KINEMATICS AND EVOLUTION OF THE SAN JACINTO FAULT ZONE IN THE SALTON TROUGH: PROGRESS REPORT FROM THE SAN FELIPE HILLS AREA

The San Jacinto fault zone (SJFZ) may slip as rapidly as the S. San Andreas fault yet the connection between the strand with the largest separation (Clark fault) and other plate-boundary faults is uncertain. Geologic mapping, gravity and magnetic data, structural analyses, LANDSAT and SPOT data, magnetostratigraphy and basin analysis in the Salton Trough all show that the kinematics and geometry of the San Jacinto fault zone change along strike. The zone widens SEward (in map view), and the current configuration of the SJFZ appears to be less than 0.57 Ma + 0.03 Ma to 0.39 Ma + 0.06 Ma old. The Clark fault probably persists into the central San Felipe Hills (SFH) as an incompletely mapped horsetail fan and en echelon zone. Gravity data show a SW-deepening step in the bedrock surface coincident with the Clark strand near the Santa Rosa Mtns. Gravity data and magnetic lineaments show the bedrock step bifurcating and persisting SE into the central SFH, SE of the tip-line identified by prior mapping.

The SE SFH preserve the most intensely folded sedimentary rocks in the area. We interpret this folding as the boundary between the domain of dextral slip and wrench folding to the NW and a broad domain of transrotation to the SE. The transrotational domain transfers slip from the Coyote Creek (CCF) and Superstition strands to the Imperial and Brawley zones in the SE (e.g., Hudnut et al., 1989) but our work shows that it also interacts the Clark strand. The NW boundary of the transrotational zone is well exposed in the SE SFH. A NE to NNE alignment of faulted en echelon domes near the NW boundary formed atop the NE-striking sinistral Extra fault zone. SE of the domain boundary around San Felipe Creek, Landsat+ Spot imagery reveal a complex network of short NE-striking faults in Quaternary sediment. The meshlike, weakly organized geometry of the transrotating sinistral faults is characteristic of fault zones in their infancy. Gravity data show that the CCF SE of Borrego Mtn. is also in its infancy because it cuts across a preexisting SW-deepening basin and is barely detectable in the gravity field. A major dextral normal fault on the NE margin of the Fish Creek and Vallecito Mountains produced this basin between 3 and 0.5 Ma and may once have been the main strand of the SJFZ. Altogether, these data suggest that transrotation SE of the SFH initiated less than ~0.5 Ma old.