California Borderland

Map of the California Borderland, which is a broad area of basins and ranges with some islands off Southern California. Map © MBARI 2006

California borderland geology

The geologic history of the region offshore of Southern California is poorly understood. Its interpretation is complicated by the fact that it is largely submerged, is heavily sedimented, and many of the rocks from which interpretations have been made were probably erratics transported offshore from continental beaches tangled in kelp holdfasts, in tree roots, or in sea lion stomachs. The Patton Escarpment marks the edge of the continental shelf (where the orange drops off to greens and blues in the map above), and is a relict accretionary wedge from a subduction zone that was active in the Mesozoic into the Cenozoic.

Note that our work on the seamounts off of California is on the Seamounts: Continental Margin page, and our work on erratic rocks is on the Seamounts: Eclectic topics page.

Our research on California Borderland geology

Interaction of a spreading ridge with the Patton Escarpment

SOUTHERN CALIFORNIA- 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.

Reference: Marsaglia, K.M., A.S. Davis, K. Rimkus, D.A. Clague (2006) Evidence for interaction of a spreading ridge with the outer California borderland, Marine Geology, 229: 259-272. [Abstract] [Article]