Structural and metamorphic data in the nappes of the Lepontine Dome (Central Alps)

Schenker, Filippo and Ambrosi, Christian and Scapozza, Cristian and Czerski, Dorota and Maino, Matteo and Castelletti, Claudio and Gouffon, Yves (2018) Structural and metamorphic data in the nappes of the Lepontine Dome (Central Alps). UNSPECIFIED. In: A Habitable Planet. Abstract Volume 16th Swiss Geoscience Meeting 16th Swiss Geoscience Meeting, 30thNovember – 1st December 2018, Bern.

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New structural and published petrological data on the new geological map and cross sections (1:10’000) of the Osogna sheet (Geological Atlas of Switzerland no. 1293, 1:25’000) are critically discussed. The area includes most nappes of the Lepontine Dome, from bottom to top: the lower penninic units of the Leventina, Simano, Adula/Cima-Lunga and Maggia. These nappes derive from the same post-Variscan gneissic crust, this complicates their lithological distinction within the nappe pile. All lithologies show a sub-horizontal penetrative foliation that dips gently W to the NW and E to the SE due to doming at the megascale and which intensity varies between rock-types. Close to the Insubric Line to the S, the main foliation plunges with a steep angle to the S. On the foliation plane the mineral and stretching lineation dips NW or SE depending on the plane dip direction. Lineation direction averages at 134°±15° and dips at 14 °±9°. The kinematic analysis indicates a top-to-the-NW shearing.The geological map shows lithological boundaries that are locally incongruent with the tectonic contacts of the published maps. In the southern part of the Lepontine dome the boundary between the Leventina and the Simano gneisses is interpreted as a moderately deformed magmatic contact that limits the allochthonous character of the Simano unit. Folding and re-activation of the magmatic contact occurred at peak metamorphic conditions between 570 and 620 °C and < 0.6 GPa. The boundary between the Simano and the Adula/Cima-Lunga units is unclear because of internal and lateral lithological variations occurring at different scales. However, the shear zone is visible through a strain gradient recognizable with a bottom to top closing up of folds (from close to isoclinal folds) and a parallelization of the lithological and structural elements. The boundary between the Adula/Cima Lunga and the Maggia units follows a folded surface at the base of a granodioritic gneissic body (Cocco gneiss) which in the literature is diagnostic for the Maggia nappe. Along this shear zone, syn- to post-mylonitic leucosomes attest partial melting during deformation. The High Pressure (HP) metamorphism is limited to minor rock volumes of the Adula/Cima Lunga unit and in one eclogite in the Simano unit and PT conditions peak around ~750°C and 2.5 GPa. The later Barrovian metamorphism is associated with incipient partial melting in the lower units and shows PT conditions ranging from ~600 °C and 0.5 GPa in the Leventina unit to ~650°C and 0.7 GPa in the Adula/Cima Lunga unit. The correlation of these results is fundamental to better understand the thermobarometric evolution patterns of the Lepontine Dome and the formation of tectonic nappes in general.

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