The Evolution from Late Miocene West Salton Detachment Faulting to Cross-Cutting Pleistocene Oblique Strike-Slip Faults in the SW Salton Trough, Southern California

Field studies in the southwest Salton Trough between Yaqui Ridge and Borrego Mountain show that the West Salton detachment fault was active during the Pliocene and may have initiated during the latest Miocene. At Yaqui Ridge dominantly east-directed extension is recorded by slickenlines on the NW-striking detachment fault, and shows that the fault is actually a low-angle dextral oblique strike-slip fault. Crustal inheritance is responsible for the position of the fault at Yaqui Ridge, which reactivates a late Cretaceous reverse -sense mylonite zone at map scale. Late Miocene to Pliocene basin fill deposits at Borrego Mountain display progressive unconformities, contain detritus shed from the footwall and damage zone of the West Salton detachment fault, record the growth of a large hanging wall anticline, and document the initiation and evolution of the West Salton detachment fault. The Borrego Mountain anticline is a major hanging wall growth fold that trends - N60 °W and has at least 420 m of structural relief. The late Quaternary Sunset conglomerate is - 600 m thick, lies in angular unconformity on Pliocene basin fill, is bound on the SW by the dextral oblique Sunset fault, and coarsens upward and SW toward the fault. It is dominated by plutonic lithologies from nearby areas, contains up to 10% recycled sandstone clasts from Pliocene deposits, and was shed from the SW side of the then-active Sunset fault. Based on lithologic, stratigraphic, compositional similarities, we correlate this conglomerate to part of the - 1. I - 0.6 Ma Ocotillo Formation. The West Salton detachment fault was folded and deactivated at Yaqui Ridge by the dextral oblique San Felipe fault zone starting - 1. l - 1.3 Ma. The Sunset fault is in the middle of a complex left stepover between the San Felipe fault to the NW and the Fish Creek Mountains fault to the SE. Structural analyses and mapping show that syntec tonic conglomerate, the West Salton detachment fault, and footwall crystalline rocks all have similar fold geometries and record similar amounts of NE-SW shortening. The dominant SE-trending population of slip vectors on the Sunset fault is not present on the West Salton detachment fault and suggests limited or no activation of the older detachment fault by the younger fault zone