Structural Setting of the Emerson Pass Thermal Anomaly, Washoe County, Nevada

Thesis proposal for the degree of Master of Science in Geology

Ryan B. Anderson

Advisor: Dr. James Faulds

Mackay School of Earth Sciences and EngineeringUniversity of Nevada, Reno

July 27, 2012

1

INTRODUCTION

The Great Basin of the western USA has an anomalously high geothermal gradient that is

favorable for geothermal energy development. Geothermal activity is commonly associated with

volcanic activity at plate boundaries and hot spots. While extensive volcanism in the Great

Basin has occurred during the Cenozoic, activity ceased in most areas by 10 to 3 Ma. (Best et al.,

1989; Faulds et al., 2004a, 2006). Models predict that intrusions of late Miocene to Pliocene age

would have little effect on the heat flow pattern in the crust (Blackwell, 1983). Furthermore,

excluding the few exceptions at the margins of the region where recent magmatism has occurred

(Roosevelt Hot Springs, Steamboat, Coso), geothermal fluids in the Great Basin do not have a

geochemical signature indicative of a magmatic heat source and are therefore amagmatic

(Arehart et al., 2002; Arehart et al., 2003). The two main components of heat flow in amagmatic

systems are high mantle heat flow due to tectonic activity and radioactive decay of elements in

the crust. However, it is unlikely that high heat flow in the Great Basin is attributable to

radioactive decay (Blackwell et al., 2007). Rather, high heat flow in the region likely results

from tectonic activity as the central and northern Great Basin have been extended 200-300 km

during the Cenozoic, with consequential thinning of the crust to an average thickness of ~30 km

(Wernicke, 1992; Heimgartner and Louie, 2007; Henry et al., 2011). As a consequence, the

Great Basin has an elevated geothermal gradient, exceeding 50°C per kilometer in ~30% of the

region (Blackwell et al., 2007).

Faults generally provide the major conduits for deep circulation of geothermal fluids.

Non-fractured crystalline and sedimentary rocks have low permeabilities (10-2 to 10-17 darcy)

such that they would not generate observed hydrothermal outflow relative to fractured rock (101-

10-3 darcy), which can have several orders of magnitude greater permeability (Curewitz and

2

Karson, 1997). These conduits for fluid flow can be formed and maintained through continued

slip on faults (Curewitz and Karson, 1997), formation of fault damage zone structures (Caine et

al, 1996; Faulkner, 2010), generation of dilational openings (Ferrill and Morris, 2003), or

simultaneous slip and dilation.

Excluding deep basin geothermal resources (Blackwell et al., 2007, 2011; Allis et al.,

2011), most geothermal systems in the Great Basin are structurally controlled (Faulds et al.,

2006, 2011). Identifying favorable structural settings that will accommodate the aforementioned

fluid conduits is particularly crucial in amagmatic settings. It is well known that individual fields

in the northwestern Great Basin are commonly controlled by moderately to steeply dipping fault

zones, particularly N- to NNE-striking faults, yet not every fault hosts a geothermal system. It is

therefore necessary to focus on segments along individual fault zones that focus geothermal

activity (Faulds et al., 2006, 2010, 2011). Recent investigations have found that such segments

occur at 1) discrete steps in normal fault zones, 2) intersections between normal faults and

transversely oriented oblique-slip faults, 3) overlapping, oppositely dipping normal fault zones,

4) terminations of major normal faults, and 5) transtensional pull apart zones (Curewitz and

Karson, 1997; Faulds et al., 2006, 2011). These primary controls are commonly manifested

topographically as 1) major steps in range front faults, 2) interbasinal highs, 3) mountain ranges

consisting of relatively low, discontinuous ridges, and 4) lateral terminations of mountain ranges

(Faulds et al., 2006, 2011).

Additionally, geothermal systems may manifest themselves in a number of ways such as

hot spring outflow, mineral deposition, and alteration of country rock. Epithermal vein systems

are commonly associated with geothermal systems. Drilling in active geothermal systems has

revealed features characteristic of epithermal vein systems, such as deposition of base-metal

3

sulfides, vertical zoning of metals, textures indicative of boiling conditions, and propylitic and

argillic alterations (e.g., Henley, 1985; Simmons and Browne, 2000). Presence of silica sinter

and hydrothermal breccias in epithermal vein systems indicate that they were once active

geothermal systems (Henley, 1985). Geothermal systems must maintain an open conduit of flow

through enhanced permeability with minimal healing from mineral deposition. Fluid flow

commonly focuses in the propagating tip of the fault and may leave behind traces of

hydrothermal deposits and record the evolution of the fault zone (Curewitz and Karson, 1997).

Thermal springs may also form calcium carbonate tufa towers in a sub-aqueous environment or

travertine mounds in a sub-aerial environment (Coolbaugh et al., 2009).

As of 2009, Nevada was generating nearly 450 MW of geothermal power with a

projected large increase to come online in the next 5-7 years. Many of these systems have been

discovered using surficial expressions, such as hot springs and silica sinter deposits. However,

blind systems account for ~35% of geothermal power in Nevada and may represent as much as

2/3 of the geothermal resources in the Great Basin region (Coolbaugh et al., 2006a). Blind

systems may therefore have great potential for development and could potentially outpace energy

output of systems with surficial expressions. Furthermore, inexpensive methods for exploration

of blind systems, such as locating tufa mounds or 2-meter shallow-temperature surveys, can be

useful guides if caution is utilized. Tufa mounds can also accumulate from non-thermal waters if

enough calcium carbonate is present. Also, 2-meter temperature surveys commonly detect

outflow from distant geothermal upwelling zones and may therefore be misleading in

discovering a viable geothermal resource (Coolbaugh et al., 2007, 2009). Therefore, a viable

strategy for exploration of blind geothermal systems involves searching for favorable structural

settings and utilizing features, such as tufa mounds and shallow temperature anomalies, as a

4

supplemental guide. Detailed geologic mapping and structural analysis of an individual field are

then required to identify the most favorably oriented faults for slip and dilation.

