MODES OF SUPERPOSED BUCKLING IN SINGLE LAYERS CONTROLLED BY INITIAL TIGHTNESS OF EARLY FOLDS

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 F1 there 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-degrees and 90-degrees. The third mode was observed when the F1 interlimb angle was less than about 90-degrees 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.