Regional Crustal Discontinuities as Guides for Geothermal Exploration

Transcription

1 GRC Transactions, Vol. 8, 2014 Regional Crustal Discontinuities as Guides for Geothermal Exploration Drew L. Siler 1, B. Mack Kennedy 1, and Philip E. Wannamaker 2 1 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 2 Energy and Geoscience Institute, University of Utah, Salt Lake City, UT Keywords Exploration, geothermal play, structure, magnetotelluric, helium, deep circulation, basin and range Abstract In an effort to develop and demonstrate a new methodology for regional-scale geothermal exploration, we examine three data-types; crustal-scale structural data, magnetotelluric data, and helium isotope data, all of which are established indicators of deep geothermal circulation. Fault zones and other crustal structures provide the fracture permeability that allows geothermal fluids to rise from depth to the near-surface. Crustal scale magnetotelluric data indicate zones of anomalously high electrical conductivity which are commonly interpreted as evidence of active or recent geothermal upflow. Helium isotope data indicate the extent to which geothermal fluids have interacted with a robust heat in the mantle or in young mantle-derived rocks. Interpretation of these three data types in concert allows us to define areas in the crust with evidence for deep geothermal circulation. These zones of deep circulation supply heat and fluids to geothermal systems in the shallow crust and therefore define locations that are favorable for exploration for undiscovered, blind geothermal systems. Within the Great Basin, USA, areas along the Wasatch front in Utah, through central Nevada, and against the eastern side of the Sierra Nevada mountains are characterized by collocation of evidence for deep fluid circulation provided by these data-types. Along these zones, locations where fault controlled shallow permeability is likely to occur are favorable places for new undiscovered geothermal prospects. Still, crustal scale magnetotelluric data and helium isotope data are sparse, and the locations and character of many crustal scale structural zones are not well constrained in the Great Basin, warranting further data collection and analyses to test and refine this regional scale exploration methodology. Introduction Many of the known geothermal systems in the Basin and Range province, western United States have been either explored or developed. As a result, discovery of new geothermal systems, many of which may not have a geothermal expression at the surface (blind systems), is crucial to continued development of geothermal resources in the region. Tectonically mediated geothermal systems, like those in the Basin and Range, require enhanced permeability in the near-surface (shallower than ~2 km) to facilitate economically viable geothermal circulation (Blackwell et al., 1999; Faulds et al., 2006). Both geochemical and geophysical studies indicate that, in addition to shallow permeability, some of the most robust geothermal systems in the Basin and Range have a component of deep seated heat and fluid supply from the lower crust or upper mantle (Kennedy and van Soest, 2006, 2007; Wannamaker et al., 2011, 201b). Magnetotelluric studies, for example image high conductivity zones generated by flow of deep fluids (and heat) to the near-surface from areas of magmatic underplating at the base of the crust (Wannamaker et al., 2006, 2007, 2011). Additionally, a component of mantle derived fluid, as defined by He/ 4 He ratios in geothermal fluids, suggests interaction with mantle derived rocks and rapid transport of fluids the surface (Kennedy et al., 1997). A westward increase of baseline He/ 4 He ratios across the Basin and Range indicates that the supply of deep fluids to the near-surface increases from east to west and is correlated with increasing strain rates. Anomalously high He/ 4 He ratios, spikes in this baseline trend, have apparent accentuated fluid upflow from depth and were interpreted as favorable geothermal targets (Kennedy and van Soest, 2007). These studies suggests that a promising play-type for exploration for new blind geothermal systems may exist along these deep fluid flow zones. Such geothermal systems may be yet unknown because they lack the shallow permeability to carry circulating fluids all the way to the surface and therefore lack surface geothermal features. Some of the deep fluid conduits detected by geophysical and geochemical techniques may be associated with major structural discontinuities in the crust, zones of relative weakness that are taken advantage of by circulating fluids. Structural discontinuities include crustal scale suture zones, boundaries between discrete geologic terranes, cratonic boundaries, and crustal scale fault zones. These are abundant in the Earth s continental crust and, as areas of relative weakness, are commonly reactivated in subsequent tectonic events. Structural discontinuities can be identified 9

