LATE CENOZOIC STRUCTURAL AND GEOMORPHIC EVOLUTION OF THE SALTON BLOCK, SOUTHERN CALIFORNIA

Southern CA’s Salton Block (SB; Fay and Humphreys, 2006) has an enigmatic late Cenozoic geologic/geomorphic history—mainly because block boundaries have starkly different ages, structural origins, and landscapes: (1) To the NE, the block is bounded by latest Quaternary strands of the San Andreas Fault (SAF). (2) To the W, it is bounded by various strands of the San Jacinto Fault (SJF) that originated at ~1.2 Ma. (3) Internally, the block contains the high-standing San Jacinto Mts (SJM) and Santa Rosa Mts (SRM) that once formed the footwall of the West Salton Detachment (WSD). The latter formed the NE boundary of the proto SB from ~6.5 Ma to ~1.2 Ma concurrent with dextral slip on the SAF, but this dynamic boundary was abandoned with initiation of the SJF as the modern SB took shape. Late Cenozoic landscapes at the NW and SE ends of the SJM-SRM highground differ dramatically. (i) To the NW, late Miocene-Pleistocene sedimentary rocks of the San Timoteo Badlands (STB) lap nonconformably against the SJM; the buttressing onlap wraps east toward San Gorgonio Pass (SGP), and if exposed there would lap against the profound northern SJM escarpment—requiring the latter to be a mature landform at least as old as late Miocene. (ii) In the SE SRM the WSD wraps around the former SRM footwall, and near-sea-level sedimentary rocks of the WSD hangingwall are anomalously perched at 4600’ elevation. The elevated hangingwall is bounded precipitously to the SW by the deep structural basin of Clark Valley and by strands of the SJF.

These diverse structural and geomorphic relations need to be integrated into a coherent structural model for the SB. I propose that the former WSD footwall (SJM-SRM domain) has been transpressively uplifted since 1.2 Ma, when dextral slip was transferred from the SAF zone to the SJF zone. This resulted in (1) a regional left step culminating in the SGP region, (2) gentle NW-tilting of the STB sequence, and (3) extensional collapse of Clark Valley via the Santa Rosa Fault and the Clark Lake listric-fault and megalandslide complex at the NW head of Clark Valley. This model does not require NE-tilting of the WSD footwall as proposed recently by Dorsey and Langenheim (2015), but rather arching of the SJM-SRM along its longitudinal axis between the STB and the SE SRM, where Pettinga (1991) first puzzled over convergent structural relations there.