Subalpine molasses

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In geology, subalpine molasses is a narrow area of ​​pushed up, steep molasses sediments on the northern edge of the Alps. Other names of the subalpine molasse are molasse zone or fold molasse .

North of the subalpine molasse, the rocks of the foreland molasse are mostly undisturbed. Both units belong to the Molasse Basin (see there for details of the formation of today's surface, layer sequence and tectonics ).
In the south, the subalpine molasse is bounded by the chains of the Helvetic Mountains , the Flysch Zone and partially the Northern Limestone Alps.

Occurrence

View from the Hohgant to the northeast: the chain of hills that extends from the left into the center of the picture ( Marbachegg and Beichlen) consists of subalpine molasses, while the Helvetic rim chain ( Schrattenfluh ) can be seen on the right edge of the picture

The zone of the subalpine molasse extends in a narrow, mostly around 10 km wide strip along the entire northern edge of the Alps from the Geneva region ( Arvetal ) to around the point where the Inn Valley exits the Alps. Occasionally, subalpine Molasse can also be found further east into the Vienna Woods region. Beyond the Vienna Basin , a zone of folded and overturned molasse rocks extends in front of the Carpathian Mountains (Sub-Carpathian Molasse). Characteristic are relatively elongated ridges with inclined Nagelfluh ribs; In contrast to the mountains of the (Helvetic) Rim Range and the Northern Limestone Alps, they do not have any rugged limestone cliffs.

Important mountains of the subalpine Molasse in Switzerland are Mont Pèlerin (1080 m above sea level), Honegg (1546 m above sea level), Beichlen (1770 m above sea level), Rigi -Kulm (1797 m above sea level). ) and Rigi-Scheidegg (but not Rigi-Hochfluh), Rossberg (1580 m above sea level), Speer (at 1950 m above sea level the highest point of the subalpine Molasse) and Kronberg (1663 m above sea level). To the east of the Rhine Valley, the area continues in the Bregenz Forest and in the Nagelfluh chain (in the Hochgrat 1832 m above sea level).

Further to the east, the Subalpine Molasse in Bavaria is no longer continuously exposed on the surface from about the line Kempten (Allgäu) - Sonthofen , but is largely covered by the moraine formations of the Worm Ice Age; at the same time the zone becomes narrower to the east until it disappears approximately at the Inn valley. Well-known mountain ranges are the Hohe Peißenberg north of the Staffelsee , the Tischberg between Starnberger See and Isartal , the Taubenberg protruding from the Munich gravel plain and the Irschenberg at the edge of the Rosenheim basin . Just before Vienna , the Subalpine Molasse in the Neulengbach (Schlossberg, Buchberg ) - Sieghartskirchen area emerges again on a small scale.

To the north of the subalpine molasse area, there is a section of compressed and therefore slightly folded molasses. In the south, depending on the region, the subalpine molasse is bounded by the chains of the Helvetic, the flysch zone of the Penninic or the Northern Limestone Alps.

Due to the history of their origins, the mountains of the subalpine Molasse are geologically mostly counted as part of the Swiss Plateau or the Alpine Foreland . Geographically, mountains such as Rigi, Speer and others already belong to the Pre-Alps, given their height . The demarcation between the Central Plateau and the Alps is blurred here and depends on the point of view.

History of origin

Tertiary Nagelfluh vom Speer , a mountain in the Swiss Prealps

The subalpine molasse initially shows the same evolutionary history as the molasse sediments in the Swiss plateau and in the Bavarian Alpine foothills . In the second half of the folding of the Alps, i.e. in a period that began around 20 million years ago and lasted around 15 million years, the Helvetic nappes were pushed over the molasses near the edge of the Alps . The extent of the thrust is on average around 15 to 25 km in Switzerland, but it can also be significantly larger in regions. Right at the edge of the thrust ceiling part of the molasses was forced upwards, inclined and divided into individual flakes.

This zone with inclined layers that plunge into the Alps is known today as the subalpine molasse. It is allochthonous , which means that it was moved to its current position in the course of the mountain formation. The subalpine molasse consists mainly of Nagelfluh , a conglomerate of rounded rock blocks of different sizes that are consolidated into a coherent rock with a binding agent (lime, sand and clay). Fine-grained sandstone and marl layers are also embedded between the Nagelfluh layers .

Literature and maps

  • Geological map of Switzerland 1: 500 000 Swiss Geological Commission, 1980
  • Geological hiking guide Switzerland. Part 1: The geological basics . Hans Heierli, Ott Verlag, 173 pp., 2nd edition, Thun 1983. ISBN 3-7225-6282-1
  • Geological map of Bavaria 1: 500000 with explanations . Bavarian Geological State Office, Munich, 4th edition 1996.
  • Geological overview map of the Republic of Austria with tectonic breakdown 1: 1,000,000 Geological Federal Agency Vienna, revised reprint, Verlag Frytag, Berndt and Artaria, Vienna 1986.

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