Ductile Shear Bands in a Naturally Deformed Quartzite

Microscale shear bands are features that often occur oblique to the mylonitic foliation inmylonites. This paper is concerned with such structures within a quartz-mylonite. Geometricalfeatures, microstructures and fabrics associated with shear bands are described. Both opticaland transmission electron microscopy have been used. It was observed that the developmentof shear bands is closely related to (i) the onset of dynamic recrystallisation during deformation,(ii) a change of bulk deformation within the mylonites from relatively homogeneous to inhomogeneousand (iii) a marked softening of the mylonite. Across shear bands, dominant deformationmechanisms change from a dislocation creep type to grain boundary sliding. This induces strongmodification of quartz lattice preferred orientations. The asymmetry of quartz fabrics due toshear should generally be favoured by the development of shear band structures. Our resultsindicate that the production of ductile shear band structures helps to accommodate large straindeformations at low temperatures. Results also indicate that grain and sub-grain sizes are notaffected by variations in strain rate.