Significance of Newly Discovered Subduction Complex, Including Partially Serpentinized Mantle Peridotite, Southwest Arizona

Iowa State University, Ames IA

Investigators

Abstract

The Late Cretaceous to early Paleogene Pelona, Orocopia, and Rand Schists of southern California and southwest Arizona represent an extensive subduction complex underplated beneath the continental crust of southwest North America during low-angle northeastward subduction, coeval with the Laramide orogeny. Several important aspects of the low-angle subduction process are poorly understood, mainly because only a small fraction of the subducted schist is exposed at the surface. The field of schists available for study was significantly widened in 2012, when we discovered an additional exposure of Orocopia Schist unusually far inland--at Cemetery Ridge, 90 kilometers west of the outskirts of greater Phoenix. Determination of uranium-lead detrital-zircon ages and argon-argon metamorphic ages for the schist at Cemetery Ridge will reveal whether the Pelona, Orocopia, and Rand Schists decreases in age to the southeast, parallel to the former subduction margin; or to the northeast, perpendicular to the former margin. Resolution of this question will allow a choice between two widely discussed tectonic models: progressive inboard subduction erosion or migration of an aseismic ridge parallel to the subduction trench. Furthermore, field and Argon-Argon study of exhumation faults at Cemetery Ridge will contribute to understanding of processes whereby deeply subducted rocks are returned to the upper crust and surface. Unexpectedly, the Orocopia Schist at Cemetery Ridge includes numerous blocks and fragments of oceanic or continental-margin mantle peridotite, chiefly serpentinized dunite and partially serpentinized harzburgite to olivine orthopyroxenite. Much of the peridotite is converted to actinolite-rich metasomatic rocks, but some contains abundant orthopyroxene and a little relict olivine. The well-preserved ultramafic rocks and minerals at Cemetery Ridge are unique in the Pelona, Orocopia, and Rand Schists, and their presence so far inland is remarkable. We will use field, petrographic, geochemical, and electron microprobe techniques to determine the origin of the peridotite, and how it was incorporated into and transported with the Orocopia Schist. In particular, we want to know whether it more closely resembles abyssal or mantle-wedge (suprasubduction) peridotite. Study of metasomatized peridotite at Cemetery Ridge will reveal details of chemical interactions between crustally derived (meta)sedimentary rocks and mantle peridotite within a low-angle subduction complex. This project has two major goals. First, we will determine the ages of rocks in and processes that formed the subduction complex at Cemetery Ridge; these ages are fundamental to understanding its origin. Second, we will study the character and origin of the remarkable peridotites at Cemetery Ridge, in order to determine how these oceanic rocks were transported so far inland. Our project will contribute to knowledge of mechanisms of low-angle subduction in southwest North America. In particular, low-angle subduction is intimately related to formation of the Rocky Mountains, which dominate the natural and human geography of much of the American West. More generally, study of the subduction complex exposed on-land in southern California and southwest Arizona will complement evidence from marine geology and geophysics and further understanding of subduction on a global scale. In addition to the scientific goals of the research, this project is contributing to the training of undergraduate students from Iowa State University in all aspects of the research, including field and laboratory work, as well as presenting the results of the research at professional society meetings. Results of the research will also be incorporated into a workshop for middle and high school students on the tectonics of western North America that will be taught at the University of Iowa's geology field station in Wyoming. The project involves collaboration with UCSC and Stanford for geochemical and isotopic analyses.