TECTONIC EVOLUTION OF THE WESTERN PINTO MOUNTAIN FAULT ZONE, SOUTHERN BIG BEND REGION, SAN ANDREAS FAULT ZONE, SOUTHERN CALIFORNIA

The western sinistral Pinto Mountain fault zone (PMfz) is located between the San Bernardino Mountains (SBM) and Little San Bernardino Mountains (LSBM) in the Big Bend of the San Andreas fault zone (SAfz) in Southern California. The PMfz joins the Mission Creek fault strand (MCfs) of the SAfz at a left-stepping bend dominated by dextral slip and compressional faulting. Two strands form the western PMfz, the curved west- to-southwest-trending Pinto Mountain fault strand (PMfs) and the southwest-trending Morongo Valley fault strand (MVfs). We propose a simple three-stage model that links the geometry of western PMfz and MCfs to clockwise (CW) rotation of the Eastern Transverse Ranges (ETR).

Stage 1: the nascent PMfz curved around the north and west sides of the LSBM and joined the MCfs, which was oriented N45°W (Dickinson, 1996). Dextral slip on the MCfs dragged the PMfz/MCfs junction to the northwest, which created tension between the PMfz and LSBM and restricted the ETR’s ability to rotate. Stage 2: The MVfs splayed off the PMfz which restored ETR rotation and became the dominant strand (Kendrick et al., 2022; Hopson, 2012). Dextral slip continued dragging the PMfz/MCfs junction northwestward bending the PMfz CW. Sinistral slip of PMfz projected the SBM westward bowing the MCfs outward which restricted dextral slip forcing the initiation of Mill Creek fault strand (MiCfs). The PMfz intersected the MiCfs displacing it 1–1.25 km (Kendrick et al., 2015). The CW bending of the PMfz led to the formation of a north-facing concave bend that acted as a fulcrum allowing the PMfz to continue bending. Stage 3: the combination of continued dextral slip on the MCfs and tension between the PMfs and MVfs were involved in the creation of Morongo Valley.