The tectonic framework of Central and Western Europe

Abstract

The complexity of the structure of Central and Western Europe is for a large part due to the rapid succession of four important orogenic events each belonging to a major orogenic period: the Cadomian, Caledonian, Variscan and Alpine orogenies. The four successive mountain chains have partly occupied the same space, resulting in an intricate pattern of rocks formed or deformed during each orogeny. Due to this multiple orogenesis the presence of older Precambrian rocks is difficult to demonstrate. Low grade Proterozoic sediments and volcanics are known from the Armorican and Bohemian Massifs and from the Iberian peninsula. Crystalline basement rocks predating these supracrustals have been reported only from the Armorican and Bohemian Massifs. Other signs of Precambrian events are zircon ages of more than 2000 Ma. The Cadomian orogeny has been demonstrated in England, the Armorican Massif, the Bohemian Massif, the Pyrenees and in the Iberian peninsula either by the unconformity of Cambrian on folded basement, or by geochronological methods. The folding is accompanied by regional metamorphism of varying grade up to granulite facies. Postorogenic Cadomian granitic magmatism occurs in the Armorican and Bohemian Massifs. The Caledonian orogeny occurs besides the main belt in Scandinavia, Scotland, Ireland and Wales, in the Ardennes and a zone from northern Germany towards Poland and Rumania. South of this zone no Caledonian folding based on geological evidence as unconformities can be ascertained, although numerous radiometric dates on metamorphic and igneous rocks indicate a thermal event during the period corresponding to the Caledonian orogeny. The Variscan orogeny is much better known although many problems still remain unsolved. Folding, metamorphism and granitic activity started in the Devonian, but the most widespread and intense folding, low P/T type metamorphism and granitic intrusion is of Late Carboniferous age. The pre-Cadomian basement and the Caledonian event are discussed in some detail. A pre-Mesozoic drift reconstruction shows the relationships of the Cadomian, Caledonian and Variscan belt on either side of the Atlantic Ocean. A large Y-shaped pattern of orogenic belts appears, where intermittent folding, metamorphism and granite intrusion has taken place at least from the end of the Precambrian to Permian times. Central and Western Europe lying on the intersection of these belts has been a high heatflow region during that time, for which a mantle origin is proposed. Spreading and contracting movements at different places and different times are thought to be responsible for the orogenies. However, the presence of oceans other than the Proto-Atlantic or Iapetus is as yet to be proved. It is suggested that they were of minor size or that spreading did not go beyond the rift stage.