Riccardo Polino

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.

Stratigraphic evidence for Cimmerian events in NW Caucasus (Russia)

The Upper Permian and Triassic sequences of the NW Caucasus present a good record of the Cimmerian events, rather undisturbed by the subsequent Alpine deformations. Original field work with new fossil identifications, microfacies analysis, and sandstone petrography were carried out. During the late Permian, active strike-slip basins were filled by continental clastics successions. Marine ingressions during the latest Permian, led to the formation of sponge reefs and skeletal carbonate ramps. During the Triassic, several siliciclastic wedges were intercalated within the marine carbonate succession. Most important is a conglomerate body made up of serpentinite pebbles of Spathian age (early Triassic), recording the exhumation and erosion of schistose antigorite serpentinites. By the late Anisian a severe deformation affected the Peredovoy (= Fore) Range of the NW Caucasus. Lower Triassic to Anisian sediments deformed also in chevron folds were overlaid with angular unconformity by a siliciclastic, and also volcaniclastic, conglomeratic and arenitic body, up to several hundred meters thick. By the late Ladinian-earliest Carnian, marine sedimentation resumed locally, forming a carbonate ramp during part of the Norian. Towards the end of the Norian, the entire area emerged and was again mildly tilted. The subsequent post-Cimmerian transgression occurred largely during the Middle Jurassic. Consequently, the most important Cimmerian deformations appear to be of early and middle Triassic age.