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)

Ducassou, Céline; Ballèvre, Michel; Lardeux, Hubert; Robin, Cécile

doi:10.1007/s00531-010-0605-3

Cited by 21 publications

(8 citation statements)

References 64 publications

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“…Below the Paris Basin sedimentary cover, the high magnetic and gravity signatures elongated eastwards throughout the map, likely correspond to basic rocks, as interpreted by Martelet et al, (2013). These basic rocks can be variously interpreted in terms of lithology: i) basaltic or gabbroic olistostoliths as described in this unit in the Armorican Massif (Cartier and Faure, 2004), ii) interbedded basalts related to the opening of a back-arc basin (Ducassou et al, 2011) or iii) mafic rocks such as the ophiolites which sporadically crop out along the NSE fault (Marchand, 1981;Faure et al, 2008;Ballèvre et al, 2009). In the eastern part of the St-Georges-sur-Loire unit, the depth of the contact between the substratum and the sedimentary cover is defined at about 500 m, and the bodies responsible for the magnetic signal are located at more than 1500 m depth (Martelet et al, 2013).…”

Section: The St-georges-sur-loire Unitmentioning

confidence: 92%

Mapping of a buried basement combining aeromagnetic, gravity and petrophysical data: The substratum of southwest Paris Basin, France

et al. 2016

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International audienceAeromagnetic and gravity data have proven to be among the most effective methods for mapping deeply buried basin/basement interfaces. However, the data interpretation generally suffers from ambiguities, due to the non-uniqueness of the gravity and magnetic signatures. Here, we tie the gravity and magnetic signatures with a petrophysical characterization of the lithologies outcropping around the French Paris Basin. Our methodology investigates the lithology and structure of its hidden Variscan substratum at the junction between the Armorican Massif and Massif Central. Our approach is based on the combination of potential field data, magnetic susceptibilities measured in the field, density values of sample rocks and information documented in boreholes, in order to propose a new interpretative geological map of the buried substratum of the Paris Basin. The petrophysical description is combined with geophysical patterns of the substratum, mapped through statistical unsupervised classification of suitably selected magnetic and gravity maps. The first step of interpretation consists in extending the outcropping major structures below the Meso-Cenozoic sedimentary cover of the Paris Basin. The litho-structural units, in between these major structures, are then interpreted separately. The second step consists in assigning lithologies within each unit, with respect to its magnetization and density (as derived from the petrophysical compilation), and mapping its extension under cover, integrating punctual borehole information. Overall, with a special emphasis on relating geophysical signatures and petrophysical characteristics of litho-structural units, this methodology permits a precise structural and lithological cartography of a segment of the buried Variscan substratum. In the southwestern part of the Paris Basin, this approach reveals: i) the limited eastward extension of Central Brittany, ii) the eastward extension of the major Cholet fault, iii) the emphasis on N150E-N160E striking fault and their N30E conjugates, controlling the opening of Permo-Carboniferous basins, and iv) the eastward extension of the Eo-Variscan suture

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Section: The St-georges-sur-loire Unitmentioning

confidence: 92%

Mapping of a buried basement combining aeromagnetic, gravity and petrophysical data: The substratum of southwest Paris Basin, France

et al. 2016

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“…In the Armorican massif, robust P-T estimates by Pitra et al (2010) yielded peak temperature above 650°C at pressure conditions ranging from 8 to 11 kbar for migmatitic paragneisses of the eclogite and HP orthogneiss bearing Champtoceaux unit. However the structural position of this units is debated: Faure et al (2005Faure et al ( , 2008 include it into the UGU by correlation with FMC occurrences; Ballèvre et al (2009) and Pitra et al (2010) place it at the base of an Upper Allochthon while the upper parts of this allochthon involve a Cadomian basement with low grade metasediments and ophiolites (Faure et al, 2005(Faure et al, , 2008Ballèvre et al, 2009;Ducassou et al, 2011). It is however included within the Lower Allochthon by Ballèvre et al (2014) by correlation within Iberian massifs.…”

Section: Correlations With Tectono-metamorphic Units Of the Western Ementioning

confidence: 99%

Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

Benmammar

Berger

Triantafyllou

et al. 2020

BSGF - Earth Sci. Bull.

