Neogene Fault-normal Compression Revealed by a 3D Geologic Map Centered on the San Andreas Fault Zone in the Parkfield Region, California

We used a combination of surface geologic map, gravity, and aeromagnetic data, and scattered well logs to construct a three-dimensional (3D) geologic map of a volume 30 km wide centered on the San Andreas Fault (SAF) Zone in the Parkfield region. We drew five two-dimensional (2D) cross sections from surface geologic map data, developed a 2D gravity and magnetic model based on each cross section, adjusted the cross sections to better match the geophysical data, and then integrated the revised cross sections into a 3D geologic map using Move™ software. The 3D map shows Neogene fault-normal compression throughout the volume. The SAF, including various abandoned strands, is straight in the region relative to the scale of the volume, so compression is not related to fault steps or bends outside the immediate vicinity of the fault. East of the fault the 3D map reveals ~66% shortening of middle to late Miocene strata, rooting into a complex system of east and west-verging reverse faults. The faults in this system nearest to the SAF dip away from it, so the system is not a flower-structure related to the SAF. Instead, the system is probably driven by upfolding of a deep ophiolitic slab that may also be driving Franciscan wedges east beneath the Coast Ranges/Great Valley margin. Ongoing seismicity, including the 1983 M6.5 Coalinga earthquake, shows the wedge system is presently active, suggesting that the upfolding and the fault system east of the SAF are active as well. Directly adjacent to the SAF, the 3D map shows a ~3 km-wide zone of tight folding (up to 80% compression of strata as young as Plio-Pleistocene) and faulting related to the SAF, including the active Southwest Shear Zone and Buzzard Canyon Faults as well as the Gold Hill and Lang Canyon faults, possible proto-SAFs. Farther west, the 3D map reveals modest (~16%) compression related to a blind fault with about 1 km of reverse throw. The relatively undeformed nature of the overlying Pleistocene strata suggests the western system is no longer active.