A model of late Cenozoic transcurrent motion and deformation in the fore-arc of northeast Japan: Constraints from geophysical studies

We use recently acquired seismic reflection data and reprocessed sections to interpret the structure of the northern part of the NE Japan forearc. We identify a NNW-SSE trending deformation zone bounded by large transcurrent faults (trends T1–T4 from west to east) on the upper continental slope. The lower slope is characterized by a frontal prism and reverse faults. Bathymetric data reveal several en echelon topographic bulges with NW-SE elongation aligned parallel to the fault trends. The bulges are interpreted as a dextral wrench zone that has developed since the Neogene, based on structural contours of two late Cenozoic seismic horizons. Clockwise rotation associated the wrenching is indicated by the paleomagnetism of Paleogene marine sediments obtained from a drill hole within the faulted zone close to trend T1. A previous seismic refraction study shows remarkable contrast in P-wave velocity across trend T2, indicating a considerable rearrangement of crust associated with transcurrent motion along this fault. We propose a paleogeographic reconstruction assuming >200 km southward (trenchward) lateral transportation along trend T2 and its northern extension. In our model, subduction erosion at the northern Japan Trench was enhanced by trenchward migration of the fore-arc sliver, but steadied following rearrangement of the sliver along the transect. The calculated variation in subduction erosion is consistent with subduction histories based on existing well data.