Regional significance of non-cylindrical folding in the northwestern part of the High-Ardenne slate belt (Redu-Daverdisse, Belgium)
Abstract
Regional mapping and a detailed geometric analysis of complex, mixed brittle-ductile, fold-related accommodation structures, along the well-exposed banks of the Lesse river between Redu and Daverdisse (Belgium), reveal that the finite strain in the predominantly incompetent Lower Devonian rock sequence in the northwestern part of the High-Ardenne slate belt deviates from pure flattening and approximates plane strain. This finite strain is materialised by different manifestations of non-cylindrical folding, i.e. regional en-echelon and periclinal fold geometries, mesoscale non-cylindrical folds with hinges showing variable plunges, oblique flexural slip and local non-axial planar, transecting cleavage. The observed non-cylindrical folding fits in the regional framework of the Meuse Valley Recess, a transpressional corridor in the Ardenne allochthon that developed on top of a buried, buttressing oblique ramp in the pre-structural basement. Differential propagation in the overriding Ardenne allochthonuous domain to the east and west of this buttressing oblique ramp led to a component of lateral shortening on top of the ramp, resulting in the rotation of the overall structural grain and the development of en-echelon, non-cylindrical folds. Our study suggests that the Meuse Valley Recess can be continued towards the southeast, at least affecting the northern parts of the High-Ardenne slate belt.