UPPER CRUSTAL STRUCTURE OF AN OBLIQUELY EXTENDING OROGEN, CENTRAL COSO RANGE, EASTERN CALIFORNIA

The Coso Range is an extensional domain in a releasing stepover between major dextral strike-slip faults along the southeastern margin of the Sierra Nevada microplate. New multifold seismic reflection data from the Coso geothermal field in the central Coso Range image reflectors that resemble suites of structural and magmatic features exposed in many exhumed metamorphic core complexes (MCC). The Coso Wash fault, a Holocene-active normal fault that is a locus of surface geothermal activity, is imaged as a moderately dipping reflector that terminates against a low-relief reflector or reflective zone (the “A”) at approximately 4 km depth. Based on the absence of seismicity below 4 km depth, as well as production data indicating that temperatures approach 350°C at about 3 km depth in the geothermal field, we associate the A reflector with the brittle-ductile transition. Southeast-dipping listric reflectors (“B”) flatten above or terminate against the A reflector. In at least one case, the 3D geometry of a B reflector can be determined from crossing 2D reflection lines, and we correlate it with a southeast-dipping fault mapped at the surface by Whitmarsh (1997). The seismic images also reveal a prominent high-amplitude, slightly convex reflector (“C”) at 6 km depth directly beneath the northern part of the geothermal field. The C reflector is at or near the top of a magma chamber imaged by recent teleseismic studies (Wilson et al. 2003), and is interpreted to be a lens of magmatic brine or partial melt. The upper crustal structure of the Coso Range inferred from the reflection data is similar to reconstructions by Serpa and Pavlis (1996) of the Black Mountains MMC in the early stages of crustal extension in the Death Valley, California region. If current deformation rates and structural geometries persist in the Coso Range for several million years, sufficient extension may occur to exhume and expose rocks currently at depths of 4-6 km in the vicinity of the geothermal field. Speculatively, the bedrock of such a future “Coso Range” would consist of ductily deformed and intruded metamorphic rocks that structurally overlain by remnants of a hydrothermally altered brittle carapace, including blocks bounded by low-angle faults.