Sense and non-sense of shear in flanking structures

The deformation of marker horizons across slip surfaces can lead to the development of several types of flanking structures. The development of such structures was investigated using a numerical model to simulate the flow around a slip surface in a viscous medium. The modelled structures can be classified on the basis of three criteria: (1) the extensional or contractional offset of markers, (2) the co- or counter-shearing sense along the slip surfaces and (3) the normal or reverse drag of markers relative to the shear sense along the slip surfaces. As a function of the kinematic vorticity of the flow and the initial orientation of the slip surface, with respect to the shear zone boundaries, three main types of structures can develop: (1) shear bands developed along co-shearing extensional slip surfaces; (2) a-type flanking folds developed along counter-shearing slip surfaces; and (3) s-type flanking folds developed along co-shearing contractional slip surfaces. All these structures can occur with either normal or reverse drag effects. Shear bands and a-type flanking folds recording opposite shear senses can be geometrically very similar and consequently lead to kinematic misinterpretations. However, correctly identified flanking structures can provide quantitative information about the kinematic vorticity of the flow. (C) 2002 Elsevier Science Ltd. All rights reserved.