The determination of true shear senses from the deflection of passive markers in shear zones

Abstract

The sense of movement in shear zones can often be determined from the reorientation of pre-existing planar or linear features within the zone. This cannot always be done simply by examining the change in strike or trend of these features: sometimes these will show the opposite sense of rotation to the true sense of shear in the zone. This is illustrated for a generalized oblique-slip zone. Using the deflection method' described here, the true movement sense can be deduced from the apparent rotation, the emphasis being on a three-dimensional approach. Consideration of the movement on a large-scale shear zone (the Laxford Front) underlines the importance of being aware of this aspect of shear zone geometry. The method may be applied to brittle fault offsets as well as to deflections in ductile shear zones.