Geomorphic expression of graben subduction and implications for décollement evolution at the Japan Trench

Horsts and grabens are ubiquitous structures on incoming plates, yet few direct constraints exist on how the subduction of these outer-rise fault systems affects the geometry, mechanics, and structural evolution of the plate boundary interface and surrounding wall rock. In northeastern Japan, the subduction of horsts and grabens have been proposed to impart spatial variations in the composition, evolution, and geometry of the decollement, but the resolution of geophysical data can limit the area over which these properties can be mapped. However, the growing abundance of high-resolution seafloor digital elevation models (DEMs) permits the adaptation of topographic analyses commonly used in terrestrial landscapes to the submarine environment to invert for deformational processes in the shallow subduction system. Here we present new analyses of bathymetric DEMs at the Japan trench and evaluate how the upper plate and shallow decollement evolve as outer-rise normal faults are subducted. We observe elongate topographic lows on the overriding plate frontal prism that have similar length, widths, and depths as incoming plate graben. Mapping of seismic reflection data in this area confirms that these overriding plate topographic basins are a direct geomorphic response to the subduction of horsts and grabens on the lower plate. Our data suggest graben-like topographic basins in the overriding plate may develop in response to two processes: slip on an undulating décollement that steps over and down into subducted graben and deforms the overlying prism, and continued slip on subducted normal faults that offsets the frontal prism and plate boundary décollement. We also find that along-strike variations in overriding plate morphology, apparent in spatial variations in the topographic basins, correspond to overriding plate variations in frontal prism evolution and sediment flux, incoming plate variations of sediment thickness and outer rise fault throw, and potentially to different styles of megathrust rupture behavior during the 2011 Tohoku and 1896 Sanriku earthquakes.