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 paradox as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. In 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. It 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.

Evolution of the lithosphere in the area of the Rhine Rift System

“…In the ECRIS area these stresses interfered constructively with northward-directed compressional stresses emanating from the Pyrenean collision zone (Bergerat 1987;Ziegler 1994;Se´ranne 1999;Merle and Michon 2001). This stress field controlled the late Eocene early rifting phases of the Upper Rhine Graben during which Late Variscan and Permo-Carboniferous crustal discontinuities were transtensionally reactivated (Sissingh 1998;Schumacher 2002).…”

“…With this, ''Pyrenean'' intraplate compressional stresses gradually relaxed in the European foreland, whilst roll-back of the Alpine Tethys slab controlled the late Oligocene-early Miocene development of the Gulf of Lions-Valencia Trough rift system (Se´ranne 1999;Roca 2001). Conversely, we question whether slab-pull forces exerted by the southeast-to east-dipping West-Alpine subduction system contributed to the Oligocene subsidence of the Massif Central, Bresse and Valence grabens, which subparallel the Western Alps, as postulated by Stampfli et al (1998), Merle and Michon (2001) and Michon et al (2003). In this respect it should be kept in mind that the late Oligocene-early Miocene pulse of basin inversion affecting the Western Shelves, the Channel and Paris Basin (Fig.…”

“…From late Eocene times onwards, ECRIS developed in the foreland of the evolving Alpine and Pyrenean orogens, with crustal extension and continued plume activity causing further destabilisation of the lithosphere-asthenosphere system. Under the present northwest-directed stress field the RRS is tectonically still active whereas the grabens of the Massif Central and the Rhoˆne Valley became inactive during the early Miocene Merle and Michon 2001;Schumacher 2002;Se`ranne 1999;Ziegler 1990Ziegler , 1994.…”

“…Similarly, the grabens of the Massif Central and Rhoˆne Valley subsided rapidly during the Oligocene and coalesced. This was coupled with further sinistral motions along the Burgundy transfer zone that links the Limagne and Bresse Grabens with the Upper Rhine Graben (Bergerat 1977;Ziegler 1994;Merle and Michon 2001) and the activation of the more diffuse eastern Paris Basin transfer zone that links the Limagne Graben with the northern end of the Upper Rhine Graben (Coulon 1992). Intermittent Oligocene marine connections between the Alpine foreland basin and the North German Basin via the grabens of ECRIS indicate that by this time the Rhenish Massif and the Massif Central were still located close to sea level (Ziegler 1990(Ziegler , 1994Michon and Merle 2001;Merle and Michon 2001;Sissingh 1998Sissingh , 2001Sissingh , 2003.…”

“…This was coupled with further sinistral motions along the Burgundy transfer zone that links the Limagne and Bresse Grabens with the Upper Rhine Graben (Bergerat 1977;Ziegler 1994;Merle and Michon 2001) and the activation of the more diffuse eastern Paris Basin transfer zone that links the Limagne Graben with the northern end of the Upper Rhine Graben (Coulon 1992). Intermittent Oligocene marine connections between the Alpine foreland basin and the North German Basin via the grabens of ECRIS indicate that by this time the Rhenish Massif and the Massif Central were still located close to sea level (Ziegler 1990(Ziegler , 1994Michon and Merle 2001;Merle and Michon 2001;Sissingh 1998Sissingh , 2001Sissingh , 2003. However, increasing volcanism in the area of the Rhine-Roer-Hessian graben triple junction, and late Oligocene gradual uplift of the Rhenish Massif probably reflect increased activity of the Rhenish mantle plume and related thermal thinning of the mantle-lithosphere (Lippolt 1983;Jung 1999;Ritter et al 2001).…”