Abstract
Abstract The Precambrian basement of NE Brittany contains a number of NNE-trending migmatite belts of Cadomian ( c. 540 Ma) age. These are heterogeneously reworked by subvertical, ductile shear zones which occur both within individual migmatite belts, and along their boundaries with low-grade Brioverian metasediments. The shear zones are characterized by blastomylonitic fabrics, penetrative sub-horizontal or gently-plunging stretching lineations, and shear criteria which indicate a consistent subhorizontal, sinistral strike slip sense of movement. The migmatite belts, and the greenschist facies Brioverian metasediments imbricated with them along the strike-slip zones, form the St Malo Terrane deformed at c. 540 Ma. This terrane is bounded to the NW and SE by two lithologically and metamorphically distinct terranes, the St Brieuc and Mancellian terranes respectively. Boundaries between the three terranes are inferred to be crustal scale sinistral strike-slip zones. The St Brieuc Terrane was also deformed and metamorphosed in a transpressive setting either prior to or during amalgamation with the St Malo Terrane at c . 540 Ma. The Cadomian belt of northern Brittany is interpreted as a sinistrally transpressive strike-slip belt related to deformation in the hanging-wall of an oblique subduction zone located to the north, and across which there is likely to have been substantial lateral displacement.
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