Consolidation state and strength of underthrust sediments and evolution of the decollement at the Nankai accretionary margin: Results of uniaxial reconsolidation experiments

Uniaxial reconsolidation experiments conducted on Ocean Drilling Program drill cores along the Muroto Transect of the Nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the decollement zone. Tests were conducted on samples collected from similar stratigraphic levels below the decollement and its seaward projection. Consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples. The sediments, however, exhibited substantial postyield strength: up to 2.8 times the predicted in situ effective vertical stress beneath the protothrust zone. This enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section. The close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state, the sediment matrix remains sensitive to in situ stress conditions. The low yield stress of one sample, collected within similar to40 m of the decollement fault at Site 808, may reflect diagenesis under reduced effective stress conditions, due to postconsolidation increases in pore pressure along the decollement. Due to their cemented state, the strong underthrust sediments resist decollement downcutting beneath the toe of the prism, leading to preferential incorporation of weaker, continuously deforming accreted sediments during shear. Seismogenic slip along the decollement at depth may create stresses in excess of sediment strength, causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the decollement zone.