THE MISSION CREEK FAULT IN THE SAN GORGONIO PASS REGION: A LONG-ABANDONED STRAND OF THE SAN ANDREAS FAULT, OR A MAJOR PLAYER IN THE LATEST QUATERNARY SAN ANDREAS STRAIN BUDGET?

The San Andreas Fault (SAF) zone in the San Gorgonio Pass (SGP) region is geologically and geomorphically more complex than perhaps anywhere else in southern California. Here, the zone consists of at least six separate strands that, acting together, record the fault’s entire movement history since inception at ~6 Ma. In addition, interaction between the SAF and associated sinistral, contractional, and extensional fault sets has significantly impacted the geometry and slip history of individual SAF strands—especially the Mission Creek strand (MCS), which generated most of the total SAF slip during Pliocene and Pleistocene time. Many workers view the MCS in SGP as an abandoned structure that has produced no dextral slip for >0.5 Ma. This contrasts with the MCS in the Indio Hills, where the fault has generated 12-22 mm/yr of slip during the late Pleistocene. Slip-rate contrasts between SGP and the Indio Hills suggest that—between the two regions—latest Quaternary slip on the MCS does not project NW through SGP but instead must step left (and/or right) from the MCS onto other components of the SAF system. Recently, Fosdick & Blisniuk (F&B) (2018) challenged this paradigm, based on work in the Mission Creek area of NE SGP. They conclude that the MCS here has generated ~20-30 mm/yr of dextral slip over the last 100 ka. A companion abstract (Yule et al., this session) explores the basis for this proposal, and presents detailed evidence refuting the idea that the MCS in SGP has generated significant dextral slip over the last 100 ka. We also refute F&B (2018) by citing the MCS’s geologic character between the Indio Hills and San Gorgonio River in northern SGP: (1) Throughout much of this 45-km extent, the MCS trace is concealed by undisrupted Holocene surficial deposits. (2) Fault scarps and other tectonogeomorphic features occur locally along the strand’s extent, but these are discontinuous and are not convincingly youthful. (3) We see no evidence for young Quaternary dextral slip on the MCS in the critical footprint between Mission Creek and the headwaters of San Gorgonio River. (4) Some workers propose that—in NE SGP—20-30 mm/yr of slip steps abruptly south off the MCS and onto faults in crystalline rocks that connect directly to the SAF San Bernardino strand to the NW. We see no evidence for this, but we do see slip stepping left well SE of SGP.