__FOLIATION, IN SHEAR ZONES, SIGMOIDAL__

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.