Mechanical characterization of slope sediments: Constraints on in situ stress and pore pressure near the tip of the megasplay fault in the Nankai accretionary complex

We performed mechanical tests on mudstone samples cored in the hanging wall of a major out of sequence thrust fault in the Nankai accretionary complex prism, SW Japan, (1) to understand the consolidation and shear behaviors of slope sediments that form the fault zone and wall rock of this important structural feature in the upper several kilometers, and (2) to constrain in situ stresses and pore pressure, two fundamental parameters governing deformation processes in subduction zones, which are notoriously difficult to measure. We conducted deformation experiments on a suite of samples taken from depths of 20-150 mbsf, including six uniaxial consolidation tests and one isotropic consolidation test. For three of these tests, we conducted undrained triaxial compression testing following consolidation. Our results suggest that in situ pore pressure is hydrostatic and that during sedimentation and burial, the effective horizontal stress is ∼41% of the effective vertical stress. In combination with analysis of wellbore failures documented during drilling of the borehole, our experimental data allow us to define the complete stress tensor, including the magnitude of in situ minimum and maximum horizontal stresses, in the hanging wall of this major fault zone. The maximum horizontal stress magnitude is comparable to that expected for sedimentation and uniaxial burial, whereas the minimum horizontal stress lies below this value. This suggests (1) that the shallow sedimentary section was subjected to extension subparallel to the trench during or following burial and (2) that stresses associated with plate convergence are not effectively transmitted within these shallow sediments.