New map, new insights: disentangling the Tauern Window’s Matrei Zone

The Matrei Zone represents the Upper Penninic Nappes in the south of the Tauern Window and is positioned between the Lower Penninic Nappes and the Austroalpine Unit in footwall and hanging wall positions respectively. As such, it comprises a heterogeneous association of lithologies derived from the Piemont-Liguria Ocean and its southern continental margin. This continental margin is nowadays represented by the Lower Austroalpine Unit. Various genetic interpretations for the Matrei Zone have been proposed. While its geodynamic setting as accretionary wedge in a subduction zone is widely agreed on, the presence of the Austroalpine lithologies within remains a matter of discussion. Their occurrence in the Matrei Zone is explained either as olistoliths, elements scraped off the upper plate within a tectonic mélange, or as lenses formed by out-of-sequence thrusting. Additionally, the position of the lower boundary of the Matrei Zone to the Lower Penninic Nappes is uncertain as the identification of unambiguous markers is lacking. To unravel the debated tectonic situation of the Matrei Zone, revisited geological mapping of key areas in the contact zone of the Tauern Window and the Austroalpine Unit is needed. Here, we reassess the lithological content, formation, and tectonic meaning of the Matrei Zone based on new detailed geological mapping in the Upper Möll valley (Carinthia, Austria).
Our findings reveal that the basis of the Matrei Zone is marked by a phyllonitization front within the impure marbles of the Lower Penninic Nappes. This becomes apparent by a lithological upward transition from thick impure marbles and massive greenschist to a heterogeneous, diversly coloured succession of siliciclastic and carbonate-clastic metasediments. The Matrei Zone is formed of several lithological associations, which can be followed for several tens of kilometres.  We distinguish a lower unit, dominated by dark phyllite intercalated with light-coloured phyllites and carbonates. The upper unit is made of chlorite-rich phyllite, quartz-phyllite and impure quartzite with greenschist. Furthermore, a discontinuous tectonic melange containing additional serpentinite and greenschist within a metasedimentary matrix occurs. All units contain metaradiolarite and metamorphic Aptychus limestone as well as metaconglomerate and large olistholiths. These olistholiths derive from Triassic sequences found in the Austraoalpine Unit. An obvious tectonic contact occurs only at the top of the Matrei Zone towards pristine and coherent Austroalpine Nappes.
Our results are consistent with the interpretation that the Matrei Zone represents an accretionary wedge developed out of the lithosphere and the cover of the Piemont-Liguria Ocean, which was thrusted on top of rocks of the subducting Valais Ocean. The occurrence of Triassic rocks is likely to be the result of gravitative flow from the continental margin within the oceanic basin and not necessarily tectonically induced.