Large-scale Basement-involved Landslides, California Continental Borderland

Large seafloor relief, shallow metamorphic basement, and seismic activity in the California Continental Borderland combine to produce major submarine slides capable of generating local tsunamis. The Catalina Schist basement complex of the Borderland contains melange units, faults between metamorphic facies, and foliation derived from a history of deep subduction underthrusting and subsequent tectonic exhumation along regional low-angle fault systems. Neogene volcanic and sedimentary rocks covered the exhumed metamorphic basement forming an interface with a strong contrast in material properties. Neogene transtensional block-faulting formed steep escarpments that cut through and elevated the low-angle detachment surface, providing slip surfaces with free faces susceptible to failure along the rheological boundary. Two basement-involved slope failure examples west of San Diego are described, including large block-glides and progressive rotational slumps along the steep northeast-facing Thirtymile Bank escarpment and along the southwest flank of Fortymile Bank.