Seismic studies of the southern Cascadia subduction zone near the Mendocino triple junction

Offshore northern California lies the southern Cascadia subduction zone, where oblique subduction of the Gorda plate interacts with Mendocino triple junction tectonics. Seismic images of the Gorda plate, Cascadia accretionary prism, and Eel River forearc basin record the tectonic evolution of the region. These images allow examination of the northward migrating triple junction's influence on internal deformation of the Gorda plate, accretionary processes within the prism, and tectonic history of the Eel River forearc basin. Examination of the accretionary prism shows that 90% of incoming Gorda basin sediments currently are being accreted to the margin while only 10% are being subducted. The outer forearc is experiencing little effect by the triple junction. Tectonic history of the Eel River forearc basin includes: uplift of the outer margin at ∼3 Ma, widespread outer basin erosion at ∼1 Ma, local erosion of the continental shelf at ∼500 ka, and collapse of the Klamath plateau region by ∼250 ka. The northward migrating triple junction affected the Eel River basin by ∼500 ka causing uplift, northward tilting, and erosion south of the basin. In addition, broad uplift of the southern basin forced Eel River sediment to be deposited both further north and to Dow out the Eel Canyon. Northward compression by the triple junction has deformed the southern Eel River basin to the northeast forming northwest-southeast oriented transpressional structures. The southern Gorda plate is internally deformed along northeast-southwest oriented left-lateral strike-slip faults, rotating clockwise, and fragmenting near the triple junction. Flow of the Gorda lithosphere around the rigid Pacific plate results in delamination of the Gorda crust allowing for fragments of Gorda crust to enter the slab-window and accrete to the base of the North American plate. The propagation of Gorda plate northeast-southwest deformation beneath the margin contrasts with the northwest-southeast deformation in the southern forearc requiring a decoupled Gorda-North American plate boundary, which is further demonstrated by a mixed vergent deformation front and Mohr-Coulomb analysis of the basal and topographic taper of the wedge. The decoupled Gorda-North American plate boundary has allowed for propagation of Pacific-North American related strike-slip faulting north of Cape Mendocino.