PRELIMINARY LATE PLEISTOCENE SLIP RATE FOR THE WESTERN PINTO MOUNTAIN FAULT, MORONGO VALLEY, SOUTHERN CALIFORNIA

The northern Coachella Valley region of the San Andreas fault (SAF) system in southern California is complicated by overlapping, active strands and its intersection with prominent, secondary structures such as the Pinto Mountain fault. Recent work in this area proposes that strain may be transferred from the Mission Creek strand of the SAF to the Eastern California Shear Zone (ECSZ), at least partly via the Pinto Mountain fault. Like the better known Garlock fault, the Pinto Mountain fault is a major east-west trending left-lateral transverse fault. It intersects the Mission Creek strand of the SAF in the eastern San Gorgonio Pass area, and its trace east through the Morongo Valley may be a surface manifestation of strain transfer to the ECSZ. Geodetically modeled slip rates for the Pinto Mountain fault vary widely from 1 to 12.5 mm/yr, and geologic rates have been speculative because the ages of possible offsets are unconstrained. We have determined a preliminary geologic slip rate from a left-lateral offset within alluvium in Big Morongo Canyon in Morongo Valley. We measured an offset of 205 +/- 20 m within an old alluvial surface (Qoa). The offset is determined from one set of piercing points that is an offest strath between Qoa gravels and underlying gneiss. This piercing point assumes pure left-lateral strike-slip motion along the main trace of the western Pinto Mountain fault. A secondary fault trace may cut the young alluvium unit (Qa) to the north of the main trace, but any lateral offset is unconstrained and therefore not included in our rate calculation.

We obtained cosmogenic 10Be exposure ages of six monzo-granite boulders on the surface of Qoa. Assuming zero-erosion rate, boulder ages range from 64 ka to 90 ka with a preferred average of 83 ka. Combining the offset and age measurements, we calculate slip rates of 2.3 mm/yr to 3.2 mm/yr, with a preferred slip rate of 2.5 mm/yr for the Pinto Mountain fault over the last ~90 ka. Although the Pinto Mountain fault is antithetic to the SAF, we think that it likely represents a direct connection from the Mission Creek strand of the SAF to the ECSZ. Furthermore, we think that Pinto Mountain fault motion contributes to a decrease in the slip rate on the SAF system as is passes northwest through the San Gorgonio Pass region by transferring strain directly to the ECSZ.