8. Growth of Accretionary Wedges off Vancouver Island and Oregon

A simple formulation for the growth of a wedge of constant taper accreting a sediment layer of given thickness and exponential decrease of porosity with depth was used to estimate the rates of growth of the accretionary wedges off Vancouver Island and Oregon. Application to the section off Vancouver Island indicates that the accreted sediments penetrated at Site 889 were added to the toe of the wedge between 0.8 and 1.1 Ma. Application to the Oregon section shows that the thickness of the accreted layer before 0.3 Ma must have been substantially less than the present 1.5 km for Pliocene sediments at Site 892. For both wedges the accretion of a thick layer of sediment from the Cascadia Basin appears to have occurred over only the past few hundred thousand years, and no more than about a million years. Since the Eocene, the equivalent average thickness of the layer of sediment that has been accreted is about 0.5 km.