CENOZOIC RIFT SYSTEM OF WESTERN AND CENTRAL-EUROPE - AN OVERVIEW

The Cenozoic rift system of western and central Europe extends over a distance of some 1100 km from the coast of the North Sea to the western Mediterranean; its southern prolongation is formed by the Valencia Trough and a Rio-Pleistocene volcanic chain which crosses the Alboran Sea and the Atlas ranges. Development of this mega-rift was contemporaneous with the Eocene and later phases of the Alpine and Pyrenean orogenies and with the evolution of the Red Sea-Gulf of Suez and Libyan-Pelagian Shelf rift systems. Evolution of the European Cenozoic rift system is thought to be governed by the interaction of the Eurasian and African-Arabian plates and by early phases of a plate-boundary reorganization that may ultimately lead to the break-up of the present continent assembly. In western and central Europe rifting commenced during the middle and late Eocene; 20-40 Ma later major rift-related domes were uplifted, entailing subsidence reversals of the grabens transecting them. Uplift of the Rhenish Shield can be explained in terms of progressive mechanical and thermal thinning of the lithosphere. The Bohemian Massif, Vosges-Black Forest and Massif Central arches, which are located in the periphery of the Alpine fold belt, are characterized by less pronounced lithospheric thinning; low-velocity mantle-lithosphere anomalies are observed under the Vosges-Black Forest and Massif Central domes; apart from thermal loads, deflection of the lithosphere in response to the build-up of intra-plate horizontal compressional stresses and/or to thrust-loading may have contributed to the uplift of these arches. Volcanic rocks associated with the Cenozoic rift system of western and central Europe were derived by mixing of partial melts from the convecting asthenosphere and from the mantle-lithosphere; the asthenospheric component shows similarities to the source of ocean island basalts. In the face of limited lithospheric extension, it must be assumed that the upper asthenosphere has a higher than ambient temperature and that the mantle-lithosphere is volatile-enriched. The upper asthenosphere of much of Variscan Europe is characterized by low S-wave velocities, indicating the presence of partial melts. Paleogene development of this anomaly, possibly in conjunction with a reorganization of mantle convection patterns, was accompanied by thermal weakening of the lithosphere, rendering it prone to failure in response to the build-up of intra-plate stresses. The Cenozoic rift system of Europe has many features which are consistent with a 'passive' rift system. Under the present stress regime, crustal extension is limited to the Roer Valley Graben whereas the Rhine Graben and the Massif Central are subjected to transpressional and transtensional deformation. The entire rift system corresponds to a zone of increased seismic hazard. The Massif Central and the Rhenish Shield are zones of latent volcanic activity.