Offset of markers along slip surfaces in ductile shear zones

Bernhard Grasemann, Ulrike Exner, Aonghus O'Connor, and Gerhard Wiesmayr

The offset of marker planes along slip surfaces results in monoclinic or triclinic structures, which have been frequently used as shear sense criteria (e.g. shear bands, C’-type foliation, flanking structures). However, instantaneous extensional or contractional offset along slip surfaces is determined only by the spatial relationship between the slip surface and the offset marker line with respect to the principal stress directions in the arbitrarily chosen reference frame. In the different quadrants of the maximum and minimum principal stress directions the shear sense along the slip surface is reversed. Within the same reference frame and along the same slip surface, differently oriented marker lines may record either extensional or contractional offset. Two perpendicular marker lines across the same slip surface always show contrary types of instantaneous offset, i.e. one is contractional while the other one is extensional, although the sense of slip is necessarily identical. Furthermore, if during progressive deformation a slip surface rotates with respect to the principal stress axes, the shear sense may be inverted and the offset of marker lines may change from contraction to extension and vice versa. Dividing extensional and contractional slip surfaces into different deformation events with opposite kinematics may lead to misinterpretation of the bulk shear sense and erroneous conclusions on the deformation history. Structures which form by offset of two perpendicular marker layers along a slip surface look very different at small shear strain. At large shear strains these two structures become qualitatively very similar when all structural elements form small angles with the fabric attractor.