VEDDER, J. G., R. G. BOHANNON, and HUGH McLEAN, U.S. Geological Survey, Menlo Park, CA

ABSTRACT: Geology of Santa Catalina Island, California Continental Borderland

Santa Catalina Island is within the Catalina terrane of the California continental borderland. This terrane makes up most of the inner part of the borderland province and it is characterized by Miocene and younger sedimentary and volcanic rocks that lie directly on Catalina Schist. The Catalina Schist, which presumably is Late Cretaceous in age, is preserved throughout the northwestern half of the island. Metamorphic grades range from blueschist to amphibolite and, although this unit is best known for its blueschist, most exposures consist of chlorite greenschist facies rocks. The Catalina Schist probably formed as part of an accretionary wedge associated with a subduction zone. Miocene rocks form most of the southeastern half of the island and include a quartz diorite pluton, andesit and dacite extrusive rocks, and a variety of coarse to fine-grained sedimentary rocks. The 19-Ma pluton intrudes the Catalina Schist and consists largely of pervasive, sheeted, nearly vertical dikes that locally contain xenoliths of schist and schist breccia. Repetitive intrusion of dikes within the pluton caused its outward growth in a way that suggests intrusion in an active extensional environment. The 15-14-Ma dacitic rocks form domelike masses that are flanked and overlain by 15-12-Ma andesitic flows, some of which may have been subaerial. Locally, schist breccia lenses and coarse-grained volcaniclastic strata abut and drape over the flow rocks, suggesting local differential uplifts. Middle Miocene diatomaceous shale and lapilli tuff in places overlie the volcanic rocks. The diatom ceous strata were chiefly deposited in a shallow marine environment, but they also include some freshwater species. Thin remnants of upper Miocene and lower Pliocene(?) calcarenite and tuffaceous mudstone contain fossils that indicate increasing water depth with time.

The Catalina terrane is distinct from the Santa Ana terrane to its east and the Nicolas terrane to its west, where thick sequences of Cretaceous through Oligocene forearc sedimentary rocks underlie Miocene and younger strata. The pre-Miocene sedimentary section was deposited in the forearc of the volcanic arc of the Peninsular Range. A tiny remnant of the forearc section is exposed in the easternmost part of the island and suggests that those rocks may have once covered parts of the Catalina terrane as well. Separation of the three terranes may have been caused by the tectonic unloading of the Catalina terrane on large oblique-slip and detachment faults. Displacement on these faults resulted in a west-southwest migrating hinge of uplift and tectonic denudation at the boundary between he Nicolas and Catalina terranes. The extension doubled the width of the borderland province in the Miocene.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.