Overthrust shear deformation of a foreland basin; structural studies south-east of the Pelvoux massif, SE France

The Late Tertiary Champsaur-Prapic fold zone lies to the southeast of the Pelvoux basement massif in the footwall of the Frontal Pennine Thrust of the western Alps. The 0.8–1.3 km thick Late Eocene-Oligocene foreland basin succession lies directly on crystalline basement and was deformed by WSW-directed shear below the overthrusting internal alpine units on the Frontal Pennine Thrust. This deformation decouples from the basement in a top to WSW-SW basal shear zone. Above, the turbidite sequence (Grès du Champsaur) is folded in chevron-like folds. A 12 km long continuous profile across the Champsaur fold zone documents variations in chevron fold shape, axial plane dips and zonal distribution of folding which indicate progressively higher shear strains towards the Frontal Pennine Thrust in the east. Chevron fold axes trend generally NNWSSE except close to the Pelvoux massif where fold axes curve to N-S possibly due to lateral drag. In the western part of the investigated area SW-directed thrusting on the Champsaur Thrust almost doubles the Tertiary stratigraphy. The Champsaur Thrust and chevron folding in its footwall deform the overlying internal thrust sheets. The Champsaur-Prapic fold zone is a particular example of overthrust shear deformation in the footwall of internal alpine nappes in which deformation is concentrated in diffuse ramp zones. To the north, late alpine deformation changes to WNW-directed partly post-dating the WSW-SW-directed deformation further south. Therefore, in the core of the external western Alpine are, two sub-perpendicular transport directions which barely overlap can be distinguished.