GRC Transactions, Vol. 38, 2014

Prospecting for a Blind Geothermal System Utilizing Geologic and Geophysical Data, Seven Troughs Range, Northwestern

Corina Forson1, James E. Faulds1, and Phil Wannamaker2 1Nevada Bureau of Mines and Geology, University of Nevada, Reno NV 2Energy and Geoscience Institute, University of Utah, Salt Lake City UT

Keywords resources. Researchers estimate that only ~20% of the accessible Nevada, Basin and Range, structural controls, Seven Troughs geothermal resources in the Great Basin have been discovered, Range, blind geothermal system, exploration, geologic map- suggesting that opportunities abound for discovering blind or ping, Magnetotelluric, two-meter survey hidden geothermal systems (Brook et al., 1979; Richards and Blackwell, 2002). If prospecting for a truly blind geothermal system, it becomes Abstract important to distinguish between a blind, hidden, and conventional geothermal resource. All geothermal systems have three main The Seven Troughs Range in Pershing County, Nevada, has elements: heat source (magmatic or high geothermal gradient in been identified as an area with high potential for hosting a blind amagmatic settings), reservoir (to hold and sustain the hot water), geothermal system. A reconnaissance 2D magnetotelluric (MT) and fluid, which is the medium that transfers the heat. Conven- survey across the western U.S. has shown a possible correlation tional geothermal systems exhibit fluid surface manifestations, between broad low resistivity anomalies, extending from the up- such as fumaroles or hot springs. Hidden geothermal systems per to deep crust, and loci of high enthalpy geothermal systems. lack fluid features on the surface but contain more subtle features Where MT surveys have been conducted across the Basin and (or paleo manifestations), such as siliceous sinter, travertine, tufa Range, anomalous zones of high conductivity strongly correlate deposits, and/or hydrothermally altered rocks. Vegetation can also with known high temperature geothermal systems, such as Dixie serve as an indicator, as it can be concentrated in lineaments along Valley, Coso, and McGinness Hills; as well as anomalies in 3He R/ faults that leak fluids or be absent near faults leaking toxic gasses. Ra. Along the regional MT transect only a few areas of pronounced Blind geothermal systems are confined to the subsurface; lacking upwelling are not connected with any known geothermal systems any obvious fluid features and/or paleo-manifestations associated (e.g., Kumiva Valley region). These locations are hypothesized to with conventional or hidden systems. have high potential for hosting blind geothermal systems. Thus, Within both magmatic and amagmatic environments faults this project focuses on identifying favorable structural settings for and fractures within the brittle crust play an important role in geothermal activity in the Kumiva Valley area and more specifi- generating and maintaining permeability and transporting fluids cally the Seven Troughs Range, where the low resistivity anomaly to the surface. In amagmatic systems, and more specifically the reaches the near surface. Detailed geologic and structural analyses extensional setting of the Basin and Range, moderate to steeply were conducted to identify favorable structural settings for geo- dipping faults and fractures promote the transmission of meteoric thermal activity that are likely to host a blind geothermal system. fluids to depth (Blackwell, 1983). The depth of circulation re- An initial two-meter temperature survey was also conducted to quired to attain high temperatures depends on the permeability of further assess two favorable structural settings in the area. the rocks and the geothermal gradient. Areas with known recent faulting (i.e., Quaternary) are ideal for prospecting for geothermal Introduction systems, because the most substantial fluid migration in a fault system is likely during and after rupture events (Micklethwaite A majority of the operating geothermal power plants and and Cox, 2004). It comes as no surprise that a majority of the prospects have utilized the presence of surface manifestations known geothermal systems in the Great Basin region of the Basin (i.e., geysers, boiling mud pots, hot springs, sinter terraces etc.) to and Range province are spatially associated with faults, and many locate areas of high geothermal potential (e.g., Bradys geothermal are temporally associated with Quaternary faults. In the Great system; Faulds et al., 2010). However, many resources have little Basin many geothermal systems are found proximal to normal or no surface manifestations and are considered ‘hidden’ or ‘blind’ and oblique-slip faults, suggesting that such faults control or chan-

