Modes of superposed buckling in single layers controlled by initial tightness of early folds

Abstract

In a series of experiments with soft test models, superposed buckling folds were produced in a competent layer resting on a slab of incompetent painter's putty. The angle between the hinge lines of the first set of cylindrical folds (F₁) and the direction of the second compression (P₂) was varied in the different experiments. The experiments showed that with an increase in the initial tightness of F₁there was a transition from one mode of superposed buckling to another. When the interlimb angle of F₁ was very large the superposed deformation gave rise to a dome-and-basin pattern (the first mode of superposed buckling). The second mode, with small F₂ folds riding over larger F₁ folds, developed when the initial interlimb angle ranged roughly between 135° and 90°. The third mode was observed when the F₁ interlimb angle was less than about 90° but the fold was not very tight. In the third mode a set of non-plane non-cylindrical folds developed; however, the sinuous hinge line of the non-cylindrical structure was newly created by replacing the old F₁ hinge. When F₁ was very tight or isoclinal the fourth mode of superposed buckling led to the development of non-plane non-cylindrical folds without concomitant hinge replacement.