The ‘Basse-Normandie’ duplex (Boulonnais, N France): evidence for an out-of-sequence thrusting overprint

The thrust-sheets outcropping in the ‘Basse-Normandie’ quarry (near the Hydrequent village, Boulonnais, N France) represents an exceptionally well exposed section of the NW European Variscan thrust front. These structures, developed in the footwall of the main Hydrequent thrust, have been often described as a classic example of a duplex structure. Only the lower part of the structure satisfies, however, the geometric and kinematic criteria defining a duplex (and more precisely an intraformational hinterland dipping duplex). per thrust-sheets of the imbricate stack exhibit a much more complex pattern of deformation than a simple piggy-back duplication of the same rock sequence. Restoration of these thrust-sheets (based upon the definition of two marker beds as well as the analyses of fold—thrust relationships and strain markers) argues for a late NE verging thrust event that progressed within the thrust system from the tip of the upper thrust sheets towards the Hydrequent thrust in a local break-back style of thrust propagation. This out-of-sequence thrusting event induced refolding and cross-cutting of the forelimb of a hangingwall anticline developed previously above the footwall ramp of the NNE verging basal thrust of the structure. Within the whole thrust system, the lower duplex represents only a minor structure developed during the initial phase of thrusting in the foreland of the major anticline as a frontal second-order duplex. ructural data presented in this paper illustrate the tectonic processes acting within the deformed zones lying in the footwall of major thrusts and emphasize the out-of-sequence style of thrust migration that arises from the sequential blocking of thrust propagation towards the foreland.