369 Forson, et al. nel hydrothermal fluids. Faults oriented orthogonal to the least in Pershing County, northwestern Nevada, and 2) to estimate the principal stress direction are most likely to host high temperature possible locations of blind geothermal resources in that area. geothermal systems (Faulds et al., 2004, 2006, 2011). Within the The field area is located ~50 km northwest of Lovelock, Nevada northwestern Great Basin, the regional extension direction trends (Figure 1). This specific field area was chosen based on the loca- west-northwest. Accordingly, normal faults striking north-north- tion of a regional magnetotelluric (MT) anomaly (Wannamaker east host most of the geothermal systems (Coolbaugh et al., 2005; et al., 2011; Figure 2A). In addition, an ongoing study funded by Faulds et al., 2005). However, many north-northeast-striking faults the Department of Energy aims to integrate MT data, soil gas flux in this region show no signs of geothermal activity. Therefore, it and geochemistry, and structural analysis to recognize heretofore is necessary to recognize common structural settings of known undiscovered blind geothermal resources. geothermal systems to aid in the discovery of blind systems with The reconnaissance 2-D MT survey has shown a possible cor- no surface manifestations. relation between low resistivity in the deep crust and magmatic Several fault patterns have been recognized as particularly underplating in areas with high rates of recent extension (e.g., favorable structural settings for geothermal activity (Curewitz and Dixie Valley region). The deep crustal, low resistivity anomalies Karson, 1997; Faulds et al., 2006, 2011), including: 1) overlapping are linked to tapering zones of lower resistivity in the upper crust, normal faults (step-overs) with hard linkage, 2) terminations of suggesting that deep crustal fluids can upwell to remarkably major normal faults (horse-tailing), 3) intersections of normal, shallow depths (Wannamaker et al., 2011). The presumed lower strike-slip, and/or oblique-slip faults, 4) accommodation zones crustal magmatic underplating may be spatially correlated to where belts of oppositely dipping normal faults intermesh, and 5) large crustal breaks that may accommodate the efficient transfer transtensional pull apart zones. These structural controls can be of high temperature fluids from deeper levels to the near surface. identified via topographical manifestations, such as 1) major steps Where reconnaissance MT surveys have been conducted across the in rage fronts, 2) interbasinal highs, 3) mountain ranges consisting Basin and Range, anomalous zones of high conductivity strongly of relatively low, discontinuous ridges, and 4) lateral terminations correlate with known high temperature geothermal systems, such of mountain ranges (Faulds et al., 2006, 2011). as Dixie Valley, Coso, and McGinness Hills (Wannamaker et al., 2011) (Figure 2B). Commonly, these systems exhibit significant Motivation and Purpose of This Study 3He R/Ra anomalies, a further indication of magmatic input (Ken- nedy and van Soest, 2007). Along the MT transect conducted by The purpose of this study is 1) to determine the structural set- Wannamaker et al. (2011), only a few areas of pronounced up- ting and geothermal potential of the central Seven Troughs Range welling associated with deep crustal, low resistivity anomalies are not connected with any known geothermal systems (e.g., Kumiva Valley region, Figure 2B&C). These locations are hypothesized to have high geothermal potential. Thus, this project focuses on identifying favorable structural settings for geothermal activity in the Kumiva Valley area and more specifically the Seven Troughs Range, where the low resistivity anomaly reaches the near surface (Figure 2C). The field area lies in the central Seven Troughs Range (Figures1and 3), where the Kumiva Valley MT anomaly is nearest to the surface (Wannamaker et al., 2011). This area may be analogous to Dixie Valley or McGinness Hills (Figure 2B), where en- hanced geothermal activity and possible deep crustal breaks correlate with areas of elevated conductivity. These low-resistivity upwelling zones likely mark areas in which geothermal fluids circulate from depth to the surface. However, there is no known geothermal system in the Seven Troughs Range. Nonetheless, the 2-D MT anomaly in this area (Figure 2C) and a preliminary analysis for favor- able structural settings suggest that this location has relatively high potential to host a blind geothermal system. The Kumiva Valley area is therefore a green- Figure 1. The great Basin physiographic province outlined in red. The Four geothermal/struc- field exploration area without current recognized tural domains are highlighted in green (from Faulds et al., 2004). From north to south these systems. It lies in a complex structural setting cur- are: The Surprise Valley belt (SV), the belt (BRD), the Humboldt Structural rently undergoing oblique extension (Rhodes et al., Zone (HSZ), and the Walker Lane Geothermal belt (WLG). The Walker lane and Eastern Cali- fornia Shear Zone boundary is denoted with the hachured polygon on the map. The black star 2010; Hammond et al., 2011). denotes the location of the Seven Troughs field area. The inset map is a close up of the Seven To aid in the discovery and evaluation of blind Troughs field area and the surrounding mountain ranges, valleys, wells and springs. resources, it is important to utilize geologic, geo-

