Processes and Rates of Arc Crust Growth and Differentiation in the Southern Sierra Nevada Crustal Section

The construction of magmatic arcs at subduction zones plays a critical role in the creation of new continental crust on Earth. In particular, a number of different mechanisms in the lower crust (>30 km depth) of arcs are hypothesized to be critical for the formation of felsic continental crust. However the restricted number of locations worldwide where rocks formed at these depths are exposed on the surface is a fundamental limitation to improving our understanding of the chemical and physical controls on these processes and the time scales over which they occur. This proposal aims to evaluate these processes through a detailed study of a relatively little studied exposure of lower crustal rocks in the southernmost Sierra Nevada batholith, combining field observations, major, trace element and isotope geochemistry, and high precision geochronology. The research activities in this project will be integrated with field and analytical experiences for undergraduates while findings will be incorporated into undergraduate courses. Through collaboration with the Tejon Ranch Conservancy, it is planned to help develop outreach programs and education materials for the Tejon Ranch property. These programs include geologic tours of the ranch, and will cover both the basics of plate tectonics and arc magmatism, as well as the geologic history of California. The programs will be available to the general public, and be specifically targeted to public schools serving the primarily underrepresented minority populations in the Tehachapi/Bakersfield region.

The Sierra Nevada Batholith is one of the best-studied examples of continental arc plutonism, but nearly the entire exposed range samples rocks formed at shallow to middle crustal pressures, and as a result our understanding of lower crustal processes in the Sierra Nevada are limited. However, the Sierra Nevada also contains a unique crustal section, exposing a structurally continuous section extending from ~7-37 km depth, and containing a suite of igneous rocks emplaced contemporaneously at ~100 Ma, coinciding with the Late Cretaceous Sierra Nevada magmatic flare-up event. This section provides an ideal location to evaluate different models for lower crustal processes including fractional crystallization and partial melting dominated deep crustal hot zones, cyclic underthrusting and relamination of buoyant material. The combination of structurally continuous and contemporaneously emplaced outcrop will allow us to better characterize these processes and their relevant timescales, and to assess their importance for the generation of felsic continental crust. These findings will have additional important implications for our understanding of the Mesozoic history of the western US, and for validating seismic observations of continental lower crust.