Hinge migration and hinge replacement

The axial migration angle (phi) is defined as the angle between the current hinge line and a marker line which was parallel to the initial hinge line. It is generally assumed that the marker line is deformed in an entirely passive manner whereas the fold axis traces out the orientations of the long axis of the sectional strain ellipse. This concept is modified when the competence contrast between a layer and its embedding medium is taken into consideration. Large values of phi may be obtained in constrictional deformations; however, because of strong hinge line arcuations of the nonplane noncylindrical folds, the concept of hinge migration will not be meaningful. Hinge migration is greatly influenced by the competence of layers. In inextensible layers the magnitude of hinge migration may become very large even in flattening and plane strain. On the other hand, hinge migration will be very small or negligible if buckle folding is associated with large layer-parallel homogeneous strain. The concept of hinge migration and hinge replacement have been critically analysed for development of Types 0, 1 and 2 fold interference patterns.