Folding and boudinage of oblique single layers under non-plane conditions

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Folds and boudins provide information on kinematics, strain, rheology and deformation history of rocks. That’s why the conditions during which both structures develop should be well known. As most of the theory and modelling of folds and boudins are based on coaxial plane-strain, current reviews of folding and boudinage in journals and textbooks are largely restricted to two dimensions. This is unfortunate as new 3D survey techniques allow detailed geometrical recording of 3D folds and boudins resulting in an increasing number of such structures described from natural tectonites. 3D folds and boudins may also develop if the initial orientation of the competent layering is oblique with respect to the principal strain axes. While orthogonal single layers result in symmetric, cylindrical structures, oblique single layers will be converted into asymmetric folds and boudins and even into periclinal folds, although the bulk strain is plane. The situation is still more complex if the layering is initially oblique on the one hand, and bulk strain is not plane, on the other hand. In such cases of 3D strain fields, 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 are tending to be non-cylindrical and asymmetric, and both structures may develop simultaneously or even in sequence. As numerical and physical experiments focusing on coaxial deformation of oblique single layers under non-plane conditions are lacking, our understanding of the evolution of related 3D structures is restricted. For this reason we will carry out scaled experiments using non-linear viscous rock analogues to show the impact of viscosity ratio between layer and matrix and of the initial layer inclination on the deformation geometry under bulk flattening and bulk constrictional conditions. The main aim of the present proposal is to show how, under bulk constriction and bulk flattening, dome-and-basin folding on the one hand, and tablet boudinage on the other hand, will change into coeval folding and boudinage when the initial attitude of the layer varies with respect to the principal strain axes. Apart from uninterrupted studies, which imply a final strain of 60%, we will carry out incremental studies to show how fast and in which way the individual structures are growing with time. As folding and boudinage of oblique single layers is common in various geological settings where rocks are imposed by flattening or constrictional strain (e.g. gneiss domes, 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 non-plane environments.