PURPOSE OF THIS STUDY

The purpose of this study is to characterize the structural setting of the Emerson Pass area

(Figure 1) on the Pyramid Lake Paiute Indian Reservation (PLPR) in northwest Nevada to

estimate the potential and possible localities for blind geothermal systems. Emerson Pass lies

near the transition between the dextral shear zone of the Walker Lane and the actively extending

Basin and Range of northwest Nevada.

Large tufa towers and shallow-temperature surveys were previously used as a proxy to

identify potentially blind or hidden geothermal resources in other parts of the reservation

(Coolbaugh et al., 2006b, 2007; Vice, 2008). A thermal anomaly discovered with a 2-meter

temperature survey (Figure 2), tufa mounds, and argillic alteration suggests the presence of a

hidden geothermal resource in the northeastern part of the reservation at or near Emerson Pass

(Kratt et al., 2010).

Step-overs or relay ramps in normal fault zones are commonly characterized by multiple,

generally overlapping fault strands, increased fracture density, and enhanced permeability

(Faulds and Varga, 1998; Faulds et al., 2011). Field reconnaissance and reviews of air photos

suggest that the primary control of the Emerson Pass thermal anomaly is a left step between

major range-front faults bounding the west flanks of the Fox Range and Lake Range (Figure 3).

However, it is unclear whether subsidiary faults within the step-over act as a secondary control

and focus fluid conduits. Precious and base metal veins of an unknown age within altered

Miocene volcanic rocks reside in the footwall of the Fox Range fault just to the east of the tufa

5

towers and shallow-temperature anomaly in Emerson Pass (Satkoski and Berg, 1982). It is also

unknown if other geothermal sites exist in the area to the east of Emerson Pass. The step-over

between the two ranges is quite broad (~19 km wide) and contains numerous faults.

This study intends to characterize the main structural controls of a potentially blind

geothermal system in Emerson Pass, explore for the evidence of a geothermal upwelling zone

that may reside in the broad step-over between the Fox and Lake Ranges, and determine the

orientation of faults and fractures conducive to fluid flow. This project will involve detailed

geologic mapping, structural analysis, geochronologic studies, and possibly paleomagnetic

investigations. Also, studies of the mineralized veins in the Emerson Pass area and dating of

adularia or alunite in these veins, if present, may reveal a relationship between the contemporary

geothermal system and record the evolution of the system through time. Collection of these data

will be instrumental in selecting sites for temperature-gradient wells and may aid in future

geothermal development of the area. This study may also enhance strategies for exploring of

blind geothermal systems and add to the understanding of similar systems elsewhere in the Great

Basin and beyond.

TECTONIC SETTING

Early east-west extension in the region began at ~12 Ma and is recorded by the formation

of sedimentary basins (Trexler et al., 2000). Right-lateral strike-slip faulting within the Pyramid

Lake domain of the Walker Lane began ~9-3.5 Ma and has accumulated 20-30 km of cumulative

offset (Cashman and Fontaine, 2000; Faulds et al., 2005; Faulds and Henry, 2008). The Walker

Lane is a system of right-lateral, left-stepping strike slip faults that accommodate ~20% of the

relative motion between the Pacific and North American plates (Faulds and Henry, 2008).

Within the Pyramid Lake domain, the Pyramid Lake fault zone is a major structure that

6

accommodates a significant portion of shear derived from plate boundary right-lateral shear in

the northern Walker Lane (figure 3). It has accommodated 5-10 km of offset (Faulds and Henry

2008). Near the southern end of Pyramid Lake, the Pyramid Lake fault splays northward and

transfers strain into a system of north-striking, west-dipping normal faults, which accommodate

northwest-directed extension and opening of the Pyramid Lake basin (Faulds et al., 2005;

Drakos, 2007; Faulds and Henry, 2008).

The transfer of Walker Lane dextral strain to north-northeast striking normal faults

generates enhanced dilation and favors deep circulation of fluids. This may account for the

north-northeast trends of geothermal fields in northern Nevada (Figure 4), which mimic the

structural grain of the Basin and Range and are oriented approximately orthogonal to the

contemporary extension direction and the least principle stress (Faulds et al., 2004).

Emerson Pass is located in the northwestern Great Basin at the northeast end of Pyramid

Lake, Nevada. It lies roughly at the junction between the Walker Lane and the Black Rock

geothermal belts (e.g., Faulds et al., 2004, 2012). This region is referred to as the transitional

Walker Lane. It is a transtensional setting that contains both northwest- striking dextral faults,

and north- to north-northeast-striking normal faults (Faulds et al., 2006). Emerson Pass is a

narrow, northwest oriented valley, approximately 1.5 km wide. It is bound on the west by the

Terraced Hills and on the east by the northerly trending Fox Range. A through-going WNW-

trending fault appears to link the range-bounding faults of the Fox and Lake Ranges (Bonham

and Papke, 1969).

Geodetic studies confirm the transtensional setting of the region. East of the northern

Walker Lane, ~1 mm/yr of N55W° directed extension of the northern Basin and Range occurs

with respect to North America (Hammond et al., 2011). West of longitude 119⁰W, the velocities

7

increase to ~10 mm/yr, deformation becomes predominantly dextral shear, and motion is

directed to ~N40⁰W (Bennett et al., 2003; Hammond and Thatcher, 2005; Hammond and

Thatcher, 2007). The geodetically observed strain rates in the western Great Basin/Walker Lane

appear to be accommodated predominantly by strike-slip deformation; however, in the northern

Walker Lane, dextral slip decreases relative to the central and southern Walker Lane and is

distributed across several strike-slip and normal faults (Hammond et al., 2011). This transfer of

dextral shear results in shear zone parallel extension within the western Basin and Range region,

and appears to be partly accommodated across the central Nevada seismic belt (Faulds et al.,

2004; Kreemer et al., 2009; Hammond et al., 2011).

