COMPLEX CENOZOIC EXHUMATION HISTORY OF THE OROCOPIA SCHIST AND RELATED ROCKS OF SOUTHEASTERN CALIFORNIA AND SOUTHWESTERN ARIZONA

The Orocopia Schist, which is exposed along the Chocolate Mountains anticlinorium in southeastern California and southwestern Arizona, is paramount to understanding both Laramide compression and middle Cenozoic extension in the southwestern United States. The schist consists of Franciscan-like trench sediments and fragments of oceanic crust metamorphosed to albite-epidote amphibolite facies at depths of ~30−35 km. Detrital zircon studies indicate a minimum age of ca. 70 Ma for the schist protolith and thus the onset of underthrusting. The schist is overlain by Mesozoic (?) orthogneiss along the regionally-extensive, low-angle Chocolate Mountains fault. Previous ⁴⁰Ar/³⁹Ar analyses indicate that this structure is a ca. 50 Ma normal fault. A second low-angle normal fault system, the Gatuna-Sortan fault, separates the orthogneiss and Orocopia Schist from lower-greenschist-facies supracrustal units of the Winterhaven Formation. Prior thermochronology and new ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He ages indicate that slip on the Gatuna-Sortan fault system occurred at ca. 28−22 Ma. In this study, we examined the contact between orthogneiss and Winterhaven Formation in a small antiform located 30 km north of Yuma, Az. Based on regional structural relations and thermochronology, this contact is predicted to be a middle Cenozoic detachment fault. Within the study area, the orthogneiss-Winterhaven contact is marked by the presence of an altered granitoid rock. Some previous workers have inferred that this granitoid intruded both the orthogneiss and Winterhaven Formation and have further suggested that the granitoid is a correlative of the nearby Jurassic granite of Marcus Wash. If so, then this contact is too old to be a middle Cenozoic detachment fault. We have remapped the contact and have found that (1) granitoid in contact with the Winterhaven Formation tends to be highly cataclastic and (2) much of what was previously mapped as granitoid is instead highly altered metadacite from the base of the Winterhaven Formation. We conclude from these relations that the contact is most likely a detachment fault. Zircon U-Pb dating currently in progress will help determine whether the granitoid is simply a felsic lens within the orthogneiss plate or perhaps a middle Cenozoic intrusion along the detachment fault.