Ductile Shear Bands in a Naturally Deformed Quartzite

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Abstract
Microscale shear bands are features that often occur oblique to the mylonitic foliation in mylonites. This paper is concerned with such structures within a quartz‐mylonite. Geometrical features, microstructures and fabrics associated with shear bands are described. Both optical and transmission electron microscopy have been used. It was observed that the development of 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 inhomogeneous and (iii) a marked softening of the mylonite. Across shear bands, dominant deformation mechanisms change from a dislocation creep type to grain boundary sliding. This induces strong modification of quartz lattice preferred orientations. The asymmetry of quartz fabrics due to shear should generally be favoured by the development of shear band structures. Our results indicate that the production of ductile shear band structures helps to accommodate large strain deformations at low temperatures. Results also indicate that grain and sub‐grain sizes are not affected by variations in strain rate.

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