The Gran San Bernardo nappe in the Aosta Valley (Western Alps): a composite stack of distinct continental crust units

In the Aosta Valley, the Gran San Bernardo nappe, which comprises the basement sequences classically referred to as Ruitor Massif and Zona Interna, appears to be a stack of distinct basement units with distinct lithological features and contrasting tectono-metamorphic evolution piled up during the Eocene. In the former Zona Interna, two different units (Gran Nomenon and Leverogne) have been distinguished. The Gran Nomenon unit is a polymetamorphic basement unit, with a pre-Alpine epidote-amphibolite facies metamorphic imprint, intruded by granitoid rocks during the early Mississippian. It bears the evidence of a pervasive Alpine metamorphic overprint under greenschist facies conditions, and does not share common characters with any other basement sequence exposed in the Gran San Bernardo nappe. The Leverogne unit is a monometamorphic basement unit, with some analogies with the Mont Pourri basement sequence, intruded by granophyric rocks of Middle Cambrian age. It suffered epidote-blueschist and greenschist facies metamorphism during the Alpine orogenesis, and shows a deformation history partly different with respect to the Gran Nomenon unit. These units are bounded by tectonic melanges that represent Alpine shear zones, and have been juxtaposed under greenschist facies conditions during the late stages of exhumation of the belt. The Gran Nomenon unit, which does not show a significant HP/LT overprint, was probably not as deeply buried as the Leverogne and the Ruitor units during Alpine orogenesis.