Kinematic analyses are performed on a seismic profile across the deformed protothrust zone (PTZ) of the Cascadia accretionary prism, off the central coast of Oregon, in order to quantify the deformation in this zone and to explore mechanisms and controls on deformation and dewatering. The analyses rely on seismically defined stratigraphy and seismic interval velocities to constrain distortional and volumetric changes within the PTZ, permitting the calculation of the complete deformation field. Two solutions are offered: the first is based on porosity estimates derived from seismic interval velocities by Cochrane et al. <1994b> which suggest extremely high volume loss in the immediate footwall of the frontal thrust; the second solution employs a modified volume distribution in which lateral shear deformation is minimized. This latter approach is justified by evidence from seismic profiles and in drill cores for subhorizontal maximum principal stresses in many prisms. The two solutions yield total shortening of about 1.46 km (12%), with a general landward increase in horizontal shortening and vertical extension; this appears to be accompanied by increasing brittle deformation and porosity decrease, perhaps enhanced by dewatering along protothrusts. The greatest difference between the solutions lies in estimates for tectonically induced volume loss in the footwall of the frontal thrust, which is significantly reduced in the second solution. Relying on kinematic and mechanical arguments that support this result, we propose that the high seismic velocities observed in the footwall of the thrust may result from the presence of intergranular cement, perhaps preferentially precipitated adjacent to the frontal thrust fault in sediments with relatively high fracture permeability and enhanced fluid flow. A high degree of lithification in this zone is consistent with the inferred increase in brittle deformation in the footwall of the thrust.¿ 1997 American Geophysical Union