Sheath fold development with viscosity contrast: Analogue experiments in bulk simple shear

It has been shown experimentally that sheath folds can develop passively by kinematic amplification of precursor non-cylindrical deflections. However, sheath folds are most commonly multilayers where rheological contrast can be expected (non-passive sheath folding). Therefore, we studied a natural occurrence of sheath folds where layers show rheological contrast and used analogue modelling to investigate sheath fold development with viscosity contrast (μ′). The initial deflection in the experiments was always hemispherical and planar layering parallel to the shear plane. Under the chosen experimental conditions, the results show that: (1) the viscosity contrast between layers should be lower than one order of magnitude (μ′ < 10) to generate a sheath fold in both higher and lower viscosity layers; otherwise the higher viscosity layer behaves as effectively rigid. (2) The lower viscosity layer develops into a tubular fold dragged by the higher viscosity layer that undergoes mixed strain (low) and shear plane parallel translation (high). (3) The ratio between ellipticities of external and internal ellipses (R′ = Re/Ri) significantly departs from 1 as the viscosity contrast increases (e.g. R′ ≈ 0.6 for μ′ ≈ 7.5 in the case where the higher viscosity layer caps the lower viscosity layer). This is in great contrast with passively developed sheath folds, for which R′ = 1 when the precursor deflection is hemispherical.From the present experimental study and previous work investigating other variables, we conclude that R′ should be cautiously used and that extraction of kinematical information from sheath fold geometry alone can be erroneous.