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Description
The primary objective of this study is to gain an understanding of the tectonic significance of structures observed and mapped in the Pliocene-Pleistocene (?) Borrego Formation in the southeastern San Felipe Hills, Imperial Valley, California. To reach this objective structures that were mapped in the Borrego Formation were compared to those produced by experimental wrench fault deformation. A secondary objective of this study is to subdivide the Borrego Formation into a number of informal mappable members that may prove useful in future sedimentological studies. Borrego Formation exposures in the study area are composed of approximately 750 m of mudstone (60%), siltstone (3%), sandstone (36%), and conglomerate (1%). The Borrego Formation was subdivided into eighteen informal members. The stratigraphy and sedimentology of the Borrego Formation indicates that it contains lacustrine and fluvial/deltaic deposits. Sandstones present in the study area are arkosic arenites. Sandstone petrography, utilizing the Gazzi-Dickinson point-counting method, indicates a mixed source for sandstones in the Borrego Formation. Feldspathic detritus probably was derived from the Peninsular Ranges and was mixed with quartzose detritus derived initially from the Colorado River drainage basin. Structural fabrics developed in the study area consist of en echelon folds, conjugate strike-slip faults, normal faults, subvertical tension fractures, and a thrust fault. Mesoscopic to megascopic folds trend east-west, are angular parallel, close to gentle, and commonly disharmonic. Northwest trending strike-slip faults are synthetic, and northeast trending strike-slip faults are antithetic with respect to dextral slip in the northwest trending San Jacinto fault zone. Structural fabrics present in the study area are strain compatible with a north-south maximum principal stress direction, and are indicative of deformation at shallow crustal levels. The results of simple wrench faulting clay model experiments performed in this study approximately duplicate the structural fabrics mapped in the southeastern San Felipe Hills. No throughgoing wrench faults are present in the vicinity of the study area, but the distribution of earthquake epicenters associated with the San Jacinto fault zone indicates active seismicity extending beneath portions of the San Felipe Hills. These results indicate that structural fabrics observed in the southeastern San Felipe Hills are the result of dextral basement wrench faulting in the San Jacinto fault zone. These structures have probably been developing since the deposition of the Borrego Formation in the San Felipe Hills. Wrench fault deformation in the study area is still it its early stages, as no throughgoing wrench fault has developed. Shortening percentages observed in the southeastern San Felipe Hills are generally consistent with proposed rates of slip on the San Jacinto fault zone. Balanced cross-sections constructed to basement depths require bedding plane decollements near the base of the sedimentary section. These decollements result from the brittle-ductile response of sediments above brittle crystalline basement rock to shortening at shallow crustal levels. Fault-propagation folding in response to the stepping up of these thrusts is also indicated by the geometry of the cross-sections.
