Tectonic evolution of the Cadomian belt in north Brittany

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Abstract
A discrete geotectonic unit, here termed the St Brieuc Terrane, has been identified within the late Precambrian Cadomian belt of the Armorican Massif. This terrane contains syn-orogenic Brioverian supracrustal rocks which accumulated in a marginal basin and were intruded by subduction-related calc-alkaline plutonic complexes during the period c. 615–580 Ma. The Brioverian unconformably overlies orthogneisses and granitoids of the Penthièvre complex which formed during an early phase of Cadomian calc-alkaline activity. Regional transpressive deformation resulted in the internal imbrication of the St Brieuc Terrane into five tectonic blocks, mainly bounded by ductile shear zones which have both strike-slip and dip-slip senses of movement. Amalgamation of the St Brieuc Terrane with the St Malo Terrane to the SE occurred at c. 540 Ma, during regional sinistral transpression. Major subvertical faulting localised along Cadomian shear zones was probably initiated during post-Cadomian extension and continued through Variscan events.

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