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    Solid earth sciences

    Punctuated growth of an accretionary prism and the onset of a seismogenic megathrust in the Nankai Trough

    Kimura G, Koge H, Tsuji T

    Nankai Trough, Accretionary prism, Seismogenic zone, Forearc basin, Subduction zone, Megathrust, Collision, Amurian Plate, Okhotsk Plate, Kumano Basin

    A schematic geologic profile at ~ 6 Ma and present. A buoyant slab of the Philippine Sea Plate might have been stagnant beneath the crust of southwest Japan after the abnormal volcanic activity at ~ 13 Ma (Kimura et al. 2014) and the subduction of the slab restarted at ~ 6 Ma. The present rupture area in the eastern Nankai region might be controlled by the dense and stiff upper plate originating from the middle Miocene igneous rocks (Kimura et al. 2018).

    Ocean drilling in the Nankai Trough forearc suggests a new scenario for the evolution of the Nankai subduction zone. Continuous subduction since the Late Cretaceous has been a common tectonic scenario, although the plate subduction was transferred from the Pacific Plate to the Philippine Sea Plate during the Miocene. Seismic reflection studies coupled with drilling have demonstrated that two episodes have controlled the recent evolution of the Nankai forearc: a resurgence of subduction at ~ 6 Ma after cessation since ~ 12 Ma and rapid growth of the accretionary prism since ~ 2 Ma because of the influx of large amounts of terrigenous sediments from the Japan Alps in central Japan. Both episodes were synchronous with large-scale plate reorganizations. The westward subduction of the Philippine Sea Plate initiated both in the Ryukyu and the Philippine trenches at ~ 6 Ma. Rifting in the Okinawa and Mariana troughs started at ~ 6 Ma. Compressive tectonics in northeast Japan started at ~ 3–2 Ma, and resultant mountain building with active surface erosion commenced in central Japan at ~ 2 Ma. This recent compressive tectonic phase might be due to the initiation of convergence of the Amurian Plate with the Okhotsk or North American Plate along the eastern margin of the Japan Sea. In addition to this event, the strong collision and indentation of the Izu-Bonin Arc since ~ 2.5 Ma was also enhanced in central Japan.