COMPOSITE UPPER CRUSTAL SECTION OF A NASCENT METAMORPHIC CORE COMPLEX IN A TRANSTENSIONAL REGIME, COSO RANGE, CALIFORNIA

The Coso geothermal field in the central Coso Range occupies a releasing stepover between two major strike-slip fault zones of the southern Walker Lane: the Airport Lake fault zone in Indian Wells Valley to the south, and the Owens Valley fault to the north. Dextral slip transfer across the Coso Range primarily is accommodated by left-stepping, en echelon NNE-striking normal faults, and is localized above a NW-trending zone of low P-wave and S-wave velocities in the depth range of ~5–12 km beneath the geothermal field. The base of seismicity shallows to ~4-5 km depth above the low velocity zone, and seismic reflection profiles image a prominent high-amplitude reflector at ~6 km depth beneath the geothermal field that is interpreted to be associated with magma or geopressured brines below the brittle-ductile transition zone (BDT). Normal faults in the geothermal field are interpreted to terminate downward against the shallow BDT. The present-day structure of the Coso Range is similar to a reconstruction by Serpa and Pavlis (1996) of the Black Mountains metamorphic core complex in Death Valley prior to late Cenozoic unroofing and exhumation of the BDT and Miocene intrusive rocks. We suggest that the Black Mountains may expose rocks and structures analogous to those presently developing at ~4-8 km depth in the Coso Range. It is reasonable to assume that intrusive rocks at greater depths in the Coso stepover region also may be involved in the upper crustal transtension. Exhumed synkinematic Cretaceous plutons in the Sierra Nevada, interpreted to have been intruded into extensional stepovers (Titus et al., 2005), may provide analogs for magmatic processes and ductile structures between ~8-15 km depth in the Coso Range.