An analysis of "Single-layer" folds developed experimentally in viscous media

Experiments have been performed to study the development of folds in single viscous layers embedded in a less viscous matrix, shortened parallel to the layering under conditions of plane strain and pure shear. This is thought to best simulate the development of small folds in isolated competent rock layers under conditions of regional metamorphism. Biot has suggested that distinct folds will not form by buckling at viscosity contrasts of less than 100:1. Effective viscosity ratios of layer to matrix of between 10 and 100 to 1 were used in the experiments, and folds were observed to develop by buckling in all cases. Analysis of arc length and limb dip with progressive deformation shows that there is always a stage of initial layer-parallel shortening during which folds appear, that gives way to a stage of development where the arc length of the folds changes only slightly for further increase in deformation. The amount of layer-parallel shortening that takes place increases with decreasing viscosity contrast, but the transition from a stage of layer shortening to one of nearly constant arc length seems to take place when the folds have mean limb dips of 10 - 20°, irrespective of viscosity contrast. No relative thickening in the hinges or thinning in the limbs was observed in any of these experiments. Analyses of wavelength/thickness ratios, amplitude, fold shape and layer-parallel shortening show that folding in the experiments is best accounted for in the early stages by the theory of Sherwin and Chapple, and in the later stages by the mathematical models of Chappie. Experiments made at very low viscosity contrasts with folds initially present in the competent layer whose wavelengths were considerably larger than the predicted dominant wavelength, lead to the development of folds with thickening in the hinges and thinning in the limbs. These folds had geometries that can be simulated by a process of simultaneous buckling and flattening. The results of the experiments may be used as an aid in the interpretation of natural fold geometry, and estimates of viscosity contrast and amount of deformation can be made where suitable folds exist.