EXHUMATION OF THE COYOTE MOUNTAINS METAMORPHIC CORE COMPLEX (AZ): IMPLICATIONS FOR OROGENIC COLLAPSE OF THE SOUTHERN NORTH AMERICAN CORDILLERA

The Coyote Mountains is a metamorphic core complex that makes up the northern end of the 80-km long Baboquivari Mountain complex (Southern AZ). The Baboquivari Mountain complex is composed of Mesozoic and Cenozoic igneous and metasedimentary rocks, including the Pan Tak granite and associated pegmatite, a muscovite-biotite-garnet peraluminous granite. Previous work show that the Pan Tak Granite, where the Coyote Mountain detachment shear zone is localized, was emplaced in Late Cretaceous / Ealry Eocene (~58 Ma zircon crystallization age). Geochemistry of the Pan Tak granite and other intrusions within the Baboquivari Mountains suggest that it was formed by anatectic melt representing the culmination of a Laramide crustal shortening orogenic event that started in the Late Cretaceous, about ~70 Ma. Field evidence, as well as petrographic and microstructural analyses of metamorphic and deformational fabrics/structures show that the ~58 Ma intrusive/magmatic fabric is overprinted by a secondary tectonic event. This event is particularly well recorded in the northern part of the Coyote Mountains, where a north-dipping mylonitic shear zone is exposed. In this study investigates the tectonic of the Coyote Mountains through its microstructural record. Based on ⁴⁰Ar/³⁹Ar geochronology of muscovite, our results indicate that Tertiary mylonitization associated with the formation of the Coyote Mountains metamorphic core complex occurred at 29 Ma (Early Oligocene). Mylonitization is recorded by a suite of microstructures that suggest that the detachment shear zone evolved close to the brittle-ductile transition, at deformation conditions of ~500 ± 50°C, and high stress/high strain rate, and depth of ~10-13km. These preliminary results suggest that plutonism may have initiated thermal and mechanical weakening of the crust, which eventually triggered rapid extension.