Scandian/Grampian Phase
The main phase of the Caledonian orogeny (from about 425 to 395 million years ago) is called the Scandian phase in Scandinavia and the Grampian phase in the British Isles. It was caused by the collision between Laurentia and Baltica. The Iapetus Ocean first closed in the north, then in the south. Therefore the collision between Baltica and Laurentia took place a little earlier than that between Avalonia and Laurentia. Continental collision started in the Mid Silurian and mountain building took place in the Early Devonian (from 407 million years onward). In North America, the collision between Avalonia and Laurentia is called the Acadian orogeny.
According to some authors, the Caledonian continental collisions involved another microcontinent, Armorica (southern Portugal, most of the north of France and parts of southern Germany and the Czech Republic), even smaller than Avalonia. This microcontinent probably did not form one consistent unit, but was instead a series of fragments, of which the current Armorican and Bohemian Massifs are the most important. The ocean between the combined continental mass of Laurentia, Baltica and Avalonia (called Euramerica, Laurussia or Old Red Continent) and Armorica is called the Rheic Ocean.
The paleogeographic position of the Armorica crustal fragments between the Ordovician and Carboniferous is highly disputed though. There are indications that the Bohemian Massif started moving northward from the Ordovician onward, but many authors place the accretion of the Armorican terranes with the southern margin of Laurussia in the Carboniferous Hercynian orogeny (about 340 million years ago). The Rhenohercynian basin, a back-arc basin, formed at the southern margin of Euramerica just after the Caledonian orogeny. According to these authors, a small rim from Euramerica rifted off when this basin formed. The basin closed when these Caledonian deformed terranes were accreted again to Laurussia during the Hercynian orogeny.
Read more about this topic: Caledonian Orogeny, Geodynamic History
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