From n-s collision to WNW-directed post-collisional thrusting and folding: Structural study of the Frontal Penninic Units in Savoie (Western Alps, France)

The Frontal Penninic Units (FPU) south of Moutiers (Savoie), are an important geometrical link between the internal and highly metamorphic Penninic units and the external fold-and thrust-belt of the Dauphinois. They were subjected to large-scale poly-phase deformation, analysed in detail by this work. The deformation history of the FPU, after correlation with that of the more internal Penninic units and the external Dauphinois units, reveals three stages of the tectonic evolution, additionally constrained by recent metamorphic and fission track studies. A first pre- to syn-collisional stage of subduction and nappe stacking during the Eocene led to detachment of the Houiller (Zone Houillere) and Subbrianconnais units, followed by subsequent poly-phase (D1 and D2) isoclinal large-scale folding within an accretionary wedge that formed in front of the internal Brianconnais and above the previously subducted Valais ocean. In the frontal parts of this wedge, made up of the Cheval Noir flysch unit, sedimentation was contemporaneous with this deformation until final collision with Europe at the end of the Priabonian. Top-north thrusting in a sinistrally transpressive tectonic scenario is inferred from kinematic indicators. This oblique convergence indicates a lateral Position of the Dauphinois during and before collision with the Subbrianconnais-Brianconnais ribbon continent and the Adria plate. The onset of the second stage at around the Eocene-Oligocene boundary reveals an important change towards top-WNW thrusting, related to WNW-directed indentation of the Adria plate. During Oligocene to Early Miocene times the nappe stack of the FPU became re-folded by large-scale F3 folds, together with the more internal Penninic units. Simultaneously, the entire Penninic nappe stack was carried onto the Dauphinois unit along the WNW-directed Roselend Thrust. Hence, the French-Italian Western Alps, characterised by this west-directed thrusting, underwent a different tectonic evolution from that of the Swiss-Italian Alps, were top N-NW thrusting continued during this second stage. Decoupling occurred by dextral movements along the Insubric-Simplon-Rhone fault system. After a period of quiescence within the FPU during the Late Miocene, associated with progressive foreland-propagation of post-collisional deformation into the external massifs of the European foreland deformation, deformation in the FPU resumed again during a third stage, which initiated about 5 Ma ago. This stage is presently still ongoing and characterized by normal faulting.