Céline Ducassou

Géosciences Rennes

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Marc Poujol

Centre National de la Recherche Scientifique

Michel Ballèvre

Géosciences Rennes

Sylvie Bourquin

Université de Rennes

Pierre Pellenard

Biogéosciences

Laurent Beccaletto

Bureau de Recherches Géologiques et Minières

Mathilde Mercuzot

Géosciences Rennes

Camille Rossignol

University of Cagliari

Cécile Robin

Université de Rennes

Hubert Lardeux

Université catholique de l'ouest

Erwan Hallot

Centre National de la Recherche Scientifique

Cooperative Institutions

Centre National de la Recherche Scientifique

Université de Rennes

Géosciences Rennes

Sorbonne Université

Terre des Hommes

Institut des Sciences de la Terre de Paris

Universidad Complutense de Madrid

Universidad de Salamanca

Institut des Sciences de la Terre

Université de Bourgogne

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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

Michel Ballèvre Céline Ducassou Hubert Lardeux Cécile Robin

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).

Devonian

Massif

Orogeny

Basement

Source

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Citations (1)

Caractérisation pétro-géochimique et datation U/Pb du volcanisme contemporain des bassins d’âge fini-Carbonifère à Permien du Nord du Massif Central

Céline Ducassou Sylvie Bourquin Pierre Pellenard Laurent Beccaletto Mathilde Mercuzot

La formation de la chaine hercynienne, dont l’apogee a lieu au Carbonifere, participe a l’assemblage de la Pangee, qui se demantelera par la suite a partir du Trias. La limite Carbonifere- Permien correspond donc a une periode cle dans l’evolution geodynamique globale, a la transition entre une periode orogenique et une periode de rifting. Cependant, les reconstitutions paleogeographiques et paleoclimatiques pour cette epoque sont assez peu contraintes, notamment du fait du manque de datations radiometriques et de correlations stratigraphiques fiables entre les differents domaines sedimentaires. Plusieurs bassins sedimentaires fini-carboniferes a permiens sont preserves a l’affleurement dans le Nord du Massif central (secteurs d’Autun, de l’Aumance, de Decize-la Machine). Ils ont regulierement enregistre des evenements volcaniques, soit explosifs (niveaux de cendres), soit effusifs (coulees volcaniques) intercales dans les series sedimentaires. Certains de ces evenements ont pu etre localement utilises pour des correlations au sein d’un meme bassin, mais ne permettent pas de correler les series sedimentaires entre les differents bassins. Recemment, des donnees geochronologiques precises ont ete obtenues pour le Bassin d’Autun (Pellenard et al., 2017, Gondwana Res., 51, 118-136). Le centre d’emission du volcanisme n’est pas identifie, mais le complexe rhyolitique de Montreuillon, au Nord-Ouest du Morvan et au Nord des bassins pourrait constituer un candidat potentiel. Les objectifs de ce travail sont (1) d’obtenir des datations radiometriques U/Pb supplementaires pour les bassins d’Autun, de l’Aumance et de Decize-la Machine mais egalement pour le volcanisme de Montreuillon, et (2) de mieux caracteriser et de comparer la nature petrologique et geochimique des differents evenements magmatiques.

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Sedimentary record of the obduction of the Samail ophiolite in northern Oman: the Muti Formation in the Sail Hatat window

European geosciences union general assembly (2016)

Céline Ducassou Cécile Robin Marc Poujol Basim Al-Rahbi Guilhem Estournès

The obduction of the Samail Ophiolite in Oman took place during the Upper Cretaceous. Since then, the northern part of Oman has been relatively preserved from deformation and is therefore one of the best places to study obduction processes. In addition, radiometric data provide good constraints on the timing of obduction from the formation of the metamorphic sole until the exhumation of the high-pressure / low-temperature metamorphic rocks involved in the subduction zone below the oceanic lithosphere. However, the response of the continental margin during the obduction is still poorly constrained. If most of the models suggest the development of a flexural basin and an associated forebulge such as in continental collision, their recognition within the syn-tectonic deposits led to different interpretations. The geometry of the youngest syn-tectonic deposits (Fiqa Formation) is relatively well constrained by sub-surface data that suggest a southward migration of the depocenter and progressive onlaps on the southern margin of the basin. The context of sedimentation of the oldest syn-tectonic deposits (Muti Formation) preserved below the nappes in the Oman Mountains is, however, still poorly understood. The location of the sedimentation area with respect of the forebulge, for instance, remains unclear. In order to acquire better constraints on the record, on the Arabian platform, of first steps of the obduction, the analysis of several sections of the Muti Formation has been performed. We present here our main results for the north-eastern part of the Oman Mountains, in the Sail Hatat window, where the thickest successions have been described in Quryat and Bidbid area, respectively located in the eastern and western part of the Sail Hatat window. Sedimentological and structural analysis have been combined to reconstitute the evolution of depositional environments in areas strongly affected by deformation. In addition, systematic measurements and restoration of palaeocurrents, petrological determinations and geochronological analyses (LA-ICPMS) on detrital zircons have been performed in order to identify the source areas. In both studied areas, the sedimentary series are characterised by mainly carbonated slope to basin deposits. The more distal deposits identified are in the easternmost part (Quryat area). Episodes of terrigenous input are recorded in both areas and palaeocurrents indicate a source area located toward the south, in agreement with the dating obtained on detrital zircons, yielding a dominant population at ca. 800 Ma. These results suggest that the Proterozoic basement was being eroded during the sedimentation of the Muti Formation in the Sail Hatat window and an episode of uplift of the Huqf High is therefore inferred. These results allow to discuss the evolution of the north-easternmost part of the Arabian platform during the first steps of the obduction.

