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Structure de l'unité de Saint-Georges-sur-Loire et du domaine ligérien (Massif Armoricain). Implications géodynamiques pour la chaîne hercynienne.

Abstract : The Saint-Georges-sur-Loire unit crops out in the Armorican Massif, more precisely in the Ligerian Domain between the Lanvaux-les-Ponts-de-Cé anticline and the Nort-sur-Erdre fault. This peculiar unit is divided into two subunits : the sandstone-pelite subunit to the North (Early Ordovician-Early Silurian) and the blocky volcanic-clastic subunit to the South. The southern subunit is considered as an olistostrome and includes blocks of various lithologies and ages, namely : Early Silurian black chert, Late Silurian to Middle Devonian limestone, Late Silurian mafic lava, acidic lava of probably Cambrian age and sandstone blocks. These blocks are mixed in a pebbly-mudstone matrix with locally turbidite-type sandstone-mudstone alternations reaching a probable Middle to Late Devonian age. This chaotic subunit presents a S0-1 surface deformed by folds overturned to the North and a NW-SE trending southward dipping cleavage bears a N-S to N60E stretching lineation. This southern subunit overthrusts the northern sandstone-pelite subunit. When moving northward to the sandstone-pelite subunit, upright folds or recumbent folds overturned to the North are found and a NW-SE stretching and mineral lineation develops. This lineation and mylonites become more and more conspicuous when moving to the Lanvaux anticline. The blocky southern subunit constitutes an intermediate zone between the low grade Paleozoic terranes to the North and the South-verging high metamorphic grade Champtoceaux and Mauges nappes to the South. A new geodynamic interpretation is proposed to account for the Saint-Georges-sur-Loire unit in a model implying North Gondwana and Armorica plate interactions in Paleozoic times. This interpretation suggests the activity of diachronic subduction zones with opposite dips in Armorica, emphasizes the role of the Nort-sur-Erdre polyphase fault and explains the geotectonic nature of the Saint-Georges-sur-Loire unit as a chaotic unit formed in an accretionary prism linked with the « Layon rift » closure.
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Contributor : Nathalie Pothier <>
Submitted on : Wednesday, November 6, 2002 - 7:58:48 AM
Last modification on : Thursday, March 5, 2020 - 6:49:19 PM
Long-term archiving on: : Tuesday, September 11, 2012 - 6:25:10 PM


  • HAL Id : tel-00001926, version 1


Carine Cartier. Structure de l'unité de Saint-Georges-sur-Loire et du domaine ligérien (Massif Armoricain). Implications géodynamiques pour la chaîne hercynienne.. Géologie appliquée. Université d'Orléans, 2002. Français. ⟨tel-00001926⟩



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