Timing and magnitude of shortening within the inner fore arc of the Japan Trench

New structural data and kinematic modeling provide evidence for Plio-Quaternary, inner fore-arc shortening inboard of the Japan Trench, northeastern Honshu, accommodated by the Futaba fault, a high-angle, basement-involved fault that bounds the Abukuma massif on the east. Significant throw along the Futaba fault associated with exhumation of the massif is implied by a regionally extensive footwall syncline, the absence of Neogene sediments in the hanging wall, and high relief in the hanging wall adjacent to the fault. Kinematic fault-related fold modeling best reproduces fold geometry with 2.0-3.1 km of dip slip along a 40°-55° west dipping reverse fault. At the southern tip of the fault, tephra horizons of known age within units that predate and postdate deformation bracket the onset of deformation to 3.95-5.6 Ma and are used to calculate an average slip rate of 0.5-0.7 mm/yr, a throw rate of 0.3-0.5 mm/yr, and a shortening rate of 0.3-0.5 mm/yr. The northeastern Japan subduction zone is viewed as a classic example of an erosive margin, where offshore subsidence records have been used to argue for Neogene basal erosion of the upper plate. Tectonic erosion rates have been estimated from reconstructions of the paleomargin that assume no upper plate deformation and temporally constant fore-arc taper. Evidence presented here for Neogene fore-arc shortening, however, suggests that the upper plate is deformable and implies that that offshore subsidence records may reflect a combination of tectonic erosion and upper plate shortening.