FOX RANGE AND LAKE RANGEStratigraphy

Basement- The basement rocks exposed in the Lake and Fox Ranges consist of the Jurassic to

Triassic metasedimentary Nightingale sequence and Mesozoic intrusions. In the Lake Range, the

Nightingale sequence is primarily a thick package of metamorphosed, quartzite-rich, sandy and

argillaceous sedimentary rocks with thin lenses of recrystallized limestone (Bonham and Papke,

1969). The Nightingale sequence at Marble Bluff and in the Fox Range consists primarily of

carbonate rocks. The carbonate rocks are recrystallized and folded (Bonham and Papke, 1969;

Drakos, 2007). The metasedimentary rocks of both the Lake and Fox Ranges are intruded by

quartz monzonite to granodiorite, presumably associated with the Sierra Nevada Batholith

(Bonham and Papke, 1969; Satkoski and Berg, 1982; Drakos, 2007).

Tertiary- The Tertiary section of rocks, from oldest to youngest, includes 1) Oligocene ash-flow

tuffs and dacitic flows, 2) the middle Miocene Pyramid sequence, 3) an intrusive suite of rocks

8

that cuts mainly the lower part of the Pyramid sequence, and 4) a package of late Tertiary

sedimentary rocks in the northern Lake Range and pre-Lahontan gravels in the southern Fox

Range. Little mapping of the Tertiary section has been conducted in the southern Fox Range

except a small portion proximal to the Sano and Packard workings in Emerson Pass (Satkoski

and Berg, 1982). However, recent detailed mapping has been conducted in the northern Lake

Range (Rhodes, 2011), the southern Lake Range (Drakos, 2007), and in the Terraced Hills (Vice,

2008) just west of the southern Fox Range. In addition, the Tertiary section has been extensively

mapped to the southwest in the Virginia Mountains, Dogskin Mountain, and the Pah Rah Range

(Garside et al., 2003; Faulds et al., 2003, 2007; Henry et al., 2004).

The Mesozoic basement in the Lake Range is locally nonconformably overlain by

Oligocene volcanic rocks. This includes an ash-flow tuff dated at 31.28 Ma in the southern Lake

Range (Drakos, 2007) and a dacite flow dated at 24.1±0.4 Ma in the northern Lake Range

(Rhodes, 2011).

Overlying the Oligocene volcanic rocks and Mesozoic basement in the Lake Range is the

middle Miocene Pyramid sequence (Bonham and Papke, 1969; Drakos, 2007; Rhodes, 2011). It

is an areally extensive suite of volcanic and sedimentary rocks found in the mountains around

Pyramid Lake. It consists primarily of mafic to intermediate flows and flow and mudflow

breccias, all intercalated with sedimentary lenses of conglomerate, sandstone, diatomite, bedded

rhyolitic tuffs, tuff breccias, and dacitic welded tuffs (Faulds et al., 2003, 2007; Henry et al.,

2004).

The Pyramid sequence is divided into upper and lower units. The lower sequence

typically consists of multiple mafic flows that range from coarsely porphyritic, with distinctive

tabular plagioclase up to 30 mm long, to aphanitic basalt to basaltic-andesite. It ranges from ~16

9

to 14 Ma (Faulds et al., 2003; Henry et al., 2004; Drakos, 2007; Vice, 2008; Rhodes, 2011).

However, in the northern Lake Range, the base of the lower sequence consists mainly of

volcaniclastic sedimentary rocks with intercalated aphanitic olivine basalt and basaltic andesite

flows (Rhodes, 2011). The upper and lower sequences are typically separated by the dacitic tuff

of Mullen Pass, which has well constrained dates of ~13.8 to ~13.6 Ma. (Faulds et al., 2003,

2007; Garside et al., 2003; Drakos, 2007). A dacitic lava in the southern Lake Range and the

Terraced Hills is also situated at the break between the upper and lower sequences. The

composition of the flow is similar in both locations, and thought to be broadly correlative.

However, in the Terraced hills it directly overlies the tuff of Mullen Pass, yet underlies the tuff

of Mullen Pass in the southern Lake Range (Drakos, 2007; Vice, 2008). In the northern Lake

Range, the tuff of Mullen Pass does not crop out, and the uppermost unit in the Pyramid

sequence is a 14.4±1.6 Ma dacite flow overlain in angular unconformity by late Tertiary

sediments (Rhodes, 2011). Much like the lower sequence, the upper sequence in the southern

Lake Range and Terraced Hills consists of multiple flows of basalt to basaltic-andesite but lacks

the distinctive coarsely porphyritic flows common in the lower sequence. Several 40Ar/39Ar dates

place the upper part of the sequence at ~13-13.5 Ma (Drakos, 2007; Vice, 2008). In the southern

Fox Range, the metamorphic and intrusive basement rocks are overlain by basalt, andesite, and a

light color tuff of the Pyramid sequence (Satkoski and Berg, 1982). In the Terraced Hills the

lower part of the Pyramid Sequence is intruded by rhyolite and basaltic andesitic (Vice, 2008). In

the southern Lake Range an intrusive suite of basalt, rhyodacite, 15.3 Ma rhyolite domes, dikes,

and plugs cuts into the lower Pyramid sequence and the tuff of Mullen Pass (Drakos, 2007).

In the northern Lake Range and southern Fox Range the upper Pyramid sequence is

overlain by a sedimentary section. In the northern Lake Range this sedimentary package

10

contains a non-welded rhyolitic tuff yielding an age of 4.8±0.9 Ma (Rhodes, 2011). This section

temporally correlates with the Truckee River Formation (Rhodes, 2011). The southern Fox

Range also contains a section of pre-Lahontan gravels, alluvium, and fan material that are highly

dissected, faulted, and tilted.

Quaternary- The western edge of the Lake Range and the southern Fox Range are overlain by

Quaternary deposits. The highstand of Lake Lahontan forms a near continuous strand of gravel

modified by younger fans, stream channels, and colluvium (Adams et al., 1999; Drakos, 2007).

Tufa towers were precipitated in a sub-lacustrine environment in the Pyramid Lake subbasin of

ancestral Lake Lahontan (Benson, 1994). Wave form tufa was deposited on the ancestral strands

of Lake Lahontan and more recently at the modern shoreline of Pyramid Lake. The tufa forms

thick encrustations on both Mesozoic and Miocene rocks and fills the pore space in some

conglomerates (Drakos, 2007; Vice, 2008). The broad flat valley at the northeast end of Pyramid

Lake, between the Lake Range and the Terraced Hills, consists mainly of lacustrine silt and sand

derived primarily from Lake Lahontan.

