Alpine convergence record in the Carboniferous Badstub Formation, Upper Austroalpine basement nappes, Austria

Within the Carboniferous of Nötsch, the Badstub Formation is a clastic sequence of the Upper Austroalpine basement nappes exposed few kilometres north of the Periadriatic Fault System (Carinthia, Austria). Although these rocks preserve pristine sedimentary features, multi-scale struc- tural analysis disclosed a syn-metamorphic foliation in fine-grained rocks, sets of mineralized faults, veins, and corona textures. Vein fillings and coronas contain equilibrium mineral assemblages with prehnite, pumpellyite, chlorite, phengitic mica, winchite, and riebeckite. Thermodynamic model- ling and geothermometry constrain metamorphic conditions at 260–310°C and 0.25–0.50 GPa that are consistent with a temperature/depth ratio of about 20°C km−1. In a 2D thermomechanical model this thermal state is reached either in the upper or lower continental plate during conver- gence. Thus, during the Alpine convergence, the Badstub rocks were buried from the shallow crust at depths between 13 and 18 km either by ablative oceanic subduction or by continental subduc- tion of the passive margin, and eventually stacked into the orogenic wedge at the Adria margin. During the downward and upward paths, the Badstub rocks were translated as a coherent poorly strained block. This first quantitative constraint on metamorphism for these Carboniferous rocks is consistent with the Upper Austroalpine basement nappes being a tectonic system that recorded the Alpine convergence under eclogite to prehnite–pumpellyite facies conditions.