PERMIAN STRATA OF THE EL PASO MOUNTAINS, SOUTHERN CALIFORNIA, PROVIDE A RECORD OF SUBDUCTION INITIATION AND THE LOCAL EMERGENCE OF THE CORDILLERAN ARC

Permian metasedimentary strata of the El Paso Mountains, southern California, were deposited along the western margin of Laurentia during a critical plate-margin transition from a transform-dominated to a subduction system. As such, they provide an important record of this tectonic change, yet their depositional setting and provenance are not well constrained. These strata are also part of the peri-autochthonous El Paso terrane, whose tectonic assembly is debated. New sedimentological, detrital zircon U-Pb, trace element, and Lu-Hf isotopic results are used to interpret the provenance, paleogeography, and depositional environments of the El Paso terrane, and timing of the emergence of the local Cordilleran arc. Lithofacies and fossil assemblages indicate deposition of the Permian strata in a wholly marine environment that transitioned from deep marine (rise / base of slope) in Cisuralian time to shallow marine (shelf) in Lopingian time. Detrital zircon U-Pb age spectra reveal Proterozoic age modes at ca. 1.0, 1.65 and 1.8 Ga indicating connections between the lower, off-shelf strata with the continental Laurentian interior. This result is supported by modal analysis of sandstones which suggests derivation from a cratonal or recycled orogen source. The lower strata also contain cryptic Pennsylvanian to Permian-aged zircon populations (ca. 320 - 280 Ma) with highly evolved εHf isotopic values (0 to -30). The source of these grains is ambiguous, but the detrital zircon and sedimentological data are inconsistent with the El Paso terrane developing as a tectonic ribbon far removed from the southwestern Laurentian margin in Permian time. The sedimentology of the upper, near-shore strata record the emergence of a magmatic arc source, which is also reflected in unimodal detrital zircon populations that overlap with local Permian plutons in ages, Hf isotopic compositions, and zircon trace element characteristics. The absence of any pre-Permian zircon by ca. 255 Ma suggests that the El Paso Mountains region was cut off from interior Laurentian sources. These observations suggest that the emergent arc formed a topographic barrier and that strata were accumulating in a forearc setting.