Determination of stress directions from plagioclase fabrics in high grade deformed rocks (Além Paraı́ba shear zone, Ribeira fold belt, southeastern Brazil)

A new method to determine stress directions using the preferential orientation of plagioclase mechanical twins has been applied to high-temperature mylonitic rocks from the Além Paraı́ba shear zone, Ribeira fold belt, southeastern Brazil. We have measured the lattice-preferred orientation of plagioclase grains and calculated the orientation of the stress axes possible for the observed twin orientations. The maximum compressive stress direction (σ1), determined for all studied samples, is a function of the mechanical twin orientations of a number of distinct plagioclase populations. The σ1 direction is generally subperpendicular to the (010) plane. The statistical treatment for most of the plagioclase grains examined for each sample shows that σ1 is almost perpendicular to the foliation plane, suggesting a significant coaxial component in the deformation process of these rocks.