Conclusions
1) The orientation of the reference frame is essential for defining extensional and contractional offset along a finite slip surface deforming in a ductile shear zone. Traditionally, Cartesian coordinate systems with axes either parallel to the shear zone boundary or parallel to the slip surface have been used.
2) Extensional and contractional offset along a slip surface are also a function of the orientation of marker layers. The same slip surface may cause extensional and contractional offsets in two perpendicular marker layers.
3) The sense of slip along a slip surface is dependent on its orientation to the principal stress directions of the deformation in the shear zone. Therefore, differently oriented slip surfaces may cause extensional and contractional offset within the same deformation increment.
4) Marker layers parallel to the fabric attractor do not rotate and the local deformation in the vicinity of the slip surface is only controlled by the heterogeneous perturbation strain caused by the slip surface. Marker layers oblique to the fabric attractor additionally experience the background strain of the shear zone and will therefore rotate.
5) Two structures which form by offset of two perpendicular marker layers along a slip surface look very different at small strains. However, in simple shear zones the structures become qualitatively very similar at larger shear strains because all fabric elements have been rotated into the direction of the fabric attractor.