Metamorphic history and structural evolution of the Orocopia Schist in the Gavilan Hills, SE California

Abstract

The Chocolate Mountains fault in southeastern California has been considered the original structure that underplated oceanic Orocopia Schist beneath plutonic and metamorphic rocks of the continental U.S. Estimations of pressure and temperature of metamorphism in both plates of the fault in the Gavial Hills show a gap in metamorphic conditions indicative of post-subduction reactivation. Gamet-biotite and gamet-homblende geothermometry show that the Orocopia Schist originated at a lower temperature than the upper plate. Pressure determinations using the phengite and gamet-homblende-plagioclase barometers confirm a subduction-comple.x origin for the Orocopia Schist. Amphibole compositional parameters show that the schist have originated at a lower temperature and at higher pressure than the upper plate. The petrological evidence agrees with structural observations that the Chocolate Mountains fault has undergone substantial postmetamorphic reactivation and that the present contact between the Orocopia Schist and upper plate records exhumation rather than subduction. Introduction The Chocolate Mountains fault is part of the regional Vincent-Chocolate Mountains (VCM) fault system of southem California and southwestemmost Arizona, which separates a lower plate of oceanic Pelona-Orocopia-Rand (FOR) schist from an upper plate of igneous