Scaly fabrics are present in DSDP cores from the Barbados, southern Mexico, Guatemala, and Mariana forearcs. Where independently documented, the scaly fabrics occur adjacent to faults. Scaly foliation surfaces in mudstone are planes of slip comparable to surfaces experimentally produced in shear boxes at low confining pressures. Microscopic and scanning electron microscope (SEM) images of scaly folia in sediments indicate strong preferred orientation of phyllosilicates both parallel to and at low angles to the slip surfaces. The clays within the slip surfaces are not new mineral phases but are produced by the reorientation of existing minerals and perhaps the disruption of clay aggregates. SEM studies and physical property data indicate loss of porosity during the development of scaly fabrics in sediments. At DSDP sites, scaly mudstone formed at temperatures of less than 25 °C, pressures less than 4 MPa, and strain rates of about 10−13; scaly fabrics develop typically in underconsolidated sediment. Scaly fabrics preferentially occur in weak smectitic mudstones as opposed to stronger calcareous or silty mudstones. In sediments, scaly fabrics apparently develop by the lateral propagation of faults in which individual slip surfaces are formed and abandoned after a limited amount of displacement. Slip may cease on scaly folia because of increasing coefficient of friction, because of decrease in pore pressure, or because of reorientation of the slip surface relative to the stress field. The propagation of scaly fabric occurs because the surrounding undeformed sediment matrix is weaker and/or has more favorably oriented potential failure surfaces. Conversely, in hard rocks fault zones would be less likely to propagate laterally and develop broad scaly fabric zones because they would not exceed the strength of the country rock.