The West Salton Detachment Fault, SaltonTrough, California: a Primary Low-Angle Normal Fault in an Evolving DextralWrench Zone

The west Salton detachment fault (WSDF), bounded the W rift flank, and was largely coeval with the southern San Andreas fault (SSAF). The WSDF is exposed in ~E-trending folds: broad, apparently primary corrugations S. Santa Rosa Mts., Borrego Valley-Pinyon Mts., Whale Peak, Vallecito Valley, and Tiera Blanca Mts) and narrow, post-WSDF folds (e.g., adjacent to San Felipe and Earthquake Valley faults). WSDF slip may have begun at ~12+, ~8.1, 5.5 or 4.6 Ma and was probably rapid from ~5 to 2 Ma. Two (U-Th)/He vertical transects from the WSDF footwall show rapid cooling since 12 Ma, and very rapid cooling between ~5.5-4.5 and ~2 Ma. Subsidence curves from the Fish Creek Vallecito basin (FCVB; Dorsey et al., this session) show increased rates at ~8.1 Ma, 5.5, and 4.6 Ma. Syntectonic conglomerate (base ~8.1 Ma) there records earliest extension, but may have been only local. Widespread marine deposits (~6.3 to 4.25 Ma) locally contain syndetachment fault-scarp facies; eustatic sea level rise may have controlled initial marine flooding. Subsidence was most rapid from ~4.6 to 3 Ma. Upper-plate normal faults are rare but folds formed locally. At Borrego Mtn. a WNW-trending anticline formed by ~6 Ma and persisted until after 4 Ma, coeval with WSDF slip. Folding at Split Mtn may have begun earlier. The WSDF has at least 5 km of E or NE slip, from offset basement but higher WSDF strands carry syntectonic conglomerates some additional distance. (U-Th)/He apatite ages from the upper and lower plates suggest ~2.4 km of footwall exhumation, yielding 5-15 km of slip, depending upon dip assumed. WSDF striae scatter widely, but concentrate at 090-110, probably the main or most recent slip direction. CW vertical- axis rotations have occurred (Housen et al., this session): ~3-4 m.y. old FCVB strata are rotated 19° ± 12°, and footwall La Posta pluton at Whale Peak rotated perhaps 36° (relative to the Peninsular Range La Posta). Similar rotations were common in N Baja CA in latest Miocene-Pliocene time and that belt of rotation may continue into the western Salton Trough, but the FCVB rotations may reflect local Quarternary dextral shear and the Whale Peak data can be explained by SW tilting. In contrast, 2 to 0.6 Ma strata in the FCVB record no vertical-axis rotation but show syndepositional SW tilting toward the Elsinore fault. Syn-WSDF deposition (Hueso Fm.) in Vallecito Valley continued until ~0.9 Ma. At ~1.1 Ma, the supradetachment Borrego sub-basin was reorganized and uplifted, recording a switch to dextral faulting. Thus, the southern Elsinore, Earthquake Valley, San Felipe, and San Jacinto fault zones were dominant since ~1 Ma.