KINEMATICS AND TIMING OF INTRACORE SHEAR ZONES IN THE FUNERAL MOUNTAINS METAMORPHIC CORE COMPLEX, DEATH VALLEY, CA

The Funeral Mountains metamorphic core complex (FMMCC) in Death Valley, California exposes middle to lower crustal rocks of the Sevier-Laramide orogeny in the footwall of the Miocene Boundary Canyon detachment. The structurally deepest rocks in the FMMCC are exposed in Monarch Canyon, where the Meso- to Neoproterozoic metasedimentary rocks record upper amphibolite facies metamorphism with migmatites developed at the deepest levels. Distributed ductile deformation and stratigraphically localized high-strain zones, termed intracore shear zones, are responsible for attenuation and local stratigraphic omission during top-northwest non-coaxial deformation. The structurally deepest Monarch Spring shear zone (MSSZ) juxtaposes the migmatitic paragneisses below against marbles, pelitic and calcsilicate schists above, and represents a deformed anatectic front. Below the MSSZ, migmatitic paragneisses lack similar top-northwest fabrics and instead exhibit a northeast-trending mineral lineation and local, strong fabric asymmetry indicative of a top-southwest shear. The relative contributions of Late Cretaceous-Early Tertiary and Miocene extensional strains which manifest in the top-northwest fabrics remain unclear, as all Late Cretaceous intrusions are deformed, and are being addressed by ongoing coupled thermochronologic, microstructural, and EBSD analyses. Our hypothesis is a polystage extensional history in the FMMCC, with Late Cretaceous intracore shear zones accommodating earlier and higher temperature extensional strain that are locally reactivated during the Miocene.

Preliminary EBSD and microstructural quartzite data show mixed <a> to prism <a> slip and subgrain rotation to grain-boundary migration mechanisms above the MSSZ, and mixed <a> slip and lower temperature subgrain rotation within the shear zone. One quartzite that displays mixed <a> slip, subgrain rotation, and a C-axis opening angle of ~55°, which suggests deformation temperatures of ~400-500°C, lies adjacent to marble yielding a ⁴⁰Ar/³⁹Ar muscovite age of 30 Ma, suggesting pre-30 Ma deformation. Additional EBSD and microstructural analyses will address the temperatures of deformation, and ongoing ⁴⁰Ar/³⁹Ar muscovite thermochronology will address the timing of deformation along the intracore shear zones.