Moderately-dipping California Strike-slip Faults With Bends in map View and Cross-section

Abstract:

New faults in a strike-slip stress field are expected to be sub-vertical. However, continental strike-slip faults commonly reactivate arrays of pre-existing non-vertical faults with non-optimal strike. The systems of strike-slip faults that dissect the Inner California Continental Borderland , Santa Monica Bay, and south-Central California are imaged with multiple grids of closely spaced 2D multichannel seismic reflection profiles. From these and stratigraphic information, we constructed multiple digital 3D fault and fold interpretations. Quaternary strike-slip faults that dip moderately in their upper few km include the southern Hosgri fault, part of the Carlsbad fault, and the Santa Monica-Dume fault. These faults reactivate structures that developed in pre-19 Ma subduction and subsequent Miocene extension and transtension. Seismic reflection imaging reveal that they dip 40° to 55°. 3D seismic data image the Hosgri fault to locally dip less than 30° below 1 km depth. Where Pliocene-Quaternary sediment accumulated in basins, these faults tend to propagate upwards as sub-vertical faults, as expected. Thus, cross sectional fault geometry can be used together with other information to infer strike-slip motion. Imaged gently-dipping faults underlie and/or interact with the Santa Monica-Dume and Carlsbad faults. Seismicity and tectonic models suggest that the southern Hosgri fault and faults to its west do not affect the subducted oceanic lower crust. Thus, strike-slip motion there may occur on the former subduction megathrust.

These moderately-dipping faults also describe bends and stepovers in map view, resulting in significant vertical deformation. Northwest of San Diego, the Carlsbad fault bends sharply as part of a 15 km right-stepover to the Descanso and Coronado Bank fault system. N-striking fault strands, a releasing orientation for the Pliocene to present strain orientation, connect between the Newport-Inglewood and Carlsbad faults and also between the Rose Canyon fault (same fault as the Newport-Inglewood) and the Descanso fault, within a few km of the San Diego metropolitan area. It is not known whether prehistoric earthquakes rupture through these bends, connecting faults, and stepovers, but the 3D geometry should be considered in models.