To reveal evolution of the plate boundary décollement of the Nankai accretionary prism, we used 3D seismic reflection data off the Kii peninsula. We observe that geometry of the oceanic crust surface strongly influences on the décollement. Furthermore the geometry of crust surface was controlled by the displacement along the thrusts within oceanic crust; the offset at the dip of the thrust is ~1 km. These thrusts within ocean crust should be still active because their locations are consistent with the hypocenters of the 2004 earthquake off the Kii peninsula. We observe the décollement transition at the landward side of the elevated basement, and two décollement horizons exist. This transition of the décollement could originate underplating and induce prism thickening. On the seismic profile, several discontinuous reflections are observed above the décollement seaward of the elevated basement, and imbricate thrusts seem to sole down into the discontinuous reflections but do not extend down to the basal décollement. Furthermore, the accretionary prism is thickened on the seaward side of the mega-splay fault and the thickening cannot be explained only by increasing the thrust angle. From these observations, we interpret that the accretionary wedge has thickened by underplating on the seaward side of the mega-splay fault. Therefore, the crustal elevation due to thrust displacements within the oceanic crust is important in the underplating processes in our survey area.


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