Abstract
Investigation of the Coso Range using seismicity, gravity, and geochemistry of rocks and fluids, supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex. The structural setting is a releasing bend in a dextral strike-slip system that extends from the Indian Wells Valley northward into the Owens Valley. This tectonic setting results in NW-directed transtension, which is accommodated by normal and strike-slip faulting of the brittle upper 4–6 km of the crust, and shearing and ductile stretching below this depth, accompanied by shallow igneous intrusions. Focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the Coso Range exhibit depth-dependent rotation of seismic P and T axes, indicating that the local orientations of the principal stresses likely favor resolved shear stress on low-angle faults. These small earthquakes occur near the base of seismicity, which we interpret as coincident with the brittle-ductile transition. Geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids, indicating that the crust is thinned within the Coso structure. Thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less. This is consistent with observed gravity anomalies and models. The mafic body may represent cumulates left over from the fractional crystallization of rhyolite, which occurs as endogenous domes at Coso, or it could be a sheeted-dike complex in the upper mid-crustal area. Transtension began at 2–3 Ma, and continues today. Using a long-term crustal deformation rate of 2 mm/yr, we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years.
