Introduction
Slip surfaces or discontinuities may act as the nucleation site of secondary shear zones and eventually develop monoclinic or triclinic structures, which have been used to derive the overall shear sense of ductile flow (for a compilation of literature see Passchier and Trouw, 2005). In greenschist facies shear zones, one of the most successful kinematic indicators are secondary slip surfaces or so-called shear bands, which make an angle of about 15-35° with the shear zone margin and record a synthetic shear sense (e.g. Berthé et al., 1979; White, 1979; Platt and Vissers, 1980; Lister and Snoke, 1984; Platt, 1984). Usually, only one set of shear bands is developed, but occasionally a second conjugate set is described (Harris and Cobbold, 1985; Behrmann, 1987). These structures, together with other recently described geometries of different sense-of-slip and foliation drag were merged into a group called flanking structures (Passchier, 2001). What these structures have in common is that markers in the host rocks (e.g. foliation) are deflected near the slip surface, whereas the host rocks are undisturbed in the far field (Grasemann and Stüwe, 2001). The heterogeneous deformation field near the slip surface (i.e. perturbation strain) is controlled by the displacement gradient along the slip surface (Grasemann et al., 2005; Passchier et al., 2005). A number of numerical and analogue models demonstrated the complex progressive development of these structures. They caution the use of single isolated structures as kinematic indicators but highlight the potential of quantitative kinematic interpretation if several structures with variable geometries are considered (Grasemann et al., 2003; Exner et al., 2004, 2006; Wiesmayr and Grasemann, 2005; Kocher and Mancktelow, 2005, 2006; Mulchrone, 2007). Kocher and Mancktelow (2006) demonstrated that the slip surface induces a perturbation flow field in the host rocks, but this perturbation flow does not influence the rotational behavior of the fracture as a passive marker line. This observation holds for anisotropic Newtonian material and therefore it follows that the only stable (i.e. non-rotating) orientations attained by slip surfaces are those parallel to the stretching and shortening eigenvectors of the flow field.
The kinematics along the slip surface can be either syn- or antithetic with respect to the far field shear sense, dependent on the orientation of the slip surface to the principal stress direction. Furthermore, Wiesmayr and Grasemann (2005) demonstrated that the offset along the slip surface can be both contractional and extensional for all boundary conditions varying between simple shear and pure shear in narrowing (transpression) and broadening (transtension) shear zones. However, all above cited references implicitly assume a reference frame and marker lines which are parallel to the shear zone boundary (i.e. the stretching eigenvector for narrowing shear zones and the shortening eigenvector for broadening shear zones, Simpson and De Paor, 1993). In this work we demonstrate that offset is strongly dependent on the chosen reference frame or the orientation of the marker line with respect to the slip surface. Extensional and contractional offset along slip surfaces in ductile simple shear zones may co-exist and opposite kinematics should not be used a priori to discriminate between different deformation events.