Testing Models of the Oceanside Blind Thrust through High-Resolution Mapping and Age Control on Marine Terrace Deformation, Coastal Southern California

Marine terrace shorelines provide information on vertical tectonic motions, thereby yielding constraints on rates and styles of deformation for underlying structures, such as blind thrust faults. In coastal southern California, the Oceanside Blind Thrust (OBT) has been inferred from offshore seismic reflection data, and its intersection with the coast has been inferred to be the source of uplift of the San Joaquin Hills (SJH). The OBT has been interpreted to be the result of a tectonically inverted Miocene detachment fault, and has been hypothesized as a late Quaternary seismic source underlying coastal San Diego and southern Orange counties. Late Quaternary motion on the OBT should deform and uplift Quaternary marine terraces. To test OBT seismic source models, we collected over 3000 high-resolution GPS elevation data points for flights of Pleistocene marine terraces spanning the southern California coastal zone from central San Diego County through the city of Newport Beach in Orange County. We mapped the terraces by geomorphically tracing out and correlating individual shoreline exposures. In addition, we compiled subsurface geotechnical borehole data that supplemented our survey data where urban development had obscured or obliterated the original geomorphic relationships. From these new data, the shorelines for terraces below 140 m elevation are observed to remain at nearly constant elevation from San Diego northward through Camp Pendleton. The lowest two terraces that date to MIS 5.1 and 5.5 show minor variation in San Clemente, and then gently decrease a few meters in elevation towards the north in the vicinity of Newport Bay, consistent with the observation that terraces below about 60 m elevation show no significant variation along the coast from San Diego into Newport Beach. These observations do not appear to support late Quaternary activity of the OBT, and raise questions about how to reconcile recent seismicity in the SJH with published models of an underlying blind thrust.