2 by intraplate deformation, including intraplate seismicity and volcanism, as well as by the adjacency of markedly different (age or composition) crustal terranes, the occurrence of ophiolite complexes and intense mineralization among other features (e.g. Sykes, 1978; Daly et al., 1989; Butler et al., 1997). We examine spatial correlations between major structural discontinuities across the Basin and Range with crustal scale conductivity zones and high He/ 4 He anomalies in order to test whether structural discontinuities may localize crustal scale fluid upflow and the use of these features as guides for geothermal exploration. Constraining the locations and controls of these deep fluid flow conduits is crucial to the discovery of new blind systems and to the continued development of geothermal resources in the Basin and Range. Background Major structural discontinuities are relatively common in both the Earth s continental crust and lithosphere. Structural discontinuities are generated by plate-scale tectonic activity and tend to be reactivated by subsequent tectonic events throughout geologic time, commonly multiple times. Reactivation occurs at the crustalscale for example, with the reactivation of rift basin faults by subsequent orogenic faulting (e.g. Dewey et al., 1986; Butler et al., 1997) and at the lithospheric-scale, with the repeated reactivation of ancient shear zones in subsequent rifting or orogenic episodes (e.g. Sykes, 1978; Daly et al., 1989). Tectonic reactivation of structural discontinuities is primarily a function of their weakness relative to the surrounding intact crustal (or lithospheric) materials. Weakness is caused by generation of weak materials along these boundaries through alteration and deformation, and/or by high fluid pressures (White et al., 1986; Butler et al., 1997). In addition to reactivation by subsequent tectonic events, structural discontinuities also focus intraplate (distal from modern tectonic plate boundaries) seismicity and magmatism and are commonly highly mineralized zones of past fluid circulation (Sykes, 1978; Daly et al., 1989). All these characteristics indicate that structural discontinuities are important conduits for crustal-scale fluid circulation. The western Cordillera of North America has been subject to nearly continuous, collisional, al and strike-slip deformation since the Devonian (Dickinson, 2006). These deformational events have generated a series of crustal- and lithospheric-scale discontinuities of varying character and age on the western margin of the North American plate (Allmendinger et al., 1987). The diverse geologic character of these discontinuities indicates that they may also have varying potential to act as zones of deep fluid circulation. Here, we focus on structural discontinuities from the Wasatch Front westward to the eastern side of the Sierra Nevada, through the northern and central Basin and Range province. We evaluate the tendency of each structural discontinuity to facilitate crustal-scale fluid circulation and thus to supply heat and fluids to shallow geothermal systems. 40 Structural Discontinuities The Paleozoic hinge line in central Utah and extending into southern Idaho coincides with the eastern boundary of the Cenozoic Basin and Range province (Figure 1). The hinge line was probably originally utilized as a major tectonic boundary during Precambrian rifting of Rodinia and the formation of the ancestral continental margin (Bissell, 1974; Allmendinger et al., 1987; Burchfiel et al., 1992). It also marks the eastern extent of sediments deposited on the late Precambrian-Triassic margin of North America (Bissell, 1974; Stewart, 1980), and served as the fulcrum for subsidence of the passive margin to the west, during this time. The Paleozoic hinge line was subsequently reactivated in the Jurassic as the axis of the thrusting during the Sevier orogeny (DeCelles and Coogan, 2006) and again with Miocene to recent normal faulting during Basin and Range (Cluff et al., 1975; Wernicke, 1992). The Sevier belt, which is more or less coincident with the Paleozoic hinge line, consists primarily of thin-skinned, (not involving basement rocks) west dipping thrust faults, but may have been connected to a mid- to lower-crustal shear zone at depth (Oldow, 1984). The Paleozoic hinge line is also characterized by a significant increase from west to east of the depth of the lithosphere-asthenosphere boundary (Levander and Miller, 2012). To the east of the Paleozoic hinge line, the eastern extent of Mesozoic metamorphism and plutonism may be another structural discontinuity with an impact on crustal fluid flow (Figure 1). The eastern extent of Mesozoic plutonic rocks and related Mesozoic metamorphic rocks (Schweickert and Cowan, 1975) is (( (( (( ± ( ( ( ( ( (( ( (( ( ( ( ( (( ( ( ( ( ( ( (( (( ( (( (( (( ( (( (( ( ( ( (( (( ( ((( ( ( ( ( ( Black Rock Desert ( ( ( ( ( ( ( ( (( ( ( (( ( ( ( Kumiva Valley ( (((((( (( ((((( ( ( Provo ( (( 40 N 40 N ( ( (((((((((( ( (( ( ( ( ( ( ( ( ( ( ( (( (((( ( Dixie Valley Austin ( Eureka ( ( Delta Ely (((( (( ( Milford ( ((( Warm Springs ( ( (( ((((( ((( Known Geothermal Systems Structural Discontinuities Helium Data ( above baseline ( baseline Eastern Boundary of Black Rock terrane Paleozoic hinge line Eastern extent of Mesozoic plutonism/metamorphism Roberts Mountain thrust Northern Nevada rift Golconda thrust Sr = Line Luning-Fencemaker thurst Pine Nut fault Wannamaker et al., 2011 MT Line Wannamaker et al., 2011 near surface conductors km Figure 1. Map of structural discontinuities and geothermal systems across the Basin and Range province. He/ 4 He data from (Kennedy and van Soest, 2006, 2007). Geothermal systems from (Faulds et al., 2011).Magnetotelluric (MT) section line and near surface conductors from (Wannamaker et al., 2011). Structural discontinuities from (Oldow, 1984; Allmendinger et al., 1987; Hardyman and Oldow, 1991; Silberling, 1991; Dilek and Moores, 1995; Degraaff- Surpless et al., 2002; Wyld, 2002).

