STRUCTURE OF THE SAN GORGONIO PASS REGION, SOUTHERN CALIFORNIA, BASED ON ANALYSIS OF GRAVITY AND MAGNETIC DATA

Gravity and magnetic data, some just recently acquired, and surface geology provide a multi-layered image of the subsurface San Gorgonio Pass (SGP) region. This image serves as the basis for inferring fault block movements within the San Andreas Fault system that are fundamental to understanding the tectonic evolution of the SGP region.

Short-wavelength magnetic anomalies north of the Banning Fault (BF) indicate that the exposed upper-plate magnetic San Gabriel Mountains (SGM)-type rocks extend south only to the BF. Gravity data indicate Miocene-Quaternary sedimentary rocks are less than 1 km thick in SGP, but thicken dramatically west of Beaumont, comparable to those at the northern end of the Salton trough. Gravity modeling also indicates that the sedimentary rocks have been underthrust northward, 5 km in places, beneath the SGM upper plate rocks. The upper plate is modeled as a southward-thinning wedge, a shape consistent with the absence of a conspicuous edge-related magnetic anomaly at the BF. The basement topography beneath the sedimentary deposits is not reflected in the magnetic anomalies, suggesting that the outcropping non-magnetic Peninsular Ranges (PR) granitoids south of SGP have been underthrust northward beneath the BF in the shallow subsurface. This inference is consistent with interpreted, north-directed buried prongs of granitic rock whose map pattern mimics the sawtooth pattern of Quaternary thrust and tear faults in the SGP Fault zone, thus suggesting basement influence on the faulting. Subtle, long-wavelength magnetic anomalies indicate that a deeply buried magnetic rock unit originates north of SGP, extends southward under the PR granitoids, and has a southern edge that roughly parallels but lies 5 km south of the BF.

We propose a possible explanation for these observations. Initial right-slip on the BF dismembers the northern Salton Trough and transports part of the basin 25 km W to its present site, W. of Beaumont. BF movement ceases at 5 Ma, followed by NS contraction across SGP, perhaps resulting from a bend in the BF. A wedge of PR basement and its overlying sedimentary rocks is driven into the block north of the BF, peeling up the thin upper plate, cutting off the BF at shallow depth, and moving the upper plate northward, leaving the deeper ancestral BF 5 km to the south.