INFLUENCE OF THE PENINSULAR RANGES BATHOLITH ON THE SOUTHERN SAN ANDREAS FAULT SYSTEM, CALIFORNIA

We speculate that structure underpinning the Peninsular Ranges batholith (PRB) has influenced the evolution of various strands of the southern San Andreas Fault (SAF) system, including the location and development of the San Jacinto Fault, the complexity of the Elsinore Fault, and the dip of the southern SAF. Geologic, geochemical, and geophysical data indicate that the PRB intruded across a suture that juxtaposes mafic, oceanic-arc crust to the west against more felsic crust of continental affinity to the east. Aeromagnetic data indicate that the suture extends nearly 1400 km south-southeast of the San Jacinto fault (near Hemet) to the tip of Baja California. Seismic-velocity, gravity, and magnetic data indicate that the suture dips moderately to the east through the entire crust. A change in the strike of the suture in the middle of the San Jacinto Fault and the absence of a correlative feature across the fault suggests that the suture may have guided the location of the northern part of the fault as it initiated about 1.5 Ma to bypass a structural knot within the SAF in San Gorgonio Pass. The bifurcation of the Elsinore Fault south of Temecula may also be related to the suture.

Seismicity, InSAR, and 3-D mechanical modeling indicate that the southern SAF in Coachella Valley dips to the NE; seismic-reflection and aeromagnetic data also indicate a northeast dip, suggesting that the currently active strand and the strand responsible for cumulative displacement are the same. This implies that either the southern SAF has always dipped to the NE or, alternatively, the fault has been rotated into its current dipping geometry during its history of displacement. A NE dip over the lifetime of the fault may not be plausible given the absence of basins northeast of the fault. Basins would be predicted NE of the fault given the transtensional regime represented by the West Salton Detachment Fault before about 1.2 Ma. The fault dip may have been influenced by the NE edge of the mafic part of the PRB basement as it has been translated northwest. High shear-wave velocities attributed to western PRB basement are present 30 km north of the surface trace of the SAF in the lower crust beneath the San Bernardino Mountains, suggesting that the PRB may be partly responsible for rotating an original vertical SAF towards a moderate to steep NE dip.