HOLOCENE SLIP RATES ALONG THE SAN ANDREAS FAULT THROUGH THE SAN GORGONIO PASS

As it traverses NW Coachella Valley the San Andreas fault (SAF) splits into multiple, divergent strands that enter the “enigmatic” San Gorgonio Pass (SGP). Large, historical earthquakes have not ruptured through SGP, thus creating debate about the rupture length and location of a future event on the southern SAF. Relatively continuous faults have been mapped in two zones through the SGP: a southern-route (SR) involving the Banning strand, Garnet Hill strand, and San Gorgonio Pass Fault zones and a northern route (NR) involving the Mission Creek and Mill Creek strands and/or associated structures. Any large earthquake on the southern SAF through SGP must follow one of these two routes, and only the southern route shows convincing geologic and geomorphic evidence for repeated throughgoing earthquake ruptures during the Holocene. Only two sites provide published Holocene slip-rates within SGP: along the Banning strand and the San Gorgonio Pass fault zone, both along the SR. An oblique slip rate of 5.7 (+2.5, -1.5) mm/yr was calculated across fault scarps within Millard Canyon in the central SGP, where fan terraces were dated (10Be exposure) as Holocene. At the eastern edge of SGP the Banning strand has a comparable slip rate of 3.9-4.9 mm/yr from an offset mid-Holocene fan surface. Together, these data suggest that the SR is active but accommodates <50% of slip along the SAF system. In contrast, there are no published Holocene slip rate sites along the NR through SGP. Notably, fault scarps and dextral stream offsets are sparse, discontinuous and developed in alluvium of undocumented age; dextral stream offsets developed on one uplifted Pleistocene terrace NW of Mission Creek cannot convincingly be attributed to Holocene slip. At Dry Tributary, a 45 m-deep canyon reveals a strath surface beneath 25 m sedimentary fill that buries the Mission Creek strand of the SAF. Although the NR undoubtedly was active during the Pleistocene, during the Holocene it has shut down and slip has been transferred to the southern route or is accommodated by diffuse off-fault deformation.