Coupled fluid flow and deformation modeling of the frontal thrust region of the Kumano Basin transect, Japan: Implications for fluid pressures and decollement downstepping

Abstract

[1] Results of coupled fluid flow and deformation modeling illustrate the influence of frontal thrust slip on excess pore pressures and decollement downstepping in the Kumano Basin transect, offshore Japan. Although permeable trench sediments and a buried sandstone layer provide potential fluid escape pathways, the numerical modeling suggests that excess pore pressure could build beneath the frontal thrust within hemipelagic clay of the Shikoku Basin facies. Simulated excess pore pressures range between 45% and 77% of the overburden weight 3 km landward of the deformation front, and are very sensitive to the permeability of the Shikoku Basin clay and the coefficient of friction of the fault zone. Excess pore pressures increase as the fault coefficient of friction is increased, suggesting that volume change due to shear within the footwall sediments is an important contributor to excess pore pressures. Raising the coefficient of friction also increases shear within the sediments beneath the fault zone, suggesting downward migration of the decollement zone. This shearing focuses in the weaker Shikoku Basin clays rather than an underlying turbidite unit, suggesting that lithologic differences play an important role in controlling deformation location.

Shear strength of sediments approaching subduction in the Nankai Trough, Japan as constraints on forearc mechanics

“…The toe area is considered to be well drained, based on lack of observed focused fluid flow and the presence of the trench wedge turbidites , therefore we use ¼ 0.5 representing approximately hydrostatic conditions. However, numerical modeling results integrating fluid flow and fault slip suggest that in the underlying Shikoku Basin sediments beneath the frontal thrust, overpressures of 45-77% of the overburden may exist [Rowe et al, 2012]. Therefore, we also test the case where ¼ 0.65, as a lower estimate for overpressured conditions.…”

The impact of megasplay faulting and permeability contrasts on Nankai Trough subduction zone pore pressures

“…Influences of tectonic stresses have been overlooked, although they may propagate seaward of the deformation front [e.g., Shi and Wang , 1985; Stauffer and Bekins , 2001]. In addition, shear stresses may transmit beneath the decollement, depending on fault friction [ Rowe et al , 2012]. This study focuses on the accretionary prism and addresses the impact of tectonic and gravitational stresses including shear on pore pressures in the megasplay fault system.…”

“…On the basis of literature values, a Young's modulus (E) of 5 × 10 11 Pa was applied to the basalt [ Domenico and Schwartz , 1998] and a Poisson ratio of 0.3 is used throughout the model. For clay‐rich sediment, a Cam‐Clay parameter, Λ with a value of 0.32 (corresponding to a compression index, Cc, of 0.74) was assigned based on the Site C0011 porosity data [ Rowe et al , 2012]. Another Cam‐Clay parameter, κ , was assigned a value of 0.01 based on the swelling index of unloading curves from consolidation tests performed on Muroto transect samples [ Bellew , 2004].…”

Data report: permeability and microfabric of mud(stone) samples from IODP Sites C0011 and C0012, NanTroSEIZE subduction inputs

“…To quantify the hydrogeologic anisotropy of Shikoku Basin sediments, we compared horizontal (cross-core) permeability (k h ) and vertical (along-core) permeability (k v ) at the same sampling depths. These tests add to an extensive set of transect-wide hydrogeological data as part of the Nankai Trough Seismogenic Zone Experiment (e.g., Dugan and Daigle, 2011;Ekinci et al, 2011;Rowe et al, 2012;Yue et al, 2012;Dugan and Zhao, 2013;Screaton et al, 2013).…”