Simple shear deformation and quartz crystallographic fabrics: a possible natural example from the Torridon area of NW Scotland

Abstract

In this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the Torridon area of NW Scotland are described. The vein, which lies along a joint separating two blocks of Proterozoic gneiss, was deformed by crystal-plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins. Mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation. In thin section the vein is a mylonite composed of dynamically recrystallized, elongate quartz grains aligned oblique to the mylonitic foliation. Fabrics within the central part of the vein have been examined by optical and X-ray texture goniometry methods, together with Orientation Distribution Function (ODF) analysis. The a-axis fabric consists of a dominant single point maximum, orientated within the XZ plane at 9° to the lineation (X); this point maximum occupies a pole position to the corresponding single girdle c-axis fabric. For individual positions on the c-axis fabric skeleton, the orientations of potential crystallographic slip systems (slip plane and direction) relative to shear zone co-ordinates have been calculated by ODF analysis. The calculation of relative resolved shear stresses (Schmid factors) for the potential slip systems, assuming a simple shear kinematic framework, reveals an elegantly simple relationship between individual grain c-axis orientation and corresponding orientations of potential slip systems. These results are readily interpreted in terms of bulk simple shear deformation.