Stratigraphic record of subduction initiation in the Permian metasedimentary succession of the El Paso Mountains, California

Abstract

Petrologic investigation of Permian metasedimentary rocks in the El Paso Mountains reveals a rock record interpreted to be consistent with the sedimentary pattern of the upper continental plate of a nascent subduction zone, based on geodynamic modeling and comparison with a Cenozoic example (Puysegur Ridge, New Zealand). Facies changes reveal a history of uplift (conglomerate), followed by subsidence (carbonate turbidite deposits) and deeper-water sedimentation (argillite, with portions deposited below the carbonate compensation depth [CCD]), and then gradual shallowing accompanied by the onset of nearby intermediate volcanism (volcaniclastic and bioclastic sediments) and construction of a volcanic edifice (andesitic lavas) in a shallow-marine environment. Comparison with Permian global sea-level curves indicates that initial uplift (relative sea-level fall) followed by deep subsidence (relative sea-level rise) are likely due to tectonic rather than eustatic effects. Shallowing during volcaniclastic sedimentation could have been due to both arc edifice building and global sea-level fall. Sandstone modal analysis suggests that the basin evolved from a tectonic setting involving compressive uplift to an arc basin setting. Geodynamic modeling implies the involvement of a transform/truncation fault in subduction initiation. Magmatic trends based on Permian paleogeography and timing suggest a limited nucleation of subduction in the El Paso Mountains followed by propagation southward. Furthermore, subduction initiation modeling suggests regional lithospheric flexure that may be reflected in coeval basins and uplift in the northern Mojave, Death Valley, and Inyo Mountains regions as well as in coeval facies changes on the western edge of the Colorado Plateau. Overall, the Permian section of the El Paso Mountains may be one of the few preserved Paleozoic sedimentary records of subduction inception along a continental margin.