Discussion
The model results in Figure 4 and 5 have some important implications, which should be considered when interpreting the kinematics of flow from slip surfaces in ductile shear zones. In order to keep the discussions of the complex behavior of rotating slip surfaces in ductile flow as clear as possible, we restrict our models to simple shear boundary conditions, which have the important simplification of only one non-rotating direction, which is parallel to the shear zone boundary and which experiences no length change during deformation. All markers and the slip surface, which creates a heterogeneous perturbation strain but behaves as a passive marker in terms of rotation rate, are rotating into the shear direction.
One of the major results of previous numerical and analogue models of slip surfaces deforming in ductile shear zones is that contractional and extensional offset along the slip surfaces is just a function of its orientation to the instantaneous stretching axes (i.e. the principal stress directions) of the background deformation (Wiesmayr and Grasemann, 2005). Therefore, extensional and contractional offset along slip surfaces may occur during the same deformation event within a shear zone, if slip surfaces form at different angles (e.g. as conjugate systems) or form at different increments and therefore rotate into different orientations (Exner et al., 2006). Moreover, when the slip surfaces rotate through a principal stress direction of the background deformation, the shear sense reverses and extensional offset may be “reactivated” with contractional offset and vice versa (Exner et al., 2004; Kocher and Mancktelow, 2005). Conjugate sets of slip systems (mostly shear bands) and possible reactivations have been observed in natural rocks leading to different and sometimes conflicting kinematic interpretations (e.g. Harris and Cobbold, 1985; Behrmann, 1987; several examples in Snoke et al., 1998). This study extends this previous work and emphasizes that extensional versus contractional offset along slip surfaces is furthermore a function of the orientation of the marker line with the slip surface and the chosen reference frame. Although the assumption that the foliation in highly strained rocks forms a marker parallel to the shear zone boundary is in many cases justified (e.g. Passchier and Trouw, 2005), the fabric attractor in broadening general shear zones is oriented oblique to the shear zone boundary (Simpson and De Paor, 1993). Additionally other markers like secondary foliations or veins, which may form at high angles with respect to the shear zone boundary, can be offset by slip surfaces. Therefore the interpretation of local extensional or contractional offset along slip surfaces in terms of narrowing and broadening shear zones independent of other structural observations should be avoided. Furthermore we suggest to avoid terms like extensional crenulation cleavage (Platt and Vissers, 1980) and preferentially use non-genetic terms like C’-type cleavage (Passchier and Trouw, 2005). Similarly, positive and negative slip (Coelho et al. 2005) along slip surfaces may be misleading, and should only be used if the reference frame is clearly defined and/or the markers are parallel to the shear zone boundary.
Slip systems with various orientations may be used in combination with the recorded offset and rotational behavior in order to estimate the rotational quality of the flow type (e.g. Wiesmayr and Grasemann, 2005). Such quantitative kinematic investigations are based on the fact that (Simpson and De Paor, 1993): (i) the non-rotating eigenvectors of flow separate sectors where material lines either co- or counter-rotate with respect to the shear direction; (ii) the principal stress directions separate quadrants where the slip surfaces record syn- and antithetic shear with respect to the overall shear sense. Kocher and Mancktelow (2005) demonstrated that structures developed around a single slip surface can be backward restored using a dynamic reverse model based on analytical solutions derived by Schmid and Podladchikov (2003), even if the structure records large shear strain. However, this technique requires the knowledge or (justified) assumptions about the initial configuration of the marker lines. As shown by the two simple models in Figure 5, the structures outlined by marker layers parallel and perpendicular to the shear zone boundary look qualitatively very similar and without the knowledge of the initial orientation of the layer before deformation, mechanic or kinematic backward balancing techniques will probably result in plausible but not necessarily unique solutions.