Preliminary Apatite (U-Th)/he Thermochronology of the Eastern Santa Rosa and San Jacinto Blocks, Peninsular Ranges, Southern California

The Santa Rosa Mts (SRM) are cut by ~4 NE-dipping Cretaceous to Cenozoic low-angle thrusts and/or normal faults that overlie the eastern Peninsular Ranges mylonite zone and Mt San Jacinto (MSJ) block. The structurally highest faults in the southern SRM belong to the late Cenozoic west Salton detachment system (WSDS), slipped concurrently with the southern San Andreas fault, and carry late Cenozoic synorogenic deposits and older basement. Before San Andreas offset, the area lay near the mid-Cenozoic B&R of southwestern Arizona.

We report apatite (U-Th)/He ages from two vertical transects that constrain fault-related cooling through ~60°C: (1) the immediate footwall of the Zosel fault (ZF; south of Martinez Can, southern SRM), a strand of the WSDS, and (2) the lower ~1 km of the eastern MSJ block. Five ZF footwall ages range from 25.7±1.5 Ma (960 m) to 6.0±0.4 Ma (190 m), with a steep age gradient below ~600 m and a possible inflection at ~10 Ma that may record onset of ZF-related cooling. Six MSJ ages range from 18.6±1.1 Ma (975 m) to 10.6±0.6 Ma (195 and 480 m), with a steep age gradient below ~600 m and a possible inflection at ~10 Ma. Adding two ages from higher eastern MSJ samples (Wolf et al. 1997) suggest a moderate age gradient from ~10 to ~30 Ma (~600 m to ~2500-3000 m), and a low age gradient higher up. These results are consistent with (U-Th)/He ages from farther south that also suggest WSDS-related exhumation began at or before ~10 Ma (Shirvell et al. 2007). Additional SRM and MSJ ages should be done before the meeting, to test these tentative conclusions. The inferred moderate age gradient from MSJ may record earlier B&R tectonics; we plan to collect and date the upper 2/3 of the MSJ transect in the future.