Relationship Between Active Faulting, Strain Transfer, and Relief on the Brittle-Ductile Transition Zone, Coso Range, Eastern California

The Coso Range is located within a right-releasing stepover between the dextral Airport Lake and Owens Valley fault zones along the southeastern margin of the Sierra Nevada microplate. Upper crustal extension in the stepover region is accommodated by the Coso Wash graben at the southeast end of the stepover region, and by a series of left-stepping, north-northeast-striking, right-normal oblique faults in the northern Coso Range at the northwest end of the stepover region. The upper crustal faults overlie a NW-trending zone in the 5 to 10 km depth range beneath the central and northern Coso Range that is characterized by high heat flow and low P-wave and S-wave velocities. The brittle-ductile transition zone (BDT) is arched upward over the low velocity zone as evidenced by shallowing of the base of seismicity to 4-5 km depth beneath the Coso geothermal field. The positive relief on the BDT may guide or control the down-dip geometry of active faults in the step-over region. South of the geothermal field, the Coso Wash fault dips east, toward the east-dipping flank of the low velocity zone and in the direction that the BDT is deepening away from the zone of highest heat flow. At the latitude of the geothermal field, the traces of active normal faults dip west and they likely terminate against or sole into the BDT as it deepens westward toward Rose Valley. The right-normal faults in the northwestern Coso Range graben similarly sole into or terminate against the elevated BDT above the low velocity zone. The low velocity zone may be acting as a strain guide for the transmission of NW dextral shear across the stepover region. Brittle faulting and seismogenic deformation above the shallow BDT contribute to development of permeability in the geothermal reservoir, and provide pathways for upward circulation of hydrothermal fluids.