3 coincident with a belt of Cenozoic metamorphic core complexes (Allmendinger et al., 1987), mid- to lower-crustal rocks exhumed on abnormally weak normal fault zones in eastern Nevada and western Utah. This discontinuity is also the eastern extent of the pronounced, relatively shallow (~0 km) Moho (the geophysical boundary interpreted as the base of the crust) in the central Basin and Range (Allmendinger et al., 1987). Additionally, a north-south striking zone of late Tertiary basaltic volcanism lies ~ 50 km east of the eastern extent of Mesozoic plutonism and volcanism (Nelson and Tingey, 1997; Wannamaker et al., 201a). These relationships indicate that the eastern extent of Mesozoic plutonism and metamorphism is manifest as a structural discontinuity and a zone of weakness at least to the base of the crust and relatively the local Tertiary volcanism may be evidence of fluid flow along this boundary. Further to the west, the Roberts Mountain thrust system (Figure 1) was active as the frontal thrust during the Devonian- Mississippian Antler orogeny and accretion of the Roberts Mountain terrane. Roberts Mountain thrusting is associated with eastward over thrusting of Cambrian-Devonian deep marine turbidites (Speed, 1977; Speed and Sleep, 1982). The younger Golconda thrust system to the west of the Roberts Mountain thrust (Figure 1) is associated with accretion of the Golconda terrane during the Permian-Triassic Sonoma orogeny. The Golconda thrust system was also associated with eastward over thrusting, but primarily of Devonian-Permian turbidites, chert, argillite and metabasalt (Oldow, 1984). In northern and central Nevada, the northern Nevada rift lies between the Roberts Mountain and Golconda thrusts (Figure 1). The northern Nevada rift is a Miocene rift zone characterized by injection of mafic dikes, eruption of mafic lava flows and tectonic rifting. It is also associated with the initiation of the Yellowstone hot spot and eruption of the Columbia River basalts. North-northwest striking rift structures likely resulted from prevailing tectonic stress conditions rather than a pre-existing weakness, and these structures were taken advantage of by coeval and subsequent Basin and Range deformation (Zoback and Mckee, 1994). Further to the west, the Sr = line (Figure 1) marks the boundary between rocks characteristic of the Proterozoic North American craton and those rocks associated with or derived from terranes which have been subsequently accreted to the North American craton. The Sr = line is thus interpreted as the western edge of the Precambrian North American craton (Kistler, 1990; Oldow, 1992). Lying to the west of the Sr = line, the Luning- Fencemaker thrust belt (Figure 1) was active during a series of orogenic events in the Jurassic-Cretaceous and primarily transported shallow marine facies to the east on shallowly-west-dipping thrust faults (Wyld, 2002). It has been hypothesized that, despite their relatively thin skinned character, the contemporaneous Sevier and Luning-Fencemaker fault systems were linked to a common mid- to lower-crustal shear zone (Oldow, 1984). The Luning-Fencemaker fault system is coincident with the Sr = line throughout much of Nevada indicating that this Mesozoic thrust system may have taken advantage of the preexisting weakness at the edge of the North American craton. He/ 4 He Ra W Depth (km) 41 Lying to the west of the Luning-Fencemaker thrust system is the Black Rock terrane, an accreted Paleozoic-Mesozoic magmatic arc with associated sedimentary rocks (Figure 1; Oldow, 1984; Wyld, 2002). The Black Rock terrane was probably accreted to North America on a west-dipping thrust fault along its eastern boundary in the Jurassic, during the earliest stages of development of the Luning-Fencemaker thrust system. The location and geometry of this boundary, however, is obscured in many locations basin fill, and thus is not particularly well constrained (Wyld, 2002). The western boundary of the northern Basin and Range is coincident with the Pine Nut fault, a Jurassic-Cretaceous age inferred sinestral strike-slip system which bounded the eastern side of the Sierran magmatic arc (Oldow, 1984). This boundary along east side of the modern Sierra Nevada has been exploited by subsequent Miocene to recent Basin and Range and dextral strike-slip deformation in the Walker Lane (Figure 1). Crustal Conductivity Zones Zones of relatively high electrical conductivity in the crust can be generated by increased fluid content and/or by the development of conductive alteration minerals and thus are indicative of modern or past geothermal fluid circulation (Wannamaker et al., 2006, 2007; Newman et al., 2008; Spichak and Manzella, 2009) W 121 W Pine Nut Fault 120 W CA NV 119 W Eastern edge of Black Rock Terrane Dixie Valley 118 W eastern edge of Mesozoic metamorphism/plutonism Luning- Fencemaker Fault 117 W Sr 706 Line McGinness Hills Golconda Thrust 116 W 115 W Roberts Mountain Thrust Northern NV rift zone 114 W NV UT Thermo 11 W Roosevelt H.S. 112 W Paleozoic Hinge Line Kumiva Valley Dixie Valley Austin, NV Eureka, NV Ely, NV Delta, UT ~100 km Figure 2. He/ 4 He ratio vs. latitude and conductivity across the Great Basin. He/ 4 He data from (Kennedy and van Soest, 2006, 2007) symbol size indicates map distance from the magnetotelluric (MT) section (Figure 1), large symbols are closer to the line. MT inversion model modified from Wannamaker et al., (2011); the section is masked to indicate loss of resolution below ~75 km. Warm colors indicate conductive areas. Blue line indicates baseline He/ 4 He trend from (Kennedy and van Soest, 2007). Thin grey lines indicate the latitude where structural discontinuities cross the MT section line. 111 W