Structural Framework

Emerson Pass lies at the northern end of a left step between the Lake Range and the Fox

Range and several kilometers northeast of the poorly defined northern termination of the

Pyramid Lake fault beneath Pyramid Lake (Figure 3). The Lake Range is 60 km long and

characterized by a series of primarily east-tilted fault blocks and north-trending ridges bounded

by moderately to steeply west-dipping normal faults. Major north-northeast striking faults cut the

central and northern Lake Range, effectively dividing the range into three structural blocks

(Bonham and Papke, 1969; Drakos, 2007; Rhodes 2011).

11

The Fox Range is a north-trending, convex to the west range approximately 50 km long.

It is a fault block, tilted to the east and bounded on the west by northwest- to northeast-striking

high-angle faults accommodating several kilometers of dip slip. However, abundant northwest-

to northeast-striking high-angle normal faults cut the interior of the range. The originally flat-

lying Tertiary rocks are now tilted ~30⁰ to the east. At the southern end of the range a set of

oppositely dipping northwest- and northeast-striking faults accommodate motion, which exposes

a horst of the Nightingale sequence. Similarly, a nearly north-striking normal fault

accommodates motion that exposes Cretaceous granodiorite (Bonham and Papke, 1969).

At the northern end of Pyramid Lake a broad left step lies between the Lake and Fox

Ranges. The overlap of the two synthetically west-dipping normal faults connecting the footwall

of one fault to the hanging wall of another has led to the formation of a relay ramp (Figure 5)

(cf., Larsen, 1988). The strata of the relay ramp dip gently to the south, and the ramp is cut by

faults of multiple orientations ranging from WNW to NNE. Step-overs or relay ramps serve as

the most favorable structural setting for geothermal systems in the Great Basin (Faulds et al.,

2011) and are likely the primary control for the Emerson Pass thermal anomaly. Multiple faults

intersect and/or terminate in the vicinity of the shallow-temperature anomaly and the tufa towers.

The fault within the step-over may be a secondary control that acts to focus geothermal fluid

flow.

Paleomagnetic studies indicate slight counterclockwise rotation since 9-12 Ma in the

Pyramid Lake domain of the Walker Lane (Cashman and Fontaine, 2000; Faulds et al., 2004).

The southern Lake Range has undergone negligible vertical axis rotation (Drakos, 2007),

whereas the southern Terraced Hills show minor clockwise vertical-axis rotation of ~12⁰ (Vice,

2008).

12

OBJECTIVES AND METHODS

The primary objectives of this study are to: 1) define the Tertiary stratigraphy, structural

framework, and alteration patterns in the central Lake Range and southern Fox Range, 2) provide

a more complete understanding of the geometry and kinematics of faults as well as the kinematic

linkage between the Lake Range and southern Fox Range, 3) determine the structural controls of

the Emerson Pass geothermal area and define features potentially favorable for fluid flow, 4)

examine the evolution of the Emerson Pass geothermal system and its relationship to alteration

and mineralization of the Packard and Sano mine workings, and 5) quantify vertical-axis

rotations in the area through paleomagnetic studies.

This study will include: 1) detailed geologic mapping at 1:24,000 scale to define

stratigraphy and faults, 2) petrographic analysis to identify and correlate stratigrpahic units and

styles of hydrothermal alteration, 3) structural analysis and characterization of fault zones to

define the geometry and kinematics of faults, 4) 40Ar/39Ar geochronology and tephrochronology

to correlate strata, quantify separation across faults, and constrain the timing of deformation,

alteration, and mineralization, 5) collection of paleomagnetic samples to constrain vertical-axis

rotations, and 6) GIS compilation to produce a geologic map and geodatabase for the

northeastern part of the Pyramid Lake Reservation.

Geologic mapping

Approximately 85 km2 of the central Lake Range and southern Fox Range will be

mapped at 1:24,000 scale. This will include portions of the Emerson Pass, Fox Canyon, San

Emidio Desert South, and Pyramid Northeast 7.5’ quadrangles. Mapping will be conducted

using 1:24,000 scale, color stereographic air photos. Mapping will be digitized and compiled

using Vr software and ArcGIS 10.1. Each stereo pair will be ortho-rectified and georeferenced

13

using Vr software, which will produce a three-dimensional model for each stereo pair. The

digitized field map will then be exported to ArcGIS 10.1 for final cartographic production.

Several cross sections will be constructed in key areas of interest in the study area.

Petrographic analysis

Petrographic analysis will be conducted to aid in identifying and constraining

stratighraphy, as well as in correlating offset rock units. It will also assist identification of

alteration styles and minerals. Furthermore, the petrographic work will be crucial to identifying

rock samples that are suitable for geochronologic analysis. This analysis will use hand samples

collected in the field and roughly 30-40 thin sections. A research-grade petrographic microscope

using transmitted, reflected, and polarized light will be employed for thin-section work.

Structural analysis

Understanding the geometries and kinematics of faults in the central Lake Range and

southern Fox Range will be crucial in understanding the structural controls of the Emerson Pass

geothermal system. Attitudes of bedding, flow foliation, and metamorphic foliation will be

collected throughout the field area and plotted on equal-area stereographic projections. Slip

sense of faults will be determined by measuring fault planes and kinematic indicators, such as

slickenlines, rough facets, and Riedel shears (Angelier et al., 1985; Petit, 1987; Angelier, 1994).

In the absence of the aforementioned kinematic indicators, slip sense can also be estimated by

the deformation of beds proximal to faults (drag folds) and offset of markers, such as intersecting

dikes and bedding, which can provide piercing points in rare cases.

Rough facets are elongated perpendicular to slickenlines. The facets step down and face

in the direction of motion of the missing block (figure 6a). Riedel shears typically form at an

angle of 5°-25° to the fault plane. Sense of motion is oriented nearly perpendicular to the line of

14

intersection between the fault plane and Riedel shear plane (figure 6b). Drag folds (6c) and

piercing points (6d) can also indicate slip sense.

