STRATIGRAPHY AND DETRITAL ZIRCON GEOCHRONOLOGY OF PERMIAN DEPOSITS WITHIN THE EL PASO MOUNTAINS, SOUTHERN CALIFORNIA: A UNIQUE RECORD OF SUBDUCTION INITIATION ALONG SOUTHWESTERN LAURENTIA

A metasedimentary and metavolcanic sequence exposed in the El Paso Mountains (EPM), southern California, was deposited contemporaneously with late Permian continental truncation along the southwestern margin of Laurentia. This sequence may be the only stratigraphic record of this important event in Cordilleran tectonics. We present new stratigraphic and U-Pb detrital zircon data that elucidate the evolving depositional and paleogeographic setting for Permian strata within the EPM, and contribute to tectonic reconstructions for the North American Cordillera.

Over 2.5 km of Permian strata were measured in three sections in the central EPM and were divided into five lithofacies associations: PhI - continental rise deposits consisting of massive conglomerates, coarse-grained carbonate turbidites, and argillites; PhII - slope to distal shelf deposits with massive conglomerates, litharenites, and argillites; PhIII-PhIV - shelf deposits consisting of carbonate turbidites, argillites, bedded fossiliferous limestones, feldspathic arenites, and volcaniclastic sandstones and conglomerates; and Pgg - lava flows of the Goler Gulch Andesite, interfingered with PhIV beds. Conglomerate and sandstone components become more compositionally diverse up-section, with primarily chert clasts in PhI, mixed sedimentary clasts in PhII, and a steady increase in arc-derived detritus throughout PhIII and PhIV.

New LA-ICP-MS ages of detrital zircons from Permian EPM sandstones show significant late Mississippian to late Permian populations, as well as minor Proterozoic peaks. Ages from a lower PhII litharenite are limited to a broad 280-330 Ma range that is possibly associated with an older, outboard arc. An upper PhII litharenite yielded a small group of ca. 1750 Ma grains, reflecting partial derivation from Yavapai continental basement and thus, proximity to Laurentia. The most prominent signature, from PhIV feldspathic and volcaniclastic beds, includes 255-275 Ma ages, indicating that arc-related magmatism had commenced in the vicinity of this basin by late Permian time. Thus, Permian EPM strata were likely derived from a marginal arc and isolated from the continent until the influx of cratonic and Permian zircons by ca. 275 Ma, implying tectonic reorganization or migration of the basin toward Laurentia.