Basement-involved thin-skinned and thick-skinned tectonics in the Alps

Abstract

The deformation of continental crust during continental collision by folding and thrusting follows three types of structural styles: (1) in a true thin-skinned style only cover rocks are involved; (2) in the case of a thin-skinned basement-involved style, thin slabs of crystalline basement rocks are piled up into a nappe stack; (3) in a true thick-skinned style, the entire upper crust is involved in the deformation. In the Alps all three styles can be recognized. The Helvetic nappes and parts of the Penninic nappes exhibit true thin-skinned style tectonics. Triassic evaporites, Jurassic shales and Cretaceous marls acted as detachment horizons. Basement-involved thin-skinned tectonics is typical for the Penninic nappes in the core of the orogen. The thickness of the basement thrust sheets is controlled by the effects of Mesozoic rifting, by deep burial and heating of the subducting crust and by the presence of Late Palaeozoic structures. Thick-skinned style is observed in the more external parts of the orogen, the external massifs and the Southalpine nappe system. It occurred in the late phase of collision and involved the entire upper crust. The basal detachment occurred possibly along phyllonites generated by the breakdown of load-bearing feldspar. Considering the Alpine orogen as a whole, the lower crust deformed seemingly independently from the upper crust. The detachment of the cover units by thin-skinned tectonics occurred prior to thrusting related to basement-involved thin-skinned tectonics. Thrust faults of both types were overprinted by 'post-nappe folding'.