The state of stress can be determined under the assumption of Coulomb failure in the

shallow crust. The orientations of σ1, σ2, and σ3, which are mutually orthogonal to each other, can

be estimated given a fault orientation and sense of motion. The intermediate stress (σ2) will lie

within the fault plane and be oriented perpendicular to the slip direction, whereas greatest

principal stress and the least principal stress (σ1 and σ3) define a plane that is perpendicular to σ2

(Angelier, 1994; Davis and Reynolds, 1996). The orientation of σ1 and σ3 can then be

determined assuming the internal angle of friction of most material in the shallow crust is ~30°

(Byerlee, 1978), and that the internal angle of friction determines the angle between the normal

to the fault surface and σ1 (Anderson, 1951).

Fault size is insignificant in determining the stress orientations; therefore, measurements

of kinematic indicators on small faults should be representative of the paleostress of all

surrounding faults (Angelier et al., 1985). This study will use numerical methods to find the best

fit to all fault slip data that belong to the same tectonic event and share a common stress tensor

(Angelier, 1979, 1994).

Slip and dilation tendency analysis is a method used to determine the orientation of faults

most favorably oriented for slip and/or dilation. Slip and dilation can both generate conduits for

fluid flow (Curewitz and Karson, 1997; Caine et al, 1996; Ferrill and Morris, 2003; Faulkner et

al., 2010). However, surfaces that are favorably oriented for both slip and dilation are the most

desirable target due to the likelihood that they will have produced zones of the highest

permeability.

15

Slip occurs on a plane when the resolved shear stress, the shear stress in the direction of

slip, is greater than or equal to the frictional resistance to sliding (which is proportional to the

normal stress). Slip tendency is the ratio of shear stress to normal stress on a given plane and

depends completely on the both the orientation of the plane and the stress field. The normal

stress, shear stress magnitude, and the shear stress direction can be calculated if the orientation

and magnitudes of the principal stress axes are known (Morris et al., 1996). Dilation of faults and

fractures is mainly a function of the resolved normal stress. Dilation tendency is calculated by

taking the difference between the maximum principle stress and normal stress, and dividing it by

the differential stress (Moeck et al., 2009). The dilation and slip tendency stereoplots are

obtained by resolving the slip and dilation tendency for all planes in 3D space. The results are

then applied to mapped faults (Morris et al., 2003; Moeck et al., 2009).

Paleomagnetic Investigations

The aim of paleomagnetic sampling is to collect a quasi-instantaneous snapshot of the

geomagnetic field direction at the time of the rock formation. Sampling will be conducted in

unaltered flows within the Pyramid sequence. A rock unit or sequence has multiple flows, each

of which will record a natural remnant magnetization (NRM). A site is an individual bed or flow

within the rock unit. Several sites are used within a rock unit so as to span an interval of time

that will average paleosecular variation. Within a site, about eight separately oriented samples

are collected over an area of several meters to tens of meters. This allows for within site

homogeneity of the NRM. Sample cores will be collected using a portable drill. Orientation will

be recorded by measurement of the inclination and azimuth of the core. Correction for tilt will

be made by recording the attitude of flow sequences.

16

Partial demagnetization procedures will remove the low stability components of the

NRM, and isolate the high stability component. The highest-stability component of the NRM

that is isolated by partial demagnetization is generally referred to as the characteristic component

of the NRM, abbreviated ChRM (Kirschvink, 1980).

A virtual geomagnetic pole (VGP) position is calculated with a series of equations using

the ChRM site-mean direction measured at a particular site. A paleomagnetic pole is then found

by taking the mean position of the set of VGPs.

Comparison between the observed pole that will be found in this study and a Miocene

reference pole will determine the amount of vertical-axis rotation (e.g., Mankinen et al., 1987).

This may elucidate fault kinematics in the area and allow for estimates of which segments may

be dilational. In addition, estimates of vertical-axis rotations may help to further quantify the

magnitude of dextral shear in this portion of the transitional Walker Lane. The results will be

compared to results of previous palemagnetic investigations (Cashman and Fontaine, 2000;

Faulds et al., 2004b; Drakos, 2007; Vice, 2008).

40Ar/39Ar geochronology

Radiogenic ages will be determined using the 40Ar/39Ar dating technique. This technique

measures the ratio between parent and daughter isotopes of 40Ar (daughter) and 39Ar. 39Ar is

produced from 39K in the rock through irradiation of the sample. The Ar isotopes are then

extracted by step-heating with a furnace or laser. Apparent ages derived from the 40Ar/39Ar ratios

are plotted against the cumulative percent of 39Ar released during step heating or laser fusion of

individual grains or small groups of grains. The age of crystallization is then interpreted from the

plateau of age spectra on the diagram or from ideograms.

17

Several minerals are suitable for dating. In general, these are materials that include

potassium in lattice sites and have measureable potassium and radiogenic argon. This will likely

include sanidine, plagioclase, or biotite in this study. In aphanitic mafic flows, analysis of

groundmass concentrates will be conducted, with care to remove phenocrysts that are not of the

potassium-bearing phase of the sample (e.g., olivine and pyroxene; McDougall and Harrison,

1999).

The first step in preparation of samples will be to examine thin sections with a

petrographic microscope for evidence of alteration, which will lead to the loss of radiogenic

argon. The sample will also be searched for the presence of glass, which typically does not

retain radiogenic argon. Suitable samples will then be crushed and sieved to obtain the desired

grain size for dating. Separations will then be carried out through use of a Frantz magnetic

separator and heavy liquids. Samples will be rinsed with HCL and/or HF solutions.

Tephrochronology

Glass shard analysis can be used to correlate silicic ash fall tuffs based on their

geochemical signature. This method is applicable for the late Neogene, from ~16 Ma to the late

Pleistocene (Perkins and Nash, 2002). Tephra will be collected and correlated using this

technique for further age constraints on units in the map area.

