BASIN AND FAULT EVOLUTION IN THE INNER BORDERLAND, OFFSHORE SOUTHERN CALIFORNIA

Seismic and bathymetric data show that the San Pedro Basin fault (SPBF) and San Diego Trough fault (SDTF) are continuous and thus form a >300 km offshore shear zone in the California Continental Borderland. This continuous shear zone has existed since 600-900 ka. Previously, slip on the SDTF was transferred to the Catalina fault (CF), forming a restraining bend that caused the uplift of Catalina Ridge. Cessation of that uplift is indicated by submarine terraces surrounding Santa Catalina Island at <350 m depth.

The current SPBF-SDTF shear zone separates discordant terranes of the Inner Borderland (IB), whose basement consists of the Catalina Schist that was exhumed during rifting and transrotation of the Western Transverse Ranges (WTR) after about 20 Ma. We propose that prior to about 5 Ma the northern part of the IB consisted of a large basin that was subsequently broken up by basin inversion along through-going faults such as the SPBF. Older sediments in San Pedro Basin continue under adjacent Catalina Ridge and Palos Verdes Peninsula. San Pedro Basin is thus a remnant of the earlier, larger basin; Los Angeles Basin and parts of Santa Monica Basin could also be remnants. Sequence stratigraphic analysis in San Pedro Basin reveals incipient uplift and basin inversion (5-14 Ma), and syn- or post-breakup development of a sigmoidal pull-apart basin bounded by the SPBF, contemporaneous with the uplift of Catalina Ridge (1-5 Ma).

Restoration of WTR rotation and slip on the San Clemente fault and SDTF back to 16 Ma brings six volcanic islands, including the Northern Channel Islands presently on the south margin of the WTR, into proximity with a group of large seafloor calderas. This and another volcanic chain (El Modeno, Glendora and Palos Verdes) seem to flank the ancient large IB basin; each lies near a right step in the break-away zone at the eastern margin of the IB.

IB history thus involves formation of a large basin behind the rotating WTR, flanked by two volcanic chains, and characterized by fault systems accommodating significant extension and dextral shear. Evolution of the continental margin (such as changes in plate motion vector, locus of plate margin) caused episodic changes in shear zones leading to basin breakup and inversion.