Late Cretaceous Extensional Collapse of the Southern Cordillera: Evidence from the Bristol and Granite Mountains, SE California

Demonstrating the regional extent of Late Cretaceous extensional collapse of western North America’s southern Cordillera is important to understanding the tectonic evolution of the Sevier-Laramide orogens, and the geodynamics of ancient and modern orogens and synconvergent extension. Documenting Late Cretaceous extension in the southern Cordillera requires looking through the extensive overprint by Cenozoic structures. Late Cretaceous extension in the eastern Mojave region has been inferred from geochronology and thermochronology studies, which document Late Cretaceous cooling of Mesozoic granitoid rocks, some of which were emplaced in the middle crust. Cooling histories from these rocks have also been interpreted to be a result of lithospheric refrigeration, as well as erosional exhumation during the Laramide. New data from this study, combined with results from other studies, demonstrate that Late Cretaceous extension and exhumation of mid-crustal rocks was a major event in the southern Cordillera. A newly described (herein) mylonitic shear zone in the Bristol Mountains records a top-to-the-SW, down-dip, non-coaxial sense of shear. Furthermore, microstructural analysis indicates the shear zone recorded deformation temperatures at upper greenschist to lower amphibolite conditions (~350 to ~550 ˚C). Low-temperature overprint suggests progressive denudation of the shear zone into shallow crustal levels. U/Pb geochronology and 40Ar/39Ar thermochronology demonstrate that Cretaceous plutons were emplaced at ~75 Ma and cooled below K-feldspar MDD small domain closure temperatures by ~65 Ma. MDD modeling of K- feldspar from plutonic rocks show that following ductile shearing and rapid cooling, rocks continued to cool at rates of ~22 ˚C/m.y (footwall) and ~16 ˚C/m.y. (shear zone) Mylonitic deformation in the Bristol Mountains is bracketed from ~75 Ma to 65 Ma. Kinematic indicators from the Granite Mountains shear zones show a top-to-the-NW, down-dip, non-coaxial sense of shear. Distinct banding of dynamically recrystallized quartz and feldspar suggests the shear zone recorded deformation temperatures in the lower amphibolite facies (~400 to ~600˚C). Furthermore, U/Pb geochronology and 40Ar/39Ar thermochronology indicate that Cretaceous plutons were emplaced from ~80 Ma to ~75 Ma and were rapidly cooled through K-feldspar MDD closure temperatures by ~66 Ma. MDD modeling of K-feldpsar, within the footwall, suggest that Cretaceous plutons cooled at rates ranging from ~16 to 67 ˚C/m.y. Mylonitic deformation in the Granite Mountains is bracketed from ~80 Ma to 66 Ma. Data from the Bristol and Granite mountains indicate ductile shear zones unequivocally document extensional collapse of the Sevier retroarc in the Late Cretaceous. We advocate the removal of the North American lithospheric mantle as the root cause for synconvergent extension in the southern Cordillera as it best fits the geologic constraints. The process of delamination is considered the most viable mechanism for removal of the mantle lithosphere, leading to crustal anatexis, peraluminuous magmatism, and extensional collapse of the southern Cordillera, which was synchronous with continued contraction in the Sevier fold-thrust belt as well as the newly developed Laramide deformational belts