Thrust‐related mudstone fabrics from the Barbados Forearc: A backscattered scanning electron microscope study

Abstract

Backscattered electron imaging on the scanning electron microscope has been used to examine the nature of microfabrics in mudstones from the Barbados forearc. Samples from two important tectonic junctions in hole 671B of Ocean Drilling Program leg 110 have been examined in detail. At thrust A (128 m depth) clays and clasts are pervasively aligned at about 50° to the thrust surface, and a macroscopic scaly fabric is defined by anastamosing fractures. In the décollement zone (500–550 m depth) there is no preferred alignment of clays or clasts even though an irregular, macroscopic scaly fabric is present. Scaly fabric in hole 671B is in part an artifact of specimen desiccation but may also reflect the release of anisotropic, elastic strain relating to in situ stress conditions. The disclosure that scaly fabric does not relate to grain alignment has profound implications for the nature of the Barbados décollement seismic reflector. The reflector does not correspond to a fabric anisotropy and is most likely to relate to the documented fluid flow anomaly in the décollement. In contrast to scaly fabrics, the microfabrics are the product of sediment deformation history. The differences between the microfabrics at thrust A and those in the décollement probably relate to contrasts in deformation conditions. In the décollement, deformation probably occurred by paniculate flow under high pore fluid pressures, where the individual panicles were aggregates of phyllosilicates. At thrust A, high pore fluid pressures were not maintained, due to surface proximity, enabling preconsolidation stresses to be exceeded. As a result, a pervasive phyllosilicate alignment has developed.