SHEAR ZONES, DISPLACEMENT OF MARKERS THROUGH

 A shows that the vein (black) is oriented in such a manner that it always falls in the negative extensional area of the strain ellipse and therefore gets shortened or buckled. B shows quite the opposite with the vein orientation always falling in the area of positive extension and the vein gets boudinaged. C shows    situation when the vein is oriented in such a manner that it first undergoes shortening and then extends so that the structures produced are boudinaged folds. If the angle that the vein makes with shear zone walls outside the shear zone is  a  and the angle it makes inside the shear zone is  a ', then the shear strain    can be computed from the simple relationship:      

 cot  a '  =   cot   a - g