SHEAR ZONES, DUCTILE
The C-surfaces are the shear (cissaillement in French) surfaces while the S surfaces in the less-deformed S zones are parallel to the local XY-plane of the strain ellipsoid. The acute angle between the C- and S-surfaces points towards the direction of shear displacement. With progressive deformation. the C-surfaces maintain a constant orientation but the S-surfaces rotate so that the angle between the two sets is reduced. As the ultramylonite stage is approached, the C- and S-surfaces become parallel. so that their separate identity is lost.
Just as there are sigmoidal extension cracks developed under brittle-ductile inhomogeneous simple shear, so can sigmoidal cleavage trajectories develop if the simple shear progresses under ductile regime of deformation. A shear zone in the early stages may contain two planes, both discrete and with noticeable spacing between them, the planes of shear or CISALLEMENT or C surfaces and those of fabric or cleavage or SCHISTOSITIE or S surfaces. The angle between the two gets reduced with progressive shear and under high amount of shear, both surfaces may merge with each other. If displaced markers are not available, one may have to rely on the angular departures between C and S surfaces. Both C and S surfaces may also be brought close to each other owing to volume loss and may give exaggerated amount of shear strain if ignored. If the shear zone is high grade, only S planes trajectories may be seen. The amount of displacement across the shear zone in this case can be computed by construction of g /d curves. This is shown in the figure below. The area below the curve can be computed by a planimeter or by using Simpson's rule or trapezoidal rule. If the amount of shear varies along a zone at different places then several such curves may be constructed along different evenly or nonevenly spaced profiles and the average shear strain and variability of it can be known. The two photographs given below are natural examples of sinistral ductile shear zones developed in the Central Indian Bundelkhand granitoids massif.