Numerical modelling of viscous folding in a layered sedimentary cover above a basement fault

A monocline fold forming in a sedimentary cover above a fault in a rigid basement is an example of fault-related folding and is often associated with fault propagation. We developed a custom implementation of finite element method model of folding in a viscous medium above a fault with an arbitrary dip (θ) and sense of slip. We explored three rheological scenarios: (1) a homogeneous isotropic cover, (2) a heterogeneous cover consisting of initially flat, alternating low- and high-viscosity (denoted by µ₁ and µ₂ respectively) isotropic layers, (3) an initially homogeneous anisotropic cover. Anisotropic fluids are characterised by shear and normal viscosity, i.e., viscosity under layer-parallel shearing and layer parallel shortening or extension, respectively. The model of an anisotropic fluid approximates the behaviour of a layered media in the limit of fine layering.

We performed systematic numerical experiments for fault dip angles ranging from θ=10° to θ=90°, number of layers n=8, 16, 32, 64, 128, viscosity ratios µ₂/ µ₁=10, 25, 50, 100 and shortening or extensional regime. Results demonstrated that an anisotropic viscous medium effectively approximates a finely layered sedimentary cover at both the onset of deformation and under large finite strain. However, the observations regarding the trends of structure evolution (e.g., fold amplitude growth rate) made at the onset or after a few initial time steps of deformation cannot be extrapolated for further stages of deformation. For sufficiently fine layering (e.g., n=64, 128), the simulated folds tend to be chevron-like. Two major geometrical types of folds can be described in the reverse fault case, i.e. in the shortening regime. A forelimb monocline alone forms above a basement fault with dip angles larger than θ=30-40°, but an additional pop-up anticline emerges in the case of a gentler dipping fault. In general, greater viscosity contrasts favour the amplification of the pop-up anticline. The anticline grows in time for most of studied cases, but its evolution is more complex for folds formed above a fault dipping close to the threshold value between two geometrical types. In these cases, the amplitude of the pop-up anticline decreases with progressive shortening at late stages of deformation. In the normal fault case (extensional regime), the covering layers tend to deform more or less parallel to the top basement boundary and fold geometries are rather similar regardless of the fault dip angle.

The work was supported by the National Science Centre, Poland, under research project “Numerical and field studies of anisotropic rocks under large strain: applying micro-POLAR mechanIcS in structural geology (POLARIS)”, no UMO-2020/39/I/ST10/00818.