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

The Late Cretaceous-early Tertiary Orocopia Schist (OS) of SE California is an oceanic unit that is overlain by an upper plate of continental affinity. Both units are in contact along the Chocolate Mountains fault, which has been interpreted as a subduction thrust responsible for the burial and prograde metamorphism of the OS. The oceanic origin of the OS and observation of NE transport of upper-plate rocks were interpreted as evidence of a collision between North America and an outboard continental fragment along a SW-dipping subduction zone. An alternative model proposes that the Chocolate Mountains fault is related to unroofing of the OS. This study provides new evidence to confirm this latter view;A zone of deformation is present at top of the OS with structures similarly oriented to those in the upper plate but different from ones in structurally deep schist. This zone of deformation is associated with several generation of shear zones and mylonites that originated in a ductile environment and were progressively overprinted by brittle deformation. These structures developed during retrogressive metamorphism as indicated by replacement of garnet by chlorite and amphibole by actinolite, and are considered to record the exhumation rather than the subduction of the OS, probably during early Tertiary time. The Gatuna fault, which places gneisses of the upper plate against low-grade supracrustal metasediments of the Winterhaven Formation, also seems to have been active at this time;Estimation of pressure and temperature of metamorphism using traditional thermobarometry and also qualitative indicators (amphibole composition) indicates a gap in metamorphic conditions between upper-plate rocks and the OS. The schist belongs to a higher pressure facies series and its peak metamorphic temperature was lower than in the upper plate. These findings indicate that the Chocolate Mountains fault has a complex reactivation history and that the present contact originated during exhumation of the OS. A two-stage exhumation history of the OS is proposed, with onset of extension and emplacement of OS and upper-plate rocks at middle-crustal levels in early Tertiary time. The second stage was the final unroofing of the OS, probably during the middle to late Tertiary