ORIGIN AND STRUCTURATION OF THE NORTH ARM ORICAN DYKE SWARM

A detailed study of a small part of the aeromagnetic map, recently sur veyed between northern Brittany and the Channel Islands, shows that th e Dinantian dyke swarm known in the St-Male region (Northern Brittany) extends offshore. It is also superimposed with a large gravity high. Gravity modelling shows that the contribution of the dyke swarm to the gravity anomaly is very weak. The same modelling suggests that a magm atic chamber may exist at depth beneath the dykes. This interpretation is reinforced by geochemical analysis, the distribution of the maximu m anisotropy of the magnetic susceptibility and the measurement of the dip of the dykes in the field. This dyke swarm has been affected afte r its emplacement by a series of sinistral shear zones oriented north 60 degrees.