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