Anisotropy of magnetic susceptibility investigations of the St Malo dyke swarm (Brittany, France): Emplacement mechanism of doleritic intrusions

Abstract

A high-quality aeromagnetic survey of northern Brittany (line spacing 250 m; flight elevation 150 m) has been used to delineate the Lower Carboniferous St Malo dyke swarm in detail. The dyke swarm fans at its northern and southern ends, and is affected by N60°sinistral transcurrent faults. After restoration of these offsets, the full structure trends in a N-S direction. Small dykes are not imaged, and only one-third of the swarm is evidenced by magnetism. Gravity and magnetic modelling shows that the swarm overlies a single N-S elongated magma chamber. The distribution of K(max) inclinations of anisotropy of magnetic susceptibility (AMS) suggests that the dykes display a fanning magma flow in section. Computed K(max) inclinations are usually shallower than the measured geological dips, probably because the flow becomes more disturbed as the dyke becomes shallower. We observe that the mean susceptibility values increase when the magma flow is steeper than about 70°. A detailed cross-section of St Briac dyke, which is part of the St Malo dyke swarm, reveals that the main carrier of magnetization is magnetite in the centre of the dyke and magnetite + maghaemite on its rims. The emplacement of the St Malo dyke swarm could have been contemporaneous with the N60°shearing which displaced the dyke swarm by about 20 km. The dyke swarm is cut at its southern end by an E-W-orientated fault which probably acted during Late Carboniferous times.