A Preliminary Structural Model for the Blue Mountain Geothermal Field, Humboldt County, Nevada

Abstract

The Blue Mountain geothermal field is a blind geothermal prospect (i.e., no surface hot springs) along the west flank of Blue Mountain in southern Humboldt County, Nevada. Development wells in the system have high flow rates and temperatures above 190ºC at depths of ~600 to 1,070 m. Blue Mountain is a small ~8-km-long east-tilted fault block situated between the Eugene Mountains and Slumbering Hills. The geothermal field occupies the intersection between a regional NNE- to ENE-striking, west-dipping normal-sinistral fault system, which bounds the west flanks of Blue Mountain and the Eugene Mountains, and a more local WNW-striking, SW-dipping normal-dextral fault on the southwest side of Blue Mountain. The WNW-striking fault appears to mark a large left step in the more regionally extensive northeasterly striking fault zone. Quaternary fault scarps, faceted spurs, and steep mountain fronts, with ~1,000 m of topographic relief, mark these faults. The NE-striking fault zone swings to a more easterly strike in the northern Eugene Mountains and in the northern part of Blue Mountain probably due to a strong ENEstriking fabric in the Triassic metasedimentary basement. In the vicinity of the geothermal field, the intersection between the NEand WNW-striking faults is marked by multiple fault splays and fault intersections, which have generated a >1 km wide, westerly plunging zone of highly fractured, relatively permeable rock. The inferred kinematics of these fault zones further implies a dilatant, wedge-shaped zone at their intersection. Thus, the Blue Mountain geothermal field appears to be controlled by a dilatant fault intersection, partly induced by the influence of strong basement fabrics on the orientation of Neogene faults.