ABSTRACT: Tectonic Setting of the Coso Geothermal Reservoir

M. C. Erskine

The Coso geothermal reservoir is being developed in Sierran-type crystalline bedrock of the Coso Mountains, a small desert mountain range just to the east of the Sierra Nevada and Rose Valley, which is the southern extension of the Owens Valley of eastern California. Optimum development of this reservoir requires an understanding of the fracture hydrology of the Coso Mountains crystalline terrain and its hydrologic connection to regional groundwater and thermal sources. An interpreted, conceptually balanced regional cross section that extends from the Sierra Nevada through the geothermal reservoir to the Panamint Mountains is presented. The cross section is constrained by new reflection and refraction seismic data, gravity and magnetic modeling, drilling data from the geo hermal reservoir, and published regional geologic mapping. The interpretation presented in the cross section and the geochemistry of the reservoir fluids is used to argue that the geothermal system is a thermal bulge on an otherwise normal fracture-controlled regional groundwater flow. This groundwater flow starts from recharge areas in the high Sierra Nevada and Coso Mountains and moves toward the very low desert valleys to the southeast. The hypothesis also suggest that the low-angle intracrustal detachment of the Basin Ranges may be controlled by thrust structures developed during the Cordilleran, Sevier plus Laramide, orogenic event.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990