The Glarus thrust (German: Glarner Überschiebung) is a major fault in the Alps of eastern Switzerland. This fault caused the Helvetic rock layers to move more than 100 km north over the Aarmassif and Infrahelvetic complex. The thrust marks the boundary between older Permo-Triassic rocks of the Verrucano group and younger Jurassic and Cretaceous limestones, as well as Paleogene flysch and molasse.

The Glarus thrust is visible across a large area in the cantons of Glarus, St. Gallen, and Graubünden. Its horizontal shape and the region’s steep terrain allow it to be seen clearly. Notable locations where the thrust is exposed include the Lochsite near the town of Glarus and a mountain cliff called Tschingelhörner between Elm and Flims. This cliff also contains a natural cave known as Martinsloch.

World Heritage

Thrust faults like the Glarus thrust are common in many mountain ranges worldwide. However, the Glarus thrust is easy to visit and has helped scientists learn a lot about how mountains form. Because of its importance, the area where the thrust is located was named a geotope, a geological UNESCO World Heritage Site, called "Swiss Tectonic Arena Sardona." This area covers 32,850 hectares of mainly mountainous land in 19 communities between Surselva, Linthtal, and Walensee. The region includes several peaks over 3,000 meters high, such as Surenstock (also known as Piz Sardona, the source of the name), Ringelspitz, and Pizol.

In 2006, Switzerland first asked the International Union for Conservation of Nature (IUCN) to declare the area a World Heritage Site. The IUCN said the area did not have enough special value and refused the request. Switzerland then submitted a new proposal in March 2008, which was accepted. The area became a World Heritage Site in July 2008 because it shows an excellent example of mountain building caused by the collision of continents and has clear geological features from tectonic thrusts.

The American Museum of Natural History in New York displays a full-scale model of the Glarus thrust.

History

The first naturalist to study the Glarus thrust was Hans Conrad Escher von der Linth (1767–1823). He found that older rocks are on top of younger ones in some rock formations in Glarus, which contradicted Steno's law of superposition. His son, Arnold Escher von der Linth (1807–1872), who was the first professor of geology at the ETH in Zürich, mapped the structure in more detail and suggested it might be a large thrust. At that time, many geologists believed the theory of geosynclines, which claimed mountains form from vertical movements in the Earth's crust. This made it hard for Escher von der Linth to explain the size of the thrust. In 1848, he invited the British geologist Roderick Murchison, an expert in geology, to examine the structure. Murchison had studied large thrust faults in Scotland and agreed with Escher's idea. However, Escher himself was unsure about his conclusion. When he published his findings in 1866, he instead described the Glarus thrust as two large overturned narrow anticlines. He later admitted this idea was not very reasonable.

Escher's successor as a professor in Zürich, Albert Heim (1849–1937), first believed the Glarus thrust was two anticlines. However, some geologists supported the idea of a thrust. One of them, Marcel Alexandre Bertrand (1847–1907), studied Heim's observations and in 1884 interpreted the structure as a thrust. Bertrand had studied the Faille du Midi, a large thrust fault in the Belgian Ardennes. At the same time, British geologists began recognizing thrust faults in the Scottish Highlands. In 1883, Archibald Geikie accepted that the Highlands form a thrust system. In 1893, Swiss geologists Hans Schardt and Maurice Lugeon found that in western Switzerland, Jurassic rock layers are on top of younger molasse layers. They argued that the Alps are made of a large stack of nappes, which are large sheets of rock that have been pushed over one another. By the early 1900s, Heim also accepted this new theory. He and other Swiss geologists then mapped the nappes of Switzerland in more detail. From that time, geologists began identifying large thrusts in mountain ranges worldwide.

It was still unclear where the forces that moved the nappes came from. This mystery was solved with the development of plate tectonic theory in the 1950s. According to this theory, the horizontal movement of tectonic plates over the Earth's soft asthenosphere creates horizontal forces within the crust. Today, geologists explain most mountain ranges as forming at convergent boundaries between tectonic plates, where one plate dives beneath another in a process called subduction. The heavier plate sinks into the Earth's mantle, while the lighter plate moves upward, creating mountain chains.

Literature

• "Glarus, Alpine Nappes." The Geological Society. Accessed on May 28, 2024.

• Franks, Sibylle; Rudolf Trümpy (September 2005). "The Sixth International Geological Congress: Zürich, 1894" (PDF). Journal of International Geoscience. Episodes. 28 (3). International Union of Geological Sciences: 187–192. Archived from the original (PDF) on September 28, 2011. Accessed on July 10, 2008.

• Poulet, Thomas; Veveakis, Manolis; Herwegh, Marco; Buckingham, Thomas; Regenauer‐Lieb, Klaus (October 28, 2014). "Modeling episodic fluid‐release events in the ductile carbonates of the Glarus thrust" (PDF). Geophysical Research Letters. 41 (20): 7121–7128. doi: 10.1002/2014GL061715. hdl: 1959.4/unsworks_37154. ISSN 0094-8276.

• Badertscher, Nicolas P.; Beaudoin, Georges; Therrien, René; Burkhard, Martin (2002). "Glarus overthrust: A major pathway for the escape of fluids out of the Alpine orogen." Geology. 30 (10): 875–878. doi: 10.1130/0091-7613(2002)030<0875:GOAMPF>2.0.CO;2. ISSN 0091-7613.

• Herwegh, Marco; Hürzeler, Jean-Pierre; Pfiffner, O. Adrian; Schmid, Stefan M.; Abart, Rainer; Ebert, Andreas (2008). "The Glarus thrust: Excursion guide and report of a field trip of the Swiss Tectonic Studies Group (Swiss Geological Society, 14–16. 09. 2006)." Swiss Journal of Geosciences. 101 (2): 323–340. doi: 10.1007/s00015-008-1259-z. ISSN 1661-8726.