Fault rocks as indicators of progressive shear deformation in the Guingamp region, Brittany

The North Armorican Shear Zone is a major structural feature running from the island of Moiene in the west to Moncontour in the east of the Armorican Massif. In the region of Guingamp it cuts through a Precambrian migmatite complex and granitoid rocks of both Precambrian and Hercynian age. A variety of fault rocks are present in this part of the shear zone, and are thought to represent a time sequence in which deep level, ductile deformation gave way to higher level brittle displacements. Mylonite and cataclasite series rocks, and pseudotachylites are described and their conditions of formation considered. The Hercynian Quintin granite post-dates the main movement of the shear zone but is itself dextrally displaced during the late stages of shear movement.