Numerical models of Alpine-type cover nappes

Abstract

We investigate the dynamics of Alpine cover nappes derived from basins with inherited geometries using a two-dimensional finite element code. Model studies are based on a natural analogue, the Klippen nappe in the Swiss Alps, which detached from the basement over a basal evaporite horizon and was transported onto the foreland in a thin-skinned tectonic style. Finite-element models are used to examine the influence of specific features on the structural style developing in the models. Experiments dealing with the influence of wedge geometry and mechanical properties show modifications in wavelength patterns and structures in detail but do not change the general trends. The control that different layer thicknesses, multilayered rocks and lateral heterogeneities in the underlying basement and within the sediments have on the style of deformation is investigated. Results suggest that the development of fold versus thrust structures in a cover nappe is controlled by the thickness of the weak detachment horizon and the relative thickness of individual layers (thickness ratio n=thickness of weak/thickness of strong layer). If n>1, folds with variable and irregular wavelengths form, if 0.5< n<1, regularly spaced detachment folds develop. Imbricate thrusting is observed for values of n≤0.5. Lateral heterogeneities in both basement and rock formations cause stress concentrations, control the geometry of the evolving folds and thrusts significantly and inhibit the growth of dominant wavelengths predicted by viscosity contrasts and thickness ratios.