4 Magnetotelluric methods in particular, are commonly used at both the geothermal-field scale and the regional-scale in order to identify conductive zones and therefore locate and characterize geothermal fluid flow conduits (e.g. Wannamaker et al., 2006, 2007, 2008, 2011, 201a; Cumming and Mackie, 2007; Uchida and Sasaki, 2006; Bertrand et al., 2012). At the regional-scale, sun-horizontal electrically conductive zones in the middle and lower crust appear to represent zones of deep crustal magmatic underplating, hybridization and fluid release (Wannamaker et al., 2006, 2011). These areas appear to be connected to the nearsurface through dipping conductivity zones (e.g. Figure 2), and thus a fluid and heat connection between the deep crust and near surface geothermal systems is implied. These dipping conductivity zones may be related to structural discontinuities in the crust. Systems that are associated with these deep upwelling zones are highly prospective for geothermal exploration as they may be the most favorable areas for sustained heat and fluid supply to the near-surface. The near-surface locations of conductive bodies extending from the mid- to lower-crust from Wannamaker et al., (2011) are shown on Figure 1 and the Basin and Range-wide magnetotelluric section of Wannamaker et al., (2011) is shown on Figure 2. Conductivity zones vary in character and location north and south of the Wannamaker et al., (2011) section (e.g. Meqbel et al., 2014), so a lack of deep conductivity in a certain location along the transect, does not necessarily preclude conductivity out of the plane of the transect. Helium Anomalies 42 Fluids with crustal signatures have a He/ 4 He ratio of ~0.02 Ra (Ra is the He/ 4 He ratio in air), while mantle helium has a He/ 4 He ratio of ~8 Ra (Kennedy et al., 1997). As such, the He/ 4 He signature of surface geothermal fluids indicates the extent to which geothermal fluids have a mantle derived component. From east to west across the Basin and Range, the baseline He/ 4 He ratios in geothermal fluids increase from ~ Ra along the Wasatch Front to >1.0 Ra along the eastern side of the Sierra Nevada is (Figure 2), suggesting a similar east to west increase in a mantle derived component in the geothermal fluids (Kennedy and van Soest, 2007). The east to west increase in the baseline He/ 4 He trend across the Basin and Range is likely associated with a correlative east to west increase in both total strain and dextral shear strain. Increased permeability associated with this increase in strain enhances fluid-flow rates, allowing for rapid transport of mantle derived fluids to the near surface and retention of the mantle helium signature despite extensive degassing (Kennedy and van Soest, 2007). This baseline trend is interrupted by anomalously high spikes in the He/ 4 He ratios of geothermal fluids (Figure 2). These spikes are associated with both amagmatic and magmatic geothermal systems. Magmatic geothermal systems inherit their high He/ 4 He ratios from fluid circulating through active, shallow, mantle derived magmatic material. Anomalously high helium spikes in amagmatic geothermal fluids, on the other hand, acquire their high He/ 4 He ratios from a relatively large component of mantle derived ( He-rich) fluids. High He/ 4 He ratios also indicate high fluid upflow rates, as high upflow rates are required in order for fluids to retain the mantle helium signature despite extensive degassing (Kennedy and van Soest, 2007). These spikes in the baseline trend are therefore highly prospective geothermal targets (Kennedy and van Soest, 2007), as high fluid upflow rates (accentuated supply of heat and fluid to the surface) are favorable for economical-scale geothermal systems. Basin and Range He/ 4 He data (from Kennedy and van Soest, 2006, 2007) are shown on Figure 2. Discussion Correlation of Structural, Helium and Conductivity Data A coincidence between the trace of the Paleozoic hinge line and a zone of relatively high conductivity is evident (Figure 2). The conductive zone extends from the near surface near Delta, Utah, dipping east, to Moho depths (~0-40 km) beneath the transition to the Colorado Plateau. The anomalously high He/ 4 He ratios occurring along the Paleozoic hinge line are primarily associated with geothermal systems to the east of the Snake River Plain in Idaho and the magmatic Roosevelt Hot Springs and Thermo geothermal areas near Milford, Utah (Figure 2). Elevated al strain as compared to eastern Nevada (Figure ; Kreemer et al., 2012) (( (( (( (( ± ( ( ( ( ( (( ( (( ( ( ( ( (( ( ( ( ( ( ( (( (( ( (( (( (( ( (( (( ( ( ( (( (( ( ((( ( ( ( ( ( Black Rock