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in the field area. A two-meter temperature survey was conducted to determine if shallow temperature anoma- lies were present in areas with favorable structural settings. Upon completion of geologic and structural analysis a more detailed MT survey and soil gas study will be conducted.

Stratigraphic Framework The Seven Troughs Range is composed of Me- sozoic metasedimentary rocks, Cretaceous plutonic rocks, Tertiary volcanic and sedimentary rocks, and Quaternary sediments, alluvium, landslide, and spring deposits (Figure 3). Basement rocks in the Seven Troughs Range consist of upper Triassic to Jurassic, low-grade metasedimentary rocks, primarily inter- fingering argillite, mudstone, phyllite, quartzite and carbonates (Burke and Silberling, 1973; Hudson et al., 2006) (Figure 4). The mudstone, argillite, and quartzite locally contain abundant quartz veins and quartz brec- cias. The metasedimentary rocks are part of the Auld Lang Syne group, a very thick (up to ~7.5 km) package of argillaceous and sandy strata deposited in a shallow marine, deltaic environment (Burke and Silberling, 1973). In the middle or late Jurassic and early Creta- ceous the Auld Lang Syne sequence was deformed in a back-arc fold and thrust belt (the Luning-Fencemaker thrust system) (Oldow, 1983, 1984; Wallace, 1987). The deformation associated with thrusting likely caused much of the low-grade metamorphism in this sequence. The metasedimentary rocks of the Auld Lang Syne group are intruded by Cretaceous granodiorite (Johnson, 1977) associated with emplacement of the Sierra Nevada batholith (Wernicke et al., 1987). Small tungsten deposits are locally found in scheelite-bearing Figure 2. A) Reconnaissance MT transect coverage of the Great Basin and neighboring tactite along the margins of the granodiorite intrusions provinces by University of Utah. Major physiographic provinces are southern Modoc (Wallace, 1987). Plateau (SMP), western, central and eastern Great Basin (WGB, CGB, EGB), Wasatch Transition Zone (TZ) and Colorado Plateau interior (CPI). Geothermal systems or districts The Mesozoic basement rocks are locally non- pertinent to this proposal are Kumiva Valley area (KV), Dixie Valley (DV) and McGinness conformably overlain by Oligocene (?) non-welded Hills (MG) (Wannamaker et al., 2011). B) 2-D MT resistivity inversion model across the to welded ash-flow tuffs and tuffaceous sedimentary Great Basin showing localized shallowing of high-temperature, lower crustal conductors rocks. The tuffs crop out extensively in the western in warmer colors. The Seven Troughs Range is denoted by 7T and has the shallowest part portion of the field area but are scarce in the eastern of the large conductive anomaly under the Black Rock-Kumiva Valley area (BR-KV). The lower crustal conductor under Buena Vista and Dixie Valley and its crustal-scale connec- part. Overlying the Oligocene (?) tuffs and Mesozoic tion to the geothermal system is marked by BV-DV (blue) (Wannamaker et al., 2011). C) basement in the Seven Troughs Range is a suite of The central Seven Troughs Range field area (pink star) with 2-D MT model overlaid on top middle Miocene bimodal volcanic rocks (Figures 3 and to show where the shallowest upwelling occurs in relation to the Seven Troughs Range; 4). These interfingering volcanic rocks can be broken warmer colors indicate areas of low resistivity. The MT survey stations are denoted with out into five main sequences: (1) older basalt flows; (2) white dots. All MT data from Wannamaker et al., 2011. middle rhyolite flows, domes, dikes, and interbedded epiclastic sedimentary rocks, (3) younger basalt flows, physical, and geochemical techniques to find the required elements cinders, and necks, (4) capping rhyolite flows and domes, and (e.g., heat source, fluid to transport the heat, and permeability in a (5) alternating flows of upper rhyolite and basaltic andesite that reservoir) for geothermal energy production. We have conducted locally overlies the basalt flows, cinders, and necks. The upper detailed geologic mapping, structural analysis, and a 2 m tem- rhyolite locally includes lenses of both vitrophyre and tuffaceous perature survey to delineate the most likely areas in the central sedimentary rocks. Seven Troughs Range for blind geothermal activity. Detailed The Seven Troughs mining district resides on the east side of geologic mapping was conducted at a scale of 1:24,000 on color the range within a bimodal eruptive center and contains both basal- stereographic air photos over ~80 km2 of the central Seven Troughs tic andesite and rhyolite lava flows, felsic domes, and interbedded Range. Mesozoic through Quaternary stratigraphy was defined clastic sedimentary rocks from sequences 1-5 above. Basalt

