Detailed geologic mapping (1:24,000 scale), structural and geochemical analyses, and integration of available geophysical and well-field data were utilized to assess the structural controls of the Neal Hot Springs geothermal field in eastern Oregon. The geothermal field lies within the intersection of two regional grabens, the middle-late Miocene, north-trending, Oregon-Idaho graben and younger late Miocene to Holocene, northwest-trending, western Snake River Plain graben. It is marked by Neal Hot Springs, which effuse from opaline sinter mounds just north of Bully Creek. Production and injection wells, with temperatures up to 142 degrees C, intersect the Neal fault zone at depths of 680-1900 m and subsidiary faults within a relay ramp or step-over within the fault zone. The stratigraphy at Neal correlates with four regional packages. Basement rocks, discovered in one well, are granite, tentatively correlated with Jurassic Olds Ferry-Izee terrane. Nonconformably above is a thick package of middle Miocene Columbia River Basalt Group lavas, regionally known as the basalt of Malheur Gorge. Conformably above are middle to late Miocene Oregon-Idaho graben lavas, volcaniclastics, fluvial and lacustrine rocks. Overlying are the youngest rocks at Neal, which are late Miocene to Pliocene, western Snake River Plain lacustrine, fluvial, and volcaniclastic rocks. The structural framework at Neal is characterized by northerly to northweststriking normal faults, including the geothermally related Neal fault zone. Stress inversion of kinematic data reveal an extensional stress regime, including an interpreted younger, southwest-trending (~243 degrees), least principal stress and an older, west-trending (~265 degrees) least principal stress. The geothermal field is bounded on the east by the Neal fault, a major, westdipping, north-northwest-striking, steeply dipping normal to oblique-slip fault, along which geothermal fluids ascend, and on the west by the concealed north-northweststriking, west-dipping Sugarloaf Butte fault. The Neal fault zone can be modeled into two structural settings: an interpreted older, left-stepping, normal-slip fault zone and a younger, oblique sinistral-normal zone, suggested by the earlier west-trending and later southwest-trending extensional stress regimes. Recent sinistral-normal displacement may have generated a small pull-apart basin in the Neal area and facilitated development of the geothermal system. 'Hard-linkage' between the Neal and Sugarloaf Butte faults occurs through concealed, west-northwest-striking faults, including the Cottonwood Creek subvertical fault, along which lateral fluid-flow is likely. An inferred northplunging fault intersection at the Neal Hot Springs likely controls the location of the hot springs and sinter terraces. Young structural features are evident at Neal. The Neal fault zone cuts Quaternary fans and late Miocene lower and upper Bully Creek Formation sedimentary rocks. In addition, the geothermal field is 4 km west of the active, north- to northweststriking, normal-slip Cottonwood Mountain fault. Furthermore, the field is within several kilometers of recently detected seismicity. This, coupled with its active hot springs (~90 degrees C), opaline sinter mounds, and geothermal fluid flow, suggest that the geothermal field lies within an active (Quaternary), southward-terminating, left-stepping fault zone, which locally acts as a pull-apart basin with sinistral- and normal-slip components.

In many hydrothermal systems, fracture permeability along faults provides pathways for groundwater to transport heat from depth. Faulting generates a range of deformation styles that cross-cut heterogeneous geology, resulting in complex patterns of permeability, porosity, and hydraulic conductivity. Vertical connectivity (a through going network of permeable areas that allows advection of heat from depth to the shallow subsurface) is rare and is confined to relatively small volumes that have highly variable spatial distribution. This local compartmentalization of connectivity represents a significant challenge to understanding hydrothermal circulation and for exploring, developing, and managing hydrothermal resources. Here, we present an evaluation of the geologic characteristics that control this compartmentalization in hydrothermal systems through 3-D analysis of the Brady geothermal field in western Nevada. A published 3-D geologic map of the Brady area is used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity. The 3-D distribution of these variables is compared to the distribution of productive and non-productive fluid flow intervals along production wells and non-productive wells via principal component analysis (PCA). This comparison elucidates which geologic and structural variables are most closely associated with productive fluid flow intervals. Results indicate that production intervals at Brady are located: (1) within or near to known and stress-loaded macro-scale faults, and (2) in areas of high fault and fracture density. This submission includes the published journal article detailing this work, the published 3-D geologic map of the Brady Geothermal Area used as a basis to develop structural and geological variables that are hypothesized to control or effect permeability or connectivity, 3-D well data, along which geologic data were sampled for PCA analyses, and associated metadata file. This work was done using existing R programs.

