Glarus thrust


Glarus thrust

Infobox World Heritage Site
WHS = Swiss Tectonic Arena Sardona


Image caption = The Tschingelhörner on the border of Swiss cantons Glarus and Graubünden. The Glarus thrust can be seen as a horizontal line in the cliffs.
State Party = SUI
Type = Natural
Criteria = viii
ID = 1179
Region = Europe and North America
Year = 2008
Session = 32nd
Link = http://whc.unesco.org/en/list/1179
The Glarus thrust (German: "Glarner Überschiebung") is a major thrust fault in the Alps of eastern Switzerland. Along the thrust the Helvetic nappes were thrusted more than 100 km to the north over the external Aarmassif and Infrahelvetic complex. The thrust forms the contact between older (Helvetic) Permo-Triassic rock layers of the Verrucano group and younger (external) Jurassic and Cretaceous limestones and Paleogene flysch and molasse.

The Glarus thrust crops out over a relatively large area in the cantons Glarus, St. Gallen and Graubünden, due to its horizontal orientation and the high local relief. Famous outcrops include those at Lochsite near Glarus (the town) and in a mountain cliff called "Tschingelhörner" between Elm and Flims (in the same cliff is a natural hole called the Martinsloch).

World heritage

Thrust faults of this kind are not uncommon in many mountain chains around the world, but the Glarus thrust is a well accessible example and has as such played an important role in the development of geological knowledge on mountain building. For this region the area in which the thrust is found was declared a geotope, a geologic UNESCO world heritage site, under the name Swiss Tectonic Arena Sardona. The area of this "tectonic arena" encompasses 32.850 hectare of mainly mountainous landscape in 19 communities between the Surselva, Linthtal and Walensee. In the arena are a number of peaks higher than 3000 meters, such as Surenstock (its Romansh name is "Piz Sardona", from which the name comes), Ringelspitz and Pizol.

In 2006 the Swiss government made a first proposal to declare the region world heritage to the International Union for Conservation of Nature (IUCN). The IUCN then did not find the area to have an extraordinary or universal value and denied the proposal. The Swiss made a new, this time successful proposal in March 2008. The region was declared world heritage in July 2008, because "the area displays an exceptional example of mountain building through continental collision and features excellent geological sections through tectonic thrust." [ [http://whc.unesco.org/en/list/1179 Swiss Tectonic Arena Sardona - UNESCO World Heritage Centre] ]

The American Museum of Natural History in New York exposes a full-scale reconstruction of the Glarus thrust. [ [http://www.glarusoverthrust.org/inhalt.asp?fHigherChapter_ID=150&fChapters_id=150&fSubChapters_id=153&fTemplatefile=inhalt.asp&ID_Postcards=&fPassword=# geopark association] ]

History

The first naturalist to examine the Glarus thrust was Hans Conrad Escher von der Linth (1767-1823). Escher von der Linth discovered that, contradictionary to Steno's law of superposition, older rocks are on top of younger ones in certain outcrops in Glarus. His son Arnold Escher von der Linth (1807-1872), the first professor in geology at the ETH at Zürich, mapped the structure in more detail and concluded that it could be a huge thrust. At the time, most geologists believed in the theory of geosynclines, which states that mountains are formed by vertical movements within the Earth's crust. Escher von der Linth had therefore difficulty with explaining the size of the thrust fault. In 1848 he invited the British geologist Roderick Murchison, an international authority, to come and look at the structure. Murchison was familiar with larger thrust faults in Scotland and agreed with Eschers interpretation. However, Escher himself felt insecure about his idea and when he published his observations in 1866 he instead interpreted the Glarus thrust as two large overturned narrow anticlines. This hypothesis was rather absurd, as he admitted himself in private.

Eschers successor as professor at Zürich, Albert Heim (1849-1937), initially stuck to his predecessors interpretation of two anticlines. However, some geologists favoured the idea of a thrust. One of them was Marcel Alexandre Bertrand (1847-1907), who interpreted the structure as a thrust in 1884, after reading Heims observations. [aut|Bertrand, M.; 1884: "Rapports de structure des Alpes de Glaris et du bassinhouiller du Nord", Société Géologique de France Bulletin, 3rd series, v. 12, pp. 318–330.] Bertrand was familiar with the Faille du Midi, a large thrust fault in the Belgian Ardennes. Meanwhile, British geologists began to recognize the nature of thrust faults in the Scottish Highlands. In 1883, Archibald Geikie accepted that the Highlands are a thrust system. [aut|Geikie, A.; 1883: "On the Supposed Pre-Cambrian Rocks of St. David's", Quarterly Journal of the Geological Society 39(1-4), p. 261-333. ( [http://jgslegacy.lyellcollection.org/cgi/content/abstract/39/1-4/261] )] Swiss geologists Hans Schardt and Maurice Lugeon then discovered in 1893 that in western Switzerland, Jurassic rock layers are on top of younger molasse too, and argued that the structure of the Alps is a large stack of nappes, large sheets of rock that had been thrusted on top of each other. [aut|Schardt, H.; 1893: "Sur l’origine des Préalpes romandes", Eclogae geologicae Helvetiae 4, pp. 129–142.] At the turn of the century, Heim was also convinced of the new theory. He and other Swiss geologists now started mapping the nappes of Switzerland in more detail. From that moment on, geologists began recognizing large thrusts in many mountain chains around the world.

However, it was still not understood where the huge forces that moved the nappes came from. Only with the arrival of plate tectonic theory in the 1950's an explanation was found. In plate tectonics, the horizontal movement of tectonic plates over the Earth's soft asthenosphere causes horizontal forces within the crust. Presently, geologists believe most mountain chains are formed by convergent movements between tectonic plates.

References

Literature

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