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

Abstract

Geofluids (2010) 10, 114–131AbstractAt many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate, and excess pore pressures commonly develop as low‐permeability marine sediments undergo rapid tectonically driven loading. Mechanical models demonstrate that pore pressure controls the overall geometry of these systems by modifying shear strength both within the accretionary wedge and along its base. At the Nankai margin offshore SW Japan, the taper angle of the accretionary wedge varies markedly along‐strike, from ∼4° along an eastern (Muroto) transect, to 8–10°along a western (Ashizuri) transect. Sediment stratigraphy on the subducting plate also varies: along the Ashizuri transect, the lowermost part of the section includes abundant sandy turbidites, whereas along the Muroto transect it is composed of monotonous hemipelagic mudstone. Here, I use a numerical model of fluid flow, together with laboratory measurements that constrain the bulk mudstone permeability, to quantitatively test the hypothesis that the turbidite‐rich section along the Ashizuri transect allows drainage at the base of the accretionary complex, resulting in differences in mechanical strength sufficient to cause the differences in taper angle. My results demonstrate that if the turbidite‐rich units are 2–100 times more permeable than the mudstone units, the variation in stratigraphy can indeed explain the observed taper angles. In contrast, permeability anisotropy within the turbidite‐rich units has only a minor effect; anisotropy ratios of ∼1000:1 would be required to cause the differences in taper angle. Along the Ashizuri transect, simulated pore pressures result in a basal shear strength ranging from a few MPa at the trench to ∼20 MPa by 30 km arcward; along the Muroto transect shear strength is substantially lower, reaching only ∼5 MPa by 30 km. This work shows that lithostratigraphy can strongly influence the mechanical behavior of subduction zone faults, through its control on the distribution and magnitude of excess pore pressure.