INVESTIGATING THE DEFORMATION TIMING AND KINEMATICS OF THE MYLONITIC SHEAR ZONE IN THE NORTH AMERICAN CORDILLERAN METAMORPHIC CORE COMPLEXES

Metamorphic core complexes (MCCs) in the North American Cordillera expose metamorphosed infrastructure in the lower plate juxtaposed against the brittle suprastructure in the upper plate. The fabrics in the infrastructure record polyphase deformation and metamorphism in the lower-to-middle crust associated with regional Mesozoic contraction and Cenozoic extension, which may represent mid-crustal thrusting, crustal flow, buoyant diapirism, and/or extensional shear. The timing, extent, kinematics, and existing mid-temperature thermochronology of these shear zones are equivocal due to multiple episodes of fabric generation and overprinting, along with Mesozoic-Cenozoic plutonism. Here, we combine field observations and apatite U-Pb petrochronology coupled with electron backscatter diffraction (EBSD) microstructure to directly constrain the deformation timing, temperature, and kinematics within the infrastructure of Catalina Mountains, Chemehuevi Mountains, and Ruby Mountains-East Humboldt Range (REHR) MCCs. Geologic mapping in the central Ruby Mountains provides insights on the geometry of the transition zone between the infrastructure and suprastructure. Preliminary apatite microstructure exhibits intra-grain dislocation creep, which can lead to resetting of the apatite petrochronometer. Crystallographic vorticity axis (CVA) is being derived on both apatite and bulk phase to ensure the petrochronometer is linked with the kinematics captured in the bulk fabrics. The apatite ages from the infrastructure of REHR and Chemehuevi MCC might reflect a signal of Eo-Oligocene deformation and late Cretaceous mylonitization, respectively, that are decoupled from the Miocene regional extension. In contrast, apatite ages in the Catalina MCC might represent deformation coeval with the late Oligocene-Miocene regional extension. Fabrics predating the regional extension can instead illustrate the presence of older phases of contractional structures and/or buoyant doming due to the widespread magmatism, crustal heating, and melting. Our integrative approach, combining apatite U-Pb petrochronology with EBSD microstructural and CVA analyses can enhance direct dating of complex deformation, including MCCs and other large-scale orogenic systems.