BEDROCK APATITE (U-Th)/He THERMOCHRONOMETRY CAPTURES ~1.5 MA INCEPTION OF THE SAN FELIPE FAULT ZONE AND TECTONIC REORGANIZATION IN SOUTHERN CA

Bedrock exhumation along seismogenic continental strike-slip faults captures the time-integrated deformation history of multiple earthquake cycles. In southern CA today, plate boundary strain is accommodated by large earthquakes and interseismic deformation along the San Andreas and San Jacinto fault systems. This fault configuration initiated in a major tectonic reorganization at ~1.5-1.0 Ma. The shift from a two-fault system of the San Andreas and West Salton Detachment faults (WSDF) to a more complex system of strike-slip faults that includes the San Jacinto, Elsinore, and San Felipe (SFFZ) fault zones is recorded predominantly in syntectonic basin deposits. Here, we document focused bedrock exhumation and initiation of the SFFZ where it bounds Yaqui Ridge and dissects the WSDF with m-scale spatial resolution apatite (U-Th)/He (AHe) thermochronometry data from Cretaceous granitoid and metasedimentary basement rocks.

Individual AHe dates (n = 45, ± 2s error) from basement samples (n = 9) are 66.0 ± 1.9 Ma to 0.80 ± 0.59 Ma over an eU range of 14 to 94 ppm. Our new data support prior AHe results in the area but define distinct spatiotemporal and AHe date-eU patterns in relation to strands of the SFFZ. A subset of ≤1.5 Ma dates across the range of eU values are unique to the core of Yaqui Ridge and adjacent to SFFZ exposures, indicating spatially-focused, rapid cooling and thus exhumation from >1.5 km depth from ~1.5-0.8 Ma. Individual dates from samples at the margins of the SFFZ and footwall of the WSDF are ~11.0-1.5 Ma, recording polyphase exhumation on the WSDF and SFFZ. WSDF hanging wall dates are ~66-16 Ma and define a positive but scattered date-eU pattern, implying these rocks resided at <1-2 km depth since this time.

Our new AHe data document rapid exhumation beginning at ~1.5 Ma in samples proximal to the SFFZ and in the core of Yaqui Ridge, complementing the basin record, and suggest that the initiation and growth of this fault system involved vertical deformation during multiple seismic cycles. A similar spatiotemporal phenomenon in response to the same tectonic reorganization is documented in AHe thermochronometry along the San Andreas fault in Mecca Hills. This connection implies that a pulse of ~1.5 Ma fault-related exhumation accompanied initiation of new seismogenic structures and slip along pre-existing faults during plate boundary broadening.