STRATIGRAPHIC AND STRUCTURAL RELATIONS IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA: POSSIBLE IMPLICATIONS FOR THE LATE CENOZOIC WEST SALTON DETACHMENT SYSTEM

San Gorgonio Pass (SGP) is a well-known zone of Quaternary contractional deformation associated with a bend in the San Andreas Fault (SAF) zone. Sedimentary rocks in SGP also may record evidence for late Miocene extensional tectonics associated with the West Salton Detachment (WSD)—a low-angle normal-fault complex that evolved coevally with the southern SAF.

Miocene and Pliocene sedimentary rocks in SGP include (old to young) conglomeratic & sandy rock of the Hathaway Fm (>7 Ma), mudrock & fine sandstone of the marine Imperial Fm (~6.5 Ma), and sandy & conglomeratic rock of the Painted Hill Fm. The Hathaway contains clasts from both the Transverse Ranges and Peninsular Ranges (PR), and locally includes brecciated PR rock that formed as long-runout rock-avalanche deposits. The basal contact of the sedimentary sequence is not exposed, but geophysical data indicate that PR basement of the San Jacinto Mts extends N beneath the sedimentary sequence. Regional relations argue that the SGP sequence accumulated in the hangingwall of an early strand of the WSD system rather than directly on PR basement.

The WSD last is exposed in the southern Santa Rosa Mts (SRM), where hangingwall deposits include the syntectonic Imperial Fm and sandy & conglomeratic rocks and long-runout rock-avalanche deposits—all containing PR-type clasts. NW from the SRM, the WSD projects beneath Quaternary fill of the Coachella Valley, although its subsurface extent has not been documented. Late Miocene Imperial Fm and nonmarine sedimentary rocks at Garnet Hill probably are hangingwall deposits; conglomeratic rocks beneath the Imperial Fm here include rock-avalanche deposits containing PR-type granitoid rocks. If the Garnet Hill sequence accumulated on the WSD hangingwall, then a similar hangingwall origin for the SGP sequence just 20 km to the NW is likely.

This inference implies that (1) a late Miocene strand of the WSD projects westward through SGP and (possibly) beyond (Matti & Langenheim, 2008); (2) this strand bottoms the SGP sequence; (3) the San Jacinto Mts massif was a rising footwall block that shed debris into the Miocene-Pliocene SGP hangingwall sequence. Quaternary contraction within SGP has uplifted the marine Imperial Fm by as much as 1.1 km, assuming deposits now situated at ~800 m accumulated 300 m below sea level (McDougall & others, 1999).