THE SAN FRANCISQUITO FAULT, SOUTHERN CALIFORNIA: A NORMAL FAULT RELATED TO UPLIFT OF THE SIERRA PELONA ANTICLINE?

The San Francisquito fault in the Central Transverse Ranges, southern California, is an east-striking fault that juxtaposes high-grade, Cretaceous/Paleocene Pelona Schist on the south with non-metamorphosed Mesozoic and Cenozoic rock units on the north. In recent years, the fault has been interpreted as a strike-slip fault with 100km displacement that was active in the Miocene as part of the greater San Andreas fault system. Based on structural considerations, we propose an alternative model whereby the fault is mainly a dip-slip fault with normal-sense displacement.

Along the western part of its trace, the San Francisquito fault juxtaposes Late Oligocene/Early Miocene terrestrial strata of the Vasquez Formation with Pelona Schist on the south. In this area, the fault dips moderately northward parallel to the foliation of the adjacent schist, which forms the north limb of the Sierra Pelona anticline. Clastic evidence from Neogene sedimentary formations in the region indicates that uplift of the anticline occurred in the Early Miocene. Major displacement along the San Francisquito fault is also constrained to have been Early Miocene.

We believe the moderate dip of the San Francisquito fault in this area indicates that the fault is not a strike-slip fault as previously proposed. Instead, the structural configuration appears more compatible with dip-slip displacement. In addition, parallelism of the fault surface with the adjacent limb of the Sierra Pelona anticline and the synchronous age of the two structures, suggests that the fold and the fault are genetically linked. We propose that the San Francisquito fault is a normal fault that was active during folding and uplift of the Sierra Pelona anticline in the Early Miocene and that the fault accommodated deformation of rock units above the rising fold. The Pelona fault on the south side of the anticline appears to have a similar origin.