MAJOR GEODYNAMIC PROCESSES STUDIED FROM THE ECORS DEEP SEISMIC PROFILES IN FRANCE AND ADJACENT AREAS

For more than five years, the French ECORS project has obtained deep Seismic profiles across major structural features that are related to the Variscan, Alpine and Pyrenean belts as well as the Permo-Mesozoic subsidence in the Celtic Sea, the English Channel and the Bay of Biscay. Profiles across the orogenic belts show striking similarities such as the flexure of the down-going plate and the geometry of the frontal detachment and related thrusts and nappes whatever the age of the belt. However, straight dipping reflections cutting through large sections of the crust seem to be common in the Variscan belt and are interpreted as late features in its tectonic history. The Tertiary Alpine and Pyrenean belts show extensive crustal thickening and crust-mantle imbrication, while the layered lower crust and its bottom are flat beneath the late Palaeozoic Variscan belt. This is interpreted as a late equilibration of the crust-mantle boundary, which involved deep metamorphism in the crust and occurred between the Permian and the middle Jurassic. The Permo-Jurassic basins reveal an unattenuated lower crust, while both the upper and lower crusts are thinned beneath the Cretaceous-Tertiary ones. Observation does not support the current models of basin formation and nowhere does extension appear to have played a major role in subsidence. The lower crust of the Permo-Jurassic basins may have been reworked and equilibrated after their rifting. © 1990.