Inferences from shear zone geometry: an example from the Laxfordian shear zone at Upper Badcall, Lewisian Complex, NW Scotland

Abstract

Abstract This contribution presents a Laxfordian age shear zone near Upper Badcall, NW Scotland, as an example of using field data and theory to assess the kinematics and nature of deformation in a shear zone. The deformation zone includes quartzofeldspathic background gneiss and a dolerite dyke that cuts the gneissic banding at a high angle. A detailed field description of the deformation zone, which is critically discussed in terms of pure and simple shear, is presented. Analysis of gneissic banding and mineral lineation data, together with a consideration of the outcrop pattern, shows that the deformation zone is best described in terms of a simple shear zone with varying finite stretching direction. To analyse this deformation we introduce the concept of local plane strain. Although the deformation of the zone as a whole is 3D, at each point there is a direction in which it does not change its length. This direction is perpendicular to the local shear direction and so varies in orientation across the shear zone. In a reference frame defined at a point, the deformation can thus be understood in terms of conventional simple shear. Details of strain are hence determined according to this conclusion. A stereographic method for the determination of the reorientation of lines is used to calculate shear strain. The shear strain values across the shear zone are then used to restore the sheared dolerite dyke to its undeformed geometry. The success of the restoration provides supports for the strain calculation and also the conclusion of simple shear deformation.