Austroalpine nappes

The Austroalpine nappes are a geological nappe stack in the European Alps. The Alps contain three such stacks, of which the Austroalpine nappes are structurally on top of the other two (meaning they were thrust over the other two). The name Austroalpine means Southern Alpine, because these nappes crop out mainly in the Eastern Alps (the Alps east of the line Lake Constance - Chur – Lake Como). Because the Austroalpine nappes consist of material from the former Apulian or Adriatic plate, that was thrust over the European plate, they are called allochthon nappes. In comparison with the other nappe stacks, they have experienced lower-grade metamorphism, which distinguishes them clearly from the Penninic nappes on which they rest. The Austroalpine nappes are fragments of the former continental shelf and continental slope of the Apulian or Adriatic plate. These fragments contain rocks from the continental basement as well as from sedimentary rocks deposited in these environments. The basement rocks have experienced metamorphism related to their original depth in the Earth's crust, but in the Austroalpine nappes Alpine metamorphism (i.e. metamorphism related to the formation of the Alps) is fairly low grade to non-existent. The basement rocks can be greenschist facies to amphibolite facies, depending on their original depth. They are Paleozoic schists and (para-)gneisses intruded by granites of Variscan and Tertiary age. On top of this basement rock, Permian and Mesozoic sedimentary and volcanic rocks were deposited. Shallow marine limestones are abundant, these limestones now form the mountain chains of the northern part of the Eastern Alps, which are therefore together called the Northern Calcareous Alps. Sometimes, the limestone has been turned into dolomite, as in the Austrian region Salzkammergut and the German region Allgäu. A special unit is the greywacke zone, a band of Paleozoic metamorphosed sedimentary rocks that forms an east-west band through the Austrian Alps.