Dewatering in many accretionary prisms is inferred to occur prefrentially along discrete tectonic structures. However, these structures commonly consist of intense particle realignment in ductile shear zones rather than dilative fractures. Such alignment prompts a reduction in porosity within the zones, but there is, nevertheless, natural evidence that the shear zones are able somehow to act as dewatering channels. Experimental investigations indicate that shear zones arise at failure of the material, at which point permeability is reduced across the zones but increase by nearly 2 orders of magnitude parallel to them. The primary fabric of the host sediment also induces an anisotropy of permeability; values differ by an order of magnitude according to flow direction. The tortuosity of the fluid flow path is theoretically analyzed for shear zones produced in different combinations of uniaxial shortening and simple shear. Varying tortuosity explains both the permeability anisotropy of the host material and the ease of fluid flow along the shear zones, despite their reduced porosity. ¿American Geophysical Union 1990