QUATERNARY TECTONIC EVOLUTION OF THE CENTRAL SAN JACINTO FAULT ZONE, SOUTHERN CALIFORNIA

The San Jacinto fault zone (SJFZ) has long been studied with respect to fault segmentation for seismic hazard because it is young, seismically active, and disorganized. Recent geophysical investigations have described the mechanical nature of the crust and the regional stress field: thick strong Peninsular Ranges (PR) crust, versus thin weak Salton Trough (ST) crust. Existing models of strike-slip fault development suggest that active segments should decrease in number and increase in length through time as slip accumulates, but the present-day SJFZ displays pronounced spatial variation in geometry, with fault complexity decreasing to the NW in the PR.

The Clark (CF) and Coyote Creek (CCF) faults of the SJFZ have about 15 and 6 km of offset, respectively. Stratigraphic and structural relationships show that folding began in the San Felipe Hills in early Pleistocene time in a restraining stepover between the Imperial (IF) and Clark faults. During this early phase, slip on the CF did not significantly deform sediments exposed in the Borrego Badlands. Sometime after ~700 ka the CCF and Superstition Mountain fault began to accumulate slip, most likely due to their favorable orientation and the increasing resistance to deformation in the San Felipe Hills. N-S contraction and folding in the Borrego and Ocotillo Badlands began at about the same time (post-700 ka) in response to initiation of slip on the CCF. N-S shortening in the original CF-IF restraining stepover continued after initiation of the CCF, causing progressive northward deflection of the CCF and creation of a restraining bend that contributed to contraction in the western Borrego Badlands.

The CF may have developed as the initial strand because it was the closest and weakest link to the IF as the SJFZ propagated to the southeast. The presence of weak, thin crust in the western ST caused multiple faults to develop, creating multiple potential sources of EQ rupture and permitting a history of simultaneous slip on the CF and CCF and related faults and folds. Because of the mechanical contrast between the ST and PR crust, the central SJFZ may not evolve in the future into a well organized fault zone with few steps and long strands.