Stress state in slope sediments above a major splay fault in the Nankai accretionary complex, SW Honshu

摘要

We carried out laboratory experiments to provide constrains on in situ pore pressure and stressrnstate in slope sediments above a major out of sequence splay fault in the Nankai accretionary prism,rnSW Japan. The splay fault, termed the “megasplay” separates the actively accreting outer wedge fromrna more lithified and apparently less active inner wedge. The orientation of the maximum horizontalrnstress in shallow (< ～1 km deep) boreholes inferred from wellbore failures rotates 90 degrees acrossrnthe megasplay, from trench-normal in the seaward part of the system, to nearly trench normalrnimmmediately landward of the megasplay. We conducted a suite of uniaxial and triaxial deformationrnexperiments on mudrock core samples from IODP Site C0001, located near this transition.rnPreconsolidation pressures obtained from our experiments suggest that in situ pore pressure isrnhydrostatic. Based on the K_0 values (the ratio of effective radial stress to effective axial stressrndetermined from the uniaxial consolidation tests), we infer that the effective horizontal stress shouldrnbe ～41 % of the effective vertical stress for a sedimentation and burial loading path. A series ofrntriaxial compression tests defines a failure envelope for the core samples with an internal frictionrnangle of ～30º and a cohesion of 0.08 MPa. The failure criterion defines a lower bound for the in siturnminmum horizontal stress, and also allows us to analyze wellbore failures to constrain the magnitudesrnof both the minimum and maximum in situ horizontal stresses. Our results suggest that the maximumrnhorizontal stress magnitude remains comparable to the condition expected during sedimentation (K_0rncondition), whereas the minimum horizontal stress has been reduced, leading to extension subparallelrnto the trench. We also infer that stresses associated with plate convergence are not effectivelyrntransmitted within the weak slope sediments.