Analysis of Geophysical Evidence of Cretaceous-Paleocene Forearc Strata in the Santa Catalina Basin, California Inner Continental Borderland

The California continental borderland has a complex tectonic history but limited quality of geophysical data. By the use of multichannel seismic reflection data, geophysical studies were conducted to define the upper crustal structure and delineate the main lithotectonic belts of the region. Due to a highly reflective sea bottom, however, strong multiples were generated masking all primary reflections beyond the arrival of the first-order multiple waves. In this study, using a new prestack two-way reverse time migration (RTM) which treats multiples as signal rather than noise, the multiple artifact was significantly attenuated in the Los Angeles Region Seismic Experiment (LARSE) data. The RTM maps both the primaries and multiples to their points of origination: seabed multiples to the sea bottom and primaries to the reflecting interfaces. Based on these results, five stratigraphic units were recognized to a depth of 6 km. The upper two units are Pliocene plus younger strata and Miocene syntectonic fill. The two deeper stratigraphic units are interpreted to be forearc sedimentary strata, though previous studies had interpreted the top unit as the Catalina Schist. The seismic characteristics of these units are similar to those of the Cretaceous-Paleogene forearc section of the Outer Continental Belt. Miocene volcanic layers form the fifth unit. Shallow plutons are inferred to be present under several bathymetric high areas.

Two additional seismic profiles in the inner continental borderland (ICB), northwest of LARSE lines, revealed evidence of forearc Cretaceous-Paleogene strata. This evidence was tied to the OCS-CAL P289 well drilled in the ICB, which has 1250 m of forearc strata. In previous studies, this forearc section of Cretaceous to early Miocene sedimentary rocks was not thought to be preserved in the ICB. This newly found forearc tectonic section in ICB, bounded by the San Clemente Fault to the southwest and Santa Catalina Fault to the northeast, is mapped from the south of Catalina Island to a well near Santa Barbara Island. The existence of the forearc sedimentary belt in the Catalina Basin has important geologic significance to suggest significant revisions of the current structural and stratigraphic interpretations of the California borderland.