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The Pelona-Orocopia-Rand (POR) schists are metamorphic complexes that crop out along a belt extending from the southern Sierra Nevada to southwestern Arizona. The schists are considered to be correlated with the Franciscan subduction complex and believed to have been emplaced beneath crystalline basement of North America along a low-angle NE-dipping Late Cretaceous-early Cenozoic subduction zone related to the Laramide orogeny. Exhumation of the POR schists appears to have occurred in two main phases, as indicated by 40 Ar/ 39 Ar thermochronology. A first episode of cooling of the schists during the Late Cretaceous to early Cenozoic may have been driven by both extensional faulting and erosion. A second phase of cooling during the middle Cenozoic is thought to be related to an extensional event that affected much of western North America. One of the most critical places for understanding the tectonic evolution of the POR schists is the Gavilan Hills area of the southeasternmost California. The Orocopia Schist in that area forms the lower plate of the Chocolate Mountains fault and its exposed structural thickness is about 300 m. Above the schist is a thin slice of quartzofeldspathic to amphibolitic gneisses derived from middle to deep levels of the Mesozoic North American magmatic arc. The gneisses are, in turn, separated from overlying low-grade metasedimentary and metavolcanic rocks of the Winterhaven Formation by the Gatuna fault. Previous 40 Ar/ 39 Ar dating indicates that the Chocolate Mountains fault in the Gavilan Hills is early Cenozoic in age. The goal of this study was to constrain the age of the structurally higher Gatuna fault. To accomplish this, (U-Th)/He thermochronological analyses were performed on single detrital zircons from Orocopia Schist and Winterhaven Formation in the Gavilan Hills, an unnamed set of hills due east of the Gavilan Hills, and the Castle Dome Mountains. The ages obtained so far are 22.4-24.8 Ma for the Orocopia Schist and 39.7 Ma for the Winterhaven Formation. The dissimilar cooling histories of the Orocopia Schist and Winterhaven Formation prior to ca. 22 Ma confirm the hypothesis that the Gatuna fault is middle Cenozoic in age.

Multi-stage exhumation history of the Orocopia schist of southern California and southwestern Arizona