Abstract

The Santa Cruz Island Volcanics comprise a thick sequence of volcanic and volcaniclastic rocks that crops out north of the Santa Cruz Island fault. The volcanic field includes lava flows, flow breccia, minor pyroclastic-flow deposits, and epiclastic breccia and conglomerate that accumulated in a sub-aerial and submarine environment. The volcanic rocks overlie in subsurface probable Saucesian-age San Onofre Breccia and are overlain by lower Relizian strata of the Monterey Formation. The eruptive center for the volcanic field, a complex zone of shallowly emplaced plugs east of Devils Peak, is juxtaposed to the south against pre-Cenozoic basement rocks along the Santa Cruz Island fault, a west-northwest-trending strike-slip fault.

The volcanic rocks range petrographically from olivine basalt to pyroxene and hornblende dacite, with pyroxene andesite forming the bulk of the volcanic sequence. Andesitic and dacitic rocks contain a distinctive three-pyroxene phenocryst and microphenocryst assemblage of coexisting augite, hypersthene, and pigeonite. The volcanic suite has SiO₂ contents that range from 52 to 70 percent and is characterized by moderate Al₂O₃, low K₂O, and relatively high TiO₂. Plots of MgO versus total iron show a slight but definite iron-enrichment. The Santa Cruz Island Volcanic suite is similar in major-element chemistry to the tholeiitic suite of island-arc lavas. This similarity and the volumetric predominance of andesite in the volcanic section suggest the Santa Cruz Island Volcanics may be part of an island-arc sequence and thus record the former presence of a Miocene subduction zone off coastal southern California. However, the proximity of the East Pacific Rise to the continental margin during Miocene time suggests that the rise may have played an important but unknown role in magma generation. The volcanic rocks of Santa Cruz Island and the southern borderland province overlap in time with major change in volcanic association throughout the western United States. The predominance of andesite and the tholeiitic to calc-alkaline chemical character of the borderland lavas of the broader region.