We conducted a comprehensive analysis of the structural controls of geothermal systems within the Great Basin and adjacent regions. Our main objectives were to: 1) Produce a catalogue of favorable structural environments and models for geothermal systems. 2) Improve site-specific targeting of geothermal resources through detailed studies of representative sites, which included innovative techniques of slip tendency analysis of faults and 3D modeling. 3) Compare and contrast the structural controls and models in different tectonic settings. 4) Synthesize data and develop methodologies for enhancement of exploration strategies for conventional and EGS systems, reduction in the risk of drilling non-productive wells, and selecting the best EGS sites. Phase I (Year 1) involved a broad inventory of structural settings of geothermal systems in the Great Basin, Walker Lane, and southern Cascades, with the aim of developing conceptual structural models and a structural catalogue of the most favorable structural environments. This overview permitted selection of 5-6 representative sites for more detailed studies in Years 2 and 3. Sites were selected on the basis of quality of exposure, potential for development, availability of subsurface data, and type of system, so that major types of systems can be evaluated and compared. The detailed investigations included geologic mapping, kinematic analysis, stress determinations, gravity surveys, integration of available geophysical data, slip tendency analysis, and for some areas 3D modeling. In Year 3, the detailed studies were completed and data synthesized to a) compare structural controls in various tectonic settings, b) complete the structural catalogue, and c) apply knowledge to exploration strategies and selection of drilling sites.

Detailed geologic mapping, structural analysis, and well data have been integrated to elucidate the stratigraphic framework and structural setting of the Tuscarora geothermal area. Tuscarora is an amagmatic geothermal system that lies in the northern part of the Basin and Range province, ~15 km southeast of the Snake River Plain and ~90 km northwest of Elko, Nevada. The Tuscarora area is dominated by late Eocene to middle Miocene volcanic and sedimentary rocks, all overlying Paleozoic metasedimentary rocks. A geothermal power plant was constructed in 2011 and currently produces 18 MWe from an ~170 degrees C reservoir in metasedimentary rocks at a depth of ~1430 m. Analysis of drill core reveals that the subsurface geology is dominated to depths of ~700-1000 m by intracaldera deposits of the Eocene Big Cottonwood Canyon caldera, including blocks of basement-derived megabreccia. Furthermore, the Tertiary-Paleozoic nonconformity within the geothermal field has been recognized as the margin of this Eocene caldera. Structural relations combined with geochronologic data from previous studies indicate that Tuscarora has undergone extension since the late Eocene, with significant extension in the late Miocene-Pliocene to early Pleistocene. Kinematic analysis of fault slip data reveal an east-west-trending least principal paleostress direction, which probably reflects an earlier episode of Miocene extension. Two distinct structural settings at different scales appear to control the geothermal field. The regional structural setting is a 10-km wide complexly faulted left step or relay ramp in the west-dipping range-bounding Independence-Bull Run Mountains normal fault system. Geothermal activity occurs within the step-over where sets of east- and west-dipping normal faults overlap in a northerly trending accommodation zone. The distribution of hot wells and hydrothermal surface features, including boiling springs, fumaroles, and siliceous sinter, indicate that the geothermal system is restricted to the narrow (< 1 km) axial part of the accommodation zone, where permeability is maintained at depth around complex fault intersections. Shallow up-flow appears to be focused along several closely spaced steeply west-dipping north-northeast-striking normal faults within the axial part of the accommodation zone. These faults are favorably oriented for extension and fluid flow under the present-day northwest-trending regional extension direction indicated by previous studies of GPS geodetic data, earthquake focal mechanisms, and kinematic data from late Quaternary faults. The recognition of the axial part of an accommodation zone as a favorable structural setting for geothermal activity may be a useful exploration tool for development of drilling targets in extensional terranes, as well as for developing geologic models of known geothermal fields. Preliminary analysis of broad step-overs similar to Tuscarora reveals that geothermal activity occurs in a variety of subsidiary structural settings within these regions. In addition, the presence of several high-temperature systems in northeastern Nevada demonstrates the viability of electrical-grade geothermal activity in this region despite low present-day strain rates as indicated by GPS geodetic data. Geothermal exploration potential in northeastern Nevada may therefore be higher than previously recognized.