CRUSTAL ARCHITECTURE OF THE CASCADIA FORE-ARC

Seismic profiling data indicate that the thickness of an accreted ocea nic terrane of Paleocene and early Eocene age, which forms the basemen t of much of the forearc beneath western Oregon and Washington, varies by approximately a factor of 4 along the strike of the Cascadia subdu ction zone. Beneath the Oregon Coast Range, the accreted terrane is 25 to 35 kilometers thick, whereas offshore Vancouver Island it is about 6 kilometers thick. These variations are correlated with variations i n are magmatism, forearc seismicity, and long-term forearc deformation . It is suggested that the strength of the forearc crust increases as the thickness of the accreted terrane increases and that the geometry of the seaward edge of this terrane influences deformation within the subduction complex and controls the amount of sediment that is deeply subducted.