Tectonic underplating in the Nankai Trough off the Kii peninsula: Insight from Kumano 3D seismic reflection data

To reveal the evolution of the plate boundary decollement of the Nankai accretionary prism off the Kii peninsula, we use three-dimensional seismic reflection data across the accretionary prism. The characterization of the decollement in the front of the mega splay fault will support the broader goals of the Integrated Ocean Drilling Program to drill through the mega splay fault. On the seismic profiles, we observe several discontinuous reflections above the decollement seaward of the mega splay fault. Imbricated thrusts lie above the discontinuous reflections and these thrusts seem to sole down into the discontinuous reflections, but do not extend down to the basal decollement. Furthermore, the accretionary prism is thickened on the seaward side of the splay fault and the thickening cannot be explained only by increasing the thrust angle. From these observations, we interpret that the accretionary wedge between the splay fault and the trench has thickened by sediment underplating. Furthermore when we focus on the decollement geometry, we observe underplating structures. The underplating seems to be induced by elevation of the underlying oceanic crust. On the seismic profiles, we observe several thrusts within the oceanic crust that are causing the crustal thickening. These thrusts within the crust should be still active because they coincide with the location of the 2004 earthquake that occurred off Kii peninsula. Furthermore, although the polarity of the decollement in most parts of the survey area is positive (the same as the seafloor and the top of the oceanic crust), the polarity above the thrusts within the crust is negative or ambiguous. We interpret this as an indication that seismic velocity of underthrust sequence below the decollement is lower than that of the overlying accreted sequence. The fluid-supply through the thrusts (fractures) within the crust may reduce seismic velocity below the decollement. Therefore, the thrusts within the oceanic crust are important in the evolution of the decollement in our survey area.