371 Forson, et al. dikes, epithermal veins, and coeval hydrothermal alteration are age of 13.83±0.02 Ma. In addition, a post-ore capping rhyolite abundant in the mining district. The dikes intrude the lower basalt is 13.79±0.06 Ma (Hudson et al., 2006). Because the capping and middle rhyolite sequences (sequences 1-3) but are truncated rhyolite truncates veins and dikes and is only roughly 600 ka by sequences 4 and 5 above (Hudson et al., 2006). Epithermal younger than some of the vein host rocks, ore-forming processes gold and silver veins lace the mining district and formed during and hydrothermal alteration were probably short lived. the emplacement of sequences 1 and 2. Hydrothermal alteration (mainly argillic with some local propylitic) of the host rocks was Structural Framework coeval with the vein emplacement and is commonly spatially as- sociated with basalt dikes and faulting. The Seven Troughs field area is characterized by a series Hudson et al. (2006) used 40Ar/39Ar dating to constrain the of mainly north- to northeast-trending, west-tilted fault blocks timing of vein deposition and hydrothermal alteration. The vein bounded by moderately to steeply east-dipping normal faults. In host rocks (primarily sequence 2) range in age from 14.43±0.09 the southern portion of the field area, a large horst block composed to 13.80±0.68 Ma; adularia from one of the veins yielded an of Mesozoic basement is bound on the east and the west by range- bounding faults marked by Quaternary scarps (Adams and Sawyer, 1999; Figure 3). To the north, the range-bounding fault on the eastern flank of the Seven Troughs Range steps to the right, whereas the range- bounding fault on the west side apparently has a smaller left step (Figure 3). In the central part of the study area the northern portion of the western range-bounding fault arcs to the east and strikes east-northeast. This segment exhibits a sinistral compo- nent of slip, which is common for faults of this orientation under the current extension direction (Faulds et al., 2005). The Seven Troughs mining district and associated middle Miocene bimodal eruptive center apparently fills a north- northeast-trending synvolcanic graben (Hudson et al., 2006). Numerous normal and oblique-slip faults cut through the mining district and accommodate both offset of the middle Miocene volcanic rocks and relative uplift of the Mesozoic basement. Pervasive argillic alteration ac- companies the mineralization and less abundant propylitic alteration associated with dike emplacement. Although abun- dant hydrothermal alteration correlates with the mineralization, no known active geothermal system is associated with the mineralization.

Slip and Dilation Tendency Analysis Faults that are oriented orthogonal to the least principal stress are more likely to slip and therefore generate permeabil- ity and channel geothermal fluids from depth to the near surface (Morris et al., 1996; Curewitz and Karson, 1997; Ferrill and Morris, 2003). The likelihood of slip Figure 3. Simplified Geologic map of the central Seven Troughs Range, Pershing County, NV. Inset map shows location of the Seven Troughs field area (black box), springs and wells in the area are denoted (slip tendency) on a given fault surface is with a black triangle and the USGS Quaternary faults from the USGS Quaternary fault and fold database contingent upon the rock type, the ratio are categorized by age of faulting (Adams and Sawyer, 1999). of shear to normal stresses, the orienta-

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Figure 4. Schematic Stratigraphic column of the Seven Troughs field area. Black lines point to the units where the corresponding 40Ar/39Ar age dates were sampled from. The age dates were obtained from Don Hudson (2006).

