SGER: Testing the Viability of (U-Th)/He Thermochronology in Complex Tectonic Systems

Exhumation in the northern California Coast Ranges, and orogens in general, results from the interplay of geodynamic processes that drive uplift and surface processes that respond to the rates and patterns of uplift. A variety of approaches have been used to determine patterns of exhumation, but placing robust, quantifiable constraints on these estimates is difficult because of the feedbacks and linkages among large scale geodynamic, thermal, and landscape evolution processes. There is a critical need for a technique to constrain the trajectory of rocks at shallow- crustal (low-temperature) levels to constrain uplift and exhumation in active orogenic belts. Most recently, (U-Th)/He thermochronometry has been identified as a potential tool to help quantitatively constrain exhumation and uplift. The (U-Th)/He system has the advantage of a low closure temperature (~ 60-70 C for apatite) and thus in principal it records the last few kilometers of unroofing. As a low-T thermochronometer the same characteristics that make (U- Th)/He so appealing for geomorphic studies also raise questions about its utility in regions of complex thermal and/or tectonic histories. It is uncertain whether this tool can extract meaningful information from a complex signal that incorporates both the exhumation history and the inevitable thermal feedbacks driven by that exhumation. In this project we will apply (U-Th)/He thermochronometry coupled with landscape evolution, thermal and geodynamic modeling, to evaluate if (U-Th)/He dating can be confidently applied in regions of active tectonism that are not in thermal or geomorphic steady-state.