plateau cooling age, lsh = laser step-heating, fs h = furnace step-heating, ltf = laser total fus ion. All cooling ages are reported in 2 sigma error.

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29 Table 3-3. Summary of leve l of chloritization of samples from the Ruby Mountains metamorphic core complex Sample Rock Classification Mineral % chloritization of biotite Samples from the 40Ar/39Ar transect in Lamoille Canyon DF02-203 2-mica quartzite ms DF02-205 bt granodiorite bt 0 5 DF02-206 bt granite bt 5 10 DF02-208 bt monzo-granite bt 15 20 DF02-209 2-mica q-rich granitoid ms DF02-210 ms quartzite ms DF02-216 ms gar q syenite ms DF02-218 ms gar syeno-granite ms H93RBY-4 bt granodiorite bt H93RBY-8 bt amphibolite bt H97RBY-42 bt granite bt H97RBY-51 bt gar granite bt H97RBY-52 biotite gneiss bt H97RBY-53 bt quartzite bt H97RBY-54 bt granite bt H97RBY-55 bt gneiss bt Samples from the 40Ar/39Ar transect in the East Humbolt Range DF02-211 bt granodiorite bt 10 15 DF02-212 bt q-rich granitoid bt 15 20 DF02-214 bt monzo-granite bt 0 5 DF02-215 ms granodiorite ms DF02-219 ms quartzite ms DF02-221 ms quartzite ms H03WH-42 ms marble ms Note: q = quartz, bt = biotite, ms = muscovite, hbl = hornblende, kfs = potassium-feldspar, gar = garnet

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30 CHAPTER 4 DISCUSSION Eocene Exhumation of the Ruby Mountains Me tamorphic Core Complex defined by 40Ar/39Ar Thermochronology The 40Ar /39Ar thermochronological data obtained fr om rock samples collected across the Ruby Mountains define a lateral cooling age grad ient, in which cooling ages young progressively to the west. Individual minera l cooling ages and transect sample localities are shown in Figure 4-1, a simplified geologic map. Thermochronological data obtained by 40Ar/39Ar thermochronology in this study are discussed below in the context of the tectonic exhumation of the Ruby Mountains metamorphic core complex. In particular, the new ther mochronologic data, combined with previous thermochronology, provide constraints on (1) the onset and duration of extension in the Ruby Mountains metamorphic core complex, and (2) th e slip rate of the bounding normal detachment fault. Samples were graphed age vs. altitude to see if there was any relationship. Based on Figure 4-2, there is no visibl e relationship between altit ude and calculated age. One complication for any discussion of this da ta is the location of the detachment fault plane. In many other metamorphic core comple xes like the Harcuvar core complex in Arizona (Foster et al., 1993), physical evidence of the su rface is preserved within the mountain range. This is not true for the Ruby-East Humboldt me tamorphic core complex. The fault plane could have been located anywhere above the current exposure. If the fault plane wa s directly above the current mountain top exposures, we would have expected to s ee a clear pattern between the altitude of the high elevation samples and the low elevation samples compared to their calculated ages. If this were the case, the higher elevati on samples would show older apparent ages, and the lower elevation samples would have passed thro ugh the closure isotherms for muscovite and

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31 biotite at younger ages as exhuma tion progressed. As Figure 4-2 il lustrates, there is no trend consistent with this pattern. This leads to the conclusion that the fault plane was located high enough above the current exposure th at the high altitude samples had not yet passed through the biotite and muscovite Ar retention isotherms. Constraints on the Timing of the Onset of Extension The me thod outlined by Foster and John (1999) was used to evaluate th e onset of extension along the Ruby Mountains metamorphic core comple x detachment fault. Figure 4-3 shows an age vs. distance in slip di rection graph using muscovite and biotite cooling ages (40Ar/39Ar thermochronology) obtained from lower plate rock samples. Figure 4-2 also shows a simplified geologic map of the Lamoille Canyon area with samp le locations along a projection line oriented at 285, which is the slip direction for the de tachment. The samples collected along Lamoille Canyon were input into ArcGIS, onto a geologic map, and project ed orthogonally back to the slip line, defining their distances in slip direction, which are sh own along the x-axis of the graph in Figure 4-3. The accuracy of this projection wa s estimated, and error values of 10 meters were assigned to the samples. The same method was used for the transect in the East Humboldt Range. The vertical error bars seen in Figures 4-3, 4-4, 4-5, and 4-6 repres ent the errors in the individual mica cooling ages, all calcula ted at 95% confidence (2 sigma). Muscovite and biotite data were plotted togeth er because at a cooling rate of > 25C/m.y. the closure temperature difference is less than about 50 C, and the error from using both types of data is, therefore, relatively small. Both mu scovite and biotite cooling ages from Lamoille Canyon young in a progressive pattern towards th e west across the Ruby Mountains following the direction of slip on the detachment fault system. At a distance in s lip direction of ~ 12 km along the transect, mica cooling ages drop from the late Eocene ( 33 Ma) to the early Oligocene ( 25 Ma). Further west along the transect, the mica cooling ages decrease more gradually to ~

