Structural Controls of the San Emidio Desert Geothermal Field, Northwestern Nevada

Abstract

Detailed geologic mapping and structural analysis have provided a more complete understanding of fault interactions and kinematics of the San Emidio geothermal system. San Emidio lies within the Basin and Range province of northwestern Nevada, ~100 km north of Reno. This area is dominated by middle to late Miocene volcanic rocks and late Miocene to recent sediments, all overlying Mesozoic metasedimentary rocks. Currently, a small power plant produces 3.6 MW from a 162°C reservoir at 520 m depth at the south end of an active fault system. Abundant altered rocks, native sulfur deposits, and boiling groundwater at depths less than 100 m suggest, however, that the San Emidio geothermal resource extends several kilometers northward from the currently producing well field. The San Emidio geothermal system appears to occupy a right step in a hard-linked, N-striking, W-dipping normal fault zone along the west flank of the northern Lake Range. Much of the hydrothermal activity is associated with a N-trending, W-dipping Holocene scarp. Abundant silicified fault and hydrothermal breccias, as well as open-space textures, are concentrated along the sinistral oblique-slip fault that links two strands of the N-striking normal fault zone within the right step. Kinematic features preserved in these silicified units indicate a WNW-trending slip direction, thus inducing minor dilation within the right-step of the northerly striking fault system. Minor dilation accompanied by high fault density within the right step likely produces the permeability necessary for deep fluid circulation within the San Emidio system.