Refolding: some structural effects of layer competence contrasts

Richard J. Lisle

Department of Earth Sciences, Cardiff University

Lisle@cardiff.ac.uk

Previous modelling studies of refolding have been based on the assumptions of a passive behaviour of lithological layering and fold growth by heterogeneous simple shear. Although this shear fold model predicts the gross geometrical features of refolded folds they are unable to throw light on features induced by the competence contrast between layers. To identify such features an end-member model is considered involving extreme competence of the competent layers. The competent layers in such situations retain their original bed thickness and, more significantly, resist distortions within the plane of the layering. For describing the folding with no stretching of the surfaces the term "isometric bending" and can borrowed from the geometry literature.

The isometric folding mechanism places severe constraints on the three-dimensional form of the folded surface, constraints that are specified by Gauss’s theorem. It is shown that the classical Ramsay refolding types 1 and 2 are inconsistent with these constraints and are therefore incompatible with the extreme competence assumption. This suggests that the presence of competent layers will exert a influence on the detailed form of refolded surfaces and lead to localized stetching/contraction. The regions of layer strain will be associated with second order structural features such as localised minor folds, boudinage and fabrics. These minor structures can be used to detect the presence of competence contrasts.

The isometrical folding mechanism can be easily modelled by paper folding experiments. Origami tectonics illustrates several aspects of refolding mechanics including the effects of first fold curvature and tightness on the form of the second phase folds.