University of Birmingham, Birmingham, United Kingdom

Ocean Drilling Program observations at many accretionary wedges indicate that fluid flow in low-permeability sediments is focused along faults rich in clay minerals. Recent ring-shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads. It is, thus, unlikely that observed focused flow occurs through the matrix of muddy faults, and we suggest that focused flow can only be explained by the development of open fracture systems. The lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot, however, exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse. We propose that the regional hydrofracture networks must propagate heterogeneously, leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions. March 1994