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32 20 Ma. The change in slope of the mica cooling ag e curve defines the posi tion of the mica partial argon retention zone at the onset of exhumation and the start of a period of rapid slip along the detachment fault. A regression plotted thr ough the collection of younger samples (Figure 4-4) shows an change in slope at th e 12 km mark to intersect the ag e line at ~ 25 Ma, indicating the age of the onset of extension in the R uby Mountains metamorphic core complex. The mica ages from the East Humboldt Range transect, while more sparse, are consistent with those of the Lamoille Canyon transect. There is a drop in age at ~ 27 Ma to ~24 Ma, and then a more gradual drop to ~ 21 Ma to the east along the transect. When a regression is plotted for only the younger samples (Figure 4-7), the regression line gives an ag e of ~ 25.5 Ma, clearly coinciding with the onset of extension in the Lamoille Canyon transect samples. Constraints on the Detachment Slip Rate Thermo chronological data from rock sample s collected along lower plate transects in metamorphic core complexes parallel to the di rection of tectonic unroofing have also been successfully used to estimate previous slip rates on bounding detachment faults (e.g., Foster et al., 1993; Foster and John, 1999; Stockli, 2005; Wells et al., 2000) The mica ages obtained in this study likewise may be used to estimate the slip rate along the detachment fault. Slip rate estimates were made using a combination of bot h muscovite and biotite cooling age and distance data. To conduct these regressions, a comput er program called Isoplot v. 3.09a (Ludwig, 1991) was used. Straight lines were fit to the thermochronological data using least-squares regressions. Note, samples falling into an intermediate age range (i.e., H97RBY-52, DF02-219) were excluded from both the older and younger regressions for both the Lamoille Canyon transect as well as the younger age grouping from the East Humboldt Ra nge transect. Figures 4-4, 4-5, and 4-7 show the results from the Isoplot least-squares regressions. For Lamoille Canyon, there were two distinct age groupings. The younger group includes eight

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33 samples (from left to right on Figure 44) DF02-206, H93RBY-4, H97RBY-53, DF02-205, DF02-203, H97RBY-54, H97RBY-55, and DF02-209. The older group includes five samples (from left to right on Figure 4-5) DF02-216, DF02-218, H97RBY-42, H97RBY-51, and H93RBY-8. H97RBY-52 fell into the middle of the apparent age range and was excluded from both the older and the younger groups. The ol der group of samples (Figure 4-5) yielded a regression slope (m) = -0.24 .22 (slope errors are 2 ). Slip rate estimates for the detachment were made from this regression by taking the inve rse of the absolute values of the regression slopes and their errors (e.g., see Foster and John 1999). The slip rate for the older group of samples was calculated to be 4.2 .5 km/m.y. (or cm/yr). The younger group of samples from Lamoille Canyon (Figure 4-4) yielded a regression slope (m) = -0.24 .26 (slope errors again at 2 ). The slip rate for the younger group of samples from the Lamoille Canyon transect was calculated to be 4.2 .8 km/m.y. (or cm/yr). It is important to note that the slip rate ca lculations are averaged over the time interval from ~ 33 Ma for the older Lamoille Canyon samples and ~ 26 Ma for the younger Lamoille Canyon samples. These ranges are the ra nge of cooling ages used in the slip rate estimates. The slip rate calculations do not account for increases or decr eases in slip along the detachment within the time interval of ~ 33 Ma and ~ 26 Ma (Foster and John, 1999; Stockli, 2005). The muscovite and biotite 40Ar/39Ar data from the slightly older group of Lamoille Canyon transect samples gave apparent ag es of ca. 33 Ma in the eastern part of the footwall. The ca. 33 Ma apparent age could indica te an Oligocene phase of extension at a poorly defined rate of ~ 4.2 4.5 km/m.y. However, the apparent ages of these samples are all within error of each

