Folding of a single layer in an effectively anisotropic host: the role of viscosity stratification and confinement on growth rates and fold patterns

Abstract

We investigate how multilayer stack geometry and viscosity stratification influences fold shapes, focusing on the case of a competent layer embedded within a layered host subject to 60% layer-parallel shortening. First, we compare the growth rate spectra calculated with an upscaled anisotropic model and a fully discrete multilayer model. We derive a novel analytical expression for growth rates of a single layer embedded in a confined anisotropic medium. The growth rates computed for the layered host case inherently split depending on whether the low- or high viscosity host layer is in contact with the main layer. For fine host layering, their average converges to the growth rates calculated for the equivalent anisotropic host. Further, using a series of two-dimensional finite element numerical simulations, we vary the internal structure of host layering and analyse the resulting fold patterns. To isolate the influence of geometric configuration, we adjust host layer viscosities to maintain a fixed effective host anisotropy factor across the simulations. To quantify fold shapes in deforming multilayer sequences, we propose a new method that is based on reconstructing an average fold shape for each folded layer. Using the fold classification diagram of Srivastava and Lisle (2004), we obtain a robust characterization of the simulated fold geometries and analyse shape variations within each model and across studied multilayer configurations. Our results demonstrate that even for the same effective anisotropy factor, variations in host layer thicknesses produce a wide spectrum of fold shapes, emphasising the important role of host layer geometry in controlling fold development.