Influence of Thrusts within Ocean Crust to the Décollement: Insight from 3D Seismic Data off Kii Peninsula

To reveal evolution of the plate boundary decollementof the Nankai accretionary prism, we used 3D seismic reflection data off the Kii peninsula. We observe that geometry of the oceanic crustsurface strongly influences on the decollement. Furthermore the geometry of crustsurface was controlled by the displacement along the thrustswithin oceanic crust; the offset at the dip of the thrustis ~1 km. These thrustswithin ocean crustshould be still active because their locations are consistent with the hypocentersof the 2004 earthquake off the Kii peninsula. We observe the decollementtransition at the landward side of the elevated basement, and two decollementhorizons exist. This transition of the decollementcould originate underplatingand induce prism thickening. On the seismic profile, several discontinuous reflections are observed above the decollementseaward of the elevated basement, and imbricate thrustsseem to sole down into the discontinuous reflections but do not extend down to the basal decollement. Furthermore, the accretionary prismis thickenedon the seaward side of the mega-splay fault and the thickeningcannot be explained only by increasing the thrustangle. From these observations, we interpret that the accretionary wedgehas thickenedby underplatingon the seaward side of the mega-splay fault. Therefore, the crustal elevation due to thrustdisplacements within the oceanic crustis important in the underplatingprocesses in our survey area.