Transpressional deformations at lateral boundaries of propagating thrust-sheets: the example of the Meuse Valley Recess within the Ardennes Variscan fold-and-thrust belt (N France-S Belgium)

The 3D kinematics of the fold-thrust structures within the Meuse Valley Recess (MVR) (Ardennes Variscan thrust belt, N France-S Belgium) have been investigated using an integrated approach combining geological mapping, cross-section analyses and studies of magnetic fabrics and paleomagnetic data at a total of 52 sites distributed within the study area. Fold-thrust geometries, strain distribution and thrust-sheet rotational patterns lead to a conceptual structural model illustrating natural deformation processes at lateral boundaries of propagating thrust-sheets. The MVR is shown to result from distributed transpressional deformation of the Ardennes Basal Thrust hanging wall above a buried lateral/oblique ramp inherited from the Lower-Middle Devonian extensional tectonics of the sedimentary wedge. This deep lateral/oblique discontinuity basically controls the thickness and basal friction of the Ardennes thrust wedge, thus resulting in contrasting thrust propagation west and east of the MVR. Out-of-plane strain within the transfer zone includes some varying components of normal-to-bedding shear and lateral shortening depending on the proximity with the buried lateral discontinuity. A gradient of lateral shortening is evidenced towards this discontinuity, thus acting as a buttress zone. We emphasize the high friction conditions prevailing along the lateral borders of the thrust sheets, which potentially result in drag effects during foreland-directed thrust propagation. Such additional shear strain is suggested to be accommodated by late pervasive thrust-sheet rotation and lateral broadening of the deformed zone subsequent to oblique folding close to the lateral/oblique boundary.