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Description
The San Felipe Hills lie within the Kane Springs NW quadrangle, approximately 10 km west of the Salton Sea. The hills are located in the Salton Trough, a classical transtensional rift basin, and are transected by several intraplate, right-lateral transcurrent faults associated with the development of the San Andreas transform fault system. Approximately 6 km of marine and non-marine sediments have accumulated in the trough since Miocene time. Underlying the San Felipe Hills is comprised of the Plio-Pleistocene Ocotillo Conglomerate, the Borrego Formation, and the entirely fault-bounded Palm Spring Formation. The Plio-Pleistocene section is ~4.27 km (14,000 feet) in thickness and was deposited as a series of alluvial fans, fan-deltas, and braided streams draining into the barrier beaches of a large, shallow lake. Detailed mapping within the San Felipe Hills, and construction of four cross-sections, indicates that deformation of Plio-Pleistocene sediments occurred in response to a maximum compressive stress direction oriented ~N02W. The initial phases of deformation were plastic, resulting in a series of east-west trending, upright, gently plunging open to close folds. In the northern and southern portions of the map area, these folds verge to the south and north, respectively. Interlimb angles vary from 32° -134°. The folds described above comprise the limbs of the San Felipe Hills anticline, and are truncated by conjugate right-lateral and left-lateral strike-slip faults associated with the south-eastward propagation of the San Felipe Hills Fault. The map area is divided into four, fault-bounded structural domains; each is characterized by a relatively unique internal structural fabric. These domains surround the southern termination of the left-stepping, right-lateral San Felipe Hills fault. Based on the following two observations deformation within the San Felipe Hills is believed to be due to the development of a transitory left-step in the San Felipe Hills fault. First, orientations of structures, and a natural rheological contrast between Plio-Pleistocene sediments and crystalline basement rocks suggest the presence of a major decollement responsible for fault-propagation folding and up to 58° of clockwise rotation of large fault-bounded wedges. Second, en echelon folds and faults represent the surface expressions of wrench faulting and fault propagation folding associated with a deformation swath along the termination of a major strike-slip fault. Additionally, seismic trends suggest that at depth propagation of the San Felipe Hills fault may extend beyond the map area to the southeast, further implying the eventual development of a left step and the continuation of a major strike-slip fault.
