Neo-Alpine structural features at the boundary between the Penninic and Helvetic domains (Pre S. Didier - Entreves, Aosta valley, Italy)

A several-kilometres wide ductile to brittle shear zone (Courmayeur deformation zone, CDZ), between the Sion-Courmayeur Zone and the Mont Blanc crystalline basement corresponds in the study area with the so-called Penninic frontal thrust (PFT). The CDZ underwent a long deformational history characterized by the development of a regional syn-metamorphic (greenschist facies) foliation (S2), which was successively reactivated by semi-brittle, transprssive-dextral, NW-vergent shear zones, giving rise to a younger (S3) foliation and often associated with large-scale open folds. NW-SE compression was predominant during this deformational stage. Brittle tectonics was later super-imposed on the S3-related structures. The onset of this later cataclastic deformation is apparently not related with significant changes in the kinematics of the CDZ and adjoining units, except for the development of a SE-vergent back thrust which transported the Mt. Blanc crystalline basement onto the Ultrahelvetic covers. The proposed kinematic interpretations are compared with the available fission track ages of the main units at both sides of the Penninic frontal thrust in order to suggest a possible model for the neo-Alpine exhumation of the Mont Blanc and Mt. Chetif units, in the light of the kinematics of the contiguous Rhone-Simplon Fault.