Obduction

Continental Margin

Passive margin

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The Late Paleozoic Ice Age in western equatorial Pangea: Context for complex interactions among aeolian, alluvial, and shoreface sedimentary environments during the Late Pennsylvanian – early Permian

Gondwana Research (2023)

Marie Olivier Sylvie Bourquin Guy Désaubliaux Céline Ducassou Camille Rossignol

Red beds

Progradation

Aggradation

Paleosol

10.1016/j.gr.2023.07.004

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Citations (3)

Chronostratigraphical constraints in the intramountain late-Paleozoic basins of the northern Massif Central (France) and synthesis of their implication on geodynamics, paleogeography and paleoclimate at the Carboniferous-Permian transition

Mathilde Mercuzot Sylvie Bourquin Pierre Pellenard Laurent Beccaletto Camille Rossignol

Massif

Palaeogeography

Paleoclimatology

geodynamics

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U–Pb age constraints on the Carboniferous–Permian transition in continental basins of eastern equatorial Pangaea (France): implications for the depositional history and correlations across the late Variscan Belt

Figshare (2023)

Mathilde Mercuzot Camille Rossignol Sylvie Bourquin Jahandar Ramezani Céline Ducassou

Supplementary Material Table 1. ICP–MS and ICP–OES geochemistry results for the LV, LYF-416.5 and LYF-567.7 samples.

Pangaea

Continental Margin

10.6084/m9.figshare.24032557

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L'enregistrement sédimentaire et structural de la bordure orientale du bassin d'Ancenis (Massif armoricain, France) : identification d'une phase de rifting au Dévonien inférieur et de l'érosion d'un relief montagneux au Dévonien moyen

Céline Ducassou Cécile Robin Hubert Lardeux Michel Ballèvre

L'enregistrement sedimentaire pre-carbonifere dans le Massif Armoricain est principalement documente dans le domaine Centre-Armoricain, relativement epargne par les deformations varisques. Au contraire, dans la zone axiale de la chaine, susceptible d'avoir enregistre les premiers evenements de la convergence entre la microplaque Armorica et Gondwana, les formations sedimentaires non metamorphisees sont rarement preservees. Cependant, dans le domaine Ligerien, les formations sedimentaires ordoviciennes a carboniferes sont non metamorphisees et pourraient nous offrir l'opportunite de discuter de l'evolution de la paleogeographie de ce domaine au cours des premices de la convergence varisque grâce a l'analyse des milieux de depots. Sur la bordure du bassin carbonifere d'Ancenis, des formations sedimentaires ordoviciennes a devoniennes dont la faune et la flore ont ete longuement etudiees, sont interpretees comme des olistolithes a la base du bassin carbonifere (Dubreuil, 1980). Les principaux resultats de notre etude stratigraphique, sedimentologique et structurale, sont en opposition avec cette interpretation : 1) Les sediments pre-carboniferes ne sont pas des olistolithes a la base du Bassin d'Ancenis mais definissent deux unites coherentes qui constituent le socle du bassin carbonifere d'Ancenis. 2) L'unite Sud est constituee de sediments marins ordoviciens (d'âge Arenig a Ashgill) recouverts en discordance par des calcaires recifaux devoniens (Emsien) suivis de gres feldspathiques (Emsien). L'unite Nord est constituee d'une serie marine condensee d'âge Ordovicien superieur (Ashgill) a Devonien (Emsien). L'unite Nord est en polarite inverse et est separee de l'unite Sud par un chevauchement a vergence Sud. 3) La discordance angulaire des calcaires emsiens sur les siltites ordoviciennes est interpretee ici comme une phase de rifting au Devonien inferieur. La difference de facies entre les calcaires emsiens d'arriere recif de l'unite Sud et ceux pelagiques de l'unite Nord precise la polarite de cette paleomarge. Ce rifting du Devonien inferieur pourrait etre mis en relation avec l'ouverture du bassin arriere-arc de Saint-Georges-sur-Loire. 4) Une etude petrologique et sedimentologique des gres feldspathiques surmontant les calcaires recifaux indique qu'il s'agit de depots de crues au front d'un delta sous-marin. La presence de debris de plantes terrestres et de sediments immatures (fragments lithiques, feldspaths) implique la mise en place d'une terre emergee soumise a l'erosion. Il s'agit du premier enregistrement de l'erosion d'un relief (chaine de montagne ?) lors de l'orogenese varisque. L'etude petrologique des fragments lithiques pourrait permettre de discuter de la provenance du materiel.

Conglomerate

Source

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Evidence for pre-orogenic, Early Devonian rifting in the Variscan belt: stratigraphy and structure of the Palaeozoic cover of the Mauges Unit (Upper Allochthon, Armorican massif, France)

International Journal of Earth Sciences (2010)

Céline Ducassou Michel Ballèvre Hubert Lardeux Cécile Robin

Allochthon

Massif

Devonian

Stratigraphic unit

10.1007/s00531-010-0605-3