TRANSITION FROM BASEMENT TO COVER - A GEOMETRIC MODEL

A new simple geometric model is proposed that explains the possible re lationships between superficial thrust sheets and deeper fold nappes. Tt demonstrates that the simultaneous formation of these two types of structures at different levels of the crust is kinematically and geome trically possible. The model is based on the difference of rheological behaviour between a lower tectonic unit that will suffer mainly a duc tile deformation along a shallow dipping shear zone and an upper and r igid tectonic unit that will deform mostly by bedding parallel slip at constant bed length. The difference in theologic behaviour implies th at the horizontal shortening that can be produced within the two units is unequal and therefore leads to a relative movement between them. T his movement takes place within a detachment level of variable thickne ss. The most efficient layers to enable detachment will consist of duc tile sediments like evaporites, shales or flysch-type sediments. Recen t geophysical data have confirmed the view that compressional mountain belts are mainly the result of continental lithosphere subduction. Th e deformation is essentially localized in the upper part of the down-g oing continental crust. This fundamental basally-driven mechanism cons trains the geometry and kinematics of the proposed model. Examples fro m the Alps show that it can be applied to many cases where cover thrus t sheets are directly related to basement fold nappes. Copyright (C) 1 996 Elsevier Science Ltd.