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A sigma-type of tail develops when the rate of dynamic

Recrystallization is large. While the rate of rotation of the porphyroclast is small. The finite strain of the rock should also be smal1. A porphyroclast may undergo a small rotation if ratio on the XZ plane is large. Hence the development of a sigma-type of tail is favoured when the porphyroclast has an oblong shape (Passchier & Simpson 1986 p. 842). The rotation of the oblong porphyroclast will be further restricted if the nature of bulk deformation is such that the porphyroclast can attain a stable orientatio


When the wings that develop on both sides of a porphyroblastic mineral are not in alignment and the geometry of wing on one side is different from that on the other, this is called side stepping, also called stair-stepping. The step that is relatively upwards indicates the direction of shear. The figure here is reproduced after online lecture notes by Jessel et al. Monash University. An oblong porphyroclast lying parallel to the foliation may not show any evidence of rotation; However, if the deformation is non-coaxial, the foliation around it may be distorted in an asymmetric pattern. The sense of shear can then be determined from the stair-stepping rule as in the case of porphyroclast-tail systems.