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, transpressive-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 UI-trahelvetic 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.