DEFINITION OF CALEDONIDES

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 4,100 miles (6,600 km). The history

of the opening and closing of the early Paleozoic Iapetus

Ocean and 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 Laurentia (proto-

North America) from Baltica and Avalonia, and the Tornquist

Sea separated Baltica from Avalonia. The eastern margin of

Laurentia has Neoproterozoic 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 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. Importantly, 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. This conclusion is supported

by paleomagnetic data. 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 Avalonia (still attached to Gondwana) 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

Avalonia from Gondwana and shrinking the Iapetus as ridges

were subducted and terranes were transferred from one margin

to another. Avalonia and Baltica 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.

See also APPALACHIANS; PLATE TECTONICS; SCOTTISH

HIGHLANDS.

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