APPARENT SHEAR-BAND GEOMETRY RESULTING FROM OBLIQUE FOLD SECTIONS

Abstract

Small-scale shear zones inclined at intermediate angles to an earlier anisotropy are often observed in deformed rocks. They are traditionally described as shear-bands, C-bands, extensional crenulation cleavage or normal kink-bands formed as a result of extension along the anisotropy. Their asymmetries are widely used to describe the large-scale kinematics of deformation and the deformational history of a given area. We demonstrate that when various three-dimensional fold structures are observed on two-dimensional outcrop surfaces or in thin section, they can appear geometrically identical. We have developed a simple technique that allows the geometrical evaluation of any section across a cylindrical fold of arbitrary geometry. The ranges of planar sections on which a fold exhibits shear-band like geometry are presented on a stereographic projection in order to simplify the determination of critical orientations. We demonstrate that for any fold geometry, there are two distinct groups of sections showing shear-band like geometry with opposite 'senses of shear' criteria systematically arranged around the axial plane and which are inclined at a high angle to the major anisotropy. We provide a field example from Western Carpathians, where kinematic analysis, mainly based on apparent extensional shear-bands, led to overemphasis of the role of post-orogenic extension on the final structural pattern of the belt.