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Investigation of the Coso Range using seismicity, gravity, and geochemistry of rocks and fl uids, 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 signifi cant asthenospheric infl uence in the isotopic composition of rocks and fl uids, indicating that the crust is thinned within the Coso structure. Thinned upper crust is underlain by a more dense mafi c body at depths of 10 km or less. This is consistent with observed gravity anomalies and models. The mafi c 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.

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Space-based geodetic observations show that the Coso Range and Indian WellsValley lie along the southeastern margin of the Sierra Nevada-Central Valley (i.e., "Sierran") microplate, which moves ϳ13-14 mm/yr northwest with respect to stable North America. Detailed kinematic analysis of seismicity indicates that active crustal extension in the Coso Range occurs in a right-lateral transtensional regime along the eastern border of the Sierran microplate. The Airport Lake fault in the northern Indian Wells Valley and the Owens Valley fault are the major strike-slip faults along the eastern margin of the Sierran microplate south and north, respectively, of the Coso Range. Patterns of seismogenic deformation and Quaternary faulting indicate that dextral shear passes through the Coso Range in a right-releasing stepover between the Airport Lake and Owens Valley faults. Extension within the stepover region is accommodated in part by opening of Coso Wash as a pull-apart basin. The stepover is bounded on the east by a blind, northwest-striking dextral fault that is well expressed by patterns of microseismicity. Comparison with analogue sandbox models of pull-apart basins suggests that the Coso stepover is a relatively immature structure, consistent with models for a westward step in the locus of dextral shear along the eastern margin of the Sierran microplate to the Indian Wells Valley and Owens Valley in the past 2-3 m.y.

The Garlock fault is an integral part of the plate-boundary deformation system inboard of the San Andreas fault (California, USA); however, the Garlock is transversely oriented and has the opposite sense of shear. The slip history of the Garlock is critical for interpreting the deformation of the through-going dextral shear of the Walker Lane belt-Eastern California shear zone. The Lava Mountains-Summit Range (LMSR), located along the central Garlock fault, is a Miocene volcanic center that holds the key to unraveling the fault slip and development of the Garlock. The LMSR is also located at the intersection of the NNW-striking dextral Blackwater fault and contains several sinistral WSW-striking structures that provide a framework for establishing the relationship between the sinistral Garlock fault system and the dextral Eastern California shear zone. New fi eld mapping and geochronology data ( 40 Ar/ 39 Ar and U-Pb) show fi ve distinct suites of volcanic-sedimentary rock units in the LMSR overlain by Pliocene exotic-clast conglomerates. This stratigraphy coupled with fi fteen fault slip markers defi ne a three-stage history for the central Garlock fault system of 11-7 Ma, 7-3.8 Ma, and 3.8-0 Ma. Pliocene to recent slip occurs in a ~12-km-wide zone and accounts for ~33 km or 51% of the total 64 km of left-lateral offset on the Garlock fault in the vicinity of the LMSR since 3.8 Ma. This history yields slip rates of 6-9 mm/yr for the younger stage and slower rates for older stages. The LMSR internally accommodates northwest-directed dextral slip associated with the Eastern California shear zone-Walker Lane belt via multiple processes of lateral tectonic escape, folding, normal faulting, and the creation of new faults. The geologic slip rates for the Garlock fault in the LMSR match with and explain along-strike variations in neotectonic rates.