We have completed a comprehensive inventory of the structural settings of known geothermal systems (>200 total; ≥37°C) and late Miocene (~8 Ma) to Quaternary epithermal mineral deposits in the extensional to transtensional region of Nevada. The structural settings are important to characterize, because many geothermal systems and epithermal mineral deposits are hidden beneath the surface, and thus these settings can be used as an exploration tool to find new systems or deposits. Of the known geothermal systems in Nevada, for example, ~37% are blind or hidden (no surface hot springs or fumaroles).
We catalogued geothermal systems into eight major groups, based on the dominant pattern of faulting. Of the ~214 known systems in Nevada, we found that step-overs or relay ramps in normal fault zones are the most common setting, hosting ~39% of the systems. Step-overs are characterized by multiple, commonly overlapping fault strands, increased fracture density, and thus enhanced permeability. Other common settings include a) normal fault terminations (~23%), where horsetailing generates a myriad of closely spaced faults and thus increased fracture permeability; and b) fault intersections between normal faults or between normal faults and transverse oblique-slip faults (~21%), where multiple minor faults typically connect major structures, and fluids can flow readily through highly fractured, dilational quadrants. Less common settings include: a) accommodation zones (~5%); b) displacement transfer zones (~4%) at the ends of strike-slip faults; c) pull-aparts in strike-slip faults (~4%); d) bends in normal faults (~1%); and e) major range-front normal faults (~1%). Pull-aparts and displacement transfer zones are more abundant in the transtensional western part of the region within and near the dextral shear zone of the Walker Lane. Quaternary faults typically lie within or near most of the geothermal systems. Controlling faults in the systems most commonly strike north-northeast (~54% of systems) approximately orthogonal to the regional extension direction. Northerly striking faults (~N10°W to N10°E) are also relatively common and serve as the primary controlling structure in ~32% of the systems.
Most late Miocene to recent epithermal mineral deposits in Nevada occupy similar structural settings. Step-overs are the most common type of setting, followed by fault terminations, accommodation zones, and displacement transfer zones. Similar to active geothermal systems, the formation of epithermal deposits is favored by complex structural settings. These data can be used to guide exploration strategies, because economic concentrations of precious metals typically occupy only a small percentage of their respective districts.
The favorable structural settings for geothermal systems and epithermal mineral deposits (e.g., step-overs, fault terminations, and accommodation zone) correspond to long-term, critically stressed areas, where fluid pathways more likely remain open in networks of closely spaced, breccia-dominated fractures. Hydrothermal systems are rare along the main segments of normal faults due to reduced permeability in zones of clay gouge and periodic release of stress in major earthquakes. Accommodation zones, displacement transfer zones, and pull-aparts are disproportionately associated with higher temperature and/or power-producing geothermal systems. These settings appear to be especially favorable for enabling fluids to circulate to and from relatively great depths (up to 5+ km). Notably, many higher temperature systems are hybrids containing more than one type of favorable setting.
James Faulds, M. Coolbaugh, N. Hinz· 8 citations· ⚡4
Curious about Yucca Mountain? This report provides a summary of the geologic setting and previous studies of the area, including discussion of existing data gaps that limit understanding of seismic hazards in the region.
Yucca Mountain is a potential geological repository for high-level radioactive waste. The site is situated on federal land adjacent to the Nevada National Security Site (formerly Nevada Test Site) in Nye County, Nevada, about 130 km (80 miles) northwest of Las Vegas Valley. The site lies in an active tectonic setting within a segment of the Pacific–North American plate boundary. As such, active faults and recent volcanic activity characterize the area. Seismic activity could affect the suitability and safety of storage of nuclear waste. Thus, a thorough and accurate assessment of previous and potential future seismic activity in the region surrounding the proposed repository is critical.
From the late 1970s to mid-1990s, deformation related to active faulting in the Yucca Mountain region was extensively studied by the U.S. Department of Energy, U.S. Geological Survey, both the Nevada Bureau of Mines and Geology and Nevada Seismological Laboratory at the University of Nevada, Reno, and other institutions to assist in the evaluation and site characterization of the area for the proposed high-level radioactive waste repository. These studies documented multiple faults with recent activity, significant current seismicity, and ongoing crustal motion related to the Pacific–North American plate boundary and San Andreas fault system. Although these studies applied the most advanced techniques existing at the time, the ages of faulted deposits, essential for precise earthquake hazard estimates, were limited by the available experimental techniques. Significant advances in analysis and dating of faulted materials (i.e., paleoseismology), fault identification by remotely sensed methods, GPS geodetic data, and the scientific community’s general understanding of the regional tectonic setting over the last two decades suggest that the previous studies may not completely describe the seismic hazards in the region.
