CENOZOIC EXHUMATION OF THE MOJAVE AND SONORAN DESERT REGION: INSIGHTS INTO LARAMIDE BULLDOZING IN THE SOUTHWESTERN US

The Late Jurassic–Eocene time in the North American Cordillera is characterized by dramatic tectonic, paleogeographic, and paleoenvironmental changes including the transition from normal to flat-slab subduction. The development of the southwestern segment of the North American Cordillera during Late Cretaceous–Eocene Laramide flat slab subduction remains relatively poorly understood compared to the rest of the Cordilleran system. The Cretaceous McCoy basin (~136–74 Ma), Maria fold-thrust belt, Mule Mountains thrust, and underplated Pelona-Orocopia-Rand schists provide natural laboratories for our understanding of the Late Cretaceous–Eocene tectono-thermal history of the southwestern U.S. The amount of post-depositional heating due to burial and the timing of regional cooling and exhumation of the McCoy basin and surrounding regions in southeastern California-southwestern Arizona can be resolved using zircon (U-Th)/He (ZHe) thermochronology, which allows for determination of the time-temperature history of basin strata through the ~130–190 ºC temperature window. We present 15 new ZHe dates from sandstone samples collected at the base, middle, and top of the ~7 km thick McCoy Mountains Formation and 71 new dates from basement samples in the Mule, Little Maria, and Big Maria Mountains. These results reveal a major phase of regional cooling and exhumation during the early Cenozoic (~60–55 Ma), consistent with contemporaneous underplating and initial exhumation of the Pelona-Orocopia-Rand schists. Together with recent work identifying rapid cooling in the Transition Zone (~70–50 Ma) and the presence of arclogitic xenoliths of possible Mojave batholith keel origin in ~25 Ma west-central Arizona Transition Zone volcanic rocks, we interpret the Paleocene–Eocene phase of cooling to be associated with northeastward bulldozing of continental lower crust and mantle lithosphere by the Farallon flat slab.