FACILITATION OF PLEISTOCENE MAGMATISM THROUGH PLIOCENE THERMAL SOFTENING OF THE CRUST AT THE COSO GEOTHERMAL AREA, EAST-CENTRAL CALIFORNIA

The eastern flank of the Sierra Nevada hosts several volcanic fields that interact with faults along the western margin of the Basin and Range. The Coso volcanic and geothermal field is located in a complicated releasing bend in the tectonically active zone along the eastern escarpment of the southern Sierra Nevada. Dextral strike-slip faulting in southern Owens Valley and Indian Wells Valley is linked through the Coso Range by a combination of normal and strike-slip faulting with attendant crustal thinning. The heat source for the Coso geothermal field is relatively shallow intrusion of Pleistocene/Holocene rhyolite which erupted with asthenospherically derived basalts. We present evidence that localization of Pleistocene magmatism in the Coso Range may be the result of thermal softening of the crust by an ~1.5 million year-long effusion of Pliocene calc-alkaline lavas. Thermobarometry indicates that Pliocene basalts erupted from midcrustal magma bodies, whereas Pleistocene basalts erupted from lower-crustal sources. Trapping of Pliocene basalts in the mid-crust led to extensive crustal interaction and production of a mixed-magma suite. Thermal softening from this Pliocene event may have sufficiently perturbed crustal strength in the Coso Range to produce a stepover in the dextral strike-slip system. This stepover subsequently became the locus of crustal thinning which facilitated bimodal volcanism and resulted in formation of the shallow geothermal system.