The microstructural features of extensional faults developed in slope sediments from the Mariana trench (D.S.D.P. Leg 60) and the Japan Trench (Leg 87) are reported. The study provides information on faulting mechanisms, fluid flow along faults, and processes of fault zone widening. A wide variety of fault geometries and deformation fabrics are preserved along the faults studied, although the deformation mechanisms involved in each case appear to be disaggregation and particulate flow with and without fracturing. In most cases studied the fault zones are composed of anastomosing micro-movement zones of finite porosity collapse. It is suggested that the different fault rock fabrics arise primarily from differences in the strain rate history experienced. Individual faults (e.g. 760 m down hole 584, Leg 87) show some evidence of deformation under a range of strain rates and a range of fault zone widening processes. Evidence for disaggregation and particle sinking along a fault from Leg 60 indicates dewatering via slow fluid expulsion.