Three-dimensional geologic model of the northern Nevada rift and the Beowawe geothermal system, north-central Nevada

Abstract

A three-dimensional (3D) geologic model of part of the northern Nevada rift encompassing the Beowawe geothermal system was developed from a series of two-dimensional (2D) geologic and geophysical models. The 3D model was constrained by local geophysical, geologic, and drill-hole information and integrates geologic and tectonic interpretations for the region. It places important geologic constraints on the extent and confi guration of the active Beowawe geothermal system. The geologic framework represented in this model facilitates hydrologic modeling of the Beowawe geothermal system and evaluation of fl uid fl ow in faults and adjacent rock units.Basin depths were determined using an iterative gravity-inversion technique that calculates the thickness of low-density, basin-fi lling deposits. The remaining subsurface structure was modeled using 2D potential-fi eld modeling software. Crustal cross sections from the 2D models were generalized for use in the 3D model and consist of six stratigraphic layers defi ned as low-density basin sediments, volcanic rocks, basalt-andesite rocks of the northern Nevada rift, Jurassic and Cretaceous intrusive rocks, and Paleozoic siliceous and carbonate sedimentary rocks of the upper and lower plates of the Roberts Mountains allochthon, respectively. This simplifi ed stratigraphy was combined with mapped surface geology and was extrapolated across the 3D model area. Features along the northern Nevada rift depicted by the model may represent preexisting crustal structures that controlled the locations and character of Ter-tiary tectonic and magmatic events related to Basin and Range extension and emplacement of the middle Miocene northern Nevada rift. Several of the geologic features represented are important components of the Beowawe geothermal system. Prominent ENE-trending faults (e.g., Malpais fault) that bound the southern edge of Whirlwind Valley, and older NNW-striking faults (e.g., Dunphy Pass and Muleshoe faults) that form major features of the model, are likely important pathways for geothermal fl uids and groundwater fl ow from the Humboldt River, which may recharge the Beowawe system.

Evidence for long timescale (>103 years) changes in hydrothermal activity induced by seismic events

“…Based upon geophysical, geologic, and drilling information, the source reservoir for the geothermal fluids is interpreted to be Paleozoic carbonate 3–5 km below the surface (Watt et al . ). This interpretation is confirmed by the δ 13 C compositions of dissolved inorganic carbon in Beowawe geothermal fluids (−2 to 1‰), which is typical of Paleozoic marine limestones (Day ).…”

Analytical and numerical models of hydrothermal fluid flow at fault intersections

“…11B). The field is adjacent Fluid flow at fault intersections 323 to an intersection of the Malpais fault with the Dunphy Pass fault zone, an older northwest-striking normal fault within the Miocene Northern Nevada Rift (Watt et al 2007) (Fig. 11B).…”

A prominent geophysical feature along the northern Nevada rift and its geologic implications, north-central Nevada

“…2) indicate additional magnetic sources associated with the northern Nevada rift. This is particularly evident along the east side of the northern Shoshone Range, as revealed by two-and threedimensional modeling (Watt et al, 2007). These modeling studies indicate that the eastern northern Nevada rift refl ects a major crustal feature that extends to at least mid-crustal depths, and, combined with geologic data, they indicate that the northern Nevada rift and vicinity is characterized by complex structures that could play an important role in localizing fl uid fl ow and mineralization.…”

“…6) and geophysical modeling indicate that this feature is unusually abrupt and represents a near-vertical crustalscale structure or fault. Two-dimensional geophysical modeling (Watt et al, 2007) indicates that the northern Nevada rift can be exclusively modeled by upper-and mid-crustal sources that are more extensive than just the northern Nevada rift and associated intrusions, and it cannot be solely accounted for by lower-crustal sources. Geophysical modeling is consistent with the possibility that the northern Nevada rift is contemporaneous with mafi c intrusions and perhaps underplating and represents a major lithologic boundary, or both.…”