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.