Structural Controls of Geothermal Activity in the Northern Hot Springs Mountains, Western Nevada: The Tale of Three Geothermal Systems (Brady's, Desert Peak, and Desert Queen)

Abstract

The northwestern Great Basin in the western USA hosts abundant, generally amagmatic geothermal activity. Significant geothermal exploration is ongoing, but controls on fluid flow in the geothermal systems are generally poorly understood. To better elucidate the controls on fluid flow, we have been conducting a detailed structural assessment (detailed geologic mapping, structural analysis, gravity surveys, and stress determinations) of the northern Hot Springs Mountains ~80 km east-northeast of Reno, Nevada. Three major NNE-striking normal fault systems dissect the northern Hot Springs Mountains, and each is associated with a distinct geothermal anomaly. From west to east, these are the Brady’s, Desert Peak, and Desert Queen geothermal systems. The surface expression of the Brady’s system is a 4-km-long, NNE-trending zone of extensive sinter, warm ground, fumaroles, and mud pots along the Brady’s fault. In contrast, both Desert Peak and Desert Queen are blind geothermal systems with no obvious surface expression of hydrothermal activity. Kinematic data gleaned from fault surfaces in the area indicate essentially dip-slip motion along controlling faults and a WNW-trending extension direction, which is compatible with regional GPS geodetic data. Although spacing between the controlling fault zones is only ~3 to 6 km, available geochemical data suggest that each system is independent from the other. Both Brady’s and Desert Peak are high enthalpy systems (175-215°C) that have operating power plants and are currently under study for expansion utilizing EGS technology. Both of these systems occupy left steps in the NNE-striking, west-dipping normal fault systems. The left steps appear to be linked by multiple minor, more northerly striking faults and thus mark steeply plunging conduits of highly fractured rock. The high fracture density in these steps enhances permeability and therefore accommodates the ascent of hydrothermal fluids. The Desert Queen system is marked by a 6-km-long shallow temperature anomaly with 2-meter temperatures as high as 42°C. These shallow temperatures overlie a shallow thermal aquifer at depths of roughly 70 m with temperatures as high as 90°C measured in shallow gradient holes. The Desert Queen system appears to occupy the southern, horse-tailing end of an east-dipping, NNE-striking normal fault zone, possibly where it intersects a major west-dipping antithetic fault. Increased fracture density associated with horse-tailing presumably generates a zone of high permeability conducive for deep circulation and fluid flow. The proximity of three independent geothermal systems in the northern Hot Springs Mountains demonstrates the high potential for geothermal development within the western Great Basin and suggests that favorable structural settings can be found along many normal fault zones.