Faulds, James E.¹, Coolbaugh, Mark F.², Hinz, Nicholas H.¹, (1) Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, Reno, NV 89557, (2) Great Basin Center for Geothermal Energy, University of Nevada, Reno, NV 89557
Structural studies are critical to assessing geothermal systems and selecting drilling targets for geothermal wells, because fractures and faults are commonly the primary pathway for deeply circulating hydrothermal fluids. Important tools for structural analysis include detailed geologic mapping, kinematic analysis of faults, and estimations of stress orientations. Structural assessments are especially useful for evaluating geothermal fields in the Great Basin of the western USA, where regional extension and transtension combine with high heat flow to generate abundant geothermal activity in regions having no recent volcanic activity. The northwestern Great Basin is one of the most geothermally active areas in the USA. The prolific geothermal activity is probably due to enhanced dilation on N- to NNE-striking normal faults induced by a transfer of NW-directed dextral shear from the Walker Lane to NW-directed extension. Our findings from analysis of several geothermal fields suggest that most systems occupy discrete steps in normal fault zones or lie in belts of intersecting, overlapping, and/or terminating faults. In addition, most fields are associated with steeply dipping faults and, in many cases, with Quaternary faults. The structural settings favoring geothermal activity are characterized by subvertical conduits of highly fractured rock along fault zones oriented approximately perpendicular to the WNW-trending least principal stress. Features indicative of these settings that may be helpful in guiding exploration for geothermal resources include: 1) major steps in normal faults, 2) interbasinal highs, 3) groups of relatively low, discontinuous ridges, and 4) lateral jogs or terminations of mountain ranges.