Structural variation and landward vergence at the toe of the Oregon accretionary prism

The Oregon margin near 45°N spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike‐slip faults in the Cascadia Basin. Because sediments thicken toward the east, translation along the strike‐slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the décollement and concomitant change in structural style across the faults. Although the Oregon‐Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping décollement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are favored.