LATE CRETACEOUS-EOCENE EXTENSIONAL SHEAR ZONES IN THE EASTERN MOJAVE REGION: EVALUATING POSSIBLE DRIVERS OF EXTENSION

In the eastern Mojave Region, Miocene-aged metamorphic core complexes and normal faults have exhumed rocks that preserve a complex geologic history that includes Jurassic, Cretaceous, and Cretaceous-Eocene events associated with Central and Southern Cordilleran tectonics. Of particular interest is the record of Cretaceous-Eocene tectonics, as the plate organization and geodynamic configuration of this time remain debated. The two end-member models are a) the onset of and/or change in flat-slab subduction of the Farallon plate and b) the oblique convergence, collision, and dextral transform motion of the Insular Terrane (the so-called “hit-and-run” scenario). We weigh these models, and others, by considering our low- and high-temperature thermochronology, geochronology, and structural analysis of dominantly top-NE extensional shear zones in the Big Maria and Riverside Mountains of southeast California. Localized shear zones in the Big Maria Mountains yield exclusively Late Cretaceous-Eocene (ca. 75-58 Ma) apatite U-pb cooling ages (closure temperature ca. 450°C). In-situ EBSD analyses and U-Pb petrochronology of pre-kinematic titanite from the same shear zones also partially preserve Late Cretaceous U-Pb ages (~75-65 Ma) and greenschist-amphibolite facies deformation. However, the Riverside Mountains, ca. 5 km to the north, lack this cooling age population and instead yield two different apatite U-Pb age populations in the Jurassic (ca. 155-145 Ma) and Cretaceous (ca. 125-105 Ma) within Jurassic and Mesoproterozoic rocks with distributed gneissic and lower-temperature top-NE normal-sense shear fabrics. This suggests that this block was significantly cooler and shallower than the Big Maria Mountains by the Late Cretaceous. We present possible expectations from the end member geodynamic models, evaluate whether our record of deformation could be explained with either or both, and point to future avenues of research to build the geologic record of this crucial time.