GEOLOGIC STRUCTURE OF THE BEAUMONT-BANNING AREA, CALIFORNIA FROM GRAVITY PROFILES

Geologic mapping, groundwater contouring and gravity studies in the Banning-Beaumont area were used to interpret the geologic structure of the western San Gorgonio Pass. A sliver of San Gabriel terrane is sandwiched in map view between the San Jacinto Mountain bedrock to the south and San Bernardino Mountain bedrock to the north. North over south low angle thrust faults place San Gabriel terrane over sediments, including the Imperial Formation, beneath the southern part of the Banning Bench. Kyanite schist in the San Gabriel terrane correlate with similar schist in the Cargo Muchacho Mountains and are offset approximately 180 kilometers along the Mission Creek branch of the San Andreas Fault. The Banning fault bounds the south side of the San Gabriel terrane, but appears to truncate at shallow depth at a low angle thrust fault. The Banning fault may be the offset surface outcrop of the South Branch of the San Andreas Fault. Northwest trending faults in the Peninsular Ranges Block form groundwater barriers across the Pass. The northwest trending McMullen fault has strike-slip reactivation along the west flank of the Banning Bench allochthon. Recent thrusting appears to cross the McMullen fault along the west flank of the Banning Bench. Inactive listric normal faults along the north flank of the San Jacinto Mountains are similar in style to the San Pedro Martir fault associated with the opening of the Gulf of California. A west-northwest trending fault offsets basement beneath the Banning Bench allochthon and may correlate with the "South Pass fault".