Structure du plateau continental armoricain

Abstract

Between 1965 and 1970 there has been conducted, in collaboration with the hydrographic service of the French navy, a magnetic survey of the Armorican continental plateau. The zone, covered with 48000 km of profiles, stretches between the parallels of 46° 10' N and 49° 30' N and between the meridians 4° W and 7° 15' W. Position fixing was carried out by means of Toran, with an average accuracy of 50 m. Maps of the total field intensity, reduced to epoch 1967.5, and the magnetic anomalies have been drawn to the scale 1:250000. The magnetic anomalies have been calculated by means of a magnetic field model defined by Jensen and Cain. Two major zones of anomaly can be identified: a northern one, bounded to the south by the 48° N parallel, with strong relief, where the magnetic anomalies are numerous and include major ones at the entrance to the Channel; a southern zone, with considerably less relief, but where the anomalies present several major directions. Several of these anomalies have been interpreted by three different methods with the intention of determining the depth of the sources of disturbance and of specifying possible relationships between these sources and the structural geology of the Armorican plateau. From the south to the north of the region studied, the floor of the continental plateau appears to be divided into compartments by large SW-NE horst and graben structures of Hercynian age. In each compartment the depth to basement increases from east to west by step-faulting with a NW-SE orientation, such faulting has played a role throughout the geological history of the continental plateau. Finally, it emerges from this study that the magnetic anomalies are, for the most part, related to the major structural and tectonic lines of the metamorphic basement visible at ground level. Several areas of high positive anomaly may be associated with basic intrusions emplaced along the lines of major faults in the basement rocks of the Armorican platform.