LOCAL VARIATIONS IN QUARTZ [C]-AXIS ORIENTATIONS IN NONCOAXIAL REGIMES AND THEIR SIGNIFICANCE FOR THE MECHANICS OF S-C FABRICS

The quartz [c]-axis orientations of quartz-bearing S-C mylonites vary on the mm scale. Two specimens from different tectonic settings (the Variscan Schwarzwald, West Germany, and the Eastern Alps, Austria) are investigated. In the first specimen prism glide along (a) predominates, and in the second specimen basal glide along (a) predominates. In both specimens the pattern of [c]-axis variations is similar-it is suggested to be characteristic of S-C mylonites. On the local scale, there are two patterns of preferred orientation: (1) a crossed-girdle, 'type I', which is inclined in a sense opposite to the inferred overall sense of shear. This occurs in moderately strained domains; (2) a single-girdle, which is slightly inclined in the same sense as the inferred overall sense of shear. This occurs in domains that suffered higher strain. Both types are considered to reflect stable orientations of the 'easy glide direction; (crystallographic (a) direction), at different stages of the deformational history. Two kinematic interpretations are discussed. Quartz [c]-axis fabric variations reflect a spatial (and possibly also temporal) partitioning in flow path between domains that underwent either: (1) spinning coaxial flow and non-spinning non-coaxial flow; or (2) spinning non-coaxial flow and non-spinning non-coaxial flow, associated with variations in parameters of the deformation (such as magnitude of strain and strain rate) and perhaps also rate of recrystallization and recovery. In either interpretation, in moderately strained domains oblique [c]-axis girdles are generated that cannot be used as shear-sense indicators. © 1990.