DIRECTLY DATING COMPLEX DEFORMATION VIA STRUCTURALLY INTEGRATED MULTI-MINERAL U-PB PETROCHRONOLOGY AND (U-TH)/HE THERMOCHRONOLOGY (SE CALIFORNIA AND WEST-CENTRAL ARIZONA, USA)

Across the globe, many orogens preserve multiple generations of deformation, often as plastic fabrics with similar kinematics, direction, and conditions of deformation. The temporal differentiation of such fabrics in these regions is paramount to discerning strain accumulation through time. Recent advances in geochronology and microstructural analytical techniques have made it possible to unravel the timing of similar fabrics in complex or poly-deformed regions. We apply an integrated approach of in-situ and grain-mount petrochronology, thermochronology, and multi-scale structural context to date deformation in the Cretaceous top-NE and top-SW Maria fold-and-thrust belt (MFTB) of southeastern California and west-central Arizona, as well as understand their relationship to the Miocene formation of metamorphic core complexes in the top-NE Colorado River extensional corridor. In particular, we focus on top-NE greenschist-amphibolite facies mylonitic shear zones in the Big Maria, Riverside, and Dome Rock/Moon Mountains.

Structural petrography and electron backscatter diffraction (EBSD) analyses of quartz and feldspar in these shear zones, coupled with upper greenschist-amphibolite facies mineral assemblages, are indicative of deformation conditions > 500°C. The apatite U-Pb system has a closure temperature ~450°C, making it an ideal thermochronometer to constrain shear zone deformation and formation of amphibolite- to upper greenschist-facies mylonitic fabrics. Apatite U-Pb cooling ages within Jurassic and Mesoproterozoic gneisses range from ~155-145 and ~125-105 Ma in the Riverside Mountains, ~75-58 Ma in the Big Maria Mountains, and ~85 Ma in the Dome Rock/Moon Mountains, indicating variable and distinct phases of mid-crustal cooling in different parts of the MFTB. Further, in-situ EBSD analyses and U-Pb petrochronology of pre-kinematic (magmatic) titanite also indicate the potential to constrain Late Cretaceous greenschist-amphibolite facies deformation. Zircon and apatite (U-Th)/He low-temperature thermochronology indicate shallow-crustal exhumation via a series of temporally and structurally distinct detachment systems in the Big Maria and Riverside Mountains in the Late Cretaceous-Paleocene and the Miocene.