Arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes. The method involves the solution of the Lagrangian form of the conservation of mass equation for the two-dimensional finite deformation gradient field within discrete deforming domains. We apply this technique to the unfaulted protothrust zone (PTZ) along a western transect across the Nankai accretionary prism, in the vicinity of Deep Sea Drilling Project sites 582 and 583. The results yield horizontal shortening estimates of about 1.7 km, or about 20%. This shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion. Shallow sediments respond to horizontal shortening dominantly by expelling pore fluids, producing volume strains up to 30%, while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70% in the footwall of the frontal thrust. Horizontal shear strains within the PTZ are not well resolved by this solution due to uncertainties in the porosity distribution, but they generally appear to be quite small.