The Ancenis Terrane: an exotic duplex in the Hercynian belt of Armorica, western France

The Ancenis Terrane, bordering the Loire, France, consists of Precambrian Brioverian basement and three Paleozoic sedimentary sequences: marine Ordovician to Devonian shelf sediments; Lower Carboniferous synorogenic intermontane sediments (the ‘Culm’); and coal-bearing Namurian deposits. The terrane is completely bound by faults, and its geology is in marked contrast with that of its neighbours. New data and observations along the terraneʹs southern boundary show that the Ordovician–Devonian sequence occupies a transpressional shear zone, and can be described as a tectonic mélange. The Culm outcrop, previously interpreted as a sinistral pull-apart structure, is reinterpreted as a Lower Carboniferous dextral pull-apart, and it is shown that Carboniferous transpression and thrusting remodelled the pull-apart basin into its present ‘mica-fish’ shape. With its remarkable concentric structure, and within a plate tectonic setting of mega dextral displacements along the northern boundary of Gondwana, the Ancenis Terrane can be interpreted as an exotic duplex.