DOES A “STRUCTURAL KNOT” IN THE QUATERNARY SAN ANDREAS FAULT EXIST IN SAN GORGONIO PASS? TRADITIONAL AND RECENT VIEWS PROVIDE CONFLICTING ANSWERS

Traditional geologic models for Quaternary (Q) strain within the southern San Andreas Fault (SAF) zone invoke a “structural knot” centered on San Gorgonio Pass (SGP). The “knot” concept grew out of Allen’s (1957) recognition that Q thrusting now predominates in SGP: older dextral-slip faults have been disrupted, and the “modern” SAF trace is disguised. We now know that several SAF strands occur in the SGP region and have evolved sequentially (“switched on and off”) throughout the zone’s ~6-Ma lifespan. Later workers also have shown that late Q SAF motion carries through SGP in complex but understandable ways via oblique thrust and tear faults of the SGP Fault Zone (SGPFZ). Oblique convergence in SGP also is supported by the first-order observation that its mountainous landscape rises abruptly above adjacent lowlands. These factors all led to a traditional time-space model where the SAF system at SGP is a left-oblique regime within which Q slip steps progressively from the Mission Creek strand (MCS, on the NE) to the Banning and Garnet Hill strands (to the SW), accompanied by the following constraints: (1) in mid-Pleistocene (PL) time the MCS in SGP was abandoned, leading to mid-PL contraction within the SGPFZ and uplift of high-standing parts of the SGP landscape; (2) in mid-PL time the Mill Creek strand bypassed this older “knot” in the SAF, but ultimately even it was abandoned by the SAF; (3) today, the late Q framework of SGP reflects left-oblique SAF slip trying to work through SGP in order to “connect” young SAF strands to the NW and SE. Recent reports by Fosdick and Blisniuk, 2018 (https://doi.org/10.1038/s41598-018-30622-3) and Balco et al., 2019 (https://doi.org/10.5194/gchron-1-1-2019) challenge this paradigm. These reports assert that—over the last ~100 ka years—20-30 mm/yr of SAF slip projects through SGP on the MCS and on fault structures associated with the MCS. To us, this poses a pressing question: is there any need for a late Q San Gorgonio Pass “knot” in the SAF? We answer “yes”: (1) interaction among ALL geologic structures in the SGP region is consistent with a long-lived Q knot; (2) in SGP we see no evidence for youthful dextral offset on the MCS itself; (3) in SGP we see no evidence for dextral-slip faults SW of the MCS (e.g., Waco et al., 2017, https://files.scec.org/s3fs-public/SCEC2017Proceedings.pdf).