Determination of stress directions from plagioclase fabrics in high grade deformed rocks (Além Paraíba 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 Paraíba 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.