4th Swiss Geoscience Meeting, Bern 2006 Alteration at the base of the Verrucano-hangingwall along the Glarus thrust, eastern Switzerland

Along the Glarus thrust different lithologies that are part of the footwall are in contact with one single but lithologically rather heterogeneous siliciclastic lithological unit forming the hangingwall, referred to as “Verrucano”. In the northern part of the thrust North Helvetic Flysch, Blattengrat Units and Sardona Units in the footwall are in contact with a facies of Verrucano that contains abundant lithic clasts from metamorphic, volcanic and granitoid sources. In contrast, in the south the Mesozoic para-autochthonous cover of the Aaremassif was cut by a Verrucano-hangingwall of mostly volcanoclastic composition. Pre-existing fold structures in the footwall were decapitated along a sharp tectonic contact. The decapitated Cretaceous to Jurassic carbonate units from the footwall can be found further north as tectonic slivers along the thrust and are referred to as “Infrahelvetic slices”. The infrahelvetic carbonates have been preserved largely undeformed in the central parts of the Infrahelvetic slices but become progressively more tectonized towards the margins. The oxygen isotope signal of the Infrahelvetic slices is between 23 and 26 ‰ d18O(V-SMOV). This indicates that they are derived from primary marine carbonates, which largel