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 decollement 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 mu(r) = similar to0.25 and mu(r) = similar to0.11 in clay horizons, respectively. Such values agree with theoretical estimates from critical wedge theory (Nankai: mu(b) = similar to0.16-0.26 and Barbados: mu(b) = similar to0.06-0.09) and fault spacing geometries from seismic profiles (Nankai mu(b) = similar to0.12-0.23 and Barbados: mu(b) = similar to0.11-0.19). Maximum pore pressure ratios of lambda* = 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 similar to10 MPa or less in the frontal similar to50 km of the decollement 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 decollement. 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. (C) 2003 Elsevier B.V. All rights reserved.