Stratigraphy and Detrital Zircon U-Pb Geochronology of the El Paso Mountains Permian Metasedimentary Sequence, Southern California

Permian metasedimentary strata in the central El Paso Mountains, southern California, were deposited contemporaneously with the inception of subduction and the development of a volcanic arc along southwestern Laurentia, and may be one of the only records 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 prior to east-west contraction by subduction zone tectonics. In this study, new stratigraphy, sedimentary provenance, and U-Pb detrital zircon data elucidate the paleogeography of this tectonically active continental margin, and contribute to broader reconstructions of the North American Cordillera. Over 2500 m of Permian strata in three stratigraphic sections were divided into five informal units: Ph1-Ph4 and Pgg, from oldest to youngest. Ph1 consists of argillite, conglomerate and conglomeratic and sandy limestone rich in recycled chert clasts, interpreted as a submarine fan deposits. Ph2 consists primarily of conglomerate and litharenite lenses derived from recycled marine sedimentary rocks, interpreted as channelized submarine canyon, and fan and/or base-of-slope deposits. Ph3 consists of silty, normally-graded carbonate beds and fossiliferous limestone, interpreted as a turbidite sequence deposited above the carbonate compensation depth. Ph4 consists of feldsarenite, volcaniclastic arenite and volcanic-bearing conglomerate, carbonate beds, and argillite, interpreted as continental shelf deposits. Ph4 is capped by and interbedded with Pgg andesitic lavas. Conglomerate clast counts and sandstone point counts show an up-section diversification of detritus entering the basin, from primarily recycled chert in Ph1 to a mixture of recycled sedimentary and volcanic lithic clasts in Ph4 deposits. Detrital zircon data from six sandstone horizons indicate that the Permian strata of the El Paso Mountains were deposited from ca. 280-255 Ma. Detrital zircon populations, which are interpreted to derive from three sources, show changing provenance up-section. Ph2 sedimentary litharenites yielded primarily ca. 330-280 Ma zircons derived from a Panthalassan island arc system and Precambrian grains recycled from Paleozoic sedimentary rocks of the Cordilleran passive margin. In contrast, Ph4 feldsarenites yielded primarily ca. 277-255 Ma zircons, interpreted to derive from a nascent continental arc along southwestern Laurentia. The disappearance of Precambrian grains in Ph4 sandstones was likely due to subsidence of their source(s) and/or the construction of a topographical barrier along the new continental arc axis. The results of this study support the hypothesis that this sequence is composed of allochthonous marine sedimentary rocks and lavas with equivocal transport history, recording a shift from transform to subduction zone tectonics along southwestern Laurentia from middle to late Permian time.