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34 other and could simply be due to slow cooling of the shallower part of the footwall through the biotite partial retention in terval, or a crustal therma l event at about 30 Ma. Figures 4-6 shows the mica 40Ar/39Ar ages (Ma) from the East Humboldt Range transect plotted versus their distance along slip direc tion (km). The group includes five samples DF02219, DF02-212, DF02-214, DF02-211, and DF02-215, as we ll as 2 samples from Dallmeyer et al. (1986) and 6 samples from McGrew and Snee (1994). It appears that the East Hu mboldt Range might have older and younger grouping similar to the transect in Lamoille Canyon, but the structural complexity of the region and the geographical scattering of the samples doesnt allow a regression for the data.

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35 Figure 4-1. Simplified geologic map showing mine ral cooling ages from samples, collected along transects across Lamoille Canyon a nd the East Humboldt Range, using 40Ar/39Ar thermochronology (2 ).

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36 M B B M B B B B B B B B M M 18 20 22 24 26 28 30 32 34 18002000220024002600280030003200Altitude (m)Age (Ma)data-point error crosses are 2 Figure 4-2. Plot of sample calc ulated ages vs. the altitude at which samples were collected.

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37 Figure 4-3. All of the mica 40Ar/39Ar thermochronology ages from the Lamoille Canyon transect were plotted versus their dist ance along slip direction. M B B B M B B B B B B B B B M M18 20 22 24 26 28 30 32 34 4 8 12 16 20 24Distance in Slip Direction (km)Age (Ma)data-point error crosses are 2 Onset of extension

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38 B M B B B M B B B B B 18 20 22 24 26 28 12 14 16 18 20 22 24 26Distance in Slip Direction (km)Age (Ma)data-point error crosses are 2 Slope = -0.40.16 (95% conf) Inter = 30.3.9 Xbar = 18.3986, Ybar =22.8644 MSWD = 4.8, Probability = 0.000 Figure 4-4. Mica 40Ar/39Ar thermochronology ages versus dist ance in slip direction from the Lamoille Canyon transect, younger age grouping. A regression was calculated for the sample set.

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39 B B B M M 29 30 31 32 33 5791 11 3Distance in Slip Direction (km)Age (Ma)data-point error crosses are 2 Slope = -0.24.22 (2 ) Inter = 33.761.5 Xbar = 6.77129, Ybar =32.1386 MSWD = 0.24, Probability = 0.87 Figure 4-5. Mica 40Ar/39Ar thermochronology ages versus dist ance in slip direction from the Lamoille Canyon transect, older age grouping. A regression was calculated for the sample set.

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40 M B M B B M B M B B B B M 20 22 24 26 28 21 23 25 27 29 31 33 35Distance in Slip Direction (km)Age (Ma)data-point error crosses are 2 Figure 4-6. All of the mica 40Ar/39Ar thermochronology ages from the East Humboldt Range transect were plotted versus thei r distance along slip direction.

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41 CHAPTER 5 CONCLUSIONS The thermochronological data set obtained from the Ruby Mountains metamorphic core complex transect across Lamoille Canyon shown in this study provide several constraints on the exhumation onset and slip rate during the Eocen e and Miocene: (1) The age of the onset of extension in the Ruby Mountains is indicated to be ~25 Ma by the marked break in the slope of the cooling age curve on the age vs. distance di agram constructed from the mica cooling ages (Fig. 4-3). This thermochronology-based age cons traint is in good agreement with the previous thermochronology done in the area (e.g., Dallmeyer et al., 1986; Dokka et al., 1986; Howard, 2003; McGrew and Snee, 1994). Thus, the onset of extension in the Ruby Mountains is now well constrained and confirmed to be at ~ 25 Ma. Furthermore, (2) the cooling ages from micas show that extension in the Ruby Mountains across Lamoille Canyon continued until at least ~ 20 Ma. (3) Muscovite and biotite 40Ar/39Ar thermochronological data obtained during this study were also used to constrain the s lip rate on the detachment fault. These data show that between ~ 26 Ma, the averaged slip rate on the detachment was ~ 4.2 3.8 km/m.y. (4) 40Ar/39Ar data from the eastern part of the foot wall may indicate an Oligocene phase of extension at a rate of ~ 4.2 4.5 km/m.y. (or cm/yr) from ~ 33 Ma, or reco rd the graduate cooling before the onset of extension.

