Structural Controls of a Blind Geothermal System in the Northern Pyramid Lake Area, Northwestern Nevada

Most geothermal systems in the Great Basin are fault controlled. Ongoing studies suggest that many of these systems occupy discrete steps in fault zones or lie in belts of intersecting, overlapping, and/or terminating faults. Fault interactions within these settings generate subvertical conduits of highly fractured rock and increase dilation, thereby allowing geothermal fluids to approach the surface. Understanding the interaction between faults and geothermal systems can facilitate the identification of hidden or blind geothermal reservoirs. Detailed geologic mapping and structural analysis has elucidated the links between faulting and a blind geothermal system in the northern Pyramid Lake region, northwestern Nevada. This system lies near the terminus of the Pyramid Lake fault zone, a transitional region between northwest-trending dextral shear in the Walker Lane and north-northeast striking normal faults in the northern Great Basin. The region is composed of intercalated Miocene basalt, rhyolite, and dacite flows with subordinate lenses of breccia, conglomerate, and diatomite. Structurally, the northern Pyramid Lake region contains north to north-northeast striking normal and north-northwest to westnorthwest striking dextral-normal faults. Linear tufa towers mark some faults and fault intersections. Hot springs upwelling into Pleistocene Lake Lahontan probably formed many tufa towers, which were used as indicators for locating blind geothermal systems. The blind geothermal system at northern Pyramid Lake has no surficial hot springs but is marked by the intersection of linear north-northwest and west-northwest trending tufa towers that follow dextral-normal faults. Recent drilling adjacent to a large tufa tower confirms the presence of a geothermal system with temperatures of at least 90°C. Analysis of cuttings shows that the upper part of the reservoir lies in highly fractured, hydrothermally altered basalt and rhyolite units. Introduction The Great Basin in the western U.S.A. contains many geothermal fields. The majority are located in northern Nevada, northeastern California, and southern Oregon (Blackwell et al., 2002; Coolbaugh et al., 2002; Coolbaugh and Shevenell, 2004; Structural Controls of a Blind Geothermal System in the Northern Pyramid Lake Area, Northwestern Nevada Garrett S. Vice1, James E. Faulds1, William J. Ehni2, and Mark F. Coolbaugh3 1Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557 2Ehni Enterprises, Inc. P. O. Box 4228 Carson City, Nevada 89702-4228 3Great Basin Center for Geothermal Energy, University of Nevada, Reno, NV 89557 Figure 1. (from Faulds et al., 2004): Geothermal occurrences along structurally controlled belts in Utah, Nevada, and California. (green colored bands). BRDBlack Rock Desert; ECSZEastern California Shear Zone; HSZHumboldt Shear Zone; SDSevier Desert; SV-Surprise Valley; WLGWalker Lane (blue colored band). Red circlesGeothermal temperatures >160oC; yellow circlesgeothermal temperatures = 100160oC