European Cenozoic Rift System Research Articles

Cenozoic uplift of Variscan Massifs in the Alpine foreland: Timing and controlling mechanisms

The European Cenozoic Rift System (ECRIS) and associated fault systems transect all Variscan Massifs in the foreland of the Alps. ECRIS was activated during the Eocene in the foreland of the Pyrenees and Alps in response to the build-up of collision-related intraplate stresses. During Oligocene and Neogene times ECRIS evolved by passive rifting under changing stress fields, reflecting end Oligocene consolidation of the Pyrenees and increasing coupling of the Alpine Orogen with its foreland. ECRIS is presently still active, as evidenced by its seismicity and geodetic data.Uplift of the Massif Central and the Rhenish Massif, commencing at the Oligocene–Miocene transition, is mainly attributed to plume-related thermal thinning of the mantle–lithosphere. Mid-Burdigalian uplift of the SW–NE-striking Vosges–Black Forest Arch, that has the geometry of a doubly plunging anticline breached by the Upper Rhine Graben, involved folding of the lithosphere. Late Burdigalian broad uplift of the northern parts of the Bohemian Massif reflects lithospheric buckling whereas late Miocene–Pliocene uplift of its marginal blocks involved transpressional reactivation of pre-existing crustal discontinuities. Crustal extension across ECRIS, amounting to no more than 7 km, was compensated by a finite clockwise rotation of the Paris Basin block, up warping of the Weald–Artois axis and reactivation of the Armorican shear zones. Intermittent, though progressive uplift of the Armorican Massif, commencing in the Miocene, is attributed to transpressional deformation of the lithosphere.Under the present-day NW-directed compressional stress field, that came into evidence during the early Miocene and further intensified during the Pliocene, the Armorican Massif, the Massif Central, the western parts of the Rhenish Massif and the northern parts of the Bohemian Massif continue to rise at rates of up to 1.75 mm/y whilst the Vosges–Black Forest arch is relatively stable.Uplift of the Variscan Massifs and development of ECRIS exerted strong controls on the Neogene evolution of drainage systems in the Alpine foreland.

Analysis of central western Europe deformation using GPS and seismic data

Abstract The kinematic field of central western Europe is characterized by relatively small movements (around 1–2 mm/year) and diffuse seismicity with earthquakes occurring mostly in the shallow crust (within 15 km), prevalently concentrated along the Alps and the European Cenozoic Rift System (ECRIS). In order to study and constrain the current crustal kinematic field we reconstructed the velocity and the strain field using permanent GPS stations, belonging to different networks (AGNES, EUREF, REGAL, RGP). The 2D strain rate tensor has been calculated using the method of least-squares collocation. Our results show that the area of maximum compression is located along the Alpine chain, where maximum values of 7 ± 2 nstrain/year are found, while maximum extension is measured between the Armorican Massif and the Massif Central, where values of 4 ± 2 nstrain/year are reached. The earthquakes with M > 3.0, have been used to estimate the seismic strain rates, while the style of the seismic deformation was reconstructed from the fault plane solutions (FPS) available from the literature. Relatively high values of seismic strain rates (around 10 nstrain/year) are measured along the Alpine Chain and the ECRIS. Results obtained by geodetic and seismic data are quite in agreement and reflect the different tectonic evolution of the geological features characterizing the area of study. The orientation of the compressional geodetic and seismic strain axes are NW-SE in most of the area of study, on account of the action of plate boundary forces. A rotation of the same axes to N-S direction along the eastern Alps, possibly related to the Adria convergence, is found.

