Caledonides

The Caledonides are an early Paleozoic orogenic belt in north and east Greenland, scandinavia, and the northern British Isles. The Caledonides were continuous with the Appalachian Mountains before the opening of the Atlantic Ocean, together extending more than 4,101 miles (6,600 km). The history of the opening and closing of the Early Paleozoic Iape-tus Ocean and the Tornquist Sea is preserved in the Caledonian-Appalachian orogen, which is one of the best-known and studied Paleozoic orogenic belts in the world. The name is derived from the Roman name for the part of the British Isles north of the firths of Clyde and Forth, used in modern times for Scotland and the Scottish Highlands.

The Paleozoic Iapetus Ocean separated Lauren-tia (proto-North America) from Baltica and Avalo-nia, and the Tornquist Sea separated Baltica from Avalonia. The eastern margin of Laurentia has Neo-proterozoic and Cambrian rift basins overlain by Cambro-Ordovician carbonate platforms, representing a rifting to trailing or passive margin sequence developed as the Iapetus Ocean opened. Similarly Baltica has Neoproterozoic rift basins overlain by Cambro-Ordovician shelf sequences, whereas the Avalonian margin in Germany and Poland records Neoproterozoic volcanism and deformation, overlain by a Cambro-Silurian shelf sequences, with an arc accretion event in the Ordovician. Gondwana sequences include Neoproterozoic orogens overlain by Ordovician shelf rocks deformed in the Devonian and Carboniferous. Significantly, faunal assemblages in Laurentia, Baltica, Gondwana, and Avalonia all show very different assemblages, interpreted to reveal a wide ocean between these regions in the early Paleozoic. Paleomagnetic data support this conclusion. Middle Ordovician ophiolites and flysch basins on Laurentia and Baltica reflect an arc accretion event in the Middle Ordovician, with probable arc polarity reversal leading to volcanism and thin-skinned thrusting preceding ocean closure in the Silurian.

From these and many other detailed studies, a brief tectonic history of the Appalachian-Caledonide orogen is as follows. Rifting of the Late Proterozoic supercontinent Rodinia at 750-600 million years ago led to the formation of rift to passive margin sequences as Gondwana and Baltica drifted away from Laurentia, forming the wide Iapetus Ocean and the Tornquist Sea. Oceanic arcs collided with each other in the Iapetus in the Cambrian and with the margin of Laurentia and Avalon (still attached to Gond-wana) in the Ordovician. These collisions formed the well-known Taconic orogeny on Laurentia, ophiolite obduction, and the formation of thick foreland basin sequences. Late Ordovician and Silurian volcanism on Laurentia reflects arc polarity reversal and subduction beneath Laurentia and Gondwana, rifting Avalon from Gondwana and shrinking the Iapetus as ridges were subducted and terranes were transferred from one margin to another. Avalonia and Baltic collided in the Silurian (430-400 million years ago), and Gondwana collided with Avalon and the southern Appalachians by 300 million years ago, during the Carboniferous Appalachian Orogeny. At this time the southern Rheic Ocean also closed, as preserved in the Variscan Orogen in Europe.

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