The Hercynian orogeny in the South Armorican Massif (Saint-Georges-sur-Loire Unit, Ligerian Domain, France): rifting and welding of continental stripes

A new approach to explain the Western Hercynian Belt is given by a lithological and structural study of the St-Georges-sur-Loire Unit (southern part of the French Armorican Massif). This unit is interpreted as a Devonian olistostrome sheared to the North. It is formed in an accretionary prism linked with a south-dipping subduction zone, called the Layon subduction zone. It results from a chaotic sedimentation including exotic blocks of various lithologies and ages in a Middle Devonian pebbly mudstone matrix. An inovative geodynamic model involving microblock rifting and collision is discussed to explain the presence of the St-Georges-sur-Loire Unit in the Armorican Massif. This model involves two Palaeozoic subductions and emphasizes the role of the Nort-sur-Erdre fault, considered as a major polyphase suture zone separating South and Central Armorica.