Free gases as tracers for mantle-related geodynamic processes in the European Cenozoic rift system

Free gases as tracers for mantle-related geodynamic processes in the European Cenozoic rift system

Kinematic sequence stratigraphy of the European Cenozoic Rift System and Alpine Foreland Basin: correlation with Mediterranean and Atlantic plate-boundary events

AbstractA review of the sequence stratigraphic development of the Tertiary basins of the North and West Alpine Foreland domains shows that their structural and depositional history was episodically affected by brief tectonic phases. These were associated with intermittent deformation events induced by the collisional convergence and compressional coupling of the Apulian and Iberian microplates with the European Plate. The plate kinematics-related episodicity was essentially isochronously recorded in the basin fills of the Alpine Foreland region. These are generally correlative with changes in eustatic sea level. The ensuing correlative successions of so-called Cenozoic Rift and Foredeep (CRF) sequences and phases can be traced throughout the European Cenozoic Rift System and Alpine Foreland Basin. Their temporal correlation indicates that, apparently, the changes in the plate collision-related stress regime of the Alpine Foreland were repeatedly accompanied by coeval changes in eustatic sea level. To test and substantiate the validity of this inferred causal relationship between intraplate deposition, plate kinematics and eustacy, the tectono-sedimentary evolution of the basins of the Mediterranean plate-boundary zone has been analysed in conjunction with a review of the plate-boundary events in the North Atlantic. Within the uncertainty range of available datings, synchroneity could thus be demonstrated for the punctuated tectonostratigraphic development of basins of the western Mediterranean (comprising the Liguro-Provençal Basin, Valencia Trough, Sardinia Rift and Tyrrhenian Basin), the Apenninic-Calabrian Arc, the Betic domain (including the Alboran Basin) and the North and West Alpine Foreland regions. Similar temporal correlations of plate tectonicsrelated events near the Mid-Atlantic Ridge in the North Atlantic and tectonostratigraphic sequences and phases of the Alpino-Pyrenean Foreland basins are further evidence of a common causal mechanism. The driving mechanisms appear to have been the northward drift of Africa and the resulting mechanical coupling of Apulia and Iberia with the southern passive margin of Europe, as well as the stepwise opening of the North Atlantic and accompanying episodic plate re-organisations of the Mid-Atlantic Ridge.

Post-Variscan evolution of the lithosphere in the area of the European Cenozoic Rift System

Abstract The European Cenozoic Rift System (ECRIS) transects Variscan basement, Permo-Carboniferous troughs and Late Permian to Mesozoic series, deposited in thermal sag basins, which are exposed on rift-related arches. We have analysed processes governing the transformation of the orogenically destabilized Variscan lithosphere into end-Cretaceous stabilized cratonic lithosphere, prior to its renewed Cenozoic rift-related destabilization. In the ECRIS area, crustal and lithospheric thicknesses at present are in the range of 24-35 km and 60-120 km, respectively. The Variscan orogen was characterized at the time of its end-Westphalian consolidation by 45-60 km deep crustal roots, marking major sutures. During the Stephanian-Early Permian wrench-induced collapse of the Variscan orogen, subducted lithospheric slabs were detached, causing upwelling of the asthenosphere, thermal thinning and/or partial delamina-tion of the lithospheric mantle, and regional uplift. By mid-Permian times, the crust was thinned to 28-35 km owing to its regional erosional unroofing, localized mechanical stretching and the interaction of mantle-derived melts with its basal parts. By mid-Permian times, when the temperature of the asthenosphere returned to ambient levels, thermal subsidence of the lithosphere commenced, controlling development of a system of Late Permian and Mesozoic intracratonic basins. These experienced repeated minor subsidence accelerations, related to the build-up of far-field stresses, which did not involve renewed lithospheric destabilization. Modelling of observed subsidence curves indicates that during the mid-Permian lithospheric thicknesses ranged in the ECRIS area between 40 and 80 km, but had increased by the end of the Cretaceous to 100-120 km. Cenozoic rifting and mantle-plume activity caused renewed lithospheric thinning.

