The Helvetic Nappes Funny to think that the French word for tablecloth gave us the geological term f
The Helvetic NappesFunny to think that the French word for tablecloth gave us the geological term for huge thrust piles of rock that have detached from the lower crust and been rumpled up in continental collisions induced by the inexorable grinding motion of the world’s plates through geological history. But push one across a table and ruck it up and that’s essentially what happens, though the rocks are a very different material to cloth in how they behave under pressure.Here we have a view of some tormented rocks near the Glarus thrust zone in Switzerland (see http://bit.ly/2m9ye8r) with the Mezsozoic marine sedimentary rocks (252-66 million years old here) that overlie the metamorphic gneiss of the 300 Ma Variscan basement to the left from which all the younger surface cover has also been stripped during the mountain building process. A layer of orange dolomite marks the thrust sediments and separates the geological domains. The Helvetic Nappe (also known as the daufinois in france) system is one of the half dozen major geological zones composing this complex mountain chain, and the rocks to the right started off as sediments deposited in the Valais ocean on the southern boundary of the European plate before the Adriatic plate came crashing in from the southeast.A mixture of sediments (mailny limestone, shales and marls, lime rich mudstones) was then detached along a fault known as a decollement (from the French for unsticking) from its basement in the contiental keel and thrust over the European landmass, ending up as a messy pile of mountains stretching east to west. These nappes are in turn overthrust by 2 other huge systems of ground up rocks.LozImage credit: Kurt Stuewe/ imageo.egu.euhttp://bit.ly/2mbDQPUhttp://bit.ly/2An7ljW -- source link
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