The structural and sedimentary record within the Ancenis basin (Armorican Massif, France) : from early Devonian rifting to middle Devonian Collision. Mechanics of Variscan Orogeny : a modern view on orogenic research
Résumé
Isotopic ages from high-pressure metamorphic rocks in the Variscan belt (Massif central and Armorican Massif) indicate an Early - to Middle Devonian age for the early stages of the plate convergence. However, the sedimentary record of this event is poorly known, because Devonian sediments from the internal zones were largely deformed and metamorphosed during the Variscan orogeny. The Ligerian Domain, located along the suture zone between Armorica and Gondwana, preserves uncleaved and unmetamorphosed sedimentary sequences covering the Ordovician to Carboniferous period. It thus offers us a unique opportunity for correlating the nature and timing of deformation recorded in the sedimentary sequences with the metamorphic record of the internal zones. Ordovician to Devonian sediments outcrop along the boundary of the Ancenis Carboniferous basin. Several models for their structure have been proposed, the prevailing interpretation being one of huge olistoliths at the bottom of the Carboniferous basin (Dubreuil, 1980). The main results of our combined stratigraphical and structural study of the pre-Carboniferous sediments drastically challenge this view. 1. The pre-Carboniferous sediments are not olistoliths at the base of the Carboniferous basin, but define coherent sequences that can be followed for several kilometres along strike and constitute the basement of the Carboniferous of the Ancenis basin. 2. Two units are identified. The southern unit consists of Ordovician marine sediments unconformably overlained by Devonian reefal carbonates (Emsian) followed by arkosic sandstones (Emsian to Eifelian). The northern unit consists of an Upper Ordovician (Ashgill) to Early Devonian (Emsian) condensed sequence. 3. The southern unit represents the Palaeozoic cover of the Proterozoic basement of the Mauges Unit. The northern unit, in reverse position, has been thrust over the southern unit. 4. The angular unconformity identified between the Ordovician (Caradoc-Ashgill) siltstones and the Devonian reefal limestones is interpreted here as evidence for an Early Devonian rifting, possibly related to the opening of a back-arc basin (Saint-Georges-sur-Loire) associated to a north-dipping subduction zone. 5. Petrographical (coarse- to fine-grained, immature, feldspar-rich sandstones), sedimentological (flood deposits in the front of a submarine delta) and palaeontological (abundant debris of early tracheophytes) data on the Early to Middle Devonian sandstones imply the proximity of an emerged land, thus recording the earliest evidence for mountain erosion in the Variscan belt. This is consistent with the isotopic ages of the oldest eclogites from the Variscan belt (e.g. la Bessenoits: 408 ± 7 Ma according to Paquette et al., 1995).