Shear zones developed in an experimentally deformed quartz mylonite

Abstract

Specimens of an experimentally deformed quartz mylonite show the development of shear zones. These shear zones are of two types: Type A shear zones are discrete and within the specimen, whereas Type B shear zones occur where the specimen has been sheared past the corner of the piston. Type A shear zones are comprised of elongate recrystallized grains with long axes oriented 30–40° to the shear zone boundary. Type B shear zones consist of strongly flattened and recrystallized ribbon grains. Augen occur where the deformation history is coaxial; around the piston corner where the deformation history is non-coaxial only strongly flattened grains occur. The c-axis fabrics which develop in the shear zones comprise girdles or maxima approximately normal to the foliation defined by the grain elongation (shape fabric). An asymmetry with respect to the shape fabric is present in many of the fabrics. This asymmetry results from either using a quartz mylonite with a strong preferred orientation as the starting material, or as a result of the non-coaxial deformation. Where it can be demonstrated that the asymmetry results from the non-coaxial deformation, the asymmetry is a leading edge with respect to the sense of shear. The results of this study suggest that care must be taken in interpreting asymmetric quartz c-axis fabrics in natural shear zones, particularly if it is suspected that the protolith to the shear zone had an initially strong preferred orientation. Type A shear zones originate as brittle zones extending from the specimen corners, which become recrystallized as pressure and temperature are increased. These shear zones show initial non-coaxial deformation (dominantly simple shearing), followed by later flattening parallel to the shortening direction. Type B shear zones form when the piston punches into the barreling specimen. The strain history of these shear zones is characterized by initial flattening (up to 30%), followed by shearing past the piston corner.