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

Abstract

The ramp–flat thrust model and other classical models based on Rocky Mountains or Appalachians structures allow a better understanding of fault and fold interaction in a sequence dominated by competent rocks, in which, in most cases, thrusts propagate forward in-sequence. This pattern, however, fails to explain the deformation history of rock successions with low proportions of competent rocks. In such lithologies, out-of-sequence faults are more common features. The Variscan Front Zone in eastern Belgium serves as a case 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 drastically, and the tectonic history can best be explained by a composite structural model. Continuous and discontinuous deformations took place in successive steps. A large 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 the ramp–flat thrust model are emphasized. Particularly, the Vesdre Nappe, which is the highest thrust sheet, is not the youngest event of deformation. Common elements of the Variscan Front Zone between northern France and western 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 compared with other wedges and thrust sheets.