Comparing Geologic and Geodetic Patterns of Deformation across the San Andreas Fault System in Central California

Geologic and geodetic data from the San Andreas fault system in central California provide an opportunity to examine distributed deformation across a major transform fault. The fault system geometry is relatively simple in this region, with three sub-parallel faults separating crustal blocks of different rock types. Field mapping and kinematic models of folding allow characterization of geologic strain, which we compare with modern strain rates derived from GPS data. We focus on deformation near the central creeping segment of the San Andreas fault and its transition to partially locked behavior near Parkfield.

A simple elastic model of the creeping – locked transition predicts regions of shortening northeast of and extension southwest of the San Andreas fault, which is similar to observed strain rates derived from GPS data. This modern strain rate field is also consistent with large geologic structures such as the Coalinga and Kettleman Hills anticlines to the northeast and Quaternary basins to the southwest of the fault. Furthermore, the GPS-derived maximum extension directions are roughly parallel to smaller-scale, en echelon fold hinges throughout the region. Features of the GPS strain rate field that are not captured by the simple elastic model are likely due in part to different deformation regimes in the borderlands between faults. For example, rocks on Franciscan basement seem to have accommodated more deformation than those on Salinian basement, which is consistent with seismicity records indicating that more earthquakes occur within Franciscan rocks. These data suggest that both the creeping – locked transition of the fault and the basement geology of borderland blocks are important controls on the spatial distribution of deformation across the plate boundary. The consistency of spatial strain patterns across geologic and geodetic time scales implies that distributed, off-fault deformation is an important aspect of the San Andreas fault system in this area.