GIS compilation

The detailed geologic map, alteration patterns, age data, shallow-temperature survey, and

any temperature gradient well data will be overlaid on a digital elevation model. The data

compilation in ArcGIS will provide a convenient and cartographically aesthetic means for

18

visualizing the overall structure and geologic features of the relay ramp between the two ranges

and how it relates to the geothermal anomaly in Emerson Pass.

PRELIMINARY SCHEDULE

Semester GoalFall 2011 Begin coursework

Begin literature reviewSpring 2012 Reconnaissance field work: define study area, work out

stratigraphy, initial mappingContinue literature reviewApply for GSA (UNR), NPGS, AAPG grantsSubmit draft thesis proposalContinue course work

Summer 2012 Continue field work: map, sample, fault analysis, paleomagnetic sampling

Submit program of studyFall 2012 Continue field work

Begin draft of mapFinish course workBegin lab work: 40Ar/39Ar dating, petrography, and

paleomagnetic dataPresent work at AGU

Spring 2013 Present work to NPGSFinalize map and write thesisDefend thesis

EXPECTED PRODUCTS

A geologic map of the central Lake Range and southern Fox Range, with particular focus

on the Emerson Pass area.

A GIS database that will contain the geologic map, cross sections, alteration patterns,

data from shallow-temperature surveys, and deeper temperature gradient wells.

Publication in a peer-reviewed journal

19

Nevada Bureau of Mines and Geology open file report of geologic map.

REFERENCES CITED

Adams, K.D., and Wesnousky, S.G., 1999, The Lake Lahontan highstand: age, surficialCharacteristics, soil development, and regional shoreline correlation: Geomorphology,v. 30, p. 357-392.

Anderson, E. M., 1951, The dynamics of faulting and dyke formation with applications toBritain: Edinburgh, Oliver and Boyd, 206 p.

Allis, R., Moore, J., Blackett, B., Gwynn, M., Kirby, S., and Sprinkel, D., 2011, The potentialFor basin-centered geothermal resources in the Great Basin: Geothermal Resouces Council Transactions, v. 35, p. 683-688

Angelier, J., 1979, Determination of the mean principal stress for a give fault population:Tectonophysics, v. 56, p. T17-T26.

Angelier, J., Colletta, B, and Anderson, E., 1985, Neogene paleostress changes in the Basin and Range: A case study at Hoover Dam, Nevada-Arizona: Geological Society of America Bulletin v. 96, p. 347-361.

Angelier, J., 1994, Fault slip analysis and paleostress field reconstruction, in Hancock, P.L., ed., Continental Deformation, Pergamon Press, Terrytown, New York, p. 53-100.

Arehart, G., Coolbaugh, M., and Poulson, S., 2002, Geochemical characterization of GeothermalSystems in the Great Basin: Implications for exploration, exploitation, and environmental issues: Geothermal Resources Council Transactions, v. 26, p. 479-481.

Arehart, G., Coolbaugh, M., Poulson, S., 2003, Evidence for a magmatic source of heat for the Steamboat Springs Geothermal System using trace elements and gas geochemistry: Geothermal Resources Council Transactions, v. 27, p. 269-274.

Benson, L., 1994, Carbonate deposition, Pyramid Lake Subbasisn, Nevada: 1. Sequence offormation and elevational distribution of carbonate deposits (Tufas): Palaeogeography,Palaeoclimatology, Palaeoecology, v. 109, p. 55-87.

Bennett, R.A., Wernicke, B.P., Niemi, N.A., Friedrich, A.M., and Davis, J.L., 2003, Contemporary strain rates in the northern Basin and Range province from GPS data: Tectonics, v. 22, No. 2, p. 1-31.

Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., Mckee, E.H., and Noble, D.C., 1989, Eocene through Miocene volcanism in the Great Basin of the Western United states: New Mexico Bureau of Mines and Mineral Resources Memoir 47, p. 91-130.

20

Blackwell, D.D., 1983, Heat flow in the northern Basin and Range province: Geothermal Resources Council Special Report 13, p.81-93.

Blackwell, D.D., P. Negraru, and M. Richards, 2007, Assessment of the enhanced geothermalsystem resource base of the United States: Natural Resources Research, doi:10:1007/s11053-007-9028-7, 2007.

Blackwell, D., Richards, M., Frone, Z., Batir, J., Ruzo, A., Dingwall, R., and Williams, M., 2011, Temperature-at-depth maps for the conterminous U.S. and geothermal resourceEstimates: Geothermal Resources Council Transactions, v. 35, p. 1545-1550.

Byerlee, J.D., 1978, Friction of rocks: Pure and Applied Geophysics, v. 116, p. 615-626.

Bonham, H.F., and Papke, K.G., 1969, Geology and mineral deposits of Washoe and StoreyCounties, Nevada: Nevada Bureau of Mines and Geology Bulletin 70, scale 1:250,000, 140 p.

Caine, J., Evans, J., Forster, C., 1996, Fault zone architecture and permeability structure:Geology, v. 24, no. 11, p. 1025-1028.

Cashman, P., Fontaine, S., 2000, Strain partitioning in the northern Walker Lane, westernNevada and northeastern California: Tectonophysics, v. 326, p. 111-130.

Coolbaugh, M.F., Raines, G.L., Zehner, R.E., Shevenell, L., and Williams, C.F., 2006a, Prediction and discovery of new geothermal resources in the Great Basin: Multiple evidence of a large undiscovered resource base: Geothermal Resources Council Transactions, v. 30, p. 867-873.

Coolbaugh, M.F., Faulds, J.E., Kratt, C., Oppliger, G.L., Shevenell, L., Calvin, W., Ehni, W.J., and Zehner, R.E., 2006b, Geothermal potential of the Pyramid Lake Paiute Reservation, Nevada, USA: Evidence of previously unrecognized moderate-temperature (150⁰-70⁰C) geothermal systems: Geothermal Resources Council Transactions, v. 30, p. 59-68.

Coolbaugh, M., Sladek, C., Faulds, J., Zehner, R., Oppliger, G., 2007, Use of rapid temperature measurements at a 2-meter depth to augment deeper temperature gradient drilling,Proceeding, Thirty-Second Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, SGP-TR-183

Coolbaugh, M., Lechler, P., Sladek, C., Kratt, C., 2009, Carbonate tufa columns as exploration guides for geothermal systems in the Great Basin: Geothermal Resources Council Transactions, v. 33, p. 461-466.

