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 th e Hydrequent village, Boulonnais, N France) represents an exceptionall y 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 intra formational hinterland dipping duplex). The upper thrust-sheets of the imbricate stack exhibit a much more complex pattern of deformation th an a simple piggy-back duplication of the same rock sequence. Restorat ion of these thrust-sheets (based upon the definition of two marker be ds as well as the analyses of fold-thrust relationships and strain mar kers) 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-cutti ng of the forelimb of a hangingwall anticline developed previously abo ve the footwall ramp of the NNE verging basal thrust of the structure. Within the whole thrust system, the lower duplex represents only a mi nor structure developed during the initial phase of thrusting in the f oreland of the major anticline as a frontal second-order duplex. The s tructural data presented in this paper illustrate the tectonic process es acting within the deformed zones lying in the footwall of major thr usts and emphasize the out-of-sequence style of thrust migration that arises from the sequential blocking of thrust propagation towards the foreland. (C) 1998 Elsevier Science Ltd.