Active Faulting in the Coso Geothermal Field, Eastern California

Abstract

New mapping documents a series of late Quaternary NNE-striking normal faults in the central Coso Range that dip northwest, toward and into the main production area of the Coso geothermal field. The faults exhibit geomorphic features characteristic of Holocene activity, and locally are associated with fumaroles and hydothermal alteration. The active faults sole into or terminate against the brittle-ductile transition zone (BDT) at a depth of about 4 to 5 km. The BDT is arched upward over a volume of crust in the 5 to 15 km depth range beneath the Coso geothermal field that is characterized by high heat flow and low P-wave velocities, and which may contain magma and/or hot, lithostatically pressured brines. The positive relief on the BDT may guide or control the down-dip geometry of active faults. Brittle faulting and seismogenic deformation above the shallow BDT may contribute to development of permeability in the geothermal reservoir, and provide pathways for upward circulation of hydrothermal fluids.