Plate to Backstop: A Geophysical Investigation of Two Japanese Subduction Zones, from the Outer Rise to the Forearc Slope

Abstract

Subduction zones are among the most tectonically dynamic regions. To help resolve the tectonics of such a complex and poorly understood region, this dissertation focuses on three different settings within subduction zones, from seaward to landward, at two Japanese subduction zones, the Japan Trench and the Nankai Trough. I use multiple geophysical methods and datasets including 2D and 3D seismic reflection data, multibeam bathymetry, and logging-while-drilling data to investigate the outer rise to the forearc slope. The first part of this dissertation starts with the incoming plate’s structures influencing the deformation front along the Japan Trench. I imaged 199 basement-cutting normal faults finding faults first from striking parallel to the trench axis and reactivate pre-existing seafloor structures as the plate moves landward. The sediment thickness of the incoming Pacific Plate fluctuates between 0 to 600 m with late stage deposition occurring in multiple low relief regions. I find that the incoming outer-rise faults directly influence the location and geometry of the up-dip décollement at the Japan Trench. The second part of this dissertation moves landward to the inner accretionary prism of the Nankai Trough. I find multiple preserved structures of the outer prism with reactivation of one buried thrust estimated to have ~580 m of slip since ~1.04 Ma, after forearc basin deposition started, based on kinematic modeling. I show that the interior of the inner prism has steeply dipping beds with multiple fracture populations. The final part of this dissertation looks at deformational processes at the forearc slope of the Japan Trench. I find forearc basins that dip landward in both bathymetry analysis and in the seismic reflection data that follows basement uplift. The regional basal unconformity is mapped throughout the survey region and exhibits >1 km of relief with slope fill varying by up to ~3000 m and thinning on the seaward edge of the upper slope setting, the same location where the regional basal unconformity consistently dips seaward. This pattern indicates a landward limit to major subduction erosion and that the upper slope has deformed from additional processes at the Japan Trench margin.