MIOCENE TO RECENT TECTONIC EVOLUTION OF SAN PEDRO BASIN AND SANTA CATALINA ISLAND: EVIDENCE FROM HIGH-RESOLUTION SEISMIC REFLECTION IMAGES

Seismic imaging reveals two major sediment sequences in San Pedro Basin, suggesting a two-stage history for the basin and its surrounding uplift features, extending from Mohnian time to the present. Approximate dating of the sediments is derived from industry wells on San Pedro Shelf and sequence dating in Santa Monica Basin, carried into San Pedro Basin by means of newly acquired high-resolution seismic images. The deformed deeper sequence is imaged to depths up to 2 km below the 900 m deep seafloor. The late Quaternary upper sequence is relatively undeformed.

A narrow, multi-sequence sediment package underlies the Catalina Shelf, and probably surrounds Santa Catalina Island entirely. The new seismic profiles show the package resting on probable schist basement. A gently-sloping surface on the basement has an outer edge resembling that of a wave cut terrace, currently at a depth of 400-500 m subsea. A working hypothesis is that this surface is associated with the 600 ka stillstand; if so, the subsidence rate would have been about 1/2 to 1 mm/y. Several benches (in multibeam bathymetry images; Cal State Monterey Bay) have been eroded into the surface of the package (seafloor).

These data suggest that the island has been subsiding for approximately the last 600,000 years, which more or less corresponds to the second stage of San Pedro Basin history. That the area is subsiding is consistent with the lack of well-defined terraces on the island itself. In contrast, Palos Verdes Peninsula, with its many terraces, has been uplifting for at least 600,000 years. Thus San Pedro Basin is asymmetric.

The first stage of San Pedro Basin history was markedly different than the second, with significant uplift of Catalina, and small to moderate uplift of Palos Verdes and the adjoining San Pedro shelf. Onlapping strata along the NE margin of the basin, adjacent to Palos Verdes, are younger than those along the Catalina margin.

The San Pedro Basin fault cuts across the basin, has seafloor expression, and has offset both sequences. Other faults in the region show restraining geometries in uplift areas such as Palos Verdes, Catalina, and Lasuen Knoll. Possible interaction between San Pedro Basin fault and these other faults, and implications for the apparent changes in uplift rates in different locales are currently under study.