Balanced Cross Sections, Shortening Estimates, and the Magnitude of Out-of-Sequence Thrusting in the Nankai Trough Accretionary Prism, Japan:

There has been much recent interest in out-of-sequence thrusting (OOST) in the Kumano Basin region of the Nankai Trough accretionary prism. Previous estimates of the amount of shortening accommodated by OOST in the area have only been able to constrain the movement on a single OOST and have been upable to constrain the amount of shortening accommodated by the overall zone of OOST(Strasser, et al, 2009). Investigating the total shortening accommodated by the in-sequence portion of the outer wedge and comparing it to published plate motion models allows us to estimate the total amount of shortening accommodated by the entire zone of OOST, rather than only a single structure. Strasser and colleagues (2009) estimate that the oldest folds in the in-sequence portion of the outer wedge formed at 2.5 - 2.0 Ma. Tectonic block models for Japan (Loveless & Meade, 2010) provide estimates for the component of plate motion perpendicular to the strike of the trench between the Nankai forearc sliver and the Philippine Sea Plate in the vicinity of the study area of 42 km/Myr. Because there is no evidence for active compressional structures between the accretionary prism and the Median Tectonic Line, we assume that all shortening takes place within the accretionary prism. This predicts that a total of ~80-100 km of shortening has been accommodated by the in-sequence fold and thrust belt and the out-of-sequence thrust system over the past 2.0-2.5 Myr. We use balanced cross sections across the wedge to constrain the amount of shortening accommodated by the in-sequence fold and thrust belt within the accretionary prism. Based on this, the 30 km width of the outer wedge restores to an original width of ~80km, indicating that the outer wedge has accommodated ~50 km of shortening in the same time frame, not taking into account sub-seismic and ductile shortening. Based on the present-day thickness of the thrust sheets in the outer wedge compared to the thickness of the incoming stratigraphic section, it is unlikely that there has been more than ~25% of ductile and sub-seismic brittle thickening of the thrust sheets during their deformation. This yields a maximum estimate of ~20 km of additional shortening accommodated by the in-sequence fold and thrust belt, yielding a total estimate of shortening of ~70km. Therefore, the out-of-sequence thrust system at the rear of the outer wedge has likely accommodated ~10-30 km of shortening during its lifetime. This is a much greater amount of shortening than could be accommodated by the single prominent OOST at the rear of the outer wedge, supporting the conclusions of several previous workers (e.g. Moore, et al, 2007) that the basal, seaward-most OOST has been inactive for a considerable period of time and further deformation has been accommodated by thrusts in the hanging wall of the basal OOST.