Armorican massif

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Armorican massif

The Armorican Massif is a Paleozoic rump or basement mountain complex in northwest France . The name is derived from Aremorica , the name of the ancient Romans for the region between the Loire and Seine rivers, which was then Celtic (" Gallic ") .

geography

Position and extent

The Roche d'Oëtre in Normandy is made up of Lower Cambrian conglomerates of the Conglomérats et Grès pourprés , which are cadomid molasse.
Nez de Jobourg in Normandy , where the oldest rocks in France are exposed.

The Armorican massif extends over an approximately triangular area of ​​around 65,000 km². It occupies all of Brittany , almost all of western Normandy and all of the Pays de la Loire, as well as the northwest of Poitou-Charentes . Geologically, the British Channel Islands are also part of the massif.

relief

In contrast to the Massif Central and the Central European basement islands ( Rhenish Massif , Vosges , Black Forest , etc.), the Armorican Massif was hardly influenced by the formation of the Alps and was therefore only slightly emphasized in the course of the Cenozoic. Therefore, with heights of rarely more than 400 m, but mostly less than 200 m, morphologically predominantly hilly character. Large parts of the massif are even below the 100 m height mark and thus lower than most of the neighboring Paris basin to the east . Only the three high areas that are located in the three "corners" of the massif have low mountainous landscapes: in the north-west in Brittany, in the north-east, predominantly in western Normandy, and in the south in the territories of the Vendée (Pays de la Loire) and Deux-Sèvres (Poitou-Charentes).

North West (Brittany)

In the west of Brittany there are two relatively striking, WSW-ONO-oriented mountain ranges: the Monts d'Arrée in the north with the Roc'h Ruz (385 m), the Ménez Kador (Signal de Toussaines, 384 m), the Roc ' h Trévezel (same height) and the Roc'h Trédudon (383 m) as well as the Montagnes Noires in the south (highest point: Roc'h Toullaëron 326 m).

Northeast

With the Mont des Avaloirs (416 m) in the extreme northeast of the Mayenne department (Pays de la Loire), the north-eastern high area is the highest point of the Armorican massif. About 30 km further south-west, in the Les Coëvrons near Évron , Mont Rochard still reaches 375 m. Although large parts of the Pays de la Loire lie in the Armorican massif, there are hardly any other significant elevations there. Most of the north-eastern high area is in western Normandy. Only about 15 km northeast of Mont des Avaloirs, in the Orne department , lies the Signal d'Écouves , which at 413 m forms the second highest elevation of the Armorican massif. Also in the Orne department, at the south-eastern end of what is known as Norman Switzerland , where the Orne and its tributaries cut relatively deep into the basement, is the Roche d'Oëtre , an imposing rock face above the Rouvre Valley and a popular excursion destination. Around 25 km northwest of Roche d'Oëtre, at the opposite end of Normandy Switzerland in the Calvados department , lies Mont Pinçon, which reaches 365 m. Another tourist attraction is the Grande Cascade near Mortain in the Manche department , where the Cance created the highest waterfall in northwestern France through retreating erosion.

south

The southern one is the smallest and at the same time the lowest of the three high areas of the massif. In its north-western part, in the Vendée, the Mont Mercure reaches 290 m and the Puy Crapaud comes to 269 m. The Terrier de Saint-Martin-du-Fouilloux in the southeastern part, in the Deux-Sèvres department, is 271 m high.

geology

Geological provinces

The geological map of France shows the two great Variscan massifs (in brown and gray-violet): Massif Central in the south and Armorican Massif in the northwest.

The Armorican massif was shaped by two orogenesis: the Upper Carboniferous Variscan orogeny and the late Neoproterozoic Cadomian orogeny , which is why a Cadomian province in the north can be distinguished from a Variscan province in the south of the massif. An even older "Icartien" orogeny, which dates back to the Paleoproterozoic, has been found in Jobourg in Normandy and in Trégor in Brittany .

North American or Cadomian Province

The mountain ranges and cratons from the Icartien (see above) and the sediments of the Pentévrien were severely deformed, folded and subjected to metamorphosis . It is due to the Cadomian and Variscan orogenes . The basement contains metamorphic rocks (such as gneiss ) from the Icartien and Pentévrien . But they very rarely come to the conclusion. Traces of it can be found in Trégor (next to Saint-Brieux in Brittany , namely the beach of Pors-Raden in Trébeurden , Port-Béni, Trébeurden and Moulin-de-la-Rive), in la Hague (in Jobourg on the Cotentin peninsula in the Normandy ) but mainly in the Bailiwick of Guernsey , namely in Guernsey , Sark and Alderney for exploration. Icart Point in South Guernsey gave the Icartien its name.

