USING APATITE (U-TH)/HE DATA TO DOCUMENT THE HISTORY OF SUBSIDENCE, UPLIFT, AND THERMAL CONDITIONS OF THE FISH CREEK-VALLECITO BASIN WITHIN THE NORTHERN GULF OF CALIFORNIA RIFT

The Salton Trough is a seismically active region in the northern Gulf of California rift. This late Cenozoic right-lateral transtensional system is characterized by low-angle normal faults and crustal-scale strike-slip faults. We present new thermochronology data coupled with geologic mapping from the Fish Creek Vallecito basin (FCVB). The FCVB, a subbasin of the Salton trough, preserves a record of syn-tectonic deposition including the arrival of Colorado River detritus. The FCVB contains a complete ~6 km thick sedimentary section deposited between 8 -1 Ma that was uplifted since ~1 Ma. However, compaction of strata in the FCVB appears inconsistent with ~6 km burial, requiring an alternative structural and depositional model and/or anomalously high pore-pressure conditions. Our mapping suggests that an early breakaway of the West Salton detachment fault partitioned FCVB subsidence into two sub-basins, allowing for reduced burial of the lower FCVB section. This scenario is supported by new apatite (U-Th)/He (AHe) data from the lower half of the FCVB section. These data show only partial or no resetting of AHe ages, as indicated by preservation of detrital age signatures and sparse AHe ages younger than the depositional ages. This implies that the base of the FCVB section may not have exceeded the AHe partial retention zone (<~55-80°C). Extracting additional information from our AHe dates requires the use of forward and inverse modeling to constrain post-depositional thermal histories. We present model results that help refine the timing of burial and exhumation of the lower FCVB section, considering the effects of radiation damage and detrital inheritance. Presently the Salton Trough exhibits a geothermal gradient as high as ~40°C/km, linked to magmatic underplating, placing the 55°C isotherm at <1.5 km depth. These new data require a significantly lower heat-flow at ~8-4 Ma in this area of the FCVB due to spatial variations (off rift axis) and/or very rapid deposition in the FCVB, depressing the geothermal gradient and keeping the section cool. Overall, these new AHe data demonstrate that the FCVB was structurally segmented, accumulated <<6 km of strata, and was characterized by depressed syn-extensional geothermal gradients.