Paleozoic geodynamic evolution of mid-europa from the example of the French Massifs.
Résumé
The Paleozoic orogen that forms the backbone of Mid-Europa results from the collision between Gondwana, Laurussia and several microcontinents. The French Massif Central and Massif Armoricain belong to three tectonic and paleogeographic domains. Namely, the entire Massif Central and the south part of the Massif Armoricain belong to the North Gondwana margin, the Central Armorican Domain is a part of Armorica microcontinent and the Léon Domain is a piece of another microcontinent. The N. Gondwana margin and Léon Domain are made of a stack of syn-metamorphic nappes, conversely, the Central Armorican Domain consists of a Proterozoic basement built up by the Neoproterozoic Cadomian orogeny and a Paleozoic basement weakly deformed by upright folds related to wrench faults. The bulk architecture of the North Gondwana margin id the result of three main tectonic and metamorphic events that followed a Late Silurian high-pressure metamorphism whose associated structures are poorly documented. The Early Devonian D1 event (ca 385 Ma) is responsible for top-to-the-SW nappe stacking coeval with migmatization and exhumation of high-pressure rocks. The Late Devonian-Early Carboniferous D2 event (360-350 Ma) is associated to a medium pressure/temperature metamorphism during which top-to-the-NW shearing develops. The Visean D3 event (335-330 Ma) is characterized by contrasted tectonic regimes. A synmetamorphic top-to-the-south ductile shearing is widespread in south Massif Central whereas in north Massif Central, D3 is coeval with the onset of synorogenic extension. Orogen-parallel (E-W) extension develops coevally with Namurian-Westphalian plutonism, and orogen-transverse (N-S) extension took place in Late Carboniferous (ca 300 Ma). The Variscan Belt is also characterized by several magmatic events. The Early Cambrian and Early Ordovician pre-orogenic alkaline magmatisms correspond to the rifting events that separated Avalonia, Armorica and Léon microcontinents from Gondwana. The Early-Middle Devonian calc-alkaline magmatism is related to the Gondwana-directed subduction of the Rheic Ocean. Other magmatic events are due to the crustal melting that took place in Carboniferous times in response to D2 and D3 events. Late Visean and younger magmatic stages are coeval with extensional tectonics. These structural, metamorphic and magmatic events are replaced in a geodynamic evolution model at the scale of Western Europe.