UC Davis

The Fish Creek Vallecito basin (FCVB) provides an extensive and continuous exposure of strata equivalent to the modern Salton trough rift basin in the northernmost Gulf of California. The FCVB exposes a continuous ~6 km thick sedimentary section deposited between 8 and 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. To test these hypotheses, I present geologic mapping that documents new fault strands and associated stratigraphic facies changes between FCVB and the Vallecito Mountains. I show that basin subsidence was partitioned across two normal fault hanging walls: the newly identified Proto-Vallecito fault, active from ~8 to 4.4Ma, and the West Salton Detachment fault, which accelerated after ~4.4 Ma and ceased activity at ~1 Ma. My structural model reduces the total burial depth required of the FCVB section from ~6 km to a maximum of ~4 km and requires significantly lower and less rapid exhumation than previous models. I validate my structural model using (U-Th)/He (AHe) thermochronology. Preservation of detrital age signatures and sparse AHe ages younger than the depositional ages imply that temperature at the base of the FCVB section may not have exceeded the AHe partial retention zone (<~55-80°C). To extract additional information from the AHe data, I use a combination of forward and inverse modeling to constrain post-depositional thermal histories while considering the effects of radiation damage and detrital inheritance. From the dates and modeling, I determine that maximum burial temperature of the FCVB was about 80-90°C and confirm that the burial depth of the exhumed section was likely ≤4km. Best-fit thermal histories from inverse modeling are consisted with uplift and tilting commencing as early as 4Ma, when activity shifted to the West Salton Detachment, and 3 Myr prior to the onset of transpression at 1.2 Ma. Using this newly established structural context, I use a one-dimensional model of basin deposition and compaction to test for overpressure conditions in the FCVB. The model results show that overpressure conditions likely developed here due to both rapid sedimentation and the presence of low permeability caprock layers. Such overpressure conditions may be present in the modern Salton Trough, promoting earthquake triggering and fault creep in this region.