Seismic structure of the southern Cascadia subduction zone and accretionary prism north of the Mendocino triple junction

Abstract

Four multichannel‐seismic reflection profiles, collected as part of the Mendocino triple junction seismic experiment, image the toe of the southern Cascadia accretionary prism. Today, 250–600 m of sediment is subducting with the Gorda plate, and 1500–3200 m is accreting to the northern California margin. Faults imaged west and east of the deformation front show mixed structural vergence. A north‐south trending, 20 km long portion of the central margin is landward vergent for the outer 6–8 km of the toe of the prism. This region of landward vergence exhibits no frontal thrust, is unusually steep and narrow, and is likely caused by a seaward‐dipping backstop close to the deformation front. The lack of margin‐wide preferred seaward vergence and wedge‐taper analysis suggests the prism has low basal shear stress. The three southern lines image wedge‐shaped fragments of oceanic crust 1.1–7.3 km in width and 250–700 m thick near the deformation front. These wedges suggest shortening and thickening of the upper oceanic crust. Discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans. The southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the Gorda basin oceanic crust and overlying sediments. These high‐angle faults, bounding basement highs, are interpreted as strike‐slip faults reactivating structures originally formed at the spreading ridge. Northeast or northwest trending strike‐slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north‐south Gorda‐Pacific plate convergence.