PLEISTOCENE (~1 MA) TRANSITION FROM WEST SALTON DETACHMENT FAULTING TO CROSS-CUTTING DEXTRAL STRIKE-SLIP FAULTS IN THE SW SALTON TROUGH

Recent structural and stratigraphic analysis documents folding and deactivation of most of the West Salton detachment fault (WSDF) at Yaqui Ridge starting ~1.1 Ma. The ~5.7-km-long Sunset fault is in the middle of a complex left stepover between the more continuous San Felipe fault in the NW and Fish Creek Mts. fault in the SE. These three faults bound two fold domains. Structural analysis shows that syntectonic conglomerate, WSDF, and footwall crystalline rocks are all folded about NW-SE-trending folds, and each domain records similar amounts of NE-SW shortening. We interpret the similar geometries of these folds as resulting from a double contractional stepover in the San Felipe-Sunset-Fish Creek Mts. fault system. The Sunset fault is steep to moderately dipping, changes dip direction over short strike distances, and exhibits a range of slickenlines with mostly strike-slip and fewer purely dip-slip directions. The stepover is exhumed due to NE-side-down components of slip on the Sunset and Fish Creek Mts. faults and SW-side-down components on the San Felipe fault. The dominant SE-trending population of slip vectors on the Sunset fault is not present on the WSDF. We estimate less than ~2 km of right slip on the Sunset fault.

Poorly sorted angular boulder conglomerate to pebbly coarse sandstone, informally named the Sunset conglomerate, is ~600 m thick and lies in angular unconformity on the Pliocene Diablo and Olla formations east of Yaqui Narrows. The conglomerate is bounded on the SW by the Sunset fault, and coarsens upward and SW toward the fault. It is dominated by nearby plutonic lithologies, contains up to 10% recycled clasts of the Diablo Formation, was deposited by NE-flowing alluvial fans, and was shed from the SW side of the then-active Sunset fault. Based on lithologic, stratigraphic, compositional similarities, we correlate this unit to the 1.1- to 0.5-Ma Ocotillo Formation. The Diablo clasts record erosion of older basin fill that once covered the Vallecito and Fish Creek mts., thus constraining the age of initial uplift of these fault blocks. We conclude that folding of the WSDF at Yaqui Ridge and resultant deactivation of detachment slip on the S limb began ~1.1 Ma in the nascent, cross-cutting San Felipe-Fish Creek Mts. dextral-oblique fault zone.