Contribution of deep seismic profiling to the knowledge of the lower crust in France and neighbouring areas

In France and neighbouring areas, the crust is essentially formed of segments of the Variscan orogen (380-300 Ma) and older formations. Cratonic basins began to form in the Permian, but extensive subsidence only occurred from the Lias in basins such as the Paris and Aquitaine basins, and also along the margin of the Alpine Tethys ocean. The Pyrenees and Alps range eventually formed during the Tertiary as a result of the collision between the European and African continental plates. Large grabens were also rifted in the Rhine and Rhone valleys. Deep seismic profiles show remarkable features, common to the crust of the whole area, although it underwent such a complicated geological history. The most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust. These reflections, hereafter called laminations, have been found to occur on most profiles within a large area covering Western Europe, although they vary in depth, thickness and energy. Dipping reflections have frequently been observed in the upper crust. Some of them extend across the lower crust and even penetrate into the upper mantle. They may be reasonably interpreted as Variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions. The origin of the flat laminations which generally lie in the lower part of the crust is more controversial. This paper attempts to use the well-known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations. These features may be explained by the following tectonothermal events: 1. (1) Compression which prevailed during the Variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust, associated with melting, intrusion and metamorphism during the different stages of the crustal evolution. 2. (2) The thermal equilibration of the upper mantle which occurred in the Permian, at the end of the Variscan orogeny, may have caused metamorphism and magmatism in the overlying crust. 3. (3) A regional thermal event which might be associated with subordinate extension occurred during the rifting of the Alpine Tethys and the initiation of Mesozoic cratonic basins in Late Triassic-Middle Jurassic times. It may also have caused metamorphism and magmatism in the lower crust. While the first model implies conservation of the crust material, the other two suggest that the crust-mantle transition has been somewhat uplifted since the end of the Variscan orogeny. The latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present Moho reflection. The above three models are not mutually exclusive, and all these phenomena may well have affected the crust to some extent. However, the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the Moho. Further experiments are needed to check these hypotheses.