THE EFFECT OF INITIAL PERTURBATION SHAPE AND SYMMETRY ON FOLD DEVELOPMENT

Single-layer fold experiments were carried out to study the influence of initial perturbation symmetry on the symmetry of folds developed using paraffin wax as a rock analogue. Under experimental conditions, both matrix and layer have a power-law rheology, with an initial effective viscosity contrast of ca 17:1. The matrix flows in steady state with n = 2.7 and the layer work softens with neff = 10. Folds developed from an initially symmetric perturbation remain symmetric, reflecting the pure shear-plane strain boundary conditions. Folds developed from an initially asymmetric perturbation, however, show a progressive exaggeration of this asymmetry, such that the introduced 3.5° difference in limb dip increases to 65° at 23% bulk shortening. These experiments indicate that, for low competence contrast between layer and matrix, the geometry of initial perturbations of finite amplitude (e.g. ripple marks, channels, half-graben structures) may be an important factor in determining the geometry and distribution of folds. © 1990.