Structural controls of the Astor Pass-Terraced Hills geothermal system in a region of strain transfer in the western Great Basin, northwestern Nevada.

摘要

Detailed geologic mapping, structural analysis, a paleomagnetic investigation, 40Ar/39Ar geochronology, and the integration of gravity, well, and shallow temperature data have elucidated the structural controls of two geothermal fields in the northern Pyramid Lake region, northwestern Nevada. The two geothermal systems, the Astor Pass and Terraced Hills, lie at 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 Basin and Range province.;The Terraced Hills are composed of generally east-tilted fault blocks (∼30°) of Miocene basaltic andesite, rhyolite, and dacite flows with subordinate lenses of breccia, conglomerate, and diatomite, all of which largely correlate with the Pyramid sequence (∼15.55-13.24 Ma). Quaternary units consisting of alluvial fans and Lake Lahontan shorelines, beach, bar, and tufa deposits overlie the Pyramid sequence in valleys and along the flanks of ridges. The fault blocks are bounded by north-northeast-striking normal and north-northwest- to west-northwest-striking normal and dextral-normal faults.;Deformation began post ∼13 Ma and continues into the Quaternary. One fault scarp was identified and trenched in the eastern Terraced Hills. This fault has ruptured since ∼3640 yr B.P. and has a slip rate of 0.04 mm/yr. Offset along the fault is probably dextral-normal with a larger component of normal movement. Total normal displacement across the Terraced Hills is ∼1.3 km, which equates to ∼650 m of horizontal extension. Total dextral displacement is at least 600 m but could be as high as ∼3 km. The sum of dextral offset and horizontal extension yields a total of ∼1.25-3.65 km of northwest-directed motion in the Terraced Hill, compatible with the apparent strain budget along the Pyramid Lake fault zone. These relations also suggest that the Pyramid Lake fault zone extends into and essentially terminates in the Terraced Hills.;Dextral-normal and normal faults control the emplacement of the Astor Pass and Needle Rocks geothermal systems. The Needle Rocks geothermal field results from increased fracture density within a series of en echelon dextral-normal faults. The Astor Pass system was generated by the intersection of a dextral-normal and a normal fault. Both fields contain multiple tufa tower lineations.;The setting at Astor Pass suggests that tufa towers are promising indicators for blind geothermal systems. However, not all tufa towers are fault controlled or reflect subsurface geothermal systems. This study found that an integration of geologic and geophysical data was needed to accurately locate the blind geothermal system. Promising features for future exploration include multiple structurally controlled linear tufa towers, intersecting faults, and kinematic data suggestive of pull-apart zones or dilation at the fault intersection.