Hydrostratigraphy as a control on subduction zone mechanics through its effects on drainage: an example from the Nankai Margin, SW Japan

摘要

At many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate,nand excess pore pressures commonly develop as low-permeability marine sediments undergo rapid tectonicallyndriven loading. Mechanical models demonstrate that pore pressure controls the overall geometry of these systemsnby modifying shear strength both within the accretionary wedge and along its base. At the Nankai margin off-nshore SW Japan, the taper angle of the accretionary wedge varies markedly along-strike, from u00024u0002 along an east-nern (Muroto) transect, to 8–10u0002along a western (Ashizuri) transect. Sediment stratigraphy on the subductingnplate also varies: along the Ashizuri transect, the lowermost part of the section includes abundant sandy turbi-ndites, whereas along the Muroto transect it is composed of monotonous hemipelagic mudstone. Here, I use annumerical model of fluid flow, together with laboratory measurements that constrain the bulk mudstone perme-nability, to quantitatively test the hypothesis that the turbidite-rich section along the Ashizuri transect allows drain-nage at the base of the accretionary complex, resulting in differences in mechanical strength sufficient to causenthe differences in taper angle. My results demonstrate that if the turbidite-rich units are 2–100 times more per-nmeable than the mudstone units, the variation in stratigraphy can indeed explain the observed taper angles. Inncontrast, permeability anisotropy within the turbidite-rich units has only a minor effect; anisotropy ratios ofnu00021000:1 would be required to cause the differences in taper angle. Along the Ashizuri transect, simulated porenpressures result in a basal shear strength ranging from a few MPa at the trench to u000220 MPa by 30 km arcward;nalong the Muroto transect shear strength is substantially lower, reaching only u00025 MPa by 30 km. This worknshows that lithostratigraphy can strongly influence the mechanical behavior of subduction zone faults, through itsncontrol on the distribution and magnitude of excess pore pressure.