Introduction to the geological zones of Austria

Austria can be separated into three main geological parts: - Austria divisa est in partes tres ;-)

• Alps (most parts of the Eastern Alps, northern rim of Southern Alps)
• Molasse Zone and Tertiary basins in the East
• Southern parts of the Bohemian Massif

Alps

The Alps represent an orogeny consisting of folds and thrusts, which was created by the collision of Africa and N-Europe. This collision is, of course, a slow process, which started in the Upper Jurassic (ca. 150 Ma ago) and is still in action today. As a consequence, the strata of a sea basin ("Tethys"), that had been deposited since Permian times (230 Ma), were folded and thrust on top of each other (thrust sheets or nappes). Those nappes which used to be situated in the south were thrust above the ones in the north.

So the southernmost part (the Upper Austroalpine Nappe System), is on top, below the Middle Austroalpine and the Lower Austroalpine Nappes. The Austroalpine Nappe System was thrust over the Penninic Nappes, which were situated beyond a mid-oceanic ridge (like the Atlantic today) during Jurassic times. The oceanic crust was almost totally subducted under the Austroalpine Nappes. Only some Prasinites remained in the Bündnerschieferformation ("phyllite formation of the Grisons") (e.g. Großglockner).

On the other hand, the Penninic Nappes thrust over the Helvetic Nappe System, which leads to the autochthonous sediments of the massifs in the north (Bohemian Massif, Black Forest, Vosges).

Geology of the Northern Alps in Austria (cross-section and interactive map)

In the Eastern Alps (Austria), the Austroalpine was thrust nearly to the northern rim of the Alps, so it covers almost the whole of Austria's Alps (hence the name). Only geological "windows" like the Tauern Window and the Window of Rechnitz (both Penninic) are excepted. The Helvetic System is rather rare in Austria's Alps, whereas it dominates in central Switzerland (hence the name). That is because most parts of the Austroalpine have been eroded away there (except for the Dent Blanche Nappe - Matterhorn). Also the Penninic Nappes are frequent there (south of the line Rhône Valley - Upper Rhine - Chur).

The younger strata of the Austroalpine Nappes consist of Mesozoic limestones and dolostones (230-65 Ma), which on the one hand represent the Northern Calcareous Alps (north of the line Bludenz - Arlberg- Innsbruck - St.Johann - Bischofshofen - Liezen - Semmering), and the Southern Calcareous Alps (Drau Range, northern Karawanken Mountains; do not mix up with Southern Alps!) on the other, which avoided transportation. A small deposit lies in the Nock Mountains.

Below, there are low-grade metamorphites (Caledonian and Variscian orogenies): greywackes, slates, phyllites and paleozoic limestones (600-230 Ma). One deposit lies at the southern rim of the Northern Calcareous Alps (called Greywacke Zone), another is the Paleozoic of Graz and the one of Murau and the Gurk Valley (Nock Mountains).

The lowermost rocks are gneisses and schists (amphibolite facies) of the Caledonian and Variscian orogenies. They build up e.g. the Silvretta Group, the Oetz Valley Alps, the W Stubai Valley Alps, the Defreggen Alps, the Schober and Kreuzeck Groups, the Small Tauern Range and the SE Alps rim (Saualpe, Kor-, Pack-, Stub-, Gleinalpe: Middle Austroalpine, Wechsel area, Fischbach Alps: Lower Austroalpine).

The Penninic Nappes can be divided into four formations: firstly, the "Old Roof" (Precambrian and Cambrian gneisses - older than 500 Ma), which is the countryrock of granites which intruded during the Carboniferous. These granites became metamorph during the First Alpine Orogeny and now represent (secondly) the Central Gneisses (FUCHS thinks that both formations are Helvetic (compare Aare and Gotthard Massifs!)); thirdly, the Habach Formation (Lower Paleozoic); and fourthly, Triassic sediments and Jurassic Bündnerschiefer / "Phyllites of the Grisons" (calcareous phyllites with ophiolites / prasinites).
The youngest penninic sediments date from the Cretaceous and Lower Tertiary. They represent turbidites of a deep sea trench that was caused by the Austroalpine Thrust (like the Alëutian or Andes trenches). During the Middle Tertiary, the Austroalpine Nappes thrust them to the north so that they were seperated from the original Penninic Nappes. Now they remain as the Flysch Zone at the northern rim of the alps.

The Helvetic Nappes only occur as a narrow zone at the north, mingled with the Flysch Zone. They consist of sandstones, slates and limestones.

Note: The identity and comparability of some nappes is being disputed
(e.g. Allgäu, Ternberg, Frankenfels nappes = Middle Austro Alpine (FUCHS)?
basement rocks and Central Gneisses of the High Tauern Range = Helvetic / Central Massif (FUCHS)?
several shear zones between MAA. and Penn. = LAA. or S-Penn. (Matrei Zone, Lizumer Reckner)?)

Molasse Zone and tertiary basins

After the Austroalpine Nappes had been thrust over the Flysch Zone (see above), the remaining sea basin further in the north was filled up with sand and gravel. It was produced by the erosion of the Alps, which had been lifting up isostatically since the Middle Tertiary. The area of this basin is called Molasse Zone (v. Etymology). The closer the sediments are to the northern rim of the Alps, the stronger they have been faulted and even folded (e.g. Bregenz Wood, N Vienna Woods: Buchberg near Neulengbach): "Subalpine Molasse".

The sea basin used to extend from the Black Forest as far as to the Black Sea and was called Paratethys. The sea regressed from the W to the E: In Lower Austria, sediments of the Badenian are still marine, the Sarmatian is brackish and the Pannonian was dominated by freshwater. The climate at these times was still warm, you can find reefs (Leitha Limestone) and fossils of sharks and manatees, for example (Eggenburg). In some areas (Hausruck Hills, Weinviertel), the molasse sands comprise the host rocks of petroleum and natural gas from the underlying autochthonous mesozoic carbonates. Consequently, there are "abundant" (in respect to Austrian conditions) petroleum and natural gas deposits there.

Bohemian Massif

The Bohemian Massif is an old orogen, that has been dominating Middle Europe since the Paleozoic. It suffered deformation and metamorphism during the Caledonian Orogeny and the Variscian orogeny, which ended up in the formation of the supercontinent Pangaea (300 Ma ago). It is situated at the E end of a huge mountain chain that reached from the Appalachian Mountains and E-Canada to N-Spain, the Brittany, S-Ireland, S-GB and finally across the Massif Central to Middle Germany and Bohemia. This orogen could well have had the dimensions of today's Himalayas.

One manifestation of this orogeny is the thrust of the Moldanubicum (Mühlviertel, Waldviertel) over the Moravicum (E-Waldviertel). The contour of the thrust is visible on a satellite image very well, leading from Krems across Manharts Mountain, bending around Horn towards the W and finally back to NE. The Moravicum was subducted below the Moldanubicum so that the former was molten up in the depths of the mountains. The magma welled up and crystallized in huge plutones as the Weinsberg Granite Plutone.

The collision of Africa during the Tertiary did not fold the massif any further, but it was faulted into many pieces and lifted (v. course of River Danube).

Back to Christof Kuhn's Homepage

Responsible for the content:
Christof Kuhn office@geol-info.at
Last modified: Apr 3, 2006