HETEROGENEOUS HYDROFRACTURE DEVELOPMENT AND ACCRETIONARY FAULT DYNAMICS

Ocean Drilling Program observations at many accretionary wedges indica te that fluid flow in low-permeability sediments is focused along faul ts rich in clay minerals. Recent ring-shear experiments show that shea r failure generally reduces permeability in muddy faults to values wel l 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 fr acture systems. The lithostatic fluid pressures necessary for hydrofra cturing in weak sediments cannot, however, exist throughout the imbric ate thrust system and basal decollement zone because a zero basal shea r stress condition would result in regional instability and accretiona ry wedge collapse. We propose that the regional hydrofracture networks must propagate heterogeneously, leaving relatively strong asperity re gions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions.