HIGH-RESOLUTION GEOPHYSICAL MAPPING OF THE OUTER CALIFORNIA BORDERLAND: TECTONICS AND REPEATED SUBMARINE LANDSLIDES IN CORTES BASIN

Repeated mass transport deposits (MTDs) are identified within fault-bounded basins of the Outer California Continental Borderland and are likely associated with intermittent seismicity and/or sea level variations of the Quaternary. These types of mass transport and episodic slope failures may be capable of triggering tsunamis, which may pose a hazard to the coastline of southern California. Here, we present a novel high-resolution bathymetric and sub-bottom dataset to map and reconstruct the history of repeated MTDs in relationship with the tectonics of the Cortes Basin, located ~170 km offshore west of San Diego, California. The Cortes Basin is a ~3600 km² double-depocenter composed by steep bounding slopes and a central Cortes Ridge separating the east and west basin, and aligned NW-SE, parallel to the regional fabric. We mapped and dated ten distinct MTD events during the last ~760 ka using oxygen isotopic measurements of benthic foraminifers from the ODP Leg 167 (Site 1012). The MTDs extend across the East and West Cortes basins with an estimated total volume of >4 km³ of basinward mobilized sediment. The most recent MTD architecture observed in the West Cortes Basin shows an erosional base truncating the surficial Holocene drape, a thicker proximal complex composed by slumps and compressional ridges, and a thinner distal flow deposit with parallel bedding pinching out at the toe scarp. The ten stacked MTD events mapped along fault segments likely occurred during earthquakes within the basin, where tectonic subsidence and slope steepening facilitated the slope failure, alternating with the pelagic drape deposited during interseismic phases. Using our age model, we calculated an event recurrence interval of ~84 ka over a period of 760 ka. Additionally, we mapped new fault segments across Cortes Basin observed in sub-bottom profiles and bathymetric sea floor expression indicating Holocene activity. The recurrence of such earthquake-induced slope failures and the fault structures presented here are currently not included in dynamic models of strike-slip earthquake ruptures in the Southern California region and have important implications for coastal seismic and tsunamigenic hazard.