The Cadomian area consists of scraps of the former Cadomian mountain range. It consists of granite plutons ( Perros-Guirec , Bay of Saint-Brieuc , region around Sartilly , Mayenne ) which go back to the Proterozoic . They are accompanied by sediments from the Proterozoic (South Cotentin , West Calvados Department , North Mayenne Department , region around the Rance River ). In addition, there are sediments from the Paleozoic Era , which are a continuation of these deposits. They can be found in the middle of Brittany from Douarnenez to Angers via Rennes . Or the sediments do not form a continuation, were easily folded during the Variscan orogeny. Variscan plutons made of granite from the Carboniferous are exposed (region around Bourbriac , Plœuc-sur-Lié and Dinan ). The whole was cut through by a Variscan fault running from the Rade de Brest to the Département Sarthe .

The border in the south corresponds to the North Armorican shear zone and in the south part of the Cadomian area there are Paleozoic deposits of the Central Armorican Synclinory (( Syncline of the Menez-Belair, of Vieux-Vy-sur-Couesnon and of Vitré, all in the Ille-et-Vilaine department in the Brittany )).

South Armorican or Variscan Province

In the South African area, the rocks consist mostly of Variscan granites. It is found along a fault that runs from the Pointe du Raz to the Loire . Sedimentary rocks from the Cambrian , Ordovician or Silurian ( Belle-Île-en-Mer , west coast of the Vendée , region south of Angers , and La grande Brière ) that were folded or became metamorphic during the Variscan orogeny form the Paleozoic sedimentary rocks of the region.

In the Proterozoic there is a lively volcanic and orogenic activity , which can be traced back to the Cadomian orogeny , and there is a powerful accumulation of sediments ( Briovérien ).

These granites can be traced back to the Variscan orogeny that began 330 million years ago and form the backbone of a mighty mountain range. The sandstones of the Ordovician and the slate from the Silurian slide into the synclines down.

The shift to the north of the Iberian plate produces significant folds during the Silurian . She dives under the armorican plate. The subduction results in the Loire mountain range ( cordillère ligérienne ) and also a mountain range in the south of the Armorican massif. The whole thing is accompanied by weathered rocks ( molasses ) and granites from the Devonian and then from the Carboniferous .

After the varicose orogeny took place, the massif was exposed to erosion in the Mesozoic Era . In the Paleocene , erosion accelerated due to the subtropical climate. This created a peneplain . The Armorican Massif hardly knew how to exploit the Pyrenees and Alpine orogenesis and was not raised by it. Nevertheless, the effects of both orogenes are visible in the form of faults.

Structures

The difference can be made between:

  • domnonéen range: basement before the Cambrian has undergone several orogenies (the last is the Cadomian Orogeny) hardly busy during the Variscan orogeny;
  • mancellien or Le Mans area: basement before the Cambrian but younger ( Briovérien ) such as flysch and Paleozoic rocks that were metamorphosed and folded during the Cadomian orogeny; in addition Variscan plutons;
  • Middle-Armorican area: Paleozoic sedimentary rocks;
  • ligérien Loire area ( Pays de la Loire ): metomorphized before the Cambrian basement;
  • West Vendée  : from the Cambrian it has to do with the Massif Central .

The first two areas are part of the Cadomian area, the other three are part of the Variscan area. There is also the North Armorican Shear Zone and the South Armorican Shear Zone.

See also

Individual evidence

  1. ^ La Roche d'Oëtre, le Synclinal de la Forêt-Auvray - Page 1 Massif d'Athis (Suisse Normande) . In: Jacques Aubry, Françoise Gigot: Lithothèque de Normandie. Version 1.5.7, last updated April 23, 2014
  2. ^ Armorican Massif ( English ). Encyclopædia Britannica , (Retrieved March 12, 2014).
  3. ^ Jean-François Moyen: Les grands traits de l'histoire géologique de la France (métropolitaine). Version 3.1, 2005 (French)
  4. Guide geologique Normandie Maine. 2nd edition. Page 85. ISBN 2-10-050695-1 .
  5. les plus vieilles Roches de France sont dans la Hague (article) ( French ). La Presse de la Manche (daily newspaper), Paris August 27, 2009, p. 5: “ The oldest rocks in France are declared in la Hague , these rocks are not only exposed in Jobourg , but also in Omonville-la-Rogue , Eculleville and Gréville-Hague , d. H. from Pointe Jardeheu to Omonville-la-Rogue , then from Gréville-Hague to Landemer . "
  6. Le tour de France d'un geologue. François Michel. Page 18. ISBN 978-2-603-01546-9 .
  7. Michel Colchen and Patrick Rolin: La chaîne hercynienne en Vendée ( French ), volume number 1-2 .. Géologie de la France ., 2001, pp. 53-85.
  8. ^ C. Le Corre et al .: Le Massif Armoricain ( French ), No. 1-2. Edition, Volume Volume 44, Scientifical Geological Bulletin, 1991, pp. 31-57.