QUATERNARY SUBSIDENCE IN THE INNER CALIFORNIA CONTINENTAL BORDERLAND: ECHO OF MIOCENE CRUSTAL THINNING AND THRUST-FOLDING OF THE NEAR-SHORE SLOPE

Extinction or subduction of the Pacific-microplate spreading centers resulted in extreme Miocene extension of offshore southern California as the Pacific Plate diverged from the North American Plate. This extension facilitated the clockwise rotation or the western Transverse Ranges, denuding the Catalina terrane of the Inner Borderland. High pressure accretionary rocks of the Catalina schist now exposed on submerged ridges and Catalina Island were uplifted tens of km. The main reason the Borderland is mostly submerged is that thinned crust eventually stands lower. Most thinning and volcanism occurred during early and middle Miocene time and initial rapid subsidence should have occurred shortly thereafter. Miocene subsidence and deposition in the Inner Borderland Santa Monica and northern San Pedro Basins, and beneath the Gulf of Santa Catalina (offshore San Clemente-San Diego) was localized to half grabens above low-angle normal faults. The current basins had previously been relatively high-standing footwalls of certain of these Miocene faults. Subsidence of the modern basins initiated, reorganized, or accelerated after Miocene time as existing basins inverted to become the Santa Monica Mountains, and Long Beach to San Diego shelf and slope. Tectonic denudation and rock uplift of the Catalina Schist may have contributed to a 10 million year delay in regional subsidence of parts of the Inner Borderland.

A late episode of subsidence beneath the Gulf of Santa Catalina is suggested by Quaternary angular unconformities reminiscent of wave-cut platforms that now are at depths from 500 m to more than 1 km. Progressive tilting of turbidites from Long Beach to San Clemente City show that if the outer shelf has no vertical motion, the slope must be subsiding with rates increasing SW towards the basin. This tilting is at a much larger scale than expected from restraining or releasing double bends on strike-slip faults and suggest a contribution from regional blind thrust faults. Such faults have been proposed, but generally with SW dip and wide offshore limbs being back limbs. We propose instead that NE-dipping low-angle Miocene normal faults along the base of the slope have been reactivated as blind (oblique?) thrust faults. Progressive tilting is thus on the forelimb of the related folds.