The formation of the West European rift: A new model as exemplified by theMassif Central area

In this paper, we use mainly field data from the Massif Central area, which have been presented in a companion payer [Michon and Merle, 2001], to disc uss the origin and the evolution of the West European Rift system. It is sh own that the tectonic event in the Tertiary is two-stage. The overall geolo gical evolution reveal a tectonic paradoxe as the first stage strongly sugg ests passive rifting, whereas the second stage displays the first stage of active rifting. In the north, crustal thinning, graben formation and sedime ntation at sea level without volcanism during the Lower Oligocene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensi onal stresses within the lithosphere (i.e. passive rifting). In the south, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the lack of crustal thinning, may be easi ly interpreted in terms of the first stage of active rifting due to the asc ent of a mantle plume. This active rifting process would have been inhibite d before stretching of the crust, as asthenospheric rise associated with up lift and volcanism are the only tectonic events observed. The diachronism o f these two events is emphasized by two clearly distinct orientations of cr ustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into a ccount the Tertiary evolution of the Alpine chain. It is shown that the For mation of a deep lithospheric root may have important mechanical consequenc es on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval extension in the surrounding lithosph ere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurren tly, the descending lithospheric flow induces a flow pattern in the astheno sphere which can bring up hot mantle to the base of the adjacent lithospher e. Slow thermal erosion of the base of the lithosphere may lead to a late-s tage volcanism and uplift as expected for active rifting.