Transition from basement to cover: a geometric model

Abstract

A new simple geometric model is proposed that explains the possible relationships between superficial thrust sheets and deeper fold nappes. It demonstrates that the simultaneous formation of these two types of structures at different levels of the crust is kinematically and geometrically possible. The model is based on the difference of rheological behaviour between a lower tectonic unit that will suffer mainly a ductile deformation along a shallow dipping shear zone and an upper and rigid tectonic unit that will deform mostly by bedding parallel slip at constant bed length. The difference in rheologic behaviour implies that the horizontal shortening that can be produced within the two units is unequal and therefore leads to a relative movement between them. This movement takes place within a detachment level of variable thickness. The most efficient layers to enable detachment will consist of ductile sediments like evaporites, shales or flysch-type sediments. Recent geophysical data have confirmed the view that compressional mountain belts are mainly the result of continental lithosphere subduction. The deformation is essentially localized in the upper part of the down-going continental crust. This fundamental basally-driven mechanism constrains the geometry and kinematics of the proposed model. Examples from the Alps show that it can be applied to many cases where cover thrust sheets are directly related to basement fold nappes.