Single layer buckle folding in non-linear materials—I. Experimental study of fold development from an isolated initial perturbation

Abstract

Scale-model experiments, involving pure shear compression of a single layer embedded in a weaker matrix, were carried out to study the influence of a pre-existing isolated perturbation in the shape of an otherwise planar layer on the location and shape of folds. Different grades of paraffin wax were used as analogues for rocks with power-law rheology. The wax used for the matrix flows in steady state with a stress exponent of about 3.8. The layer has a stress exponent of about 3 and strain softens. Viscosity ratios (considered at yield stress for the layer and matrix) of 30:1 and 8:1 were used. Bell-shaped perturbations were introduced into the layer with initial average wavelengths of 8, 16 and 32 times layer thickness and corresponding initial limb dips of 12 °, 5° and 3° respectively. The fold packet develops at the site of the introduced perturbation and spreads slowly along the layer with increasing shortening. The shape of the initial perturbation strongly influences the fold geometry: broader initial perturbations develop broader folds, undergo more layer parallel shortening and develop a wider zone of contact strain. The influence of the introduced perturbation shape is most marked when its average wavelength component is larger than the dominant wavelength (i.e. the wavelength of maximum growth rate). The degree of bonding between layer and matrix also influences fold development. Folds with welded layer—matrix boundaries amplify much more slowly, with a greater proportion of layer parallel shortening, than folds where slip between layer and matrix is promoted.