Palaeozoic evolution of the Armorican Massif: from Gondwana break-up to continental collision. Mechanics of Variscn Orogeny: a modern view on orogenic research.
Résumé
The Armorican Massif (western France) is divided in four main domains (namely the Léon domain to the NW, and the Northern, Central and Southern domains) by late-Variscan, transcurrent, shear zones. The Armorican massif preserves an undeformed to slightly deformed Proterozoic basement in the Northern and Central Armorican domains and is located between two main suture zones, namely the Lizard ocean to the North and the Galice-Massif Central (GMC) ocean to the South. Resorption of the Lizard ocean should be recorded by the tectono-thermal evolution of the Léon domain, while subduction of the GMC ocean leads to the building of the South-Armorican domain. The Northern and Central-Armorican domains are pieces of crust that were shortened and sheared in between the two main suture zones. The main stages of the tectonic evolution of the Armorican Massif are as follows. During the Cambrian and Ordovician periods, the Armorica microplate was located at a palaeo-latitude close to the South Pole, as shown by palaeomagnetic data and Ordovician faunal communities (trilobites, ostracods, ...). The Cambrian and Ordovician sedimentary sequences record two main episodes of continental rifting, leading to widespread continental rifts coexisting with narrow oceanic domains (Gondwana break-up). Latitudinal migration of the Armorica microplate from the Upper Ordovician to the Middle Devonian is recorded by late Ordovician dropstones associated to the Hirnantian glaciation, followed by development of reefal build-ups during the Early Devonian. The nature and timing of the early stages of convergence are still disputed. Some metamorphic rocks, including eclogites and high-pressure granulites, whose ages span the 440-380 Ma range, indicate an earlier event. However, critical assessment of age reliability and independent geological data from the Loire valley (see Ducassou et al., this volume) suggest a two-stage evolution, i.e. continental rifting during the Lower Devonian (possibly associated to back-arc opening) followed by the earliest mountain-building event at about the Lower/Middle Devonian (i.e. 400 Ma.). Continuing convergence during the Upper Devonian (at about 370-360 Ma) is recorded by high-pressure metamorphic rocks from both continental (e.g. lower unit of the Champtoceaux nappe stack) and oceanic (Groix - Bois-de-Cené) units. The late Devonian to early Carboniferous is marked (i) by progressive stacking and exhumation of crustal sheets (e.g. the Champtoceaux Complex) and (ii) southeastward thrusting (i.e. backthrusting) of the Léon domain over the Central-Armorican Domain. Therefore, the dominantly lacustrine, early Carboniferous, deposits are unconformably resting over slightly deformed Devonian sequences (e.g. the Laval and Chateaulin basins in the Central Armorican domain) or on top of exhumed metamorphic units (e.g. the Ancenis basin on top of the Champtoceaux-Mauges stack). At about 330 Ma, the whole Armorican massif was submitted to horizontal shortening associated with dextral shearing, that culminated with the development of the South-Armorican Shear Zone (about 200km of left-lateral displacement). The late Carboniferous shear zones either rework or cut across previous plate boundaries, and deform the early Carboniferous basins as well as their Variscan basement. Moreover, the SASZ bounds a northerly domain where erosion was the dominant process during the Upper Carboniferous from a southerly domain where EW-trending crustal extension took place at about 310-300Ma.