Robert D. Francis , California State University (Long Beach)
San Pedro Basin fault - San Diego Trough fault system: a recently integrated fault complex (150-600 ka), offshore southern California Inner Borderland
Robert Francis, Mark Legg, Chris Castillo, and Luke Shafer
This paper reports the results of our investigation using seismic reflection data collected and interpreted using the funding provided by the Petroleum Research Fund. Chris Castillo and Luke Shafer are the two undergraduate students who worked on the project, supported by the PRF grant. Both students participated in our cruises, in reducing and interpreting seismic data, and in preparing images and topographic maps from publicly available multibeam bathymetry raw data. A third student, Charles Parrish, participated in the early phases of the project. All three students have since graduated from CSULB. Luke is now in the graduate program at the University of Wyoming, and Chris has recently been accepted into the graduate program at Stanford University with full support for Fall 2012. Luke’s graduate project involves use of seismic data and well logs to develop a fracture model for the Rock Springs uplift in Wyoming, a potential sequestration site for carbon dioxide. Chris is interested in pursuing a geophysics-related project at Stanford. Without a doubt, working on the San Pedro Basin project has been excellent preparation for the future careers of these very promising students.
Our data, combined with industry 2D seismic data, acquired in 1981-82 and recently made public, show that the San Pedro Basin fault (SPBF) is continuous with the San Diego Trough fault to the south (SDTF), forming a 350km major, active fault system in the California continental Borderland. High resolution multibeam bathymetry images show offset channels and scarps that, when correlated with seismic images, were shown to be the result of a throughgoing fault. These features are in line with the SPBF and SDTF, and show a continuous fault. The previously held view is that the SDTF was linked with the Catalina fault, which runs in a more northwesterly direction along the southwest side of Catalina Ridge. This fault geometry would form a restraining bend, and Catalina Ridge (with Santa Catalina Island as the portion of the ridge above sealevel) would be a pop-up structure resulting from this restraining bend. Previous data and mapping had the SPBF not extending far enough south to link with the SDTF.
High resolution multibeam bathymetry around Santa Catalina Island reveal at least five seafloor benches or terraces. These terraces, and the lack of well-defined terraces on the island itself, indicate that Catalina Ridge has been subsiding for the last several hundred thousand years. Of two major sediment sequences in San Pedro Basin, the upper one is relatively undeformed, while the lower dips steeply away from the flank of the Catalina Ridge, which forms the southwest margin of the basin. The date of the sequence boundary appears to be around 150-600ka (an age correlated by seismic from the Ocean Drilling Program site in Santa Monica Basin to the north), similar to the oldest of the marine terraces.
We propose that translation along the SDTF continued on the CF, forming the restraining bend that caused uplift of Catalina Ridge in Miocene to mid-Pleistocene time, and that this translation was at least partially transferred from the CF to the SPBF in the late Pleistocene (150 to 600ka). This would have largely eliminated the shortening that created the pop-up, and in its absence the dense rocks of the metamorphic Catalina Schist and Miocene igneous rock basement of the uplift would be expected to sink due to isostatic adjustment in the crust. Thus the marine terraces around the island and the upper sedimentary sequence in San Pedro Basin are contemporaneous and owe their origin to the same event, namely the transfer of motion from the CF to the SPBF.
The SPBF-SDTF system is 350 km long, extending from Point Dume on the Malibu coast into Baja California, coming ashore again near Punta Banda as the Maximinos or transpeninsular Agua Blanca faults. This length is similar to those of better known faults such as the San Clemente or Newport-Inglewood-Rose Canyon fault systems. As such, the SPBF-SDTF system represents potential for large (M>7) earthquakes and associated tsunamis. This fault system has been largely overlooked in most previous seismic hazard investigations for southern California. Large landslides along the steep escarpments surrounding the island and submerged ridge, some revealed by our seismic data, demonstrate that the tsunami threat persists today. Deformation (slip) rates, which help to quantify earthquake and tsunami potential requires, are difficult to determine for offshore faults, although recent efforts with remotely-operated vehicles (ROV) and autonomous underwater vehicles (AUV) have begun to bear results. This type of work is important, and it requires accurate prior mapping of the faults to be studied.