Poly-metamorphic evolution of a slice of the Briançonnais terrane: structural and petrographic evidence from the Tambo Nappe (Central Alps)

The study of the crystalline basement nappes of the Briançonnais Domain provides not only insights into the mechanisms responsible for the Alpine orogeny but also into the pre-Alpine geodynamic evolution of the southernmost portion of the paleo-European plate. Situated at the north-eastern margin of the Lepontine Dome, the Tambo Nappe represents a clear example of poly-metamorphic basement characterized by a long-lasting and intricate tectonic history. Given the widespread preservation of pre-Alpine textures and mineral associations, detailed analysis of their relationships with the Alpine mineral parageneses and structures provides valuable constraints for deciphering the complex geological evolution of the entire nappe. In light of the outcome of the new Swiss and Italian National geological maps of this sector of the Alps, petrographic and structural evidence are presented that point to a regional medium- to high-temperature metamorphic event predating the Alpine overprint, possibly chronologically associated with the previously recognized Permian magmatism in this region. These findings are discussed in terms of their geological and geodynamic significance within the broader geodynamic framework of the adjacent nappes in the Central Alps, also highlighting the uncertainties arising from different interpretations which require further petro-chronological studies to be clarified.