A new occurrence of the Vorau Nappe in the Semmering area (Eastern Alps)

The area around the Semmering pass (Austria) is of particular importance for the geology of the Eastern Alps as it was here that the sequence from the crystalline rocks of the Central Eastern Alps to the Mesozoic sediments of the Northern Calcareous Alps was tectonically subdivided for the first time. This took place during the construction of the railway line in the 1880s, but the current work on the railway base tunnel has also brought new insights into the regional geology. This article deals with a variegated lithological association that was excavated in the course of the tunnel construction under the Hocheck Mountain, but which is also known from surface outcrops. According to the available geological maps it locally forms the base of the Stuhleck-Kirchberg Nappe, directly above the Wechsel Nappe. Both nappes are part of the Austroalpine Unit.

The variegated lithological association consists of micaschist and paragneiss with intercalations of granitic orthogneiss and frequently amphibolite. Of special interest are weakly deformed alkalifeldspar and quartz phyric dikes, which are Permian in age due to regional considerations. Further, small ore deposits containing pyrite, galena and chalcopyrite or quartz veins with haematite occur. Partly the micaschist is rich in muscovite and contains garnet up to 1.5 mm in diameter. Paragneiss often shows a layering due to a varying biotite content. Subordinate quartz and feldspar rich types with garnet and/or amphibole occur. Sometimes the latter are interlayered with amphibolite. The orthogneiss is mostly hololeucocratic with chloritisised biotite and alkalifeldspar porphyroclasts up to 3 cm in length. Based on the observed mineral assemblage upper greenschist to amphibolite facies conditions were reached at the metamorphic peak. A later greenschist facies overprint caused intense retrogression and phyllonitisation at a variable grade. Retrogression is indicated by chloritisation of biotite, garnet and amphibole in the paragneiss and sericitisation of feldspar. In the amphibolite actinolithe or chlorite formed. Phyllonitisation occurred along internal shear zones but especially along the basal nappe contact and towards the monotonous phyllonitic micaschist and orthogneiss overlying the variegated lithological association. The lithological layering as well as the main schistosity are mostly dipping towards south and a frequently observed stretching lineation is SW-NE orientated. An overprinting folding and crenulation shows E-W orientated axes.

With respect to the lithological association and the characteristics of the lithologies we argue that the investigated sequence is not part of the Stuhleck-Kirchberg Nappe, but a part of the Vorau Nappe (Silvretta-Seckau Nappe System, Austroalpine Unit), which appears in between the Wechsel and Stuhleck-Kirchberg Nappe further in the south. The Vorau Nappe is built up by the Waldbach Complex. The latter experienced an upper greenschist to amphibolite facies Variscan metamorphic imprint in the Late Devonian and Carboniferous. In Permian time, it was at the Earth’s surface and covered by Permian acidic volcanics and siliciclastic sediments. Subsequently Early Triassic sandstones (Semmering quartzite) and carbonates were deposited. During the Eoalpine event in the Cretaceous the whole sequence experienced greenschist facies conditions and deformation during nappe stacking and folding with E-W trending axes.