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The southwestern French Massif Central (Najac-Laguépie area) displays an inverted metamorphic sequence with high grade units at the top of the nappe stack. These high pressure and/or high temperature units are often grouped into the ill-defined leptyno-amphibolite complex forming the base of the Upper Gneiss unit. This paper presents new data on the pressure-temperature-time evolution of the Najac and Laguépie units and discuss the implications of the results for the structure and evolution of the Variscan French Massif central during the Upper Devonian. The Najac unit consists of phengite-garnet metapelites, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite-orthogneisses. P-T estimates on micaschists with syn-kinematic garnets yielded peak pressure at 16 kbar for 570°C, followed by retrogression in the greenschist facies. According to recently published data, the age of high pressure metamorphism is constrained between 383 and 369 Ma. The Laguépie amphibolite unit is a mafic migmatite comprising garnet-free and garnet-bearing amphibolite with felsic leucosomes. Thermobarometry and phase diagram calculation on a garnet amphibolite sample yielded peak P-T conditions at 710°C, 10 kbar with retrogression and deformation under greenschist facies conditions. New U-Pb analyses obtained on zircon rims from a leucosome yield an age of 363 ± 3 Ma, interpreted as the timing of medium-temperature metamorphism. Considering the different metamorphic evolutions and the contrasted age-deformation relationships in the two localities, it is suggested that Najac and Laguépie rocks depict two distinct tectono-metamorphic units. The Najac LT-HP domain recorded the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle allochthon, sandwiched between the Lower Gneiss unit and the Upper gneiss unit. Such an intermediate unit is still poorly defined in the French Massif Central but it can be a lateral equivalent of the Middle Allochthon (encompassing the Groix blueschist units) in the Armorican massif. The Uppermost Devonian MT-MP metamorphism recorded by the Laguépie amphibolite correlates with the P-T-t evolution of the Upper Gneiss unit in the Western French Massif Central and the Upper Allochthon in the Armorican massif. This event is contemporaneous with active margin magmatism in the Western European Variscan belt confirming that the Upper Gneiss unit, encompassing the Laguépie amphibolite, once belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.

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“…They overthrust gneiss and migmatites of the Champtoceaux complex, correlated to the Massif Central Upper Gneiss Unit. In France, blueschists are Cartier and Faure, 2004, Faure et al, 2008, Ducassou et al, 2011.…”

Section: The Eo-variscan Belt: Medio-european Ocean Closurementioning

confidence: 99%

Alpine, Variscan, eo-Variscan belts: comparison between hot and cold orogens from the examples of French segments

Faure¹

2024

Comptes Rendus. Géoscience

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The Cenozoic Alpine, and Paleozoic Variscan and eo-Variscan collisional belts are compared in the framework of the Wilson cycle considering differences between cold and hot orogens. The W. Alps result of the opening and closure of the Liguro-Piemonte ocean, whereas the Paleozoic Eovariscan and Variscan orogenies document multiple ocean openings and collisions in space and a polyorogenic history in time. Jurassic or Early Ordovician break-up of Pangea or Pannotia megacontinents led to the formation of passive continental margins, and the opening of Liguro-Piemonte, or Rheic, Tepla-Le Conquet, and Medio-European oceans, respectively. In Paleozoic or Mesozoic, microcontinents such as Apulia and Sesia or Armorica and Saxo-Thuringia were individualized. The oceanic convergence stage was associated with the development of arcs and back-arc basins in the Variscan belt but magmatic arcs are missing in the W. Alps, and inferred in the Eo-variscan one. Though the nappe stack is mainly developed in the subducted European or Gondwana crust in the western Alps and Eo-variscan cases, the Moldanubian nappes formed in the upper plate in the Variscan case. The Alpine and Variscan metamorphic evolutions occurred under ca. 8 °C/km and 30 °C/km gradients, respectively. During the late-to post-orogenic stages, all belts experienced "unthickening" accommodated by extensional tectonics, metamorphic retrogression, and intramontane basin opening. The importance of crustal melting, represented by migmatites, granites, and hydrothermal circulations in the Variscan and Eo-Variscan belts is the major difference with the W. Alpine one. The presence, or absence, of a previous Variscan or Cadomian continental basement might have also influenced the rheological behavior of the crust.

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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)

Cited by 21 publications

References 64 publications

Mapping of a buried basement combining aeromagnetic, gravity and petrophysical data: The substratum of southwest Paris Basin, France

Mapping of a buried basement combining aeromagnetic, gravity and petrophysical data: The substratum of southwest Paris Basin, France

Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

Alpine, Variscan, eo-Variscan belts: comparison between hot and cold orogens from the examples of French segments

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