GEOLOGICAL HISTORY OF THE MORVAN

VARISCAN OROGENY

Part 2

EVOLUTION IN WESTERN EUROPE

Besides classical studies involving stratigraphy and tectonics and run since the begining of this century, news tools allow to reconstruct the geodynamic evolution of the Variscan belt :

Nevertheless the geodynamic interpretation of the Variscan orogenic belt remains uncertain, fragments of oceanic and continental crust disappearing during particular stages of the orogeny.

The geologic history of the Paleozoic series cropping out in the Morvan region is a part of the evolution of the Variscan orogen, which is characterized by a succession of probably repeated events including :

The presence of microcontinents and subsequent irregularities on the Gondwana margin are responsible of the abundance of major strike-slip faults separating the main structural units or zones that are defined in the Variscan belt.

A such succession of tectonic events and their related geologic units are encountered in the Paleozoic formations of the Morvan region :

In the Western Europe and more particularly in France, five main periods are distinguished in the building of the Variscan orogenic belt [REF], [REF].

Pre-Variscan period (550 to 450 Ma): Cambrian to Ordovician distension

Under a generalized extension, the North European continent (NE) is separeted from the Gondwana continent (G) by a large oceanic domain (O), the Rheic ocean [REF]. Belonging to the northern margin of Gondwana a small continental block, the Cadomian block (C), is characterized by the extension of the Armorican Sandstones of Arenig age. In Germany, the Barrandian block is a similar microcontinent.

Prevarisque

 

Eo-Variscan period (450 to 400 Ma): Plates convergence, subduction and burying from Late Ordovician to Silurian.

Eovarisque

Medio-Variscan period (400 to 380-350 Ma), continental collision from Early Devonian to Middle Devonian.

  1. An upper gneissic unit comprises a thick series of para-derived gneisses and anatectites, the leptyno-amphibolic group and abondant eclogites and granulites remnants near the nappe bottom; mesozonal metamorphism ends the nappe deformation.
  2. A lower gneissic unit is composed of metamorphosed graywackes and shales (paragneiss) and intrusions of aluminous granites emplaced between 540 and 430 Ma and orthogneissified. The tectonic setting of the lower gneiss unit is not well known, possibly overthrusting less metamorphosed autochthonous Brioverian to Cambrian series.

Mediovarisque

In the French Massif Central, from Marche to Monts du Lyonnais, through Haut Allier region, various segments of these two nappes are southward thrusted. Similar tectonic structures are disclosed by the Sioule nappes with a possible northeastern prolongation in Montjeu gneisses that contain retromorphosed eclogites [REF].

Neo-Variscan period (380 to 300 Ma), from Late Devonian to Late Carboniferous.

In Vosges and Morvan regions, calc-alkaline volcanism [REF], [REF] extruded form Late Devonian to Early Carboniferous [REF] may be related to a subduction zone whose the site remains uncertain, probably located on the southern edge of the Rhenohercynian zone i.e. the Phyllitic zone [REF].

In South Massif Central, the southward progradation of the orogenic front is responsible of folding Lower Visean metamorphic nappes in Rouergue region and Namurian-Westphalian nappes in Montagne Noire region.

Neovarisque

Early and Middle Visean alternating limestones and conglomerates extend in the area between Brévenne series and Morvan region (Forez, Montagne Bourbonnaise).

It is inferred that in Morvan region Late Devonian and possibly Middle Devonian sediments overlap with an angular unconformity the gneissic basement although the contact has never been observed.

The geologic history of Morvan region is detailed in various chapters see : Abstract

Variscan orogeny part 3


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