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42 APPENDIX 40Ar/39Ar THERMOCHRONOLOGY METHODS Sample Preparation and Irradiation 20 samp les were collected along two transects in 2002. Billets were cut from the samples and were sent to Texas Petrographic Inc. to be cut and polished into thin sections. Examination of the thin sections provided information on the extent of alteration to various mineral phases in the rock samples to be used in 40Ar/39Ar analyses. Samples that showed un-altered micas, Kfeldspars, and hornblende were selected for mineral separation and 40Ar/39Ar analyses. The selected samples were crushed and milled into a sand sized fraction using a Sturtevant rock Jaw Crusher and Bico Pulverizer type UA disk mill. The pulverized sample was sieved. Each sample was then run through the water table to separate minerals according to density. The IV and III (lightest) water table fractions were used further to process for K-feldspar and muscovite, and the water table II and I fractions were processed for biotite and hornblende. Following water table processing, tetrabromoet hane (TBE) and methylene iodine (MI) heavy liquids (densities of 2.96 and 3.33 g/cm3, resp ectively) were used to separate the micas, K-feldspar, and hornblende from less dense minerals These separates were rinsed with ethanol (for TBE) and acetone (for MI) 2-3 times followi ng separation. Special dilute TBE mixes were used to further separate the TBE floats (conta ining quartz, K-feldspar, plagioclase feldspar, muscovite, etc.) to first sink the qua rtz and feldspars and allow the muscovite to float, and then to sink the quartz and plagioclase feldspar and allow the K-feldspar to float. A Frantz magnetic separator Model L-1 was us ed to separate the biotite and hornblende from non-magnetic phases. Only the most magne tic biotite and hornblende Frantz separates were kept to avoid incl usion-rich minerals. All mineral se parates were then hand picked under a binocular microscope for better refinement usi ng standard picking tool s (i.e., nylon brushes,

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43 Pyrex glass dishes, and wax weighing paper). Some separates were given an ultrasonic bath in de-ionized water for approximately fifteen minutes to remove any altered materials. Nine muscovite separates and seven biotite separates along with GA1550 biotite flux monitors (98.79 0.5 Ma, see Reene et al., 1998) were individually packaged in aluminum foil (~ 5 mg for mineral separate, ~ 1 mg for flux mo nitors) and sealed in a pure quartz glass. The mineral separates and flux monitors were irradi ated in 2 different batches in a 1.1 MW TRIGA MARK II research nuclear reactor at the Oregon State University Radiation Center. For a more detailed description of these faci lities and irradiation methods see http://ne.oregonstate.edu/f acilities/radiation_center/ The first batch, OS10, was irradiated for 2 hours. This included samples: DF02-203 (musc ovite), DF02-212 (biotite ), DF02-214 (biotite), DF02-215 (muscovite), DF02-216 (muscovite ), DF02-218 (muscovite), and DF02-219 (muscovite). The second batch, OS11, was irra diated for 7 hours and included samples DF02204 (biotite), DF02-205 (biotite), DF02-206 (biotite), DF02-208 (biotite), DF02-209 (muscovite), DF02-210 (muscovite), DF02-211 (b iotite), DF02-221 (muscovite), and H03WH42 (muscovite). 40Ar/39Ar Analytical Instrumentation and Procedures The 40Ar/39Ar analyses were carried out in the nobl e gas laboratory at the Department of Geological Sciences, University of Florida. A combination of both laser ablation step-heating and furnace step-heating techniques were utilized to extract Ar gas from mineral separates. Laser ablation step-heating of mica separates wa s done by a water-cooled New Wave Research model MIR10 30W CO2 laser. During laser step-heating the New Wave laser was manually controlled using LAS (laser ab lation software) version 1.3.0.1 by New Wave Research. Mica separates were ablated a to tal of 5-15 steps under a 1750 m continuous wavelength focused laser beam at 2-5.5% power. A final laser fusing-step was done at 10-12% power. The step-

