NEW GEOLOGIC MAPPING SUGGESTS UPDATED STRUCTURAL MODEL FOR THE FISH CREEK-VALLECITO BASIN IN THE SALTON TROUGH, NORTHERN GULF OF CALIFORNIA

The Salton Trough is a seismically active region in the northern Gulf of California right-lateral, transtensional rift system that is characterized by several upper-crustal, creeping strike-slip faults. We report new mapping of the Fish Creek-Vallecito basin (FCVB), a sub-basin of the Salton Trough, that preserves a record of syn-tectonic deposition, including the arrival of Colorado River detritus, and explore the influence of rapid sedimentation on the mechanical conditions of faulting. The FCVB contains a complete ~6 km sedimentary section deposited between 8 -1 Ma that was uplifted since ~1 Ma. However, the compaction of strata in the FCVB appears inconsistent with ~6 km burial, requiring an alternative structural model, anomalously high pore-pressure conditions, or both. We present new mapping in the FCVB between Fish Creek Wash and the Vallecito Mountains, along the present-day left lateral Vallecito fault, that 1) redefines the stratigraphy proximal to the Vallecito fault, 2) documents the presence of a proto-Vallecito normal fault through outcrop exposures and stratigraphic relationships, and 3) provides an updated and detailed structural analysis of the FCVB. Our new mapping was completed on lidar-derived topographic basemaps at a scale of 1:10,000 and is compiled with existing maps completed at ~1:12,000 on airphotos and USGS topographic quadrangles. This compilation creates a uniform, detailed map, on a high-resolution topographic base over a large part of the FCVB in Anza Borrego Desert State Park; shows new structural complexity; and resolves discrepancies or lack of overlap between previously published maps. A key take-away from this mapping is that the lower basin strata interfinger with facies derived from the footwall scarp of the proto-Vallecito fault, indicating that it originated as a Mio-Pliocene normal fault. This proto-Vallecito fault may have acted as an early breakaway of the West Salton detachment fault, partitioning FCVB subsidence into two sub-basins and allowing for reduced (<6 km) burial of the lower FCVB section.