C' shear bands are a common feature of ductile shear zones and are widely used as kinematic indicators. They are also enigmatic: research on naturally- and experimentally- deformed rocks has suggested several theories as to why they form, but there is little consensus. C' shear bands dip at 15-35° from the shear zone boundary in the direction of shear and usually occur in concert with C planes, which are parallel to the shear zone boundary, and S planes, which dip in the opposite direction to C' shear bands (see figures above). Previous research has suggested that C' shear bands form when rocks contain a weak phase, that is, a mineral that is weaker than the other minerals that produces anisotropy in the rock. However, it is unclear why this causes C' shear bands to develop. It has also been difficult to pinpoint how much of that weak mineral is required to cause C' shear bands to develop, and exactly how weak the weak mineral needs to be.
This is a companion discussion topic for the original entry at https://geo-down-under.geoscience.education/enigmatic-shear-bands/