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

The ramp-flat thrust model and other classical models based on Rocky ountains or Appalachians structures allow a better understanding of ault and fold interaction in a sequence dominated by competent rocks, n which, in most cases, thrusts propagate forward in-sequence. This attern, however, fails to explain the deformation history of rock uccessions with low proportions of competent rocks. In such ithologies, out-of-sequence faults are more common features. The ariscan Front Zone in eastern Belgium serves as a case study for this on-classical deformation model. Cross-sections are well-constrained by urface data covering a large area. The Devono-Carboniferous sequence, esting unconformably on Caledonian basement, has shortened drastically, nd the tectonic history can best be explained by a composite structural odel. Continuous and discontinuous deformations took place in uccessive steps. A lame anticline developed first. It evolved ubsequently into either break-thrust or translated fault-propagation olds. Out-of-sequence thrusting by the Theux-Tunnel fault formed the esdre Nappe. The latter is the northern part of the Ardenne Allochthon. he main discrepancies with the ramp-flat thrust model are emphasized. articularly, the Vesdre Nappe, which is the highest thrust sheet, is ot the youngest event of deformation. Common elements of the Variscan ront Zone between northern France and western Germany are discussed. he Theux-Tunnel fault has the same kinematics significance as the idi-Aachen fault. The footwall of the Vesdre Nappe is the eastward rolongation of the Brabant foreland. The complex structure of the Theux indow reflects imbrications under the allochthon and can be compared ith other wedges and thrust sheets. (C) 1999 Elsevier Science B.V. All ights reserved.