STRATIGRAPHY AND DETRITAL ZIRCON GEOCHRONOLOGY OF THE EL PASO MOUNTAINS PERMIAN METASEDIMENTARY SEQUENCE, SOUTHERN CALIFORNIA: A UNIQUE RECORD OF ARC EMERGENCE ALONG SOUTHWESTERN LAURENTIA

Permian metasedimentary strata in the central El Paso Mountains (EPM), southern California, were deposited contemporaneously with the inception of subduction and the development of a volcanic arc along southwestern Laurentia, and are purportedly the only record of this major transition in Californian tectonics. It is debated whether they were deposited offshore to the west, or entrained in a late Paleozoic sinistral transform system that displaced crustal blocks from the northwest. We present new stratigraphy, sedimentary provenance, and U-Pb detrital zircon data that elucidate the paleogeography of this tectonically active continental margin.

Over 2500 m of Permian strata were measured in three sections and divided into five members: Ph1 – argillite, conglomerate and coarse carbonate turbidites rich in recycled chert clasts, interpreted as submarine canyon and fan deposits; Ph2 – conglomerate and lithic arenite lenses derived from recycled marine sediments within argillite, interpreted as channelized distal shelf deposits; Ph3 – silty, calcareous turbidites and fossiliferous limestone, interpreted as shelf deposits; Ph4 – cross-bedded feldspathic arenite, epiclastic sandstone and conglomerate, and argillite, interpreted as proximal shelf deposits; and Pgg – andesitic lava flows interbedded with Ph4 shelf strata.

Detrital zircon data from seven EPM sandstone horizons indicate that deposition occurred from ca. 275-255 Ma, with an evolving provenance of three principle zircon sources. Ph2 lithic arenites yielded ca. 330-280 Ma grains and Precambrian populations resembling those of the Roberts Mountains allochthon, whereas Ph4 feldspathic arenites yielded unimodal, ca. 265-255 Ma peaks. Paleodrainages likely recycled sediment from the Roberts Mountains allochthon and an outboard arc system prior to 256 Ma. The emergence of a new volcanic arc is indicated by the predominance of late Permian zircons in Ph4. The disappearance of Precambrian grains was likely due to subsidence of their source and/or the construction of a topographical barrier at the arc highland. Our data support the hypothesis that these are allochthonous marine deposits with equivocal transport history, recording a shift from transform to subduction zone tectonics along southwestern Laurentia from middle to late Permian time.