| In response to the subduction of the young, hot, thickly sedimented Juan de Fuca plate, sediments are offscraped along the Washington-Oregon continental margin. Cascadia Basin sediments offscraped along the Washington margin during the last two million years have formed a distinct physiographic province, the lower slope or borderland. This lower slope consists of a series of anticlinal thrust-folds composed of highly dewatered terrigenous and hemiterrigenous sediments.;The seismic data indicate a 400-800 meter thick basal high velocity sediment layer extending from the basin to the lower slope with velocities increasing landward from 2.6 to 4.6 km/sec.;Several mechanisms for the formation of this high velocity basal layer are discussed. These include the diagenesis of sediments due to thermal effects of the young, hot, well insulated oceanic lithosphere; lithification due to carbonate cementation; and for slope sediments, consolidation of sediments by tectonic shearing due to the motion of the subducting crust.;Numerical models of the accretion process are developed using finite element analysis. The accreting sediments are modelled as a Newtonian viscous incompressible fluid. Results of the numerical modelling indicate that a low strength zone (decollement) may exist above the basal well consolidated sediments. In the numerical models, this weak zone allows the formation of anticlinal folds seaward of the existing slope, as is observed along the Washington margin.;In order to better define the accretion process on the Washington margin, seismic refraction and reflection data from the Cascadia Basin and lower slope are presented. These data detail the geometry of the subducting oceanic crust and mantle, and the sediment velocity structure in the basin and lower slope. Results of these data indicate that the oceanic crust and mantle dip at a shallow landward angle (less than one degree) across the Cascadia Basin and under the lower continental slope. |
