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.
Caus ...
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-grabe ...
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 (about 180 million years ago). The final stages ...
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
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
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 ''Carbonifero ...
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 languages, 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 ...
name for a large part of northwestern
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
that includes
Brittany
Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo language, Gallo: ''Bertaèyn'' ) is a peninsula, Historical region, historical country and cultural area in the west of modern France, covering the western part of what was known ...
, 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 d ...
, forming the
Armorican Massif
The Armorican Massif (french: Massif armoricain, ) is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. It is important because it is connected to Dov ...
.
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. It is t ...
,
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 C with bar, 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 ...
to
Ordovician
The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...
although the terrane was still close enough to Gondwana to be affected by the
Andean-Saharan glaciation
The Andean-Saharan glaciation, also known as the Early Palaeozoic Icehouse, the Early Palaeozoic Ice Age, the Late Ordovician glaciation, the end-Ordovician glaciation, or the Hirnantian glaciation, occurred during the Paleozoic from approximately ...
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 result ...
. One possible model includes Armorica as part of the
Hun superterrane The Hunic superterrane is a terrane that is now attached to Europe and Asia. At the end of the Ordovician or beginning of the Silurian it separated from Gondwana and joined Laurasia at the beginning of the Carboniferous, at the time of the Variscan ...
,
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, whe ...
and early
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 ''Carbonifero ...
, 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 cumu ...
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
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 ...
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