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 wester n Mediterranean; its southern prolongation is formed by the Valencia T rough and a Rio-Pleistocene volcanic chain which crosses the Alboran S ea and the Atlas ranges. Development of this mega-rift was contemporan eous with the Eocene and later phases of the Alpine and Pyrenean oroge nies and with the evolution of the Red Sea-Gulf of Suez and Libyan-Pel agian Shelf rift systems. Evolution of the European Cenozoic rift syst em is thought to be governed by the interaction of the Eurasian and Af rican-Arabian plates and by early phases of a plate-boundary reorganiz ation that may ultimately lead to the break-up of the present continen t 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 th em. Uplift of the Rhenish Shield can be explained in terms of progress ive mechanical and thermal thinning of the lithosphere. The Bohemian M assif, Vosges-Black Forest and Massif Central arches, which are locate d in the periphery of the Alpine fold belt, are characterized by less pronounced lithospheric thinning; low-velocity mantle-lithosphere anom alies are observed under the Vosges-Black Forest and Massif Central do mes; apart from thermal loads, deflection of the lithosphere in respon se to the build-up of intra-plate horizontal compressional stresses an d/or to thrust-loading may have contributed to the uplift of these arc hes. Volcanic rocks associated with the Cenozoic rift system of wester n and central Europe were derived by mixing of partial melts from the convecting asthenosphere and from the mantle-lithosphere; the asthenos pheric component shows similarities to the source of ocean island basa lts. In the face of limited lithospheric extension, it must be assumed that the upper asthenosphere has a higher than ambient temperature an d that the mantle-lithosphere is volatile-enriched. The upper asthenos phere of much of Variscan Europe is characterized by low S-wave veloci ties, indicating the presence of partial melts. Paleogene development of this anomaly, possibly in conjunction with a reorganization of mant le convection patterns, was accompanied by thermal weakening of the li thosphere, rendering it prone to failure in response to the build-up o f intra-plate stresses. The Cenozoic rift system of Europe has many fe atures which are consistent with a 'passive' rift system. Under the pr esent stress regime, crustal extension is limited to the Roer Valley G raben whereas the Rhine Graben and the Massif Central are subjected to transpressional and transtensional deformation. The entire rift syste m corresponds to a zone of increased seismic hazard. The Massif Centra l and the Rhenish Shield are zones of latent volcanic activity.