Experimental tests of buckling folds in relation to strain ellipsoid in simple shear deformations

Abstract

A series of experimental deformations with plastic materials, such as modelling clay, putty and wax, was carried out by simple shear in order to determine the geometrical relationships of buckling folds with the strain ellipsoid. It was found that the fold axis is not necessarily parallel to the intermediate or major axis of the strain ellipsoid. The fold axis develops parallel to the major axis of the strain ellipse (i.e., a section through the strain ellipsoid) on the enveloping surface of the bed. The orientation of the fold axis is partially controlled by the viscosity of the layer and its host because in rotational strain the orientation of the strain ellipsoid is dependent on the relative viscosity of the two materials. The axial planes of folds also are not necessarily parallel to the AB plane of the strain ellipsoid. However, there is a tendency of the strain ellipsoids to be roughly symmetrical about the axial plane. There is a resistance of isolated layers of competent material embedded in an incompetent host to form asymmetrical folds by simple shear alone. Asymmetrical folds, however, readily form by a combination of pure shear and simple shear, and by shearing strain parallel to the enveloping surface of a series of pre-existing folds. In some of the tests a cleavage-like structure developed symmetrical about the axial plane of the folds. The structures were strictly parallel to the plane of maximum extension of the strain ellipsoid.