WHAT DO WE KNOW ABOUT LATE CENOZOIC LANDSCAPE EVOLUTION AT THE NORTHWEST HEAD OF THE SALTON TROUGH, SOUTHERN CALIFORNIA?

The San Andreas Fault (SAF) system in southern CA includes the following elements: (1) the SAF zone, a continental transform that evolved at ~6-5 Ma; (2) the coeval West Salton Detachment (WSD) and its supradetachment fill; (3) the Salton Trough that developed as the Gulf of California Transtensional Regime (GCTR) propagated northwestward beginning at ~6.3 Ma; (4) the San Jacinto Fault (SJF) that initiated at ~1.2 Ma in response to SAF complexities manifested especially in the San Gorgonio Pass (SGP) contractional belt; (5) younger parts of the Eastern CA Shear Zone (ECSZ) that evolved as dextral shear stepped across the Eastern Transverse Ranges (ETR) and its clockwise-rotating panels. All of these events yielded distinctive landscapes, including: the SW-facing Little San Bernardino Mountains escarpment (LSBME); the N boundary of the ETR (Pinto Mountain Fault, PMF); the contractional SGP region; and landscapes on the WSD footwall of the San Jacinto Mts/Santa Rosa Mts block. Critical questions include: (1) Do late Miocene marine rocks in today’s SGP represent the onshore head of the GCTR, when SGP landscapes originally were positioned 140+ km to the SE (Matti and Morton, 1993)? (2) Pleistocene contraction in SGP led to overthrusting of low-density materials by crystalline rocks of the SE San Bernardino Mts, presumably including the marine rocks; does this mean that the NW head of the GCTR originally could have extended NW beyond SGP? (3) When and by what mechanism did uplift and exhumation of the LSBM begin (7 Ma according to cooling ages; Sabala, 2010)? Is the timing of LSBME development compatible with that of other landscapes within the distributive SAF system? (4) Did the LSBME originally extend NW of the ETR? (5) if sinistral slip on the PMF is responsible for the E-trending 15 km bend in the Mission Creek Fault in SGP (Matti and Morton, 1993), then most of the PMF’s displacement is post 1.2 Ma. Is this compatible with what is known about the PMF as a northern boundary to clockwise-rotating ETR blocks (Powell, 1993)? (6) Where and what are structures in the LSBM that facilitate strain transfer to the ECSZ (Hislop, in progress)? We explore these questions by integrating what is known about geologic, geomorphic, and geochronologic data sets from today’s northwest head of the Salton Trough as it evolved during the timeframe 7 Ma—0 Ma.