EAR-PF: Unraveling the Paleogene exhumation history of the Orocopia Schist in west-central Arizona

Seymour, Nikki M, Fort Collins CO

Investigators

Abstract

<p>Dr. Nikki M. Seymour has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education plans at Stanford University and the University of California Santa Cruz under the mentorships of Dr. Martin Grove and Dr. Jeremy Hourigan respectively. This research project will determine the exhumation history of the northern Plomosa Mountains, located in west-central Arizona. The rocks exposed in the Plomosas originated as sediments deposited on the Pacific Ocean floor west of California and were subducted and added to the base of the North American continent during the late Cretaceous-Paleogene Laramide Orogeny, a period of shallow-angle subduction from ~90-50 Ma that has been tied to the end of Sierra Nevada magmatism, construction and subsequent collapse of high plateaus, uplift of basement-cored mountain ranges, and extensive volcanism. The top of the Orocopia Schist was exposed at the surface by the Plomosa detachment fault between ~21-15 Ma, meaning it would have been exposed at the surface after traveling more than 40 km through the crust to reach shallow crustal levels in the latest Cretaceous-Paleogene (~73-23 Ma). The thermal history that follows the buildup of sediment at the base of the crust during shallow-angle subduction can reveal how these rocks moved through the crust. This project will study metamorphic zircon rims from the Orocopia Schist to document the timing of sediment subduction and addition to the bottom of North America. However at present there are no constraints on the history that brought these rocks to the shallow crust prior to Miocene faulting (~21-15 Ma). As part of the broader impacts of this project Dr. Seymour will coordinate with San Francisco-based software developers to expand the capabilities of the Augmented Reality Topographic Sandbox to integrate geologic maps. Successfully integrating maps with topography in the sandbox will allow students to experiment with the 3D geometries of sedimentary formations and structures and will be widely applicable throughout all levels of geoscience education.</p> <p>Determining the exhumation history of the Orocopia Schist (OS) is critical to understanding the transition from Cretaceous shortening to Miocene extension, as well as the processes driving sediment underplating, accretion, and syn-subduction exhumation. Dr. Seymour will systematically investigate the temporal and thermal evolution of the Orocopia Schist subduction complex using meso- and microstructural, geo- and thermochronological, petrological, and geochemical analyses to understand (1) at what depth the OS was accreted and understand the processes that exhumed it, (2) the age and geochemical affinity of the crystalline gneiss that structurally overlies the OS and the significance of its tectonic contact with the OS, and (3) the exhumation histories of the gneiss and OS. Methods will include detailed mapping and microstructural study of the OS and the overlying gneiss at key locations to document crucial cross-cutting relationships, dating of syn-kinematic phases with clear kinematic context such as titanite for U-Pb and hornblende, phengite, and biotite for Ar/Ar thermochronology, and thermobarometry using Titanium-in-quartz, Raman spectroscopy of carbonaceous material, and oxygen stable isotope ratios for thermometry and garnet-biotite-plagioclase-quartz chemistries for barometry. This approach will document the number and significance of fabric generations preserved in the Orocopia Schist, and determine whether the presently exposed mylonitic fabric is related to Paleogene metamorphism, Miocene extension, or preserves elements of both. Comparing these results to the thermal history of the crystalline gneiss will provide critical details to tectonic relationship between the two rock types. Particular attention will be paid to whether the schist and gneiss share any portion of their thermal history, and if so, at what point their histories evolved together. A shared thermal history between the OS and gneiss would provide insights into the geodynamic processes that control subduction complex exhumation during or shortly following shallow-slab subduction. These observations have implications for (1) the regional tectonics of the southwestern USA, (2) the extent of subduction underplating during shallow-angle subduction, and (3) the geodynamic processes that control subduction complex exhumation during or shortly following flat-slab subduction.</p> <p>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.</p>