Influence of heterogeneous lithostructural layering on orogenic deformation in the Variscan Front Zone (eastern Belgium)

The ramp-flat thrust model and other classical models based on Rocky Mounta ins or Appalachians structures allow a better understanding of fault and fo ld interaction in a sequence dominated by competent rocks, in which, in mos t cases, thrusts propagate forward in-sequence. This pattern, however, fail s to explain the deformation history of rock successions with low proportio ns of competent rocks. In such lithologies, out-of-sequence faults are more common features. The Variscan Front Zone in eastern Belgium serves as a ca se study for this non-classical deformation model. Cross-sections are well- constrained by surface data covering a large area. The Devono-Carboniferous sequence, resting unconformably on Caledonian basement, has shortened dras tically, and the tectonic history can best be explained by a composite stru ctural model. Continuous and discontinuous deformations took place in succe ssive steps. A lame anticline developed first. It evolved subsequently into either break-thrust or translated fault-propagation folds. Out-of-sequence thrusting by the Theux-Tunnel fault formed the Vesdre Nappe. The latter is the northern part of the Ardenne Allochthon. The main discrepancies with t he ramp-flat thrust model are emphasized. Particularly, the Vesdre Nappe, w hich is the highest thrust sheet, is not the youngest event of deformation. Common elements of the Variscan Front Zone between northern France and wes tern Germany are discussed. The Theux-Tunnel fault has the same kinematics significance as the Midi-Aachen fault. The footwall of the Vesdre Nappe is the eastward prolongation of the Brabant foreland. The complex structure of the Theux Window reflects imbrications under the allochthon and can be com pared with other wedges and thrust sheets. (C) 1999 Elsevier Science B.V. A ll rights reserved.