The deformation and diagenetic processes in sandstone blocks in underthrusted tectonic m¿lange from the Shimanto Belt, Japan, were analyzed to understand the onset of seismogenesis in subduction zones. The earliest deformation is characterized by "web" cataclasis with grain size reduction in association with ubiquitously developed microshears. A porosity of about 10 ¿ 3% is estimated for this stage. Subsequently, pore cementation of the sandstone by calcite, quartz, and chlorite occurred, together with pressure solution along the web margin. The porosity suggests a depth of about 3 km below seafloor. Finally, the sandstones were boudinaged, and tension gashes were filled by quartz veins in the neck of the boudins. At this stage, cementation was fully accomplished, and the porosity of the sandstone was reduced to less than a few percent. The final deformation was due to shear parallel to the layering. P-T conditions for the final necking were about 150--190¿C and 150--200 MPa, coinciding with the upper part of the seismogenic zone. Our study on the m¿lange suggests that relative plate motion is partitioned in time and space in a plate boundary zone.