Structural controls of the geothermal system at Gerlach, Washoe County, Nevada
AdvisorCashman, Patricia H
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Detailed geologic mapping, spring and two-meter temperature data, and gravity and magnetic data constrain the structural controls of the high-temperature (>175°C) geothermal system at Gerlach. The system lies along the termination of a major normal fault at the southern end of the Granite Range in the southern Black Rock Desert, a known favorable setting for geothermal activity. Mapping of Quaternary deposits documents Holocene movement on the north-northeast-striking, east-dipping Gerlach fault. Dominant orientations of geothermal veins in exposed fossil geothermal systems parallel this active, terminating range-front fault, suggesting that it does provide a major control on the current geothermal system.Locally, there are two areas of geothermal upwelling with separate sets of structural controls. The two areas are expressed by outflow as two sets of springs, Great Boiling Springs and Mud Springs, as well as by separate altered bedrock fossil systems above the springs. The areas are topographically distinct (neither could represent outflow from the other), are separated by relatively lower shallow temperature measurements, and are associated with different orientations of the gravity gradient. Prominent northwest-striking fractures provide areas of enhanced permeability and parallel the secondary set of veins in the altered bedrock uphill of Great Boiling Springs. These northwest-striking veins are not observed in the area above Mud Springs, but there is an east-northeast-striking set that is not present above Great Boiling Springs. The presence of east-northeast-striking structures is supported in the gravity data. These structures could represent an ever broader-scale structural connection between the southern termination of the Gerlach fault and the northern termination of the Fox Range fault to the south.