Geochemistry and Origin of Middle Miocene Volcanic Rocks from Santa Cruz and Anacapa Islands, Southern California Borderland

Major-oxide and trace-element compositions of middle Miocene volcanic rocks from north Santa Cruz and Anacapa Islands are very similar. In contrast, they are geochemically distinct from the volcanic clasts from the Blanca Formation, of similar age but located south of the Santa Cruz Island fault, which implies significant strike-slip movement on this fault. The island lavas are also compositionally distinct from the Conejo Volcanics located onshore in the Santa Monica Mountains. The island lavas are part of a larger group of about 12 similar-aged volcanic suites from the California Borderland and onshore southern California that all belong to the calc-alkaline magma series. This group is interpreted to have originated in a subduction environment in one of two possible scenarios: 1) their magmas were produced from the subduction of the Cocos plate south of the Rivera triple junction, which would imply that the Baja-Borderland allochthon onto which they were emplaced experienced northwestward translation subsequent to eruption or 2) their magmas were produced from warm, young subducted slab located north of the Rivera triple junction, which would imply that slab persisted in the Los Angeles area after the triple junction had migrated to tire south.