Shear criteria and structural symmetry

Abstract

During the last decade, it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures (e.g. pressure shadows, shear bands, C-S structures, fabrics, tension gashes, folds, veins). The correspondence between coaxial or non-coaxial deformation, and symmetric or asymmetric particle velocity fields, respectively accounts for the use of structural symmetry as an indicator of strain history. The application of this symmetry concept to various field examples emphasizes that: (i) the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system; and (ii) the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern, even where the progressive deformation results in a complex pattern which cannot be directly interpreted.