Low-temperature thermochronology of the flanks of the southern Upper Rhine Graben

The Upper Rhine Graben (URG) is the most perceptible part of the European Cenozoic Rift System. Uplifted Variscan basement of the Black Forest and the Vosges forms the flanks of the southern part of the graben. Apatite and zircon fission-track (FT) analyses indicate a complex low-temperature thermal history of the basement that was deciphered by inverse modelling of FT parameters. The models were tested against the observed data and independent geological constraints. The zircon FT ages of 28 outcrop samples taken along an E–W trending transect across the Black Forest and the Vosges range from 136 to 312 Ma, the apatite FT ages from 20 to 83 Ma. The frequency distributions of confined track lengths are broad and often bimodal in shape indicating a complex thermal history. Cooling below 120°C in the Early Cretaceous to Palaeogene was followed by a discrete heating episode during the late Eocene and subsequent cooling to surface temperature. The modelled time–temperature (t–T) paths point to a total denudation of the flanks of URG in the range of 1.0–1.7 km for a paleogeothermal gradient of 60°C/km, and 1.3–2.2 km for a paleogeothermal gradient of 45°C/km since the late Eocene.

Interplay between tectonic, fluvial and erosional processes along the Western Border Fault of the northern Upper Rhine Graben, Germany

The northern Upper Rhine Graben, situated in the central part of the European Cenozoic rift system, is currently characterized by low intra-plate seismicity. Historical earthquakes have not been large enough to produce surface rupturing. Moreover, the records of Quaternary surface processes and human modifications are presumably better preserved than the record of the relatively slow tectonic deformation. In order to gain information on the neotectonic activity and paleoseismicity in this setting, the geological and geomorphological records of fault movements along a segment of the Western Border Fault (WBF) were studied using an integration of techniques in paleoseismology, structural analysis and shallow geophysics. The WBF segment investigated follows a 20 km long linear scarp of unclear origin. A series of geophysical measurements were performed and the results suggested that near-surface deformation structures are present at the segments' southern end. Several trenches opened at this location revealed fault structures with consistent extensional style and a maximum vertical displacement of 0.5 m. In one trench, the deformation structures were dated between 19 and 8 ka. Assuming the deformation has been caused by an earthquake, a M w ∼6.5 earthquake would be implied. Aseismic deformation would point to a fault creep rate ≥ 0.04 mm/yr. A reconstruction of the sequence of events at the trench site, from Middle Pleistocene to Present, demonstrates that the morphology at the base of the scarp is the result of interplay between tectonic activity and fluvial and erosional processes. At the regional scale, a mixed origin for the WBF scarp is proposed, combining the effects of fluvial dynamics, erosion, regional uplift and localized tectonic activity on the WBF.

Discussion on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

The evolution and origin of the European Cenozoic Rift System (ECRIS) is a matter of debate for several decades (e.g., Tapponnier, 1977; Bergerat, 1987; Ziegler, 1992; Michon et al., 2003). This rift system was characterized by the development of several grabens in the Pyrenean and Alpine forelands and by a magmatic activity starting at the K/T transition. Dezes et al. (2004) propose an additional reappraisal and interpret the ECRIS formation and the associated volcanism as resulting from the Alpine and Pyrenean collision and the emplacement of a mantle plume at depth below western Europe. Our remarks on this paper will be focused on three different topics which make the final conclusions of Dezes et al. (i.e., origin of the extension in the ECRIS) highly questionable.

Reply to comments by L. Michon and O. Merle on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

Reply to comments by L. Michon and O. Merle on “Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere” by P. Dèzes, S.M. Schmid and P.A. Ziegler, Tectonophysics 389 (2004) 1–33

Evidence for ascending upper mantle‐derived melt beneath the Cheb basin, central Europe

We present new 3He/4He data of CO2‐rich gas exhalations of three presently nonvolcanic areas in Europe. The 3He/4He ratios from the Cheb basin (Czech Republic) are clearly higher than data obtained ten years ago, whereas the 3He/4He ratios from the other areas (Laacher See/Germany and Mariánské Lázně/CR) have remained nearly constant. No other locality showing such high 3He/4He ratios in free gases supplied by continental mantle degassing is known in the European Cenozoic rift system. At the northeastern edge of the Cheb basin swarm earthquakes repeatedly occur. In the context of contemporaneous periods of seismicity the increased 3He/4He ratios are interpreted as the first geochemical evidence for ascending mantle‐derived melt beneath the Cheb basin, which is related to the triggering of earthquake swarms.

