The Armorican

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...

, Armorican terrane assemblage, or simply Armorica, was a

microcontinent Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Cau ...

or group of continental fragments that

rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben wi ...

ed away from

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period The Jurassic ( ) is a geologic period and st ...

towards the end of the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

and collided with Laurussia towards the end of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonif ...

during the

Variscan orogeny The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Variscan'', comes f ...

. The name is taken from

Armorica Armorica or Aremorica (Gaulish: ; br, Arvorig, ) is the name given in ancient times to the part of Gaul between the Seine and the Loire that includes the Brittany Peninsula, extending inland to an indeterminate point and down the Atlantic Coast. ...

, the

Gaulish Gaulish was an ancient Celtic language spoken in parts of Continental Europe before and during the period of the Roman Empire. In the narrow sense, Gaulish was the language of the Celts of Gaul (now France, Luxembourg, Belgium, most of Switzerl ...

name for a large part of northwestern France that includes

Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo: ''Bertaèyn'' ) is a peninsula, historical country and cultural area in the west of modern France, covering the western part of what was known as Armorica during the period ...

, as this matches closely to the present location of the rock units that form the main part of this terrane.

Extent

The main exposures of the Armorican terrane are found throughout Brittany, the

Channel Islands The Channel Islands ( nrf, Îles d'la Manche; french: îles Anglo-Normandes or ''îles de la Manche'') are an archipelago in the English Channel, off the French coast of Normandy. They include two Crown Dependencies: the Bailiwick of Jersey, ...

, parts of

Upper Normandy Upper Normandy (french: Haute-Normandie, ; nrf, Ĥâote-Normaundie) is a former administrative region of France. On 1 January 2016, Upper and Lower Normandy merged becoming one region called Normandy. History It was created in 1956 from two ...

, forming the Armorican Massif. Other fragments thought to have originally formed part of the Armorican terrane assemblage include rock units exposed in the

Vosges The Vosges ( , ; german: Vogesen ; Franconian and gsw, Vogese) are a range of low mountains in Eastern France, near its border with Germany. Together with the Palatine Forest to the north on the German side of the border, they form a single ...

Black Forest The Black Forest (german: Schwarzwald ) is a large forested mountain range in the state of Baden-Württemberg in southwest Germany, bounded by the Rhine Valley to the west and south and close to the borders with France and Switzerland. I ...

Bohemian Massif The Bohemian Massif ( cs, Česká vysočina or ''Český masiv'', german: Böhmische Masse or ''Böhmisches Massiv'') is a geomorphological province in Central Europe. It is a large massif stretching over most of the Czech Republic, eastern Ger ...

and most of the

Iberian peninsula The Iberian Peninsula (), ** * Aragonese and Occitan: ''Peninsula Iberica'' ** ** * french: Péninsule Ibérique * mwl, Península Eibérica * eu, Iberiar penintsula also known as Iberia, is a peninsula in southwestern Europe, defi ...

History

All of the fragments that make up the Armorican terrane are thought to have originally formed part of the northern margin of Gondwana. Rifting initiated in the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

although the terrane was still close enough to Gondwana to be affected by the Andean-Saharan glaciation during the late Ordovician. Complete separation from Gondwana, across the developing Paleotethys Ocean, is thought to have occurred towards the end of the Silurian. At this time Armorica was separated from Laurussia by the

Rheic Ocean The Rheic Ocean was an ocean which separated two major palaeocontinents, Gondwana and Laurussia (Laurentia-Baltica-Avalonia). One of the principal oceans of the Palaeozoic, its sutures today stretch from Mexico to Turkey and its closure resulted ...

. One possible model includes Armorica as part of the Hun superterrane, while other show it moving separately. The Rheic Ocean closed during the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

and early

, as the

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic c ...

was

subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

. The

marks the final closure of the Rheic Ocean as the various continental fragments, including Armorica, collided with Laurussia towards the end of the Carboniferous.

References

* {{Geology of Europe Terranes Historical continents Historical tectonic plates Carboniferous geology Devonian geology Hercynian orogeny Paleozoic Europe Armorica