Gravity and aeromagnetic maps of the San Gorgonio Pass region, California: Potential insights from potential-field data on fault and basin geometry in a restraining bend

Victoria E. Langenheim, & Jonathan C. Matti

Published August 6, 2018, SCEC Contribution #8286, 2018 SCEC Annual Meeting Poster #272

Gravity and aeromagnetic maps superposed on detailed geologic mapping in San Gorgonio Pass (SGP) reveal details of fault and basin geometry where the southern San Andreas fault (SAF) breaks up into multiple strands as it curves into a restraining bend. Prominent gravity lows reflect basins beneath Coachella Valley, the Beaumont area south of the Banning fault, and San Jacinto Valley. Smaller gravity lows are strung out east-west along the SGP south of the Banning fault and beneath Morongo Valley. Gravity data indicate that the northern part of Coachella Valley is underlain by an asymmetric basin that deepens NE towards the Mission Creek strand of the SAF. The fault dips steeply NE based on the position of the steepest horizontal gravity gradient at the base of the gradient and modeling of magnetic data that inform on the downward projection of the fault dip into basement. The SW basin margin, in contrast to the linear NE basin margin, appears to be stepped. Along the SW basin margin, the SE-most alignment of density boundaries coincides with the Banning strand of the SAF. North of Garnet Hill, the maximum horizontal gradient steps ~2 km to the left (SW) for a distance of 5 km, before stepping again to the SW onto the Garnet Hill fault near the mouth of Whitewater Canyon, where the boundaries curve to a N trend. Within SGP, basin deposits are generally less than 1 km deep east of the Beaumont area. The late Miocene Banning fault forms the northern margin of the exposed basin deposits, but the presence of this fault at the base of a gentle gravity gradient indicates the basement-bounding fault dips shallowly to the north, with basin deposits extending as far as 5 km north of the fault at the surface. The gentle gradient bounding the northern part of the SGP contrasts with steep, irregular gradients that mark the south side of the SGP and that delineate buried prongs of basement that extend northward into the Pass--mirroring the sawtooth pattern of Quaternary thrust and tear faults in the SGP fault zone. These prongs may reflect undulations on the West Salton Detachment, a low-angle normal-fault complex that evolved coevally with the southern SAF. In this scenario, the late Miocene marine Imperial Formation and non-marine strata at Garnet Hill and similar-aged deposits in SGP represent deposits that formed in the hanging wall of the detachment. We speculate that the earlier extensional faulting influenced later compressional deformation.

Key Words
fault geometry, basin geometry, San Andreas fault

Citation
Langenheim, V. E., & Matti, J. C. (2018, 08). Gravity and aeromagnetic maps of the San Gorgonio Pass region, California: Potential insights from potential-field data on fault and basin geometry in a restraining bend. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)