Structural Controls of Three Blind Geothermal Resources at the Hawthorne Ammunition Depot, West-Central Nevada

Abstract

In cooperation with the U.S. Navy Geothermal Program Office, we have undertaken a multidisciplinary investigation to identify geothermal resources and select new drill target areas for future exploration at the Hawthorne Ammunition Depot (HAD). The HAD straddles the southern part of the Walker Lake basin, extending east into the northwestern Garfield Hills and west into the south-central part of the Wassuk Range. The northwestern Garfield Hills are composed mostly of Mesozoic metamorphic basement with lesser granitic plutons overlain by Cenozoic volcanic and sedimentary rocks. Cenozoic rocks are cut by a system of NW-striking dextral faults and NE-striking sinistral faults, and locally contain gentle ENE -trending folds. A complex westward-deepening half-graben separates the Wassuk Range from the Garfield Hills. In contrast to the Garfield Hills, the Wassuk Range is composed mostly of Mesozoic granite with minor metamorphic rocks or Cenozoic rocks. The Wassuk Range front fault zone bifurcates into two major strands west of the town of Hawthorne. Based on geophysical, field, and well data, the NW-striking, east-dipping Wassuk Range front fault strands dip of 30° to 55°, with an average of ~45°. The NE-striking rangefront segment at Cat Creek dips ~65° SE. Fault slip data from the range-front within the step-over indicate a NW-SE (azimuth 127°) extension direction. Temperature and geochemistry data indicate three blind geothermal systems (no surface hot springs) within the HAD. All three geothermal systems follow steeply dipping, NE-striking faults, consistent with slip and dilation tendency analysis indicating that NE-striking, 70° NW- and SE-dipping fault planes have the highest tendency to slip and dilate. One system (A) parallels the Wassuk Range front adjacent to where the range-front fault zone bifurcates with a major right step in the active part of the fault zone. A second system (B) resides along the northwest margin of the Garfield Hills at the south end of a pull-apart. The third system (C) resides ~7 km north-northeast of the city of Hawthorne where tufa mounds form a linear outcrop along part of the Hawthorne fault zone and the northwest corner of the pull-apart. Cation geothermometers for geothermal systems A and C range 90-180º C and 90-137º C, respectively, while geothermometry for system B has not been established. Our primary recommendation for drilling is the highly faulted, down-plunge projection of the right step between faults A and B in the western part of the basin near Cat Creek.