Coring of sediments in active accretionary prisms by the Ocean Drilling Program (ODP) has revealed a wide variety of deformation features. The structures throw light on processes associated with accretion and the drainage of water-saturated sediments during active deformation. However, some of these structures are rarely described from ancient rocks, partly because they are not well preserved, but perhaps also through the descriptions being scattered among literature not widely read by geologists. This article provides a synopsis of the structures and their implications, based on sediment cores recovered from the five accretionary prisms most recently targeted by the ODP.
Deformation bands are roughly planar, bedding-oblique zones of displacement that vary from resembling faults to being kink like or ductile-shear zone like. They can provide information on bulk strain geometries and dewatering regimes. Narrow zones of faulting (some including grain breakage) are common, and can grade into planar bands of breccia. Scaly fabric results from microscopic zones of flattening which become supplanted by shear-induced rotation and intensification into C–S bands. Surfaces associated with shear in the above structures commonly bear lineations, which even in these incompletely lithified materials can be used for stress tensor analysis. Hydraulic breccia is thought to be an indicator of in-situ overpressuring without shear. Web structure involves cataclasis of sand under high pore pressures and can occur at surprisingly shallow levels of burial. Sediment-filled veins may be an indicator of palaeoseismicity. All these structures have hydrological implications and record fluctuating histories of overpressuring and localized fluid flow during deformation.