Showing papers by "Laurent Michon published in 2001"

The formation of the West European Rift; a new model as exemplified by the Massif Central area

Abstract: In this paper, we use mainly field data from the Massif Central area, which have been presented in a com­panion paper (Michon and Merle, 2001), to discuss the origin and the evolution of the West European Rift system. It is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradoxe as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. ln the North, crustal thinning, graben formation and sedimentation 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 extensional 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 easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into account the Tertiary evolution of the Alpine chain. lt is shown that the formation of a deep lithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval exten­sion in the surrounding lithosphere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric flow induces a flow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal ero­sion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting.

The evolution of the Massif Central Rift; spatio-temporal distribution of the volcanism

Abstract: The Massif Central area is the largest magmatic province of the West-European Rift system.The spa­tial-temporal distribution of Tertiary-Quaternary volcanism in the Massif Central, France, shows that three magmatic phases can be defined, each of them characterized by different volumes and different locations. The first event, termed the pre-rift magmatic event, is very scarce and restricted to the north of the Massif Central. It is suggested that this could result from lithospheric bending of the European lithosphere ahead of the incipient Alpine chain during the Pa­leocene. The second event, termed the rift-related magmatic event, is located in the north of the Massif Central only and is spatially connected with zones of high crustal thinning (i.e. the Limagne graben). It immediately follows Oligo­cene graben formation and associated sedimentation, and is represented by more than 200 scattered monogenic edifices. This second event can be attributed to partial melting as a consequence of lithospheric thinning that affected the north of the Massif Central during the rifting event. The lack of volcanism in the south during the same period of time is probably related to the very slight lithospheric thinning during the Oligocene. The third event, termed the major magmatic event, started first in the South in the upper Miocene at about 15 Ma, well after the end of the sedimentation. lt is unrelated to any extensional event. This major magmatic event reached the North of the Massif Central at about 3.5 Ma, following a pause in volcanism of about 6 Ma after the rift-related magmatic event. These two episodes of the ma­jor magmatic event are spatially and temporally associated with the two main periods of uplift, suggesting a common origin of volcanism and uplift processes. The major magmatic event can be attributed to late thermal erosion of the base of the lithosphere above a mantle diapir, as suggested by seismic tomography data. This general magmatic evolution drawn from data at the Massif Central scale may apply to the Eger graben as well, as the three magmatic events described in this study (pre-rift magmatic event, rifting event and post-Miocene volcanic event) are also reported in the literature. This suggests that a single cause should explain the formation of the entire western European rift surroun­ding the Alpine mountain belt.