A study of the crystalline basement of the Cima Lunga unit (Lepontine Dome, Swiss Central Alps)

The Cima Lunga is a crystalline basement unit located in the Lepontine dome (Swiss Central Alps). It is formed by paragneisses, orthogneisses, layers of calcschists and marbles, and boudins of mafic and ultramafic rocks. These latter preserve the record of Alpine (ultra)high-pressure conditions and have been the object of numerous studies in the past years.

The presence of such diverse lithologies lead previous authors to suggest that the Cima Lunga is a subduction channel tectonic mélange. However, recent field studies redefined its internal structure and relationships with the nappes tectonically above and below. The Cima Lunga unit is sandwiched within the upper levels of the Simano and below the Maggia nappes. The deformation within the unit is intense, especially around the mafic and ultramafic bodies enclosed within the paragneisses, and can be explained by shearing affecting the whole unit. In the field, layers of orthogneisses, calcschists, and marbles extend over kilometers, suggesting that these lithotypes cannot be fragments derived from the adjacent Simano.

In this contribution we present a comprehensive map of the Cima Lunga unit. This map is the result of extensive fieldwork performed for the realization of the new Swiss National Maps 1:25.000 (fieldwork at scale 1:10.000) and of the new Tectonic Map of Switzerland 1:500.000. We present within the map a compilation of literature results comprising calculation of pressure and temperature conditions as well as age data. We highlight the spatial distribution of these data, which is fundamental for the comprehension of such an intensely deformed unit.