TECTONIC EVOLUTION AT THE INTERSECTION OF THE SAN ANDREAS, PINTO MOUNTAIN, AND MORONGO VALLEY FAULT ZONES, SOUTHERN CALIFORNIA: IMPLICATIONS FOR QUATERNARY SAN ANDREAS FAULT SLIP RATES

In the San Gorgonio Pass (SGP) region, dextral slip rates for northern strands of the San Andreas Fault Zone (SAFZ) are hotly debated. Some workers maintain that dextral slip on the Mission Creek strand (MiCS) of the northern route (NR) slowed at ~1.5 Ma as a “structural knot” evolved in the SAFZ, with slip approaching zero before ~500 ka (Matti and Morton, 1993); in SGP the MiCS has an E-W bend that formed a 9-15 km left step in the Pleistocene SAFZ. At ~500 ka, slip initiated on the Mill Creek strand (MCS) and produced ~8 km of displacement before ~100 ka, when slip ended (Kendrick and others, 2015). In contrast, recent workers contend that the SAF NR has 20-30 mm/yr of slip during the last 100 ka and longer (e.g., Blisniuk and others, 2021). Here, we argue that interaction between the SAF NR and intersecting sinistral faults (Pinto Mountain Fault [PMF] and Morongo Valley Fault [MVF]) allows only minimal slip on the SAF NR over the last 1.5 Ma. Specifically: when slip on the MCS is reconstructed, the MiCS left bend aligns with the MVF—suggesting that sinistral slip on the latter is coeval with and caused the MiCS left bend (Kendrick and others, 2022). We use these relations to propose the following tectonic scenario: (1) At ~1.5 Ma, dextral slip on the MiCS gradually was impeded by sinistral slip on the MVF. (2) During this period, the MiCS was deformed into its E-trending bend. (3) The MVF thus is as a major strand of the PMFZ. (4) The PMFZ is widely viewed as the northern boundary for clockwise-rotating blocks of the Eastern Transverse Ranges (ETR); we see the MVF as part of that boundary during middle Pleistocene time. (5) Following interaction between the MVF and the MiCS, dextral slip punched through the “structural knot” in the SAFZ, allowing the MCS to displace the MiCS left bend NW to its current location. (6) In late Pleistocene time, sinistral slip on the PMF truncated and displaced an abandoned MCS by ~1.2 km. (7) Geometric relations and sequencing of major dextral and sinistral faults at the NE portal to SGP suggest that they have interacted together for the last ~1.5 Ma to produce the current geologic framework. Implication: The original spatial arrangement of these faults has been disrupted only by 8 km of dextral slip on the MCS, requiring that the SAF NR has not produced significant dextral slip in middle and late Quaternary time.