tion of the fault plane, and the in situ stress field (Morris et al., 1996). Dilation tendency of a fault is controlled by the resolved normal stress, which relies on fluid pressure and the lithostatic and tectonic stresses (Moeck et al., 2009). Calculating which faults in the study area have the highest slip and dilation tendency is valuable for deciding where to conduct more detailed geophysical and geochemical studies. A preliminary analysis of the faults most fa- vorably oriented for slip and dilation has been conducted using 3-D stress software and then plotted in ArcGIS 10.1 (Figure 5). This analysis is a best-case scenario calculation, as it assumes that the faults dip 60° in the slip tendency calcu- lation and are vertical in the dilation tendency analysis. In both slip and dilation tendency

A B

Figure 5. Simplified fault map of the Seven Troughs Range showing some of the major faults in the field area. Color on faults indicates which faults are more likely to slip (A) and dilate (B) under the inferred current stress regime; warmer colors are most favorably oriented for slip/dilation. This analysis assumes a normal faulting regime and that the minimum principal stress direction trends 116.5°. The minimum principal stress direction for this area was gleaned from fault inversion data from the nearby Brady’s geothermal field, as well as Yucca Mountain, Hawthorne, and Dixie valley (Siler, personal communication, 2013). The slip and dilation tendency analyses assume the best-case scenario, with all faults dipping 60° for the slip tendency analysis and 90° for the dila- tion tendency analysis.

373 Forson, et al. calculations, the minimum principal stress direction is orientated the east side of the range (orange dots in Figure 6) in the vicinity 116.5°, which is the average extension direction for this region of the fault intersection and right step. Rocky ground made it based on available borehole breakout data and fault inversion data difficult to insert the probes to take measurements in all of the from several geothermal systems in the Basin and Range (Siler, desired locations. We hope to return for a follow-up survey with a personal communication; Figure 5). rock drill to obtain a more uniform grid around the areas of interest. Based on the current data, no high temperature shallow anomaly Two-Meter Temperature Survey has been found in the field area. However, the thick alluvium could be acting as a thermal insulator. Alternatively, an impermeable A two-meter temperature survey was conducted on the west clay cap could prevent fluids from reaching near surface levels. and east sides of the central Seven Troughs Range to assess whether any shallow temperature anomalies were associated with Discussion and Conclusions the favorable structural settings (Figure 6). The highest measured temperature was 19.3° C just north of Porter Spring on the west A reconnaissance 2D MT transect across the Great Basin side of the field area (red dots in Figure 6). The temperature for conducted by Wannamaker et al. (2011) shed light on a possible Porter Spring is 20° C, so this temperature is on par with the relationship between zones of low resistivity in the shallow crust upwelling of the spring fluids. The background temperature at two- that taper up to the near surface proximal to high temperature meter depth in the area is 13.5° C, and most of the temperatures geothermal systems. The low resistivity anomaly situated under recorded were near background (yellow dots in Figure 6). A few Kumiva Valley shallows under the Seven Troughs Range but is above background temperatures (~15-16° C) were recoded along not associated with any known geothermal resource. Detailed geo- logic mapping and structural analysis were conducted in the central Seven Troughs Range to determine the potential for a blind geothermal system in this area. Two areas have a favorable structural setting for generating permeability and channeling geothermal fluids to the near surface: 1) On the east side of the southern Seven Troughs Range the range-bounding fault steps to the right and intersects a major east-northeast-striking fault. Slightly elevated two-meter temperatures have been found in this vicinity. 2) On the west side of the field area in the vicinity of Porter Spring, the range steps to the left near the northern termination of an east-dipping normal. This area has the highest recorded two-meter temperatures. Although the two-meter temperature sur- vey does not reflect the presence of geothermal fluids within two-meters of the surface at these locations, the 2D low resistivity MT anomaly and favorable structural settings warrants further analysis for blind geothermal systems in the area. More detailed 3D MT coverage in the area and soil gas surveys in the area will help to constrain the potential for blind geothermal systems in the Seven Troughs Range.

Acknowledgments This work was funded by a Department of Energy grant (DE-EE0005514). I would like to acknowledge: Nick Hinz for all of his help, patience and guidance, Don Hudson for thoughtful discussions on the Seven Troughs mining district, Chris Sladek and Connor New- man for help with the two-meter temperature survey, and my field assistant Mitch Allen for all of his hard work.

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