TECTONIC EVOLUTION OF THE NORTHERN PANAMINT RANGE, SOUTHEASTERN CALIFORNIA: EVIDENCE FOR SYNCONVERGENT LATE CRETACEOUS EXTENSIONAL COLLAPSE OF THE SEVIER-LARAMIDE OROGEN

It has become increasingly evident throughout the Mojave and, to a lesser extent, the Great Basin regions that the Sevier hinterland experienced widespread Late Cretaceous extension; however, the primary mechanism(s) driving extension remains controversial. Evaluating mechanisms requires understanding the timing, kinematics and geographic distribution. Here, through integration of microstructural and field observations with U/Pb geochronology and ⁴⁰Ar/³⁹Ar thermochronology, we reinterpret a “Neogene” top-NNW extensional shear zone in the northern Panamint Range as Late Cretaceous. The Skidoo pluton and granitic dikes are locally involved with this deformation and yield zircon U/Pb ages of 85.6 and 94.1 Ma respectively. Thin sections of quartz-rich mylonitic rocks show BLG II and SGR recrystallization textures, suggesting deformation temperatures ca. 350-425°C. Samples were collected for ⁴⁰Ar/³⁹Ar thermochronology along a transect parallel to the shear direction. Muscovite from the southern outcrops of the Skidoo granite yields an 85 Ma plateau age and the K-spar age spectrum gradually climbs from 40 to 75 Ma. Muscovite from samples collected 15-20 km NNW at Tucki Mountain yield plateau ages between 75 and 68 Ma and K-spar small-domain ages are as young as 8 Ma. We applied the MDD model to both muscovite and K-spar, which provides a continuous thermal history from ~400 to 150°C. Modeled samples from Tucki Mountain show enhanced cooling (10-20°C/myr) from >400 to 250°C between 75 and 60 Ma. The modeling shows these rocks remained approximately isothermal at 250°C from 60 to 8 Ma until the onset of rapid cooling (30-50°C/myr), interpreted to be related to footwall exhumation along the Neogene Emigrant detachment fault. These findings suggest that the ductile top-NNW shear fabric at Tucki Mountain developed during Late Cretaceous cooling/exhumation. Our results revise the tectonic history of the northern Panamint Range and allow for the evaluation of mechanisms responsible for synconvergent Late Cretaceous extension. These findings are consistent with the idea that synconvergent extension may have been a response to surface uplift during removal of mantle lithosphere to the south along the Laramide flat slab corridor.