Folding and boudinage of an oblique single layer

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Folds and boudins provide information on strain, rheology and deformation history of rocks. That is why the conditions during which both structures develop should be well known. Nearly all the theory and modelling of folds and boudins are based on coaxial deformation of a layer (or layers) oriented perpendicular to the principal strain axes. Under such conditions folds and boudins grow simultaneously or develop separately as single structures. The situation, however, is entirely different and more complex if the layering is oblique to all three far-field principal strain rates. In such cases the directions of maximum shortening rate in the competent layer varies continuously with time and the base strain rate on which a buckling or necking instability would be superimposed is therefore also changing. Consequently, folds and boudins may develop in sequence. As experimental evidence for sequential growth of both structures is lacking, we will carry out scaled experiments using non-linear viscous rock analogues to show the impact of layer inclination on the deformation geometry. The main aim is to constrain the conditions (viscosity ratio, layer inclination, finite strain) which are suitable for the sequential development of folds and boudins (or pinch-and-swell structures) under bulk coaxial plane strain. It will further be shown how folding will change to boudinage (and vice versa) when the attitude of the layer is changing. As folding and boudinage of an oblique competent layer is common in various settings (e.g. orogens, salt domes), the present project will be of interest for a large number of structural geologists, who can use the new results to constrain the conditions of folding and boudinage in natural environments.