Lithostratigraphic controls on dewatering and fluid pressure in the western Nankai subduction zone: Implications for the drainage behavior and consolidation state of the underthrust sequence

In subduction zones, fluid-pressure generation in the underthrusting section is of particular interest because it governs the effective stress conditions of the footwall lining the plate interface. Only a few studies have systematically explored the role of lithological heterogeneity of underthrust sediment on the resulting fluid pressure and its distribution. We used a coupled loading and pore-pressure dissipation model with a new compilation of sand properties to investigate the role of such heterogeneity on the drainage state beneath the plate interface in the western Nankai subduction zone offshore Japan, where the incoming sediment sequence hosts numerous sand layers with a total thickness of up to ~210 m within a matrix of hemipelagic mud. Our results show that sand layers act as important conduits for both pressure translation and solute transport from greater depth to the trench and seaward. The simulated pore pressure is mainly controlled by aggregate sand-layer transmissivity, and to second order by sand-layer depth, which affects the ability of fluids to access permeable sands from the surrounding less-permeable mudstone matrix. Modeled sand permeability in the outer subduction system is in the range of previous estimates for décollement zone permeability (10 ^-13 to 10 ^-16 m ² ) and evolves to approximately three orders of magnitude lower permeability in the inner subduction system. The enhanced drainage leads to 15% lower excess pore pressures in models with sands than without sands. Thus, differences in the lithostratigraphy of the subducting sediment should have implications for the mechanical behavior along the Nankai subduction system.