Curewitz, D. and Karson, J.A., 1997, Structural settings of hydrothermal outflow: Fracture permeability maintained by fault propagation and interaction: Journal of Volcanology and Geothermal Research, v. 79, p. 149-168.

21

Davis, G., Reynolds, S., 1996, Structural geology of rocks and regions: New Jersey, John Wiley and Sons, 776 p.

Drakos, P.S., 2007, Tertiary stratigraphy and structure of the southern Lake Range, northwest Nevada: Assessment of kinematic links between strike-slip and normal faults in the northern Walker Lane [M.S. thesis]: University of Nevada, Reno, 165 p.

Faulds, J., and Varga, R., 1998, The role of accommodation zones and transfer zones in theregional segmentation of extended terranes, in Faulds J.E., and Stewart, J.H., eds.,Accomodation Zones and Transfer Zones: The Regional Segementation of the Basin andRange Province: Boulder, Colorado, Geological Society of America Special Paper 323.

Faulds, J.E., Henry, C.D., and dePolo, C.M., 2003, Preliminary geologic map of the Tule PeakQuadrangle, Washoe County, Nevada: Nevada Bureau of Mines and Geology Open-FileReport 03-10.

Faulds, J.E., Coolbaugh, M., Blewitt, G., and Henry, C.D., 2004a, Why is Nevada in hot water?structural controls and tectonic model of geothermal systems in the northwestern Great Basin: Geothermal Resources Council Transactions, v. 28, p. 649-654.

Faulds, J.E., Henry, C.D., Hinz, N.H., Delwiche, B., and Cashman, P.H., 2004b, Kinematic implications of new paleomagnetic data from the northern Walker Lane, western Nevada: Counterintuitive anticlockwise vertical-axis rotation in an incipient dextral shear zone: Eos. Trans. American Geophysical Union, v. 85(47). Fall Meet. Suppl., Abstract GP42A-08.

Faulds, J.E., Henry, C.D., and Hinz, N.H., 2005a, Kinematics of the northern Walker Lane: an incipient transform fault along the Pacific-North American plate boundary: Geology, v. 33, no.6, p. 505-508.

Faulds, J.E., Coolbaugh, M.F., Vice, G.S., and Edwards, M.L., 2006, Characrterizing structural controls of geothermal fields in the northwestern Great Basin: a progress report: Geothermal Resources Council Transactions, v. 30, p. 69-76.

Faulds, J., dePolo, C., and Henry, C., 2007, Preliminary geologic map of the Sutcliffe Quadrangle, Washoe County, Nevada open file report 03-17.

Faulds, J.E., Henry, C.D., 2008, Tectonic influences on temporal and spatial evolution of the Walker Lane: an incipient transform fault along the evolving Pacific-North American plate boundary, in Spencer, J.E., and Titley, S.R., eds., Ores and orogenesis:Circum Pacific tectonics, geologic evolution, and ore deposits: Arizona GeologicalSociety Digest 22, p. 437-470.

Faulds, J., Coolbaugh, M., Bouchot, V., Moeck, I., and Oguz, K., 2010, Charaterizing structuralcontrols of geothermal reservoirs in the Great Basin, USA, and western Turkey: Developing successful exploration strategies in extended terranes: Proceedings of theWorld Geothermal Congress, Bali, Indonesia, 25-29 April, p. 1-10.

22

Faulds, J.E., Hinz, N.H., Coolbaugh, M.F., Cashman, P.H., Kratt, C., Dering, G., Edwards, J.,Mayhew, B., McLachlan, H., 2011, Assessment of favorable structural settings of geothermal systems in the Great Basin, Western USA, Geothermal Resources CouncilTransactions, v. 35., p. 777-783.

Faulkner, D., Jackson, C., Lunn, R., Schlische, R., Shipton, Z., Wibberley, C., and Withjack, M.,2010, A review of recent developments concerning the structure, mechanics, and fluidflow properties of fault zones: Journal of Structural Geology, v. 32, p. 1557-1575.

Ferrill, D., and Morris, A., 2003, Dilational normal faults: Journal of Structural Geology, v. 25, p. 183-196.

Garside, L. J., Castor, S. B., and dePolo, C.M., 2003, Geology of the Fraser Flat Quadrangle and the west half of the Moses Rock Quadrangle, Washoe County, Nevada: Nevada Bureauof Mines and Geology, Map 146.

Hammond, W.C. and Thatcher, W., 2005, Northwest Basin and Range tectonic deformation observed with the Global Positioning System, 1999-2003, Journal of Geophysical Research, v.110, B10405, doi: 10.1029/2005JB003678.

Hammond, W., and Thatcher, W., 2007, Crustal deformation across the SierraNevada, northen Walker Lane, Basin and Range transition, 2000-2004: Journal ofGeophysical Research, v. 112, B05411, doi: 10.1029/2006JB004625.

Hammond, W., Blewitt, G., Kreemer, C., 2011, Block modeling of crustal deformation of the northern Walker Lane and Basin and Range from GPS velocities: Journal of Geophysical Research, v. 116, B04402, doi:10.1029/2010JB007817.

Heimgartner, M. and Louie, J.N., 2007, Using seismic refraction to assess geothermal potential: An updated view of crustal thickness in the Great Basin: Geothermal Resource Council Transactions, v. 31, p. 121-124.

Henley, R.W., 1985, The geothermal framework of epithermal deposits, in Berger, B.R., andBethke, P.M., eds., Geology and geochemistry of epithermal systems: Reviews inEconomic Geology, v. 2, p. 1-24.

Henry, C.D., Faulds, J.E., dePolo, C.M., and Davis, D.A., 2004, Geologic map of the

Dogskin Mountain Quadrangle, northern Walker Lane, Nevada: Nevada Bureau of Minesand Geology Map 148, scale 1:24,000, with accompanying text.

