EVIDENCE FOR EARLY PERMIAN CONTRACTION AND BASIN FORMATION, AND LATE PERMIAN CORDILLERAN ARC INITIATION IN THE INYO MOUNTAINS, EASTERN CALIFORNIA

Permian sedimentary rocks in the Inyo Mountains, eastern California, were deposited at the tectonically unstable western margin of Laurentia during major tectonic regime change. These rocks record the transition from passive margin sedimentation to transpressional tectonics followed by subduction initiation and incipient Cordilleran arc magmatism by late Permian times. A paleo-uplift separating two Permian paleo-sub-basins has been previously documented through structural analysis and stratigraphic relations: Permian strata in the eastern and western sub-basins onlap a constructional high and anticline that folds the Pennsylvanian Keeler Canyon Fm. Sakmarian fusulinids in the basal strata of both sub-basins constrain the timing of uplift initiation to ~295-290 Ma. Permian sedimentary rocks in both sub-basins were measured and sampled for detrital zircon U-Pb geochronology and trace element geochemistry as well as sandstone petrography, in order to understand the provenance, depositional environments, and tectonic setting of these rocks. Detrital zircon in both sets of strata show typical Laurentian continent spectra with prominent spikes indicating sources in Appalachia, Grenville, Yavapai/Mazatzal, and the Wyoming or Superior cratons. In both sub-basins, the uppermost unit contains the first influx of late Permian Cordilleran-arc-derived detrital zircons, which constrain arc initiation to ~265 Ma. Sandstone petrography shows that rocks in the eastern sub-basin are dominated by carbonate grains likely deposited by turbidity currents from unstable carbonate shelves to the east, while sandstones in the western sub-basin lack these carbonate grains. The differing composition of rocks in the two sub-basins provides additional evidence for paleo-uplift, suggesting faulting and contraction in late Pennsylvanian to early Permian time possibly related to transpressional faulting of the California-Coahuila transform just prior to subduction initiation.