Desert ( ( ( ( ( ( ( ( (( ( ( (( ( ( ( Kumiva Valley ( (((((( (( ((((( ( ( Provo ( (( 40 N 40 N ( ( (((((((((( ( (( ( ( ( ( ( ( ( ( ( ( (( (((( Dixie Valley ( Austin ( Eureka ( ( Delta Ely (((( (( ( Milford ( ((( Warm Springs ( ( (( ((((( ((( Known Geothermal Systems Structural Discontinuities Northern Nevada rift Total strain rate model (from Kreemer et al., 2012) Helium Data ( above baseline ( baseline Eastern Boundary of Black Rock terrane Paleozoic hinge line Eastern extent of Mesozoic plutonism/metamorphism Roberts Mountain thrust Golconda thrust Sr = Line Luning-Fencemaker thrust Pine Nut fault nd invariant strain rate tensor (nanostrain/yr) km Figure. Map of structural discontinuities, geothermal systems, and total strain-rate model across the Basin and Range province. He/ 4 He data from (Kennedy and van Soest, 2006, 2007). Geothermal systems from (Faulds et al., 2011). Structural discontinuities from (Oldow, 1984; Allmendinger et al., 1987; Hardyman and Oldow, 1991; Silberling, 1991; Dilek and Moores, 1995; Degraaff-Surpless et al., 2002; Wyld, 2002). Total strain-rate model (from Kreemer et al., 2012), is a proxy for shallow permeability.

5 (( (( (( (( ± ( ( ( ( ( (( ( (( ( ( ( ( (( ( ( ( ( ( ( (( (( ( (( (( ( ( (( (( ( ( ( (( (( ( ((( ( ( ( ( ( Black Rock Desert ( ( ( ( ( ( ( ( (( ( ( (( ( ( ( Kumiva Valley ( (((((( (( ((((( ( ( Provo ( (( 40 N 40 N ( ( (((((((((( ( (( ( ( ( ( ( ( ( ( ( ( (( (((( ( Dixie Valley Austin ( Eureka ( ( Delta Ely (((( (( ( Milford ( ((( Warm Springs ( ( (( ((((( ((( Known Geothermal Systems Structural Discontinuities Helium Data ( above baseline ( baseline Eastern Boundary of Black Rock terrane Paleozoic hinge line Eastern extent of Mesozoic plutonism/metamorphism Roberts Mountain thrust and increased amounts of Quaternary occur along the Wasatch Front. These are likely a result crustal weakness along the preexisting Paleozoic hinge indicating that the hinge line may be favorable for upflow of fluids, both magma and geothermal fluids, from depth as well. This accentuated also results in a relatively high density of geothermal systems along the Wasatch Front (Figures and 4; Faulds et al., 2011). The eastern extent of Mesozoic plutonism and metamorphism is coincident with a conductive zone extending from the near-surface near the Utah-Nevada border to the middle crust (Figure 2). Wannamaker et al., (201a) suggest that this conductive zone is associated with upper mantle upwelling and late Tertiary volcanism in a north-south trending volcanic zone in western Utah (Nelson and Tingey, 1997; Wannamaker et al., 201a). He/ 4 He data is relatively sparse in western Utah and the limited He/ 4 He data do not correlate well with the trace of the eastern extent of Mesozoic plutonism and metmorphism (Figure 1). However, collocated Cenozoic metamorphic core complexes and a step in the depth of the modern Moho (Schweickert and Cowan, 1975; Allmendinger et al., 1987) indicate that this discontinuity may be related to structures extending to mid to lower crustal depths. The late Tertiary volcanism and rifting may have taken advantage of this pre-existing structural discontinuity. The mapped trace of the Roberts Mountain thrust system (Speed, 1977; Speed and Sleep, 1982) is coincident with a conductivity anomaly that extends from Moho depths to the near-surface near Eureka, Nevada (Figure 2). The Roberts Mountain thrust Northern Nevada rift Golconda thrust Sr = Line Luning-Fencemaker thurst Pine Nut fault Quaternary Fault Density High Low km Figure 4. Map of structural discontinuities, geothermal systems, and the density of Quaternary faults across the Basin and Range province. He/ 4 He data from (Kennedy and van Soest, 2006, 2007). Geothermal systems from (Faulds et al., 2011). Structural discontinuities from (Oldow, 1984; Allmendinger et al., 1987; Hardyman and Oldow, 1991; Silberling, 1991; Dilek and Moores, 1995; Degraaff-Surpless et al., 2002; Wyld, 2002). Density of Quaternary faults (calculated basin on United States Geologic Survey, 2006) show areas of relative high fault density (warm colors) and relative low fault density (cool colors) as a proxy for shallow permeability. 4 system was probably associated with west-dipping subduction and eastward transport of materials along west-dipping thrust faults (Speed, 1977; Speed and Sleep, 1982). The Eureka conductivity anomaly extends to a lower crustal conductive zone that lies to the west of the near-surface expression of the anomaly (Wannamaker et al., 2011). Although shallow permeability along the Roberts Mountain Thrust system is probably mediated Basin and Range structures which dip in both directions, the geometry of the deep conductor would favor fluid upflow on the western side, in what would be the hanging wall of the structure. Several geothermal systems with anomalously high He/ 4 He ratios are coincident with and lie to the west of the trace of Roberts Mountain thrust (Figures 1 and 2). The traces of the Golconda thrust, the Luning- Fencemaker thrust belt, and the Sr = line all lie within ~50 km of one another through the central and northern Basin and Range (Oldow, 1984, 1992; Kistler, 1990; Wyld, 2002). These structures are