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 Outer Continental Borderland offshore southern California consists of an arkose-bearing accretionary prism or truncated forearc basin with two subsequent discrete periods of calcalkaline volcanism and a third prolonged period of alkaline seamount volcanism. The volcanics exhibit progressively less continental influence/contamination with time. The earliest period consists of calcalkaline lavas dated at 30 million years ago (Ma) that erupted prior to the Pacific–Farallon spreading ridge reaching the California margin. These are moderately contaminated by continental rocks. The
second more widespread period of calcalkaline activity, from 16 to 18 Ma, consists of lavas ranging from basaltic andesite to rhyolite that generally experienced less continental contamination than the 30 Ma volcanics. The 16 to 18 Ma lavas apparently formed as a triple junction migrated along the margin, although the offshore lavas are not midocean ridge basalt–like, as expected if generated within a slab window. The third, even younger period of volcanic activity produced Rodriguez Seamount on the Patton Escarpment; Northeast Bank within the Outer California Borderland; and San Juan, San Marcos, Little Joe, Davidson, Guide, Pioneer, and Gumdrop Seamounts on the Pacific Plate. These youngest lavas, previously described in the literature, have little to no signature of continental contamination, although lavas that built Rodriguez and Northeast Bank migrated through continental crustal materials. These results are largely based on samples either dredged or collected via remotely operated vehicle from the seafloor. The widespread occurrence of erratics in our California Borderland sample set indicates that their presence must be weighed when reconstructing the geology of the region.

Reference: Clague, D., Marsaglia, K.M., Cousens, B.L., Paduan, J.B. (2019) Oligocene and Miocene volcanics in the sedimentary forearc of the Outer California Borderland. Society for Sedimentary Geology, 110, 22-42. doi: 10.2110/sepmsp.110.04

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. doi: 10.1016/j.margeo.2006.02.006