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

The Rhine Rift System (RRS) forms part of the European Cenozoic Rift System (ECRIS) and transects the Variscan Orogen, Permo-Carboniferous troughs and Late Permian to Mesozoic thermal sag basins. Crustal and lithospheric thicknesses range in the RRS area between 24–36 km and 50–120 km, respectively. We discuss processes controlling the transformation of the orogenically destabilised Variscan lithosphere into an end-Mesozoic stabilised cratonic lithosphere, as well as its renewed destabilisation during the Cenozoic development of ECRIS. By end-Westphalian times, the major sutures of the Variscan Orogen were associated with 45–60 km deep crustal roots. During the Stephanian-Early Permian, regional exhumation of the Variscides was controlled by their wrench deformation, detachment of subducted lithospheric slabs, asthenospheric upwelling and thermal thinning of the mantle-lithosphere. By late Early Permian times, when asthenospheric temperatures returned to ambient levels, lithospheric thicknesses ranged between 40 km and 80 km, whilst the thickness of the crust was reduced to 28–35 km in response to its regional erosional and local tectonic unroofing and the interaction of mantle-derived melts with its basal parts. Re-equilibration of the lithosphere-asthenosphere system governed the subsidence of Late Permian-Mesozoic thermal sag basins that covered much of the RRS area. By end-Cretaceous times, lithospheric thicknesses had increased to 100–120 km. Paleocene mantle plumes caused renewed thermal weakening of the lithosphere. Starting in the late Eocene, ECRIS evolved in the Pyrenean and Alpine foreland by passive rifting under a collision-related north-directed compressional stress field. Following end-Oligocene consolidation of the Pyrenees, west- and northwest-directed stresses originating in the Alps controlled further development of ECRIS. The RRS remained active until the Present, whilst the southern branch of ECRIS aborted in the early Miocene. Extensional strain across ECRIS amounts to some 7 km. Plume-related thermal thinning of the lithosphere underlies uplift of the Rhenish Massif and Massif Central. Lithospheric folding controlled uplift of the Vosges-Black Forest Arch.

Determination of displacements in the upper Rhine graben Area from GPS and leveling data

The Upper Rhine graben is a north–northeast trending rift system, which belongs to the European Cenozoic rift system. Today, the southern part of the graben is seismically still active. Earthquakes of magnitude 5 have a recurrence time of approximately 30 years. In order to monitor and to determine the displacements in the study area, GPS measurements have been carried out in two campaigns (1999 and 2000), and observations of the available permanent stations have been processed in 2002. Owing to the small size of deformations expected, high accuracy requirements must be met by the GPS processing. In order to achieve these requirements, precise antenna modeling has been introduced into the processing. As expected the short time span has not enabled to detect significant displacements from the GPS measurements. The deformation analysis shows that the horizontal displacement rates do not exceed 1 mm/year, which is compatible with the geological information. Owing to the fact that the accuracy of positioning with GPS for the vertical coordinates is lower than for the horizontal coordinates, the determination of vertical displacements has been carried out using the leveling technique. In the area of Freiburg, first-order and second-order leveling lines have been chosen for the detection of local displacements on the Weinstetten, on the Lehen-Schonberg and on the Main Border Fault (MBF). Some sections of these faults are still active today. Significant vertical displacements have been observed at the Weinstetten fault in the area of Bad Krozingen and on the MBF in Freiburg. The displacement rates (1925–1984) are 0.17±0.01 mm/year and 0.25±0.02 mm/year respectively. The results agree very well with the results of seismotectonic investigations, and show that ongoing displacements can be found on the northern part of the Lehen-Schonberg fault in the vicinity of Eichstetten, and on the MBF in the vicinity of Freiburg.

