Rapid forearc basin uplift and megasplay fault development from 3D seismic images of Nankai Margin off Kii Peninsula, Japan

Offshore Kii Peninsula, Japan, a large thrust within the overriding forearc, the megasplay fault, appears to move coseismically during great earthquakes. 3D seismic images of the Kumano forearc basin that overlies the megasplay, correlated with IODP drilling data, are a potential record of the history of large-scale motion along this structure. In the early Quaternary, uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay. More extensive landward tilting of the outer basin sediments across the seismic volume occurred over ~ 300 kyr in the middle to late Quaternary (1.3–1 Ma); this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry. Extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late Quaternary tilting and in many cases offset the modern seafloor; these faults may form either due to gravitational response to the uplift or as a by-product of sediment underthusting. These results suggest that the megasplay is a recently formed and transient structure and support the idea that out-of-sequence thrusts serving as the dominant structure for convergence-driven shortening in a subduction zone may be short-lived geologically but dominate a margin during these intervals.