Active Fault Structure and Potential High Temperature Geothermal Systems: Lidar Analysis of the Gabbs Valley, Nevada, Fault System

Abstract

High temperature geothermal systems in the Great Basin have been linked to active Quaternary faults. Surface ruptures from two historical earthquakes, the 1932 Cedar Mountain (MS 7.2) and 1954 Fairview Peak (MS 7.2), overlap in the area of the Gabbs Valley, Nevada geothermal lease. New mapping of Quaternary faults is in progress utilizing low sun-angle aerial photography and Lidar remote sensing imagery. Holocene and historic fault ruptures record an actively deforming area and these faults are likely to have fractured, brecciated rock associated with their fault plane(s) at depth. Permeability associated with these fault planes is a potential geothermal fluid pathway. High-resolution hill shade and slope shade images were generated from lidar (light detection and ranging) data collected by Watershed Sciences, Inc. for GeoGlobal Energy covering their BLM geothermal lease area near Cobble Cuesta in Gabbs Valley, Nevada. Bare earth digital elevation models (DEMs) were provided by Watershed Sciences with horizontal ground resolution of 0.5 m, and vertical accuracy better than 0.05 m. The synthetic images, along with low sun-angle (sun ~10-25° above the horizon) aerial photography (LSA) were used for detailed mapping of Quaternary fault scarps to a scale of 1: 3,000 or better. The use of hill shade, slope shade and LSA imagery provides a variety of methods and repeatability to identify fault scarps. Detailed mapping depicts two fault trends. The Phillips Wash fault zone is primarily located in the wash between Cobble Cuesta (a low, broad ridge) and the Monte Cristo Mountains. These scarps are NNE striking and discontinuous. Fault segments exhibit a right stepping en echelon pattern and graben of varying size and extent. The Eastern Monte Cristo Mountains fault zone ruptures the piedmont alluvial fans striking NNE and increasing in fault zone width to the north. Typically the piedmont faults are relatively longer and more continuous. The longer length, increased height of scarps, and older ages of alluvial fan surfaces cut by these fault scarps records a longer paleoseismic history. Mapped fault data will be field verified and combined with kinematic data to discern current stress orientations at the site. Interpretations will be integrated into GeoGlobal’s structural kinematic model for the resource.