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Department of Earth Science & Engineering, Royal School of Mines, Imperial College, London SW7 2BP, UK (e-mail: j.cosgrove{at}imperial.ac.uk)
Shear zones (i.e. locally developed planar zones of ductile deformation that contain a tectonically induced fabric) are one of the most commonly used kinematic indicators (i.e. asymmetric structures that can be used to determine the sense of movement and the orientation of the stress field operating at the time of their formation). This is because of the abundant occurrence of these structures, and the assumption that there is a unique relationship between them and their causative stress field. However, there is a range of structures that, when viewed in two dimensions on an outcrop surface, display the geometry of shear zones but are formed in a variety of different ways and are oriented at various angles to the maximum principal compression. These include shear zones that form at angles between 25° and 45° to
1 and that have the same relationship to 1 as brittle shear fractures, shear zones linked to the deformation of anisotropic materials that form at angles between 45° and almost 90° to 1, and structures with the geometry of a shear zone that can be inclined at angles between c. 10° and 80° to 1 and that appear on certain sections though folded mineral fabrics. Unless the mechanisms of formation of these structures are understood, there is a strong possibility that their kinematic implications will be misinterpreted.