Towards the determination of displacements in the Upper Rhine Graben area using GPS measurements and precise antenna modelling

The Upper Rhine Graben is a north-northeast trending rift system, which belongs to the European Cenozoic Rift System. Today, the southern part of the graben is seismically still active. Earthquakes of magnitude 5 have a recurrence time of approximately 30 years. The EUCOR-URGENT project focuses on the seismic hazards and neotectonics in the Upper Rhine Graben and the surrounding areas. Within the framework of this project five institutions have created and have been maintaining a GPS network in the Upper Rhine Graben and the surrounding area in order to monitor horizontal and vertical displacements in the study area. Two GPS campaigns have been carried out in the network to monitor and to determine the horizontal and vertical displacements in the study area. The first one was observed in 1999 and the second one in 2000. In 2002, 2 weeks of data of the available permanent GPS reference stations in the area have also been included in the investigations. These permanent stations belong to the European Reference Frame Permanent Network, International GPS Service network, Satellite Positioning Service of Baden-Württemberg, Réseau GPS Permanent-France (RGP), Réseau GPS Permanent dans les Alpes (REGAL) and the AGNES (operated by the Swiss Positioning Service). Due to the small size of the displacements expected, high accuracy requirements must be met in the GPS processing. In order to achieve this, the results of the available absolute antenna calibrations have been considered in the data processing, too. The results have shown that the application of absolute antenna calibration models is crucial for the derivation of a highly accurate displacement field. Without these models large discrepancies (on the level of few centimetres) can be observed at those stations, where the observations were carried out by different antennas in the various campaigns. As expectedly, due to the short time span, no significant displacement could be detected in the study area. However, the deformation analysis shows that the horizontal displacements do not exceed the level of 1 mm/yr. This value agrees with the results of other investigations.

Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively. During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS. At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones. Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere. ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.

The deep crust of the Southern Rhine Graben: reflectivity and seismicity as images of dynamic processes

The Rhine Graben, part of the European Cenozoic rift system, deserves special attention because of its location in the foreland of the Alpine orogen. The Phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the Variscan orogeny, late-orogenic crustal re-equilibration to the interference of rifting and Alpine orogeny in Tertiary times.The Rhine Graben is one of the most detailedly studied continental grabens. Prospecting for hydrocarbons in its sedimentary fill and intensive pre-site studies for the Continental Deep Drilling Program (KTB) on its eastern crystalline flank (Black Forest) provided a comprehensive set of geoscience data. Seismic investigations ranging from a deep seismic network of near-vertical reflection to concurrent wide-angle refraction experiments were accompanied by seismological and geological surveys. Therefore, a direct observational comparison between the two depth definitions of crustal subdivisions, elastic vs. strength/rheological, was possible. In large parts of Western Europe deep seismic sounding (DSS) investigation distinguishes between a crystalline upper crust of comparatively low P-wave velocities (5.9–6.0 km/s) with little and discrete reflectivity and a strongly reflective lower crust with higher P-wave velocities (about 6.4–6.8 km/s). Both are separated by the Conrad discontinuity. In contrast, strength and rheological considerations define a brittle upper crust and a ductile lower crust. The boundary between these two realms is usually referred to as the brittle/ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres. The present brittle/ductile transition in the Rhine Graben region, as defined by the deepest hypocentres, does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies. This suggests that the laminated lower crust developed during the crustal re-equilibration phase at the end of the Variscan orogeny.The second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the Variscan lower crust. A remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern Rhine Graben and is possibly induced by the young rifting process.We suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the Alpine orogen. This allows us to propose the distinction between the influence of compression, extension, continental subduction and intra-crustal detachment during graben formation. The dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small-scale collisional escape tectonics associated with an intra-continental transform fault at the southern end of the Rhine Graben, the Rhine-Saoˆne Transform zone.

Massif Central (France): new constraints on the geodynamical evolution from teleseismic tomography

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment. During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P-wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.

