Images of the lithosphere-asthenosphere system beneath France and their geodynamic implications: a contribution from teleseismic tomography and seismic anisotropy

From seismic tomography and seismic anisotropy, images of the lithosphere-a sthenosphere system beneath France for some remarkable tectonic areas have been computed: a continental rift system (the Upper Rhinegraben), an Hercyn ian structure reactivated by Neogene volcanism (Massif central), a region o f a recent continental collision (Pyrenees) and finally a region of an anci ent orogeny (Armorican Massif). These images have a horizontal spatial reso lution of the order of 10 km and show not only the geometry of the deep geo logical structures but will also illustrate the link between surface observ ations and structures detected at depth. The images demonstrate the passive character of the Rhinegraben mainly because no low-velocity was found belo w the Moho, show the presence of a thermal anomaly beneath the Massif centr al interpreted as caused by a mantle plume in the decaying phase of its evo lution and prove the lithospheric scale of the North Pyrenean fault and of the South-Armorican shear zone. The anisotropic measurements suggest a lith ospheric deformation related to the most recent tectonic event. In the Pyre nees, the Armorican Massif or the Rhinegraben areas, the directions of the fast-polarisation azimuth (the polarisation direction of the fast shear wav e) are parallel to the tectonic texture of the last events, bur suggest als o a reactivation of inherited Hercynian discontinuities. In the Massif cent ral, the splitting parameters distinguish between two lithospheric units re gions marked by a distinct fast-polarisation azimuth on each side of the Si llon Houiller fault zone.