THE EXTERNAL ASYMMETRY OF QUARTZ C-AXIS FABRICS - A LINEAR-APPROXIMATION TO ITS STATISTICAL DESCRIPTION

When the skeletal outline of quartz c-axis fabrics is not clearly esta blished, some testing procedures can be defined to attempt to verify t he symmetry of the c-axis intensity distribution with respect to certa in external reference axes (usually the principal axes of the finite s train ellipsoid). In this paper we propose a numerical method to descr ibe that external asymmetry. Taking into account symmetry principles, a series of simplifications from the initial spherical distributions o f axial poles to a final linear data distribution must be assumed. Two different parameters are derived: the external asymmetry factor, Am, upon which a test of external symmetry is established, and the statist ical obliquity angle, alpha(f). The coupled use of both (Am and alpha( f)) on non-isotropic fabrics is thought to be a reliable new kinematic criterion. This is shown by the analysis of a number of experimental, natural and theoretical quartz c-axis fabrics taken from the literatu re, where dextral shear senses imply almost invariably positive alpha( f) angles for asymmetric fabrics. The method proposed here may also al low a quick numerical comparison of fabrics from different regions or sources.