Mafic alkaline magmatism associated with the European Cenozoic rift system

Primitive mafic alkaline volcanic rocks from the Cenozoic rift system of western and central Europe have major and trace element and Sr-Nd-Pb isotopic characteristics which suggest the involvement of both lithospheric and asthenospheric mantle source components in their petrogenesis. The geochemical characteristics of the lithospheric component are in part constrained by those of spinel lherzolite and mafic granulite xenoliths entrained within the magmas. This component appears to be the product of partial melting of phlogopite/amphibole bearing mantle which was metasomatised as a consequence of magmatic activity during and preceeding the Hercynian orogeny and during subsequent phases of Permo-Carboniferous and Cenozoic extension. Partial melting of phlogopite is required to account for the generation of potassic magmas (leucitites and leucite nephelinites) with K 2O Na 2O ratios > 1. This component appears to differ between the individual Hercynian terrane blocks of Europe, reflecting, in part, their previous magmatic histories. The asthenospheric component has affinities with the source of HIMU OIB. This could represent a zone of enriched mantle in the thermal boundary layer at the base of the subcontinental lithosphere which is preferentially partially melted during extension. Alternatively it could, in part, be 500-400 Ma oceanic lithosphere subducted during the Hercynian orogeny. Although the data do not preclude the existence of deep mantle plumes, there is no need to invoke them in order to explain the HIMU characteristics.

European Cenozoic rift system

The European Cenozoic rift system extends from the coast of the North Sea to the Mediterranean over a distance of some 1100 km; it finds its southern prolongation in the Valencia Trough and a Plio-Pleistocene volcanic chain crossing the Atlas ranges. Development of this mega-rift was paralleled by orogenic activity in the Alps and Pyrenees. Major rift domes, accompanied by subsidence reversal of their axial grabens, developed 20–40 Ma after beginning of rifting. Uplift of the Rhenish Shield is related to progressive thermal lithospheric thinning; the Vosges-Black Forest and the Massif Central domes are probably underlain by asthenoliths emplaced at the crust/mantle boundary. Evolution of this rift system, is thought to be governed by the interaction of the Eurasian and African plates and by early phases of a plate-boundary reorganization that may lead to the break-up of the present continent assembly.

Subsidence analysis and modelling of the Roer Valley Graben (SE Netherlands)

The Roer Valley Graben (southeastern Netherlands) forms the most northern branch of the European Cenozoic Rift System. Sedimentary backstripping of exploration wells provides control on vertical tectonic motions during the complex post-Paleozoic geodynamic evolution of the area. The Roer Valley Graben evolved by repeated reactivation of Permo-Carboniferous fracture systems. At least three episodes of extension-related subsidence are recorded. These occurred during the Late Permian-Early Triassic (> 255-240 Ma), the Middle Jurassic (168-165 Ma) and the late Cenozoic (36-0 Ma). A Late Jurassic-Early Cretaceous stretching phase is weakly inferred from incomplete data. Crustal stretching values calculated from the tectonic subsidence curves can account for the reflection seismic controlled crustal configuration of the Roer Valley Graben area. Results demonstrate a close relationship between the Roer Valley Graben and the basins in the North Sea during the Mesozoic. In contrast, the Cenozoic evolution of the Roer Valley Graben is independent from that of the North Sea and is related to the development of the Rhine Graben.

Lithospheric cross sections of the European Cenozoic rift system

The lithospheric structure of the European Cenozoic rift system ( ecris) is presented in transects through the southern Rhine Graben and the Rhenish Massif/Hessen depression, emphasizing the geophysical structure of the lithosphere based on seismic refraction/reflection investigations, teleseismic tomography, electromagnetic depth-sounding models, and gravity, aeromagnetic, earthquake, uplift/subsidence and heat flow data. The rift is clearly expressed in the Rhine Graben, but is not evident at the surface in the area of the Rhenish Massif where its existence is indicated by seismicity. It is characterized by abnormal crustal and upper-mantle structures which vary considerably in horizontal direction. For example, under the Rhine Graben the crust is thinned to 25 km, but at 40 km depth anomalously high velocities are observed. In contrast, beneath the Rhenish Massif the crust is thickened to 35–37 km and under its eastern part a high-velocity thin upper-mantle slice is seen at 30 km depth within the lower crust which, towards the Hessen depression, is gradually replaced by normal Variscan mantle with the Moho near 30 km depth. Under the western part of the Rhenish Massif P- and S-wave velocities are reduced below 50 km depth which is not seen east of the Rhine river. Under the Rhine Graben the existence of a low-velocity upper mantle above 100 km cannot be generalized, but is restricted to confined regions.