Smectite swelling in the Miura-Boso accretionary prism: Possible cause for incipient décollement zone formation

►Miura-Boso accretionary prism has smectite-rich homogeneous mineral composition. ►Smectite swelling produces ~ 3.0 MPa of pressures in the over-consolidated zone. ►High swelling pressure facilitates initiation of a décollement zone formation.