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44 heating schedule used for laser ablation varied and was adjusted accordingly to maximize Ar gas output for the mineral separates. A water-cooled, double vacuum, resistively heat ed furnace was used to step-heat the remaining mica separates. The furnace step-heating analyses were controlled automatically. The heating schedule was programmed into the comp uter to maximize the Ar gas output for each heating step ranging from a de-g assing step at 400 degreed to a fusing step at 1450 degrees. Ar gas extracted from mineral samples by la ser and furnace step-hea ting was transferred by vacuumed lines to a getters trap for 10 minutes to remove reactive gasses. The purified Ar gas was analyzed in a gas-sourced Mass An alyzer Products Model 215-50 mass spectrometer equipped with a filament for gas ionization a nd a magnetic sector mass discriminator followed by a Balzers Electron Multiplier collector to measure the isotopic abundances of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar. Cold laser blanks were analyzed at the beginning of each analytical session and every five steps after. The laser blanks were obtained by closing the laser chamber for two minutes and then passing the blank gas to the mass spectrometer following the same steps as a regular sample step. Heated furnace blanks we re made by closing and he ating the empty furnace to 400C, 600C, 800C, 1000C, 1200C and 1450C prior to sample step-heating. Data files produced from the mass spectrometer sample and blank analyses were imported into the program ArArCALC version 2.4 by Koppers (2002) for data reduction and 40Ar/39Ar cooling age calculations. ArArCA LC uses Microsoft Excel to plot data tables, age plateaus and isochrons. ArArCALC was also used to calcul ate J-values from laser total fusion analyses of the GA1550 biotite flux monitors. These J-values we re then applied to their individual samples to better constrain their 40Ar/39Ar cooling age calculations.

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45 LIST OF REFERENCES Angelier, J., and Coletta, B., 1983, Tension frac tures and extensional tectonics: Nature, v. 301, p. 203. Arm strong, R.L., and Ward, P., 1991, Evolving geogr aphic patterns of Cenozoic magmatism in the North American cordillera: The termoral and spatial association of magmatism and metamorphic core complexes: Journa l of Geophysical Research, v. 96, p. 13,201,224. Brun, J., Sokoutis, D., and Driessche, J. V. D ., 1994, Analogue modeling of detachment fault systems and core complexes: Geology, v. 22, p. 319. Buck, W.R., 1988, Flexural rotation of normal faults: Tectonics, v. 7, p. 959. 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 Ge ological Society of America, v. G-3, p. 407. Carter, T.J., Kohn, B.P., Foster, D.A., and Gl eadow, A.J.W., 2004, How the Harcuvar Mountains metamorphic core complex became cool: Evidence from apatite and (U-Th)/He thermochronometry: Geology, v. 32, p. 985. Chery, J., 2001, Core complex mechanics: From the Gulf of Corinth to the Snake Range: Geology, v. 21, p. 439. Coney, P.J., 1980, Cordilleran metamorphic core complexes: An overview: Geological Society of America, Memoir 153, p. 7. 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: Tectonics, v. 5, p. 931. Davis, G.H., 1983, Shear-zone model for the origin of metamorphic core complexes: Geology, v. 11, p. 342. Dokka, R.K., Mahaffie, M.J., S noke, A.W., 1986, Thermochronologic evidence of major tectonic denudation associated with detachment faulting, northern Ruby Mountains-East Humboldt Range, Nevada: Tectonics, v. 5, p. 995. Foster, D.A., Gleadow, A.J.W., Reynolds, S.J., Fitzgerald, P.G., 1993, Denudation of metamorphic core complexes and the reconstr uction of the transition zone, West Central Arizona: Constraints from apatite fission track thermochronology: Journal of Geophysical Research, v. 98, p. 2167.

