Multilayer folds initiated in bulk plane strain,with the axis of no change perpendicular to the layering

Buckle folds in internal multilayer systems will initiate and grow in a bulk plane strain condition, in which the principal axis of no change, Y (X >Y >Z), is perpendicular to the layering and to the fold axial direction, providing that the multilayer is confined both above and below. The bulk extension direction, X, is then parallel to the fold axial direction.