Relationship between dip and magma flow in the Saint-Male dolerite dyke swarm (Brittany, France)

A low elevation aeromagnetic survey has been recently carried out in Northe rn Brittany (France) revealing magnetic anomalies over both the onshore and offshore areas. The shaded aeromagnetic map displays a striped pattern rel ated to a dolerite dyke swarm dated at ca. 330 Ma. The dyke swarm is slight ly fanning at its Northern and Southern ends and is affected by some major N60 degrees sinistral transcurrent faults. After sliding the different segm ents of the dyke swarm along the actually observed offsets (about 20 km), o ne can see the dyke swarm as it was originally trending. A study of the ani sotropy of the magnetic susceptibility (AMS) has also been conducted over 2 50 specimens uniformly distributed on 13 sites. In section, the K-max plung e of the AMS is broadly the same as the dip of the dykes measured on the fi eld. On this section, the centre of the fanning pattern is superimposed, bo th, with the centre of a magma chamber, calculated after gravity and magnet ic modelling and with the axis of symmetry of the dyke swarm. It can also b e observed that the K-max plunge, which is usually vertical above the magma chamber, is oblique with respect to the rims of the dykes in the shallow d ipping dykes. This may be interpreted as a result of the crystallization of an oblique flow. Also, it seems that the magnetic susceptibility increases when the magma flow is steeper. Beyond a K-max inclination of 45 degrees t he bulk susceptibility tends to increase in a linear way. (C) 2001 Elsevier Science B.V. All rights reserved.