Tectonic Mélange

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Tectonic Mélange, Narooma , Australia

A tectonic mélange ( French : mêler : to mix) is a mappable geological rock unit that has no continuous stratification and consists of rock bodies of very different sizes that lie in a fine-grained, mostly strongly deformed base mass. The small, large and kilometer-sized rock blocks, often lying in a chaotic confusion, are of various origins , both autochthonous and allochthonous . Characteristic of tectonic mélanges are metamorphic remains of oceanic crust with ophiolites , sediments of the deep sea and rocks of the continental slope and the shelf .

Mélanges come into contact with over Schiebungs - terranes in orogenic front zones. They form in the accretion wedge over a subduction zone . The ultramafic ophiolite sequences autopsied on the continental crust are typically underlain by a mélange.

Before the advent of plate tectonics in the early 1970s, it was difficult to explain a mélange using the previously known geological mechanisms. A particularly confusing paradox was the occurrence of blueschist - a rock that is only formed by metamorphosis under high pressure - in direct contact with Grauwacken , which as sedimentary rocks can be assigned to completely different formation conditions.

Similar to a tectonic mélange are olisthostromes , which are often associated with turbidites and arise from submarine landslides. They also show a confused jumble of rock blocks of different sizes and origins in a fine-grained base mass without layers, but are of sedimentary origin.

Examples of tectonic mélanges are the Franciscan Complex in the coastal mountains of California and the Bay of Islands ophiolite complex in Newfoundland and Labrador . The Gwna-Mélange in England runs from Anglesey and the Lleyn Peninsula to Bardsey Island in North Wales . There the British geologist Edward Greenly coined the term "Mélange" in 1919. However, recent research indicates that precisely this occurrence is an olistho stream.

Other occurrences of tectonic mélanges can be found wherever subduction processes have taken place: in the Italian Apennines , in Norway, Japan or Australia.

literature

  • Harvey Blatt and Robert Tracy: Petrology: Igneous, Sedimentary, and Metamorphic . Freeman, 2nd edition, ISBN 0-7167-2438-3 , pp. 178 & 514.
  • Kenneth Jinghwa Hsü : Preliminary report and geologic guide to Franciscan melanges of the Morro Bay - San Simeon area, San Luis Obispo County, California. California Geological Survey Special Publication 35, 1970.
  • LA Raymond: Classification of melanges. Geological Society of America, Special Paper 198, 1984, pp. 7-20.
  • British Geological Survey: Geology of the country around Aberdaron. HMSO, London 1993, ISBN 0-11-884487-3 .

Individual evidence

  1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 11th edition. Elsevier / Spektrum, Heidelberg 2004, ISBN 3-8274-1445-8 , pp. 138 .
  2. ↑ about melange. Bimrock's website
  3. ↑ David C. Schuster: Gwna Melange, Upper Precambrian Olistostromal Sequence, North Wales, United Kingdom. American Association of Petroleum Geologists Bulletin, Vol. 63, 1979 ( abstract )
  4. ↑ F. Remitti, G. and P. Bettelli Vannucchi: Insights into the deformation of an under thrust tectonic mélange from the Northern Apennines, Italy. Geophysical Research Abstracts, Vol. 8, 08190, 2006 (PDF; 31 kB)
  5. ^ David C. Smith: A tectonic mélange of foreign eclogites and ultramafites in West Norway. Nature 287, pp. 366-367, September 1980
  6. ↑ Kugimiya Yasuo, Takasu Akira: Geology of the Western Iratsu mass within the tectonic melange zone in the Sambagawa metamorphic belt, Besshi district, central Shikoku, Japan. Journal of the Geological Society of Japan, Vol. 108, No. 10, pp. 644–662, 2002 ( short version  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link accordingly Instructions and then remove this notice. )@1@ 2Template: Toter Link / sciencelinks.jp  
  7. ↑ DJ Och, EC Leitch, G. Caprarelli and T. Watanabe: Blueschist and eclogite in tectonic melange, Port Macquarie, New South Wales, Australia. Mineralogical Magazine, Vol. 67, No. 4, pp. 609-624, August 2003 ( short version )

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