PERMIAN STRATA IN THE EL PASO MOUNTAINS, SOUTHERN CALIFORNIA, RECORD PALEODRAINAGE REORGANIZATION IN SOUTHWESTERN LAURENTIA AND THE EMERGENCE OF THE EARLY CORDILLERAN ARC

Permian metasedimentary strata in the central El Paso Mountains were deposited contemporaneously with the restructuring of the SW Laurentian margin from a strike-slip to a subduction regime. The tectonic setting of the basin is not well constrained; it is debated whether the strata were deposited offshore to the west, or entrained in a NW-striking sinistral transform system. Stratigraphy and detrital zircon (DZ) U-Pb ages and Hf isotope data were used to elucidate the paleogeography of the SW margin during a critical time interval. We sampled a lower member (Ph2), which includes lithic arenite lenses and conglomerate rich in deep-marine sedimentary rock fragments and an upper member (Ph4), which includes cross-bedded feldspathic arenite, volcaniclastic strata and argillite, interpreted as proximal shelf deposits. Maximum depositional ages estimated from the youngest DZ populations in each of six samples range from ca. 275-250 Ma. Ph2 lithic arenites yielded Precambrian populations at ca. 1.1 Ga, 1.8 Ga, and 2.6 Ga, and distinctive populations of grains with ca. 330-280 Ma ages. Ph4 feldspathic arenites yielded unimodal, ca. 260 Ma peaks. Precambrian DZ grains in Ph2 strata suggest a continental North America provenance, similar to those of the Roberts Mountains allochthon. Provenance of the ca. 330-280 Ma DZ grains is uncertain; they predate the emergence of igneous arc activity in the Cordillera and are not represented in other regional Paleozoic – Mesozoic strata. Hf signatures in the ca. 330-280 Ma DZ grains are evolved (EHf(T) = -5 to -20), precluding derivation from outboard fringing arc system(s). A possible source is the Ouachita – Marathon orogenic belt , sediment from which may have been distributed across or along southern Laurentia. Ph4 strata recorded a dramatic change in provenance signaling a reorganization of the sediment dispersal system with the emergence of the early Cordilleran arc. DZ grains with tightly clustered, ca. 260 Ma ages have corresponding EHf(T) values ranging from -1 to +6 and match well with local arc plutons. Ph2 strata were likely deposited in a channelized distal shelf setting with drainage connections to the continental interior. Those connections were beheaded with the emergence of an arc highland that created a topographic barrier and limited provenance to local arc sources.