Abstract: Forty-Six Kilometer Westward Lateral Propagation of the Channel Islands Thrust in the Last l Ma Revealed by Axial Surface Mapping, Santa Barbara Channel, California

Karl Mueller, Maribeth Price, John Shaw, John Suppe

The Channel Islands thrust is an active crustal-scale southwest-vergent thrust ramp that is uplifting the Channel Islands and a large area of the sea bottom to the north in its hanging wall. The slip history on this thrust is revealed by an actively widening kink band that overlies the 17-km-deep base of the thrust ramp along the Oak Ridge trend of the Santa Barbara Channel. Seismic mapping of the progressive widening of the kink band in Pliocene-Pleistocene growth strata reveals the slip history of this major thrust in remarkably fine detail. We find that the fault is divided into two segments (eastern and western), based on its slip history and on the uplifted bathymetry and topography. The eastern fault segment extends along strike for 40 km in the eastern Santa Barbara Channel and has been active for the last 5.5 Ma, during which 5 to 10 km of southwest-vergent slip occurred (Shaw and Suppe, in press, GSA Bulletin). New seismic mapping between Santa Barbara and Pt. Conception shows that the bulk of slip on the western fault began much later, less than 1 Ma ago, when the thrust ramp propagated a minimum of 46.6 km along strike to the west. Total slip on the western segment varies from 5 km at its eastern edge to 0.7 km at the western limit of mapping at the mouth of the Channel. Estimates of the magnitude of future earthquakes along the combined eastern and western ramp segments suggest a maximum Ms of 7.6. Combination of the rate of dip slip on the eastern segment with the lateral propagation rate from axial surface mapping of the western segment suggests approximately 230 m of along-strike fault propagation during seismic events having a recurrence interval of 1500 yr.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994