Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Continental Borderland

Christopher C. Sorlien, Jonathan T. Bennett, Marie-Helene Cormier, Brian A. Campbell, Craig Nicholson, & Robert L. Bauer

Published July 30, 2015, SCEC Contribution #2082

Changing conditions along plate boundaries are thought to result in the reactivation of pre-existing structures. The offshore southern California Borderland has experienced dramatic adjustments as conditions changed from subduction tectonics to transform tectonics, including major Miocene oblique extension, followed by transpressional fault reactivation. However, consensus is still lacking about stratigraphic age models, fault geometry, and slip history for the near-offshore area between southern Los Angeles and San Diego. In that area, we interpret an extensive dataset of seismic reflection, bathymetry, and stratigraphic data to determine the three-dimensional geometry and kinematic evolution of the faults and folds. The resulting structural representation reveals a moderately landward-dipping San Mateo-Carlsbad fault that converges downward with the steeper, right-lateral Newport-Inglewood fault, forming a fault wedge affected by Quaternary contractional folding. This fault wedge deformed in transtension during late Miocene through Pliocene time. Subsequently, the San Mateo-Carlsbad fault experienced 0.6 to 1.0 km displacement, spatially-varying between reverse-right-lateral to transtensional right-lateral. In contrast, shallow parts of the previously identified gently-dipping Oceanside detachment and faults above it that are located farther offshore appear to have been inactive since the early Pliocene. These observations, together with new and revised geometric representations of additional steeper faults and the evidence for a pervasive component on these near-shore faults, suggest a need to revise the earthquake hazard estimates for the coastal region.

Sorlien, C. C., Bennett, J. T., Cormier, M., Campbell, B. A., Nicholson, C., & Bauer, R. L. (2015). Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Continental Borderland. Geosphere, 11(4), 1111-1132. doi: 10.1130/GES01118.1.