Henry, C., McGrew, A., Colgan, J., Snoke, A., Bruseke, M., 2011, Timing, distribution, amount,and style of Cenozoic extension in the northern Great Basin, in Lee, J., and Evans, J.,ads., Geologic Field Trips to the Basin and Range, Rocky Mountains, Snake River Plain,and Terranes of the U.S. Cordillera: Geological Society of America Field Guide 21, p. 27-66.

23

Kirschvink, J.L, 1980, The least-squares line and plane and the analysis of paleomagnetic data: Geophys. J. Roy. Astron., v. 62, p. 699-718.

Kratt, C., Sladek, C., and Coolbaugh, M., 2010, Boom and bust with the latest 2m temperature surveys: Dead Horse Wells, Hawthorne Army Depot, Terraced Hills, and other areas in Nevada: Geothermal Resources Council Transactions, v. 34, p. 567-574.

Larsen, P., 1988, Relay structures in a Low Permian basement-involved extension system, EastGreenland: Journal of Structural Geology, V. 10, No. 1, p. 3-8.

McDougall, I., and Harrison, M., 1999,Geochronology and thermochronology by the 40Ar/39Armethod-2nd edition: Oxford University Press, 269 p.

Mankinen, E.A., Larson, E.E., Grommé, C.S., Prévot, M., and Coe, R.S., 1987, The Steens Mountain (Oregon) geomagnetic polarity transition, 3. Its regional significance:  Journal of Geophysical Research, v. 92, p. 8057-8076.

Moeck, I., Kwiatek, G., and Zimmermann, G., 2009, Slip tendency analysis, fault reactivationpotential and induced seismicity in a deep geothermal reservoir: Journal of StructuralGeology, v. 31, p. 1174-1182.

Morris, A., Ferrill, D., Henderson, D., 1996, Slip-tendency and fault reactivation: Geology, v. 24,no. 3, p. 275-278.

Perkins, M.E. and Nash, B.P., 2002, Explosive silicic volcanism of the Yellows hotspot: The ash fall tuff record: Geological Society of America Bulletin, v. 114, no. 3, p. 367-381.

Petit, J.P., 1987, Criteria for the sense of movement on fault surfaces in brittle rocks: Journal of Structural Geology, v. 9, no. 5/6, p. 597-608.

Rhodes, G., 2011, Structural controls of the San Emidio geothermal system, northwestern Nevada [M.S. thesis]: University of Nevada, Reno, 73 p.

Satkoski, J.J., Berg, A.W., 1982, Field Inventory of Mineral Resources Pyramid Lake Indian Reservation Nevada: U.S. Bureau of Indian Affairs, Report BIA No. 38-II, 45 p.

Simmons, S.F., and Browne, P.R.L., 2000, Hydrothermal minerals and precious metals in the Broadlands-Ohaaki geothermal system: Implications for understanding low-sulfidationepithermal environments: Economic Geology, v. 95, p. 971-999.

Trexler, J.H., Cashman, P.H., Henry, C.D., Muntean, T., Schwartz, K., TenBrink, A., Faulds, J.E., Perkins, M., and Kelly, T., 2000, Neogene basins in western Nevada document the tectonic history of the Sierra Nevada – Basin and Range transition zone for the last 12 Ma, in Lageson, D.R., Peters, S.G., and Lahren, M.M., eds., Great Basin and Sierra

24

Nevada: Boulder, Colorado, Geological Society ofAmerica Field Guide 2, p. 97-116.

Vice, G., 2008, Structural controls of the Astor Pass-Terraced Hills geothermal system ina region of strain transfer in the western Great Basin, northwestern Nevada [M.S. thesis]: University of Nevada, Reno, 114 p.

Wernicke, B., 1992, Cenozoic extensional tectonics of the U.S. Cordillera, in Burchfiel, B.C., et al., eds., The Cordilleran Orogen: Conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America,v. G-3, p. 553–581.

25

26

Figure 1- DEM hillshade of the Pyramid Lake region with location of major ranges and Emerson Pass. The area outlined in red is the proposed study area.

27

28

Figure 3- Main structures of the Pyramid Lake Domain of the northern Walker Lane. EP-Emerson Pass; FRF-Fox Range Fault; HLF-Honey Lake fault; LRF- Lake Range fault; OF-Olinghouse fault; PLF-Pyramid Lake fault; WSF- Warm Spring Valley fault; . Note the location of Emerson Pass at the broad step between the Lake Range and Fox Range faults. Modified from Faulds and Henry (2008).

LRF

FRF

29

Figure 4- The geothermal belts of the Great Basin . Fields are oriented along structurally controlled belts. ECSZ- Eastern California Shear Zone; WLG-Walker Lane; SD-Sevier Desert; HSZ- Humboldt structural zone; BRD- Black Rock Desert; SV-Surprise Valley. Geothermal fields indicated by circles (red>160°C, yellow=100-160°C). From Faulds et al. (2004).

Emerson Pass

30

Figure 5- a) Oblique view from Google Earth looking northeast at the relay ramp formed by the left step-over between the Lake and Fox Range Range bounding normal faults. Vertical exaggeration 2x. b) Schematic figure of overlapping synthetically dipping normal faults leading to the formation of a relay ramp. Modified from Faulds and Varga (1998).

a)

b)

31

b)a)

c)

Figure 6- Schematic illustration of various kinematic indicators. a) Rough facets. The facets step down in the direction of motion of the missing block. Modified from Angelier et al. (1985). b) Reidel Shears. The reidel shear is represented by the red line. Slip will be oriented perpendicular to the line formed by the intersection of the Reidel shear and the fault plane. Modified from Angelier (1994). The acute angle formed by the intersection of the Riedel shear and main fault plane points in the direction of motion of the host block. c) Drag folds caused by slip on a fault. d) Offset of intersection of horizontal bed and sub-vertical dike (piercing point). D, total displacement is the sum of S, displacement along slope, T, transverse horizontal component of displacement, V, vertical offset, and L, lateral component of displacement. Also shown, F, fault plane, sl, slickenlines, p, fault dip, and i, rake of slickenlines. Modified from Angelier (1994).