Abstract. – In the Aosta Valley, the Gran San Bernardo nappe, which comprises the basement sequences classically re- ferred 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 dif- ferent 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 juxtapo- sed 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.

The Gran San Bernardo nappe in the Aosta valley (western Alps):a composite stack of distinct continental crust unit

MARTIN, SILVANA
2005

Abstract

Abstract. – In the Aosta Valley, the Gran San Bernardo nappe, which comprises the basement sequences classically re- ferred 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 dif- ferent 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 juxtapo- sed 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/134125
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