Recent analyses of multichannel seismic data for the Cascadia margin off central Oregon show a systematic, landward increase in velocity (increase in sediment dewatering) within a protothrust zone about 10 km wide seaward of the toe of the accretionary wedge (Cochrane et al., 1994). Here we analyze the mechanical conditions for the transmission of compression from the rear of the accretionary wedge to the area seaward of the frontal thrust to create the protothrust zone and to cause sediment dewatering. The analysis assumes that sediments are elastoplastic (Coulomb) material; i.e., they behave elastically up to a yield limit at which failure occurs. We show that the necessary conditions for the transmission of compression from the accretionary wedge to the seaward area are (1) the presence of incipient decollements seaward of the frontal thrust and (2) the presence of high pore pressures within the incipient decollement. We also show that the magnitude of pore pressure within the incipient decollement may control the width of the protothrust zone. We suggest that the latter determines the location of the next frontal thrust and hence the ramp spacing between the imbricated thrusts within accretionary wedges. Thus pore pressures within incipient decollements may significantly influence the evolution of the internal structures of accretionary wedges. ¿ American Geophysical Union 1994