SEDIMENTATION AND GEOCHRONOLOGY OF PERMIAN STRATA, INYO MOUNTAINS, CA, AND RELATION TO ONSET OF SUBDUCTION

In Pennsylvanian to Permian time, the passive margin along Laurentia’s west coast transitioned from transpressional and transcurrent faulting into a subduction zone and magmatic arc, but the precise timing of initial arc magmatism, resulting tectonic-basin formation, and the sedimentary record of subduction initiation are not fully constrained. The Inyo Mountains in eastern California are a rare location in which exposed Permian sedimentary rocks yield detrital zircons from initial arc magmatism. A previous detrital zircon study of the upper Reward Conglomerate member of the Lone Pine Formation in the Inyo Mountains yielded a Permian maximum depositional age of 266 +/- 2 Ma, likely associated with incipient Cordilleran arc magmatism. To constrain the timing of initiation of arc magmatism, we collected and analyzed six new detrital zircon samples from underlying strata. All samples yielded only pre-Permian grains, suggesting that the Cordilleran arc did not initiate at the latitude of the Inyo Mountains until late Permian time, ~266 Ma. Detrital zircon spectra are similar among all six samples, showing prominent spikes at ~300-600 Ma, ~900-1300 Ma, and ~1500-1800 Ma, along with several Archean grains. Sources for these grains are interpreted to be Appalachia, Grenville, Yavapai/Mazatzal, and the Wyoming or Superior cratons, respectively. Preliminary trace element signatures in zircons suggest that 300-400 Ma grains are derived from Appalachian continental crust rather than from island arcs accreted to the west coast of Laurentia. Permian sedimentary facies in the Inyo Mountains indicate gradual shoaling throughout the Permian, with arc zircons appearing at ~266 Ma in strata interpreted as shallow marine to subaerial. This pattern partially aligns with models of the upper-plate sedimentary record of induced subduction initiation, which show rapid uplift followed by rapid subsidence, and finally gradual shoaling with the building of an arc.