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46 Foster, D.A., and John, B.E., 1999, Quantifying tectonic exhumation in an extensional orogen with thermochronology: examples from the s outhern Basin and Range Province: In Ring, U., Brandon, M., Lister, G.S., and Willett, S.D. (eds), Exhumation Processes: normal faulting, ductile flow, and erosion: Geologica l Society (London) Sp ecial Publication, 154, p. 356. Grice, W., 2006, Exhumation and cooling history of the middle Eocene Anaconda metamorphic core complex, western Montana: Master of Science Thesis Defense, University of Florida. Henry, C.D., and Boden, D.R., 1998, Eocene Magmatism: The heat source for Carlin-type gold deposits of northern Nevada: Geology, v. 26, p. 1067. Hofstra, A.H., and Cline, J.S., 2000, Characteris tics and models for Carlin-type gold deposits: Reviews in Economic Geology, v. 13, p. 163. Howard, K.A., 1980, Metamorphic infrastructure in the Northern Ruby Mountains, Nevada: Geological Society of America, Memoir 153, p. 335. Howard, K.A., 2003, Crustal struct ure in the Elko-Carlin regi on, Nevada, during Eocene gold mineralization: Ruby-East Humboldt metamor phic core complex as a guide to the deep crust: Economic Geology, v. 98, p. 249. Kistler, R.W., Ghent, E.D., ONeil, J.R., 1981, Pe trogenesis of garnet two-mica granites in the Ruby Mountains, Nevada: Journal of Geophysical Research, v. 86, p. 10,591,606. Koppers, A.A.P., 2002, ArArCALC softwa re for Ar-40/Ar-39 age calculations: Computers and Geosciences, v. 28, p. 605. Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic co re complexes and detachment faults formed during Tertiary continental extension in the nort hern Colorado River region, USA: Journal of Structural Geology, v. 11, p. 65. McDougall, I., and Harrison, T.M., 1999, Geochronology and Thermochronology by the 40Ar/39Ar method, 2nd edition. Oxford University Press, N.Y. McGrew, A.J., and Snee, L.W., 1994, 40Ar/39Ar thermochronologic constraints on the tectonothermal evolution of the northern East Humboldt Range metamorphic core complex, Nevada: Tectonophysics, v. 238, p. 425. Miller, E.L., Dumitru, T.A., Br own, R.W., and Gans, P.B., 1999, Rapid Miocene slip on the Snake Range-Deep Creek Range fault system, eas t-central Nevada: Geological Society of America Bulletin, v. 111, p. 886. Oldow, J.S., 1984, Evolution of a late Mesozoic back -arc fold and thrust belt, northwestern Great Basin, U.S.A.: Tectonophysics, v. 102, n. 1, p. 245.

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47 Poole, F.G., 1992, Distribution and thickness of late Proterozoic th rough middle Ordovician rocks: The Geology of North America, The Ge ological Society of America, vol. G-3, p. 407. Roberts, R.J., Ferguson, H.G., Gilluly, J., Hotz, P.E., 1958, Paleozoic rocks of north-central Nevada: Bulletin of the American Associ ation of Petroleum Geologists, v. 42, n. 12, p. 2813. Snoke, A.W., 1980, Transition from infrastructure to suprastructure in the northern Ruby Mountains, Nevada: Geological Society of America, Memoir 153, p. 287. Stockli, D.F., 2005, Application of low-temperat ure thermochronometry of extensional tectonic settings: Reviews in Mineral ogy and Geochemistry, v. 58, p. 411. Wells, M.L., Snee, L.W., and Blythe, A.L., 2000, Dating of major normal fault systems using thermochronology: An example from the Raft River Mountains, Basin and Range, western United States: Journal of Geophysical Research, v. 105, p. 16,303,327. Wernicke, B., 1981, Low-angle faults in the Basin and Range province: Nappe tectonics in an extending orogen: Nature, v. 291, p. 645. Wernicke, B., 1985, Uniform-sense normal simple sh ear of the continental lithosphere: Canadian Journal of Earth Sciences, v. 22, p. 108. Wernicke, B., and Axen, G. J., 1988, On the role of isostasy in the evolution of normal fault systems: Geology, v. 16, p. 848.

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48 BIOGRAPHICAL SKETCH Jennifer was born in Hartford, CT, in 1983. Sh e is the youngest of three sisters. She graduated from RHAM high school (Hebron, CT) in 2001. S he received a Bachelor of Science degree in geology from Syracuse University (Syracuse, NY) in 2005. While attending graduate school at the University of Florida (Gainesville, FL), Jennifer served as a teaching assistant for several courses in the Department of Geological Sciences and as a research assistant for Dr. David A. Foster. She received a masters degree in geology from the University of Florida in May of 2008. Jennifer continues to pursue her interests in the PhD program in geology at the University of Florida (Gainesville, FL).

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QUANTIFYING EOCENE AND MIOCENE EX TENSION IN THE SEVIER HINTERLAND IN NORTHEASTERN NEVADA Jennifer N. Gifford Geological Sciences David A. Foster Master of Science May 2008 This project provides a compre hensive low-temperature thermochronologic history of the Ruby Mountain metamorphic core complex in northeas t Nevada, and helps to define the Tertiary extensional history of the regi on. Samples from two transects across the range were analyzed using furnace and laser 40Ar/39Ar methods. Data generated by this project was used to: a) establish the timing of Paleogene and Neogene exhumation and rate of westward slip on the fault system that separates the upper cr ustal block that include the Car lin gold trend mineralizations from the Ruby Mountains metamorphic core co mplex, and b) better understand the geologic setting and origin of the Carlin-type depos its that form the Carlin gold trend.