PURE SHEAR

A homogeneous strain involving either plane strain (one of the principal strains is zero) or general strain, in which material lines that are parallel to the principal axes have the same orientation before and after deformation (non-rotation of principal axes)is called pure shear.

In progressive pure shear, the principal strains do not change their
orientations with the progress of deformation. The upper part of the figure
shows the fate of 4 differently oriented lines during progressive pure shear.
The red line always or at all stages of deformation falls in the contractional
area of the strain ellipse and is being shortened all the time, the geologically
realistic situation being **CONTINUOUS** **FOLDING OR BUCKING**. The blue
line, on the other hand falls at all stages of deformation in the extensional
area of the strain ellipse and is therefore continuously extended or increased
in length, the geologically realistic situation being **CONTINUOUS BOUDINAGE**.
The purple line first undergoes shortening of length and then its length is
restored to the initial length and then its length gets increased. The
geologically realistic situation is **FOLDING, UNFOLDING and then BOUDINAGE**
giving rise to **BOUDINAGED FOLDS**. Finally, the green line does not change
in length at the first increment of deformation since it lies parallel to the
line of no finite longitudinal strain but later it increases in length, the
geological situation being boudinage with development of boudins that show *en
echelon* arrangement.

The lower part of the figure shows progressive homogeneous
simple shear. The red line is continuously shortened in length and shows **ACTIVE
BUCKLING** since it always or at all stages of deformation by simple shear
lies in the negative extensional field. The green line on the other hand, always
lies in the field of positive extension and is **ACTIVELY BOUDINAGED**.

Note that the width of the zone of deformation in the lower figure is constant and all lines parallel to the direction of shear are unchanged in length. These planes are therefore lines of e=0 or 1+e=1 or the lines of no finite longitudinal strain at all stages of deformation by simple shear.