The development of sheath folds in viscously stratified materials in simple shear: an analogue and numerical modelling approach

Abstract

The aim of this PhD thesis is to provide new insights on the development of sheath folds in viscously stratified materials, in conditions of simple shear. Sheath folds are a type of non- cylindrical folds, often associated with zones that experienced a high amount of strain. Sheath folds have been found across the globe in a variety of rocks of different ages at scales ranging from less than a millimetre to 10 km or more. If a section perpendicular to the elongation direction of the sheath folds is visible, it will show a peculiar geometry made by alternating rings or omega shapes, which can be used to identify sheath folds in the field. Sheath folds have been mainly recognized in highly sheared rocks in shear zones, where the principal deformation style is simple shear. For this reason this PhD thesis focuses on sheath fold formation under simple shear conditions. According to previous studies, several parameters that control the final geometry of sheath folds are: the viscosity ratio between the layers, the amount of shear strain experienced and the initial shape of the perturbation from which the sheath folds evolves during the simple shear deformation. Although experimental results show that sheath folds can develop by a passive (same viscosity for all the layers) as well as an active (viscosity ratio between the layers > 1) folding process, the previous studies did not provide a sufficiently quantitative estimation of the influence of the viscosity ratio, and in particular they did not produce observations of the progressive stages of simple shear. In order to provide new quantitative insights on these aspects, the sheath folding process has been modelled using both analogue and numerical methods. In the first part of the thesis the analogue approach is presented. Models have been designed in order to investigate the effects of variations in the viscosity ratio and in the relative thickness of the layers, while preserving the visibility during progressive shear strain increments, which was achieved using an opaque high-viscosity layer embedded within a low-viscosity transparent matrix. The second part of the thesis illustrates the numerical modelling technique, which allowed to expand the investigation beyond the physical limitations of the analogue materials. The numerical approach also provided a more precise control on the initial conditions, in particular on the geometry. The numerical models investigated the influence of changes in the viscosity ratio between the layers, with a partial overlap with the analogue models, and the effects of change in the shape (curvature) of the initial deflection. The last part of the thesis includes a critical discussion on the two modelling methods used, comparing the experimental results obtained and highlighting the advantages and the disadvantages of both approaches, followed by suggestions for future research. The results from both analogue and numerical modelling experiments show that the viscosity ratio and the relative thickness of the layers, as well as the amount of shear strain exert a principal control on the development of sheath folds, influencing the elongation and the dip angle of these folds.