Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

Brian Boston,Gregory F Moore,Yasuyuki Nakamura,Shuichi Kodaira

doi:10.1186/1880-5981-66-135

Abstract

Multichannel seismic reflection lines image the subducting Pacific Plate to approximately 75 km seaward of the Japan Trench and document the incoming plate sediment, faults, and deformation front near the 2011 Tohoku earthquake epicenter. Sediment thickness of the incoming plate varies from <50 to >600 m with evidence of slumping near normal faults. We find recent sediment deposits in normal fault footwalls and topographic lows. We studied the development of two different classes of normal faults: faults that offset the igneous basement and faults restricted to the sediment section. Faults that cut the basement seaward of the Japan Trench also offset the seafloor and are therefore able to be well characterized from multiple bathymetric surveys. Images of 199 basement-cutting faults reveal an average throw of approximately 120 m and average fault spacing of approximately 2 km. Faults within the sediment column are poorly documented and exhibit offsets of approximately 20 m, with densely spaced populations near the trench axis. Regional seismic lines show lateral variations in location of the Japan Trench deformation front throughout the region, documenting the incoming plate’s influence on the deformation front’s location. Where horst blocks are carried into the trench, seaward propagation of the deformation front is diminished compared to areas where a graben has entered the trench. We propose that the décollement’s propagation into the trench graben may be influenced by local stress changes or displacements due to subduction of active normal faults. The location and geometry of the up-dip décollement at the Japan Trench is potentially controlled by the incoming outer-rise faults.

Highlights

When large bathymetric features, such as seamounts, fracture zones, ridges, and oceanic plateaus, are subducted at convergent margins, they strongly deform the landward trench slopes (e.g., McCann and Sykes 1984; Lallemand and Le Pichon 1987; Dominguez et al 1998; Taylor et al 2005)
Our depth-migrated regional and high-resolution seismic lines that image the incoming Pacific Plate along the Japan Trench off Tohoku show trench-parallel faults that offset the igneous crust on the outer-rise seaward of the trench
Recent sediment deposits are present in some normal fault footwalls and near-trench topographic lows

Summary

Introduction

When large bathymetric features, such as seamounts, fracture zones, ridges, and oceanic plateaus, are subducted at convergent margins, they strongly deform the landward trench slopes (e.g., McCann and Sykes 1984; Lallemand and Le Pichon 1987; Dominguez et al 1998; Taylor et al 2005). Smaller features, such as subducting horst and graben structures, were once considered to play a role in sediment subduction and upper plate abrasion by horst blocks (Hilde 1983), but better imaging of the subducting plate suggested the subduction plane was well above the top of such features (von Huene and Culotta 1989). Normal faults produced by plate flexure seaward of the trench penetrate into the mantle are thought to allow hydration of the upper mantle and crust (Ranero et al 2003; Grevemeyer et al 2007) leading to a potential role in overpressuring and hydrofracturing of the upper plate at depth (von Huene et al 2004)

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Discussion

Conclusion