The Quaternary tectonic and structural evolution of the San Felipe Hills, California

We examine the transition between extension and strike-slip in the San Felipe Hills, western Salton Trough, southern California using new and compiled geologic mapping, measured stratigraphic sections, magnetostratigraphy, and structural analysis. A 625 m measured section describes the Borrego, Ocotillo, and Brawley formations in the SE San Felipe Hills and constrains a regional disconformity and correlative angular unconformity at ∼ 1 Ma. Sedimentation rates for the Brawley Formation above the disconformity range from 1.0 to 1.2 mm/yr, paleoflow was to the ENE. The Brawley Formation consists of three interbedded lithofacies; (1) fluvial and fluvio-deltaic, (2) lacustrine, (3) and eolian deposits. Changes in facies, provenance, and paleoflow, with deposition of Ocotillo and Brawley formations record onset and evolution of cross cutting strike-slip faults other than the San Jacinto fault zone in the western Salton Trough at ∼ 1 Ma.