The west Salton detachment fault (WSDF) is a system of low-angle normal faults that can be traced for ~180 km along strike from the Santa Rosa Mts to the Tierra Blanca Mts (Axen and Fletcher, 1998). The WSDF was active from late Miocene to early Pleistocene time and accommodated NE-SW extension during dextral slip on the San Andreas fault, in a pattern of regional strain partitioning. The fault dips 15 to 40 deg to the E and NE, except where it has been backtilted on the S or SW limbs of younger E- to SE-trending folds. Slickenlines define overall top-to-the-ENE slip but range from NNE to E in trend. The detachment offsets the steeper Cretaceous Santa Rosa mylonite, and preliminary data suggest ~15 km of extension. The immediate hangingwall of the detachment typically contains pervasively brecciated crystalline rocks; some breccias are sturztstrom deposits but others are poorly understood bedrock "riders". The breakaway for the southern portion of the WSDF is located within the NE-facing front of the Tierra Blanca Mts where it was previously interpreted as a strand of the Elsinore fault. Normal fault scarps there show that strands of the breakaway zone are still active in the modern transtensional regime.

Latest Miocene to early Pleistocene Imperial, Palm Spring, and Canebrake Formations, and other units in the Imperial and Palm Spring Groups of Winker and Kidwell (1996), accumulated in a large upper-plate basin created by top-to-the-NE slip on the WSDF. There are no unconformities in ~4.5 km of uniformly SW-dipping strata in the Fish Creek section, but at least ~1.5 km of Imperial marine strata are missing across a disconformity to the NE (NE of Split Mt Gorge). These growth relationships are consistent with formation of a NW trending monocline above a listric detachment fault. Imperial and younger deposits thus thicken to the SW toward the detachment breakaway, whereas older deposits of the Split Mt. Group thicken to the NW toward NE-striking normal or oblique-slip faults (Winker, 1987). These features indicate a significant change in fault kinematics after deposition of the Elephant Trees Cgl, at ~5-6 Ma, during deposition of large rock avalanches and turbidites in the Split Mt area. Subsequent initiation and/or integration of the supradetachment basin produced widespread marine flooding and deposition of distal Imperial claystone.