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

ABSTRACT

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 structural 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 modelling 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 convergence. 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 subduction 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.

KEYWORDS:

• Alpine pumpellyite–prehnite riebeckite
• Alpine subduction
• Palaeozoic sequences
• geodynamic modelling
• Drauzug–Gurktal Nappe System

Acknowledgments

Authors are grateful to three anonymous reviewers whose comments made a significant improvement of the manuscript possible. Christian Kofler (Mineral Abbau GmbH) gave access to the Jakomini quarry and approved the publication of the data. Georg Worsche (Worsche Vermessung) provided the updated georeferenced vectorial topography of the quarry. Lucia Angiolini, Curzio Malinverno, Andrea Risplendente, and Luca Corti (Milano University) helped with fossil determination, prepared the thin sections, assisted the acquisition of microprobe analyses, and helped with fieldwork, respectively.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2206443

Additional information

Funding

This work was funded by CTE_NAZPR18ISPAL_01 and PSR2021_MRODA.