DYNAMICS OF THE NASCENT COSO METAMORPHIC CORE COMPLEX, EAST-CENTRAL CALIFORNIA

The Coso Range is situated in a releasing bend tectonic setting between two dextral strike-slip faults: the Little Lake fault to the south and an unnamed (Owens Valley?) fault to the north. Monastero et al. (1997) proposed that the Coso Range is a nascent metamorphic core complex. The range is moving with the Sierra Nevada/Great Valley microplate on a N48°W azimuth at a rate of ~12 mm/yr. This provides ~5-8° divergence across the structure inasmuch as the orientations of the bounding strike-slip faults are approximately N40-43°W. Thus, the Coso Range is currently undergoing transtensional deformation, locally partitioned into brittle upper crustal faulting underlain by ductile deformation. The interface between the two regimes is interpreted to be the brittle-ductile transition which corresponds to a seismic reflector that occurs at about 4 km beneath the geothermal production area. The reflector coincides with the seismic-aseismic boundary at this location, an argument that is strengthened by down-hole temperature measurements as high as 350°C at 3.5 km depth. GPS crustal velocity data acquired over a ten-year period show that there is a minimum of 6.5 mm yr^-1 of NW-directed dextral shear across the range. Using presumed horizontal dimensions of the entire releasing bend structure, and varying the assumption regarding how much of the crust is involved in vertical compensation, we estimate tectonic strain rates on the order of 10^-14s^-1to 10^-15s^-1, and associated subsidence rates ranging from 1 to 5 mm yr^-1. These values are comparable to those found along active plate boundaries. Vasco et al. (2002) analyzed INSAR data over that same ten-year period showing that there has been approximately ~35 mm/yr of subsidence over the area of maximum geothermal fluid withdrawal, with the largest percentage of the volume change occurring in the top 2.5 km. The corollary to this finding is that a small percentage of the vertical change is due to a broader tectonic signal which is consistent with our simple model.