DATING DUCTILE DEFORMATION IN THE MARIA FOLD-AND-THRUST BELT WITH APATITE AND ZIRCON U-PB GEOCHRONOMETRY, BIG MARIA AND RIVERSIDE MOUNTAINS, SOUTHEASTERN CALIFORNIA

The Maria fold-and-thrust belt (MFTB) in southeastern California is a unique S- to SW-verging segment of the Sevier to Laramide-age fold-and-thrust system of the North American Cordillera. The Big Maria Mountains preserve Cretaceous to Paleocene contractional and extensional deformation of the MFTB that was not overprinted by subsequent Miocene extension within the Colorado River Extensional Corridor (CREC). Ductile deformation preserved in the Big Maria Mountains provides the opportunity to robustly constrain the timing of the cessation of Late Cretaceous contraction. Preliminary new zircon U-Pb crystallization ages of variably deformed, pre- to syn-kinematic Jurassic (~170-155 Ma) granitic plutons and of post-kinematic undeformed Cretaceous-Paleocene (~70-64 Ma) andesitic and granitic pegmatite dikes which cross-cut both the Mesozoic plutons and the highly attenuated and metamorphosed Paleozoic Grand Canyon sedimentary section exposed in the overturned Big Maria syncline bracket the age of ductile deformation. Further, apatite U-Pb analyses of top-NE greenschist-facies mylonitic shear bands within the Jurassic granitic plutons of the Big Maria Mountains yield ages of ~80 to 55 Ma, suggesting that the middle crust exposed in the MFTB cooled below ~450°C in the latest Cretaceous to earliest Paleocene and that these greenschist-facies shear fabrics are likely of Laramide affinity and age. To the north of the Big Maria Mountains, the Riverside Mountains preserve MFTB fabrics within the footwall of the Miocene Whipple-Riverside detachment fault system, providing the opportunity to examine and date mylonitic fabrics via in-situ titanite and apatite U-Pb petrochronology and hence temporally differentiate between contractional and extensional fabrics associated with Sevier to Laramide deformation and Miocene extensional deformation related to metamorphic core complex formation. Constraining deformation associated with the end of orogenesis in the North American Cordillera is critical to understanding the thermal history of the mid-crust and the role of tectonic inheritance in subsequent Miocene detachment faulting and metamorphic core complex formation in the CREC and the southern Basin and Range.