coincident with a west-dipping high conductivity anomaly that reaches the near surface near Austin, Nevada and extends to the base of the crust. Since the Sr = line marks the western edge of the Precambrian North American craton, it is likely that Permian-Triassic (Golconda) and Jurassic-Cretaceous (Luning-Fencemaker) thrusting took advantage of this pre-existing structural weakness. The trace of the Golconda Thrust system is also coincident with the eastern extent of relatively high total strain-rates in the central Basin and Range (Figure ; Kreemer et al., 2012), indicating that Miocene to recent Basin and Range might also have taken advantage of this long-lived discontinuity. Additionally, anomalously high He/ 4 He values are coincident with both the trace of the Sr = line and the trace of the Luning-Fencemaker thrust system, in what would be the hanging wall of the west-dipping conductivity anomaly (Figure 2). These high He/ 4 He values include samples from the Dixie Valley wells and springs (Kennedy and van Soest, 2006) among others. An east-dipping conductor also extends from the base of the crust to the near-surface just east of Austin, Nevada. The Mc- Ginness Hills geothermal systems sits above this east-dipping conductor. Anomalously high He/ 4 He values have been found in newly collected data from the production fluids of the McGinness Hills geothermal system (Kennedy, 2014 personal communication) northeast of Austin, Nevada. The west-dipping eastern edge of the Black Rock terrane correlates relatively well with anomalously high He/ 4 He values located to the west of the terrane boundary (Figure 1) and this boundary lies on western side of the Kumiva Valley deep conductivity zone. Though the trace of the eastern edge of the Black Rock terrane is not particularly well constrained, this structural discontinuity may be associated with deep fluid flow beneath Kumiva Valley. The conductivity anomalies extending from Moho depths to the near-surface between Eureka, Nevada and Ely, Nevada do not

6 correlate with any known structural discontinuities in the crust, though anomalously high He/ 4 He values do occur north of Ely, NV and may be associated with this conductive zone (Figures 1 and 2). Deep conductivity in this area (and other areas that lack long-lived zones of crustal weakness) may be associated with either active or extinct fluid circulation along yet undiscovered structural discontinuities. It is also possible that deep reaching conductivity is associated with deep fluid circulation focused upon through-going Cenozoic structures or some unrecognized means of generating deep permeability. Regional Geothermal Prospecting First order collocation of long-lived structural discontinuities, deep conductivity anomalies, and anomalously high He/ 4 He ratios indicate that some long-lived structural discontinuities likely act as the conduits for deep fluid circulation in the crust that have been indicated by geophysical and geochemical studies (e.g. Kennedy and van Soest, 2006, 2007; Wannamaker et al., 2011, 201b). Structural weakness along these discontinuities focus modern al strain, provide fast pathways for heat and fluid transport to the near surface and thus may be highly prospective for geothermal exploration. We therefore suggest that a prospective play-type for present and future exploration, are geothermal systems along these long-lived structural discontinuities. Along these discontinuities, the most prospective geothermal plays are in areas where adequate shallow permeability allows deeply derived fluids to circulate to the near-surface. We utilize two different data sets as regional proxies for shallow permeability: modern geodetic strain-rate (Figure ) and the spatial density of Quaternary faults (Figure 4). Strain and permeability generation are intimately related (Faulds et al., 2012), as slip on deforming fault systems constantly generates and maintains fracture permeability (Curewitz and Karson, 1997). Additionally, the traces of Quaternary faults in the Basin and Range correlate with locations of high temperature geothermal systems (Bell and Ramelli, 2007, 2009) and dense networks of faults are favorable for permeability (e.g. Curewitz and Karson, 1997; Faulds et al., 2006; Siler and Faulds, 201). As defined by these constraints, the most prospective areas are those where structural discontinuities with good evidence for deep fluid flow are collocated with one or both proxies for shallow permeability (Table 1). The edge of the Precambrian North American craton (as marked by the Sr = line) and where the Golconda thrust and Luning-Fencemaker thrust system lie in close proximity to one another is one such highly prospective area. Though the Golconda and Luning-Fencemaker systems are associated with thin-skinned thrusting, they probably took advantage of the through-going crustal weakness at the edge of the Precambrian craton and thus have a connection to the deep crust. Both conductivity and helium data suggest that this area is characterized by fluid circulation into the deep crust and mantle, respectively (Figure 2). Along the trace of the Sr = line in central Nevada, strain-rates are relative to the