CHARACTERIZING THE STRUCTURAL CONTROLS OF THE TERRACED HILLS-ASTOR PASS GEOTHERMAL SYSTEM, 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. Attaining a better understanding of the interaction between faults and geothermal systems can facilitate the identification and production 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 Terraced Hills-Astor Pass area, northwestern Nevada. The Terraced Hills-Astor Pass system lies near the terminus of the Pyramid Lake fault zone, a transitional region between northwest-trending dextral shear in the Walker Lane and NNE-striking normal faults in the northern Great Basin. The Terraced Hills are composed of intercalated Miocene basalt, rhyolite, and dacite flows with subordinate lenses of breccia, conglomerate, and diatomite. Lake Lahontan beach, bar, and tufa deposits dominate the Quaternary units. Structurally, the Terraced Hills contain N- to NNE-striking normal and NNW- to WNW-striking dextral-normal faults. Linear tufa towers mark some of the faults and fault intersections. Many of the tufa towers probably formed due to the upwelling of hot springs into Pleistocene Lake Lahontan and were therefore used as an indicator of blind geothermal systems. The blind geothermal system in Astor Pass has no surficial hot springs but is marked by the intersection of linear NNW- and WNW- trending tufa towers that follow dextral-normal faults. Recent drilling adjacent to the main tufa tower at Astor Pass confirms the presence of a geothermal system with temperatures at ~90°C. Analysis of cuttings shows that the reservoir lies in highly fractured, hydrothermally altered basalt and rhyolite units.