The d¿collement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the d¿collement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this d¿collement zone may be unique from that of most thrust faults. We propose that the d¿collement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai d¿collement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the d¿collement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of ''cementation'' in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protod¿collement subsequently accommodates shear displacements along discrete fractures. ¿ American Geophysical Union 1995