Variscan orogeny

Variscan orogeny
Name	Variscan orogeny
Date	~420, 300
Duration	~120 million years
Continent	Pangaea
Type	Continental collision
Cause	Closure of the Rheic Ocean
Orogen	Variscan orogen
Area	Europe, Northwest Africa
Predecessor	Caledonian orogeny
Successor	Alpine orogeny

Variscan orogeny. The Variscan orogeny was a major Paleozoic mountain-building event caused by the continental collision between the paleocontinents of Laurussia and Gondwana, culminating in the assembly of the supercontinent Pangaea. This protracted tectonic episode, active from the Devonian into the early Permian, created a vast, continuous orogenic belt stretching from present-day Portugal and Morocco across central Europe to the Carpathians. The eroded roots of this colossal mountain range form the geological backbone of much of central and western Europe, including regions like the Massif Central, the Bohemian Massif, and the Iberian Massif.

Overview

The event represents the European segment of the broader Hercynian orogeny, a term often used interchangeably in regional geology. It followed the earlier Caledonian orogeny and preceded the Mesozoic-Cenozoic Alpine orogeny, which overprinted many of its structures. The profound crustal thickening and deformation associated with the orogeny had a lasting impact on the lithosphere of Europe, defining fundamental geological terrane boundaries and basement blocks that influenced subsequent sedimentary basin development. The final suturing of Laurussia and Gondwana along this belt was a pivotal event in the formation of Pangaea.

Tectonic setting and causes

The primary driver was the long-term closure of the Rheic Ocean, a major Paleozoic seaway situated between the continents of Laurussia (which included Baltica, Avalonia, and Laurentia) to the north and Gondwana to the south. This closure involved the northward subduction of oceanic lithosphere beneath the southern margin of Laurussia. The process entailed the accretion of numerous microcontinents and volcanic island arcs, such as those potentially represented in the Armorican Massif and the Saxothuringian Zone, onto the active continental margin. The ultimate continent-continent collision created an orogenic wedge and led to the formation of a major suture zone, such as the Saxothuringian-Moldanubian suture.

Orogenic phases and chronology

Orogenesis was not a single event but occurred in several distinct, diachronous phases across the belt. The initial stages, often termed the Bretonian phase, began in the Middle Devonian to Early Carboniferous, affecting areas like the Armorican Massif. The main phase of collision and crustal shortening, the Sudetian phase, occurred during the Late Carboniferous (Pennsylvanian). This was followed by the final consolidation and strike-slip tectonics of the Asturian phase and Saalian phase in the Late Carboniferous to Early Permian. These phases are recorded in unconformities within the stratigraphic record of regions like the Rhenish Massif and the Iberian Peninsula.

Structural features and mountain building

The orogen is characterized by large-scale, predominantly north-vergent thrusting and folding, which stacked continental crust and created a thick, hot orogenic root. Major structural trends are often east-west, as seen in the Moldanubian Zone of the Bohemian Massif. Significant late-orogenic strike-slip fault systems, like the Bayarian Pfahl and faults within the Massif Central, accommodated lateral escape of crustal blocks. The mountain belt itself, sometimes called the Variscan Mountains or Hercynian Mountains, was likely as topographically prominent as the modern Himalayas or Alps during its zenith.

Metamorphism and magmatism

The collisional process generated widespread regional metamorphism, producing high-grade gneiss and migmatite in core zones, such as the Moldanubian region, with recorded ultra-high-pressure metamorphism in locations like the Erzgebirge. Extensive granite plutonism occurred during the late- to post-orogenic stages, forming massive batholiths like the Cornubian Batholith in South West England and numerous plutons in the Massif Central and the Bohemian Massif. This magmatism is often associated with significant mineralization, including tin and tungsten.

Economic geology and legacy

The orogen is exceptionally important for its mineral deposits. Major coal basins, such as those in South Wales, the Pennines, and Ruhr, formed in foreland basins during the Carboniferous. The associated magmatic activity sourced world-class metallogenic provinces, including the Erzgebirge tin-silver deposits and the Kupferschiefer copper shale of central Europe. The eroded Variscan basement provided the stable cratonic framework for later Mesozoic cover sequences, directly controlling the location of structures like the Paris Basin and the North German Basin. Its remnants also form the high ground of regions like the Vosges and the Black Forest.

Category:Orogenies Category:Geology of Europe Category:Paleozoic