Forearc slope deformation above the Japan Trench megathrust: Implications for subduction erosion

Subduction erosion is a commonly invoked model that is used to explain the tectonic subsidence of the Japan Trench forearc slope, although other models have explained the morphology and history of the margin. New multichannel seismic reflection and bathymetric data collected after the 2011 Tohoku earthquake provide the opportunity to investigate the detailed structure of the overriding plate near the earthquake epicenter and obtain new constraints on tectonic models. We use regional-residual separation of the local bathymetry to constrain fault scarp extents and local landward-dipping forearc basins. Seismic images of these basins clearly show landward-dipping horizons in the shallow section. The strata in these basins imply a different mechanism for formation than the surrounding forearc slope, and we propose that these basins formed from local uplift. A regional basal unconformity mapped ∼150 km along-trench has highly variable relief, indicating that forearc slope subsidence occurs at multiple wavelengths in response to multiple different sources. We characterize the upper to middle slope transition and propose that this region may be the landward limit of major subduction erosion and also the main region for large mass wasting. Normal faults found in this setting have maximum lengths of ∼20 km, limiting their role in margin processes. Our results place constraints on the extent of major subduction erosion at the Japan Trench margin, and indicate that subduction erosion should be revisited as the sole model of formation to include additional tectonic processes.