Evidence for interaction of a spreading ridge with the outer California borderland

The southward passage of the Rivera triple junction and its effect on the North American plate are primary controls on the Miocene tectonic evolution of the outer borderland of California. Detrital modes of sand shed off the Patton Ridge and cored by the Deep Sea Drilling Project provide evidence of progressive tectonic erosion of the Patton accretionary prism and near-trench volcanism. Volcanic glass in the sediment is predominantly calcalkaline rhyolite and andesite, typical of subduction-related volcanism, but also includes minor low-K2O tholeiitic basalt. We attribute these compositional features to interaction with a spreading ridge associated with a possible trench–ridge–trench triple junction along the Patton Escarpment from 18 to 16 Ma. This study suggests that evidence of ridge–trench interaction may be commonly preserved along submerged plate margins, in contrast to its more limited recognition and discussion in the literature based on exposed examples in Chile, Japan and Alaska.