Remarkable crustal features appear on the ECORS profiles carried out in northern France and the Bay of Biscay as well as on the SWAT profiles shot in the western Channel and the Celtic Sea. The most striking one is the occurrence of flat laminations in the lower crust. Dipping events and laminations are also present in the upper and lower crust, especially in the SWAT profiles. They can readily be related to tectonic events, Variscan in age, some of them identified in the field. The flat laminations in the lower crust are at first interpreted as resulting from delamination, shearing, magmatism and metamorphism at the crust-mantle transition during the Variscan orogeny. This interpretation raises some difficulty concerning the space and time correlation of the laminations with the Variscan orogeny. They seem to have been emplaced after the Permian-Triassic infilling of the Plymouth Bay basin and before the early Cretaceous opening of the Bay of Biscay. An early to middle Jurassic age is suggested, a period when large cratonic basins were formed without noticeable extension. Heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations. Choosing between orogenic and non-orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination.
The ECORS project was launched in 1983 with the aim of studying fundamental mechanisms of geodynamics in France. The ECORS deep seismic profiles have concentrated on a few structures of major significance: outer zone of the Variscan orogen in northern France, the basin formed during the Bay of Biscay's opening and transects of recent orogenic ranges (Pyrenees and Alps). The seismic profiles have been carried out with all the available modern techniques of industry and completed wherever possible by additional geophysical surveys (magnetism, gravity, MT, wide-angle and refraction seismics) and geological surveys. The first results already shed new light on major geodynamic phenomena such as variations in the frontal Variscan detachment, lower crust formation, crustal behaviour during orogenesis and variations in the formation of cratonic basins.
