Precambrian in the external zones o f the Variscides: central and northern parts o f the Armorican Massif

Abstract

Tiré-à-partPrecambrian in the external zones o f the VariscidesCadomian tectonic effects appear relatively minor and are seen to diminish in importance as one goes towards the south and east away from the Domnonean ‘Cordillera’ (ridge). The deformation style of these external parts of the Cadomian chain is characterized by a lack of cleavage and a development of synsedimentary breccias and superficial recumbent folds produced by local sliding over an unknown basement.Both the Brioverian and its Palaeozoic cover are actively affected by Hercynian deformation; in fact, most of the schistosity developed in the Brioverian of Central Brittany relates to Variscan orogenesis.The southern limit of the domain is marked by the northerp branch of the South Armorican Shear Zone, which is itself guided by the Canvaux granitic-gneiss belt. Intrusives along this axis have Ordovician and Silurian ages and appear as the first signs of crustal distension that, further south, characterizes the early stages in the evolution of the Ligerian active margin