In this report, the preexisting studies are evaluated in an effort to better understand the state of knowledge of the seismic hazards, as well as the adequacy of the approaches applied to characterize the hazard. Potential information and data gaps that may not have been included in the original evaluation are also addressed. The regional tectonic setting and paleoseismic, seismic, and geodetic studies are all reviewed. The paleoseismic studies record deformation over the past tens to hundreds of thousands of years; the seismic investigations reflect deformation over the past ~150 years; and the geodetic studies account for ongoing crustal deformation in the region (i.e., past couple decades). These datasets permit an overview of recent and geologically relevant deformation important for estimating future seismic hazards in the vicinity of Yucca Mountain.
James Faulds, Rich Koehler, Kenneth D. Smith et al.· 0 citations
This report provides a detailed stratigraphic analysis, conceptual structural model, and three-dimensional (3D) geological model of the Soda Lake geothermal field, a geothermal system in the Carson Sink basin in northwestern Nevada. We first present an updated stratigraphy of the field area based on re-analysis of drill cuttings, geophysical logs, and radiometric dates from key igneous units. The stratigraphy was validated against bedrock sections in surrounding ranges and basin-fill sections derived from wells in the Carson Sink. In the second section, we present a detailed 3D geological model of the field based on three inputs: 1) well intercepts interpreted in context with the new stratigraphy, 2) block tilt estimates from borehole bedding attitude logs, and 3) a fault framework derived from drill intercepts and a 3D seismic reflection survey.
The stratigraphic framework at the Soda Lake geothermal field is dominated by 1) ~1 km of late Miocene to Quaternary basin-fill sediments and volcanic units, 2) an underlying ~1-km-thick section of middle to late Miocene volcanic and sedimentary rock, and 3) Mesozoic metamorphic and granitic basement. Unconsolidated late Miocene to Quaternary basin fill can be divided into distinctive upper and lower units. The upper unit is highly variable but dominated by pebble to silt-size quartzo-feldspathic detritus. It contains 2–3 marker beds, the most significant of which is a 30- to 60-m-thick mudstone layer at ~245–275 m depth. Basin-fill sediments below this mudstone horizon are moderately coarse grained but become increasingly lithic-rich and epiclastic down section. The lower basin-fill unit includes a lithic lapilli crystal ash tuff and an underlying ~300 to 400-m-thick section of lacustrine silt and mud. The basin fill is interrupted by a small, 5.1 Ma trachyandesite volcanic center near the heart of the Soda Lake well field. The basin-fill sediments are underlain by a ~1-km-thick section of middle to late Miocene volcanic and sedimentary strata. It is dominated by mafic flows that are similar to Miocene lavas in the surrounding highlands. The Tertiary bedrock section also includes minor intervals of intermediate to felsic tuff, clastic sediments, and limestone. Two of the thicker and more continuous non-volcanic intervals serve as stratigraphic marker units. Granitic and metasedimentary Mesozoic basement units intercepted in wells at the Soda Lake geothermal field are correlated with recognized assemblages in the surrounding region.
The structural framework at the Soda Lake geothermal field is dominated by a series of east-dipping, N5˚E-striking normal faults that are ~10 km in length. This set of faults bound a series of west-tilted fault blocks and half-grabens. The most closely spaced of these east-dipping faults are near the center of the Soda Lake well field and define the west side of a deep central graben. They step slightly to the left in tandem a few hundred meters west of one of the current production zones. Maximum throws on the largest of these faults range from 200 to 450 m at the Mesozoic basement contact, and dips range from 65˚to 75˚ east. The maximum fault block tilt in the central graben is ~35˚ west, based on well intercepts and dipmeter data. A subordinate set of north-striking, west-dipping normal faults is also present, and limited data suggest they are similar in strike length to the more dominant east-dipping fault set. A ~2-km-long, nearly vertical, southeast-dipping, east-northeast-striking cross-fault is present in the production area and appears to intersect the most prominent east-dipping faults slightly south of the step-over. Offset on all fault sets appears to be roughly contemporaneous, with extension constrained from ~13 Ma to present. Tilt versus age measurements suggest graben development slowed significantly at ~5 Ma. However, the main faults continued to accommodate minor normal slip into the late Pleistocene. The east- and west-dipping normal faults are inferred to intersect beneath the central graben.
Upwelling fluids at the field appear to be controlled by a hybrid setting that includes the minor step-over, fault intersection between the north- and east-northeast-striking faults on the west side of the central graben, and intersecting oppositely dipping normal faults beneath the central graben. Three or more historically or presently active wells intersect the east-dipping, northerly striking normal faults that form the west side of the central graben. Two currently active geothermal wells ≥700–800 m southeast of the step-over intersect the main west-dipping fault that bounds the east side of the central graben. Two notable stratigraphic features may influence the geothermal system. The ~5 Ma trachyandesite volcanic center that underlies the well field may provide subvertical fracture permeability, enhancing shallow production from wells that do not intersect the major normal faults. Production may also be enhanced by the low permeability mudstone horizon in the upper basin fill that may act as a clay cap for the geothermal upwellings along subjacent faults.
H. McLachlan, James Faulds, W. Benoit· 0 citations