Strain within ductile shear zones

Abstract

The intensity of strain has been measured from the change in shape of clusters of mafic and felsic minerals within a ductile shear zone of short finite length from Botswana and a much longer planar shear zone from North Uist, Scotland. The strain measurements from North Uist have been supported by measurements of the anisotropy of magnetic susceptibility. These measurements indicate that simple-shear movement was accompanied by pure shear across the zone. A model is proposed for the strain during the growth and propagation of shear zones. Shear zones with a high rate of shear strain relative to rate of propagation may have a sigmoidal form. Such shear zones may coalesce to enclose lozenge-shaped blocks of less deformed rocks which carry a tectonic fabric at a high angle to the actual shear zone.