Deep seismic imaging of the Cadomian thrust wedge of Northern Brittany

Abstract

The Armor seismic profile (AR1, AR2) crosscuts the major units of the Cadomian Domain (600–540Ma) of Northern Brittany and extends the offshore SWAT 10 profile (BIRPS–ECORS program). We thus have a combined 95-km-long NNW–SSE section for describing the regional-scale structure of the Cadomian crust in the area. Following reprocessing and migration of the southern end of the SWAT 10 profile so that it could be compared with the AR1–AR2 Armor profile, a structural interpretation of the composite profile was carried out taking into account gravity modelling and magnetotelluric soundings.The SWAT 10–Armor profile reveals the existence of steeply dipping thrust faults in the upper crust representing a thrust wedge with a gently northwest-dipping sole. This Cadomian thrust system continues below the unmetamorphosed Brioverian and Palaeozoic sediments of central Brittany and beyond the southern end of the profile in the Central Armorican Domain. It supports previous interpretations of dominant oblique-type thrusting during Cadomian tectonics proposed on the basis of geological field evidence and confirms the existence of a major northwest-dipping thrust fault that brings the volcanic-arc formations of the Saint-Brieuc Unit onto the metasediments of the Saint-Malo Unit. The profile also emphasises the regional extent of a thrust wedge at the scale of the whole Cadomian domain of Northern Brittany.