OFF FAULT DEFORMATION AND IMPLICATIONS FOR SLIP RATE ALONG THE SOUTHERN SAN ANDREAS FAULT IN THE SAN GORGONIO PASS, SOUTHERN CALIFORNIA

The San Andreas Fault (SAF) system in the San Gorgonio Pass (SGP) and northern Coachella Valley of southern California is comprised of discontinuous, divergent, oblique-slip fault strands with uncertain slip rates. Three important strands are the SGP fault zone, the Banning fault strand (BF), and the Garnet Hill fault strand (GH). The slip rate of the BF decreases to the northwest from ~5 mm/yr in the northern Coachella Valley to zero in the SGP. In contrast, the slip rate of the SGPF zone decreases to the southeast from ~5 mm/yr in the SGP to a lower, but poorly constrained rate, in the northern Coachella Valley, where it is referred to as the Garnet Hill strand. Whitewater Hill (WWH) is located between the BF and GH where the slip is apparently transferred within a N-S trending, doubly plunging anticline with series of N-S trending tear faults. The northern side of WWH is cut by the moderately north-dipping BF and the southern side is cut by two splays of the GH. The northern splay of the GH is a steeply north-dipping oblique-reverse fault with >50 m right-lateral offset of late Pleistocene and Holocene fans. The southern splay is a thrust fault with a 1-2 m up-on-the-north scarp in late Holocene alluvium. The surface of WWH has a maximum elevation of ~350 m and is distinguished by a thick (>3m) red-orange soil of unknown, but assumed age of 75-750 ka that formed above mid-late Quaternary fan gravels. Detailed mapping of the soil horizon gives a minimum uplift of ~140 m in the hanging wall of the northern splay GH, although the total uplift is likely much more because the base of the hill is buried by talus. Using an east-west cross-section of WWH and extrapolating the hinge of the anticline minimum shortening across the fold is ~40 m. Using these measurements and the assumed age range of the soil, we calculated ~0.2-1.9 mm/yr uplift rates and ~0.05-0.5 mm/yr shortening rates. Right-lateral motion on the northern splay of the GH may approach the ~5 mm/yr slip rate established ~15 km to the west at Millard Canyon, but confirming this rate in WWH will require the acquisition of more precise offset and age data. The local fold-and-fault system at WWH helps explain how slip on the San Andreas Fault is carried through the San Gorgonio Pass in complex, but understandable ways.