The development of sheath folds in viscously stratified materials in simple shear conditions: An analogue approach

Sheath folds are highly non-cylindrical folds occurring in a variety of geological settings, and have been studied using different approaches. With the present work, we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions. The sheath folds develop from an initial non-cylindrical deflection located on the highly viscous layer. The analogue experiments investigated the influence of (1) variations in the viscosity ratio between the high viscosity layer and the matrix (η_hvl/η_m), (2) variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer (A_f/T_hvl), and (3) progressive simple shear (γ). The results show that increases in η_hvl/η_m will produce progressively less elongated sheath folds, while increases in A_f/T_hvl will result in more elongated sheath folds. We present regime diagrams with η_hvl/η_m and A_f/T_hvl for different shear strains illustrating under which conditions sheath folds form. In case the original deflection amplitude and layer thickness as well as γ can be retrieved for sheath folds in nature, then their geometry can be used to quantify the effective η_hvl/η_m.