35 Pennsylvanian to Early Triassic Tectonics of Southwest Laurentia (Posters)

Wednesday, 16 May 2018
8:30 AM - 6:30 PM
Outdoor Tents

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

Paper number: 35-3
Program: Theme Sessions
Time: 8:30 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-314394

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