Numerical models for the control of inherited basin geometries on structures and emplacement of the Klippen nappe (Swiss Prealps)

Cover nappes commonly deform above a shallow basal detachment surface located above rigid crystalline basement rocks. Inherited basin geometries are presumed to control the kinematic evolution and detachment of cover nappes in an accretionary wedge. We compare results from two-dimensional finite element modelling with the structural style of a natural cover nappe, the Klippen nappe, which was detached from its basement along an evaporitic detachment horizon and thrust over a distance of roughly 100 km. First-order characteristics of the Klippen nappe paleo-basin include a complex distribution of weak detachment rocks and various changes in the sediment composition and layer thicknesses; two types of structural style can be distinguished: an imbricate fan at the rear end of the nappe (Préalpes Médianes Rigides) and a fold-dominated region at the front (Préalpes Médianes Plastiques). del results resemble the first-order characteristics of the Klippen nappe. The model experiments suggest that the formation of imbricates and fault-related folds are controlled by the discontinuity of detachment horizons. Discontinuities are locations where thrust ramps are triggered and layer heterogeneities control the locations for the initiation of detachment folds. Basement horsts may lead to the formation of recumbent folds, which develop a mélange zone on the highly sheared inverted limb. The experiments also suggest that the thickness of the basal detachment horizon of the Médianes Plastiques decreases gradually from the foreland to the hinterland.