Friction experiments on saturated sediments and their implications for the stress state of the Nankai and Barbados subduction thrusts

The Nankai and Barbados forearcs have low-stress subduction thrusts. The sediments entering the subduction zone, and namely the material in the décollement zones, have been well characterized by numerous deep-sea drilling legs and studies of the recovered cores. Nankai has high heat flow and significant amounts of illite, while Barbados is a smectite-dominated system. Based on results from ring shear (<2 MPa normal stress) and direct shear (<30 MPa) tests on marine sediments and mineral standards, this translates into a residual frictional resistance of μr=∼0.25 and μr=∼0.11 in clay horizons, respectively. Such values agree with theoretical estimates from critical wedge theory (Nankai: μb=∼0.16–0.26 and Barbados: μb=∼0.06–0.09) and fault spacing geometries from seismic profiles (Nankai μb=∼0.12–0.23 and Barbados: μb=∼0.11–0.19). Maximum pore pressure ratios of λ*=0.85 and 0.73 for Nankai and Barbados, respectively, allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only ∼10 MPa or less in the frontal ∼50 km of the décollement zone, respectively. Our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone, with the first probably dominating the shallow décollement. Shear velocity stepping tests show that the clay-dominated gouges strengthen velocity irrespective of the clay mineralogy, and hence suggest that clay transformation does not affect the onset of seismogenesis.