Asymmetric shape preferred orientations as shear-sense indicators

Mineral shape preferred orientations can be characterized by plotting aspect ratios against grain-length orientation, and such plots commonly display asymmetric patterns in XZ-sections (perpendicular to foliation and parallel to the direction of maximum elongation). In most cases, the asymmetries cannot be discerned by superficial examination of thin sections. The sense of asymmetry can be defined by: (a), the shape of the highest aspect-ratio per grain-length orientation curve, and (b), the relative orientations of the most elongate grain and the mean or median of the population as a whole. Observed senses of asymmetry display consistent relationships to known senses of shear, and they therefore provide a useful alternative to, or check on, other methods of shear-sense determination. Examples of quartz, plagioclase, microcline, amphibole and biotite shape preferred orientations are described, and they include recrystallized mosaics as well as populations of discrete mineral grains. The examples include mineral preferred orientations that evolved by plastic deformation, anisotropic crystal growth, and mechanical rotation of inequidimensional grains.