While the San Andreas fault system (SAFS) is one of the best studied plate margins in the world, many questions remain about the older paleotectonic history of the SAFS. At ~30 Ma, the ancestral east Pacific Rise subducted under southern California, leading to significant tectonic reconfiguration of the plate margin that includes extension, transrotation, and transtension before modern transpression and the San Andreas fault.  The timing of extension is quite broad (~27-18 Ma), and studying related features is difficult due to younger deformation and offset. However, these features are well preserved in the San Gabriel Mountains in southern California.

The San Gabriel Mountains hosts segments of the Chocolate Mountains Anticlinorium (CMA) in addition to Oligocene-Miocene extensional basins which flank the CMA. Coffey et al. (2019) demonstrated that the CMA was a topographic high before deposition of these basins initiated - thus low temperature thermochronology of the clasts and correlative source basement is one of the few techniques that will allow me to assess the tectonic history before deposition of these basins. Additionally, the CMA is cored by Pelona-Orocopia schist, of which unroofing is documented in ~18 Ma alluvial fan. However, apatite fission track ages of CMA schist are up to 9 myr younger than the deposition age, pointing to an interesting conundrum:

How is a rock at the surface at 18 Ma, but is also several km underground at 16-9 Ma? 

This chapter of my thesis  will obtain apatite and zircon U-Th/He thermochronology of basement and clasts in the San Gabriel Mountains. Understanding the thermal history will allow us to develop a better model for the structures that facilitated this uplift.