Scaly fabrics in sheared clays from the décollement zone of the Barbados accretionary prism

Abstract

Scaly fabrics in the decollement at the toe of the northern Barbados accretionary prism occur in centimeter-thick zones interpreted as the horizons where tectonic displacement is concentrated. Detailed microstructural investigations of the scaly fabrics have been carried out, using optical, scanning (secondary and backscattered modes), and transmission electron microscopy. These observations show that the scaly fabrics essentially comprise three types of microstructures, which arise from a combination of shear and flattening. In these microstructures, strain is concentrated in micrometer- to millimeter-thick deformation bands, caused by clay-particle rotation associated with porosity collapse (compactional plastic strain), resulting in the formation of domains with marked preferred orientation of clay particles. However, this preferred orientation affects only a minor part of the sediment involved in the scaly-fabric zones and only a small proportion of the total decollement thickness. On the basis of the mode of microstructure associations, we propose a model for the kinematic evolution of the scaly-fabric zones. In these, deformation initiates by the formation of a spaced foliation corresponding to flattening band arrays, then continues by concentration of shear strain in S-C (schistosite-cisaillement [schistosity-shear]) bands geometrically analogous to the S-C tectonites common in metamorphic shear zones. Partitioning of deformation results in the late formation of fracture networks at the periphery of the S-C bands, the fractures networks being possible precursors of S-C band widening. Compactional strain in the deformation bands is typical of normally or undercompacted sediments and implies expulsion of pore fluid. Preferred orientation of clay particles makes the deformation bands potential pathways for fluid circulation in deformation zones, but compactional strain requires the bands to be dilated by excess pore pressure to have significant permeability. We infer cyclic variations of stress state in the scaly-fabric zones, related to pore-pressure variations. Formation of scaly fabrics by compactional plastic shear strain would be achieved under relatively low pore pressure and significant shear stress, whereas high pore pressure would inhibit further compactional strain while increasing permeability. Tectonic displacement is likely to be favored at the sharp boundaries of the scaly-fabric zones during high pore-pressure episodes; formation of the scaly fabrics thus would account for only part of the cumulative displacement.