THREE-DIMENSIONAL GEOLOGIC MODEL OF THE BEOWAWE GEOTHERMAL AREA, NORTH-CENTRAL NEVADA

A simplified three-dimensional geologic model of the Beowawe geothermal area, including parts of Battle Mountain, Shoshone Mountains, and the Sheep Creek Range, was developed from geologic, geophysical, and drill-hole information to aid fluid flow modeling and provide a framework for tectonic interpretations of northern Nevada. The model encompasses a volume about 85-km long, 75-km wide, and 6-km deep, approximately centered on the Beowawe geothermal area in north-central Nevada. Five stratigraphic layers were defined: low-density basin-filling deposits, volcanic rocks, basalt-andesite rocks of the northern Nevada rift (NNR), and Paleozoic sedimentary rocks of the upper and lower plate of the Roberts Mountain allochthon.

The model is based on surface geology, geologic cross sections, drill-hole information, and 2D geophysical models. Using an iterative gravity inversion technique, geophysical data were particularly useful in determining the thickness of low-density basin-filling deposits. Geologic cross sections were constrained using two-dimensional geophysical (gravity and magnetic) modeling techniques. Geologic layers were extrapolated across the area of the model from the revised geologic and geophysical cross sections and imported into a geologic modeling and visualization software package that allows fully three-dimensional rendering and manipulation (EarthVision, Dynamic Graphics, Inc., Alameda, Calif.).

The Beowawe geothermal system lies within a 1.5-km thick basin, about 10 km east of the magnetically-defined northern Nevada rift, along a zone of prominent ENE-striking faults (e.g., Malpais fault) that bound the southern edge of Whirlwind Valley, and near prominent N-striking faults (Dunphy Pass fault zone). Due to the increased permeability along these faults, the faults are likely conduits for groundwater flow from the Humboldt River to Beowawe. In addition, major NNW-striking structures bounding the Shoshone Range (e.g. Muleshoe Fault) may provide another source for groundwater recharge.