eastern Basin and Range (Figure, Kreemer et al., 2012). The trace of the Golconda thrust, in particular, is coincident with a discrete boundary in modern strain-rates between eastern Nevada and central Nevada (Figure ). As modern strain-rate correlates with deep permeability (Kennedy and van Soest, 2007), deep geothermal circulation is favored along this discontinuity. Interestingly, Table 1. The characteristics of structural discontinuities across the Great Basin and the associated helium isotope, magnetotelluric, shallow permeability data and the relative geothermal prospectivity of each area. Re activated? Basement involved? He/ 4 He signature MT signature Favorable Quaternary structures? Paleozoic Hingeline Reactivated by Paleozoic and Mesozoic orogenesis and Cenozoic Eastern extent of Mezozoic metamorphism/ plutonism Reactivated metamorphic core complex formation and Cenozoic Basin and Range Roberts Mountain thrust Maybe reactivated? Northern Nevada Rift Reactivated Golconda Thrust Reactives Preacambrian edge of craton, probably reactivated by Cenozoic Sr = line/ edge of craton Probably reactivated extesion Luning- Fencemaker thrust belt Reactives Preacambrian edge of craton, probably reactivated by Cenozoic Eastern edge of the Black Rock Terrane Maybe reactivated? Pine Nut fault Reactivated and Walker Lane dextral shear Yes Yes Yes No Yes Yes Possible unclear unclear Several He/ 4 He systems. Magmatic systems in the south, amagmatic in ID? East dipping conductor to Moho High strain-rate in central Utah, high Q fault density in southern Utah None East dipping conductor to lower crust Neither high stain-rate or high fault density Several He/4He systems West dipping conductor to Moho High Q fault density north of Warm Springs, NV None None Both moderately high fault density and moderately high strain rate Several He/ 4 He systems West dipping conductor to Moho The eastern edge of strain rate in the central Great Basin, moderately high fault densities Several He/ 4 He systems West dipping conductor to Moho Both moderately high fault density and moderately high strain rate Several He/ 4 He systems West dipping conductor to Moho Both moderately high fault density and moderately high strain rate Two He/ 4 He systems Kumiva Valley?, but offset by ~75 km near the surface Moderately high strain rate, but relatively low fault densities Prospectivity High Moderate Moderate Low High High High Moderate Regionally No High strainrate and high Q fault density High? But no MT anomoly is curious 44

7 the density of Quaternary faulting (calculated based on United States Geologic Survey, 2006) along trace of the Sr = line is relatively low compared to surrounding areas (Figure 4). Counterintuitively, strain-rate and a low regional scale fault density may in fact be the ideal situation for near-surface geothermal circulation. Under these conditions, strain will be accommodated on a few structure, rather than spread amongst many structures, and therefore permeability on those few structures will have a higher likelihood of being continually generated and maintained. The Paleozoic hinge line in central and northern Utah and southern Idaho is another highly prospective area for exploration for new blind geothermal systems. Both conductivity and helium data indicate that fluid upflow from depth, in the form of young magmatic material and likely geothermal fluids as well, is focused along the hinge line. Elevated shallow permeability is indicated by relatively high strain-rates near Provo, Utah (Figure ) and a relatively high density of Quaternary faults near Milford, Utah (Figure 4). In these areas the deep fluid circulation occurring along the Paleozoic hinge line has a higher likelihood of reaching the near surface and sustaining shallow geothermal circulation. The trace of the Roberts Mountain thrust is prospective for future exploration for blind geothermal systems as well. Deep conductivity and anomalously high He/ 4 He ratios indicate that deep fluid upwelling occurs along this discontinuity. However, modern strain-rates along the trace of the Roberts Mountain thrust are low relative to the central and western Basin and Range, and the Wasatch front (Figure ; Kreemer et al., 2012), and thus accentuated shallow permeability is less likely. A zone of relatively high Quaternary fault density north of Warm Springs, Nevada is the most prospective area along the trace of the Roberts Mountain thrust system (Figure 4). The Kumiva Valley area is also an attractive prospect. The west-dipping Kumiva Valley deep conductivity anomaly may be associated with the eastern edge of the Black Rock terrane. The highly oblique nature and/or poorly constrained location of the eastern edge of the Black Rock terrane relative to the Wannamaker et al., (2011) magnetotelluric transect may be responsible for the apparent spatial disconnect between the structure and the deep conductivity zone. Anomalously high He/ 4 He ratios in the Black Rock Desert, Nevada lie west, in the hanging wall, of the westdipping eastern boundary of the Black Rock terrane, indicating that deep fluid does indeed occur along the eastern edge of the Black Rock terrane. Both strain-rate (Figure ) and Quaternary fault density (Figure 4) are moderately high relative to other parts of the Basin and Range though Kumiva Valley. Though the eastern extent of Mesozoic plutonism and volcanism through southern Nevada and western Utah has evidence for deep circulation, it is a less prospective geothermal prospect than those described above. The associated conductivity zone extends to the lower crust and Cenozoic metamorphic core complexes along this boundary suggest zones of weakness extending into at least the middle crust that may act as fluid flow conduits. However, He/ 4 He data is sparse along this discontinuity, limiting our ability to fully evaluate its fluid flow potential. Additionally, both modern strain-rates (Figure ) and Quaternary fault density (Figure 4) are low relative other areas of the Basin and Range. Neither the Pine Nut fault system nor the northern Nevada rift zone are coincident with evidence for deep fluid circulation, either zones of deep conductivity or anomalously high He/ 4 He values in geothermal fluids. The lack of anomalous helium ratios along the Pine Nut fault may be misleading, however. Although geothermal systems coincident with the Pine Nut fault/walker Lane, have baseline He/ 4 He ratios, these values are higher (exceeding ~1 Ra) than some anomalously high He/ 4 He values further to the east (Figure 2). Kennedy and van Soest (2007) suggest that the westward increase in baseline He/ 4 He ratios in the Basin and Range is associated with westward increasing strain. The baseline helium trend therefore suggests that the entire western boundary of the Great Basin/Pine Nut fault/walker Lane area is associated with deep circulation and rapid upwelling of mantle fluids. Again, the oblique nature of the Pine Nut fault/walker Lane area to the Wannamaker et al., (2011) magnetotelluric transect and/or along strike (northwest-to-southeast) conductivity variation along the Pine Nut fault/walker lane system may be responsible for that fact that there is no correlative conductivity zone (Figures 1 and 2). Elevated strain rates (Figure ) and high Quaternary fault densities (Figure 4) indicate that there is accentuated shallow permeability along the Pine Nut fault/walker Lane system. The Walker Lane also hosts a relative high density of known and production geothermal systems, relative to the rest of the Great Basin (Faulds et al., 2012). If fluid and heat are indeed supplied by deep fluid circulation, the Pine Nut fault/walker Lane system, like several of the structural discontinuities mentioned above, is also highly prospective for exploration for blind geothermal systems. Conclusions Structural discontinuities in the crust are weaknesses generated during tectonic events that tend to focus subsequent tectonic activity including orogenic and rifting episodes, magmatism, seismicity, and mineralization. As indicated by correlative crustal scale conductivity anomalies and mantle helium signatures in geothermal fluids, some structural discontinuities may act as conduits for deep fluid upwelling as well, supplying heat and fluids to near-surface geothermal circulation systems. Across the Basin and Range province, western United States several Precambrian through Miocene structural discontinuities show multiple lines of evidence for deep fluid circulation and thus are prospective for exploration for undiscovered geothermal systems. Near-surface geothermal circulation is most favorable where these zones of deep fluid upwelling are capped by accentuated shallow permeability. In the Basin and Range the most prospective areas include; 1) areas of high strain-rate and low Quaternary fault density along the western edge of the Precambrian North American craton (Sr = 0.706) in central Nevada, and 2) high Quaternary fault density near Milford, Utah and high strain-rate near Provo, Utah both along the Paleozoic hinge line of North America. Other areas including Kumiva Valley, the Roberts Mountain Thrust system, and the eastern extent of Mesozoic plutonism and metamorphism require additional geochemical, geophysical and structural characterization to evaluate deep fluid flow potential. Although no deep conductivity anomaly is evident, He/ 4 He ratios along with both high strain-rate and high Quaternary fault density indicate that the western boundary of the Great Basin/Pine Nut fault/walker 45

8 Lane system is prospective as well. Despite these intriguing initial results, additional helium isotopic, magnetotelluric analyses, and surface and subsurface structural characterization are necessary to further constrain the locations and character of these crustal upwelling zones and their controls on crustal-scale fluid upwelling. Acknowledgements We thank Corné Kreemer for providing the strain-rate model, and Jim Faulds for sharing his database of geothermal systems in the Great Basin. This work was supported by Lawrence Berkeley National Laboratory under U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Geothermal Technologies Program, under the U.S. Department of Energy Contract No. DE-AC02-05CH1121 and by DOE/EERE/GTP contract DE-EE to Wannamaker. 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