Michel Heeremans

Carboniferous-Permian rifting and magmatism in southern Scandinavia, the North Sea and northern Germany: a review

Abstract During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120 000 km3) and in the NE German Basin (ca. 48 000 km3). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source component for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism. These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.

Permo-Carboniferous magmatism and rifting in Europe: introduction

An extensive rift system developed within the northern foreland of the Variscan orogenic belt during Late Carboniferous-Early Permian times, post-dating the Devonian-Early Carboniferous accretion of various Neoproterozoic Gondwana-derived terranes on to the southern margin of Laurussia (Laurentia-Baltica; Fig. 1). Rifting was associated with widespread magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field affecting western and central Europe (Ziegler 1990; Ziegler & Cloetingh 2003). The change in regional stress patterns was coincident with the termination of orogenic activity in the Variscan fold belt, followed by major dextral translation between North Africa and Europe. Rifting propagated across a collage of basement terranes with different ages and thermal histories. Whilst most of the Carboniferous-Permian rift basins of NW Europe developed on relatively thin lithosphere, the highly magmatic Oslo Graben in southern Norway initiated within the thick, stable and, presumably, strong (cold) lithosphere of the Fennoscandian craton. The rift basins in the North Sea, in contrast, developed in younger Caledonian age lithosphere, which was both thinner and warmer than the lithosphere of the craton to the east. A regional hiatus, corresponding to the Early Stephanian, is evident in much of the Variscan foreland, with Stephanian and Early Permian red beds unconformably overlying truncated Westphalian series (e.g. McCann 1996) (Fig. 2). Regional uplift coincides with the onset of voluminous magmatism across the region, raising the possibility that uplift could have been related to the presence of a widespread thermal anomaly within the upper mantle (i.e. a mantle plume or, possibly, several plumes). In

Paleostress reconstruction from kinematic indicators in the Oslo Graben, southern Norway: new constraints on the mode of rifting

Abstract The Oslo Graben, southern Norway, is a N-S-trending Carboniferous-Permian rift system, characterized by major mafic to silicic magmatism, N-S-trending faults, reactivation of preexisting Precambrian faults and formation of half grabens. Magmatism is expressed by the presence of lavas, dyke injections, cauldron formation and the intrusion of batholiths. Paleostress analyses, mainly based on slickensides, have been performed in the area, both within and outside the rift structure. Combined with the current tectono-magmatic model of the Oslo Graben area, the analyses show the following stress evolution from the Caledonian Orogeny, with its compressional tectonics in the Silurian to the late stages of the Carboniferous-Permian rifting. NW-SE compression occurred in Silurian times, due to the continental collision of the Caledonian Orogeny. After a long period of a missing geological record, the area was affected by N-S compression during the Late Carboniferous. A shallow sedimentary basin developed, indicative for the pre- and proto-rift phases. The transition from the proto-rift phase to the initial rift phase, is marked by a transition from a transpressional regime into a transtensional regime. During the Early Permian, the stress regime changed from pure extension to radial extension. We suggest that the Oslo Graben can not simply be explained in terms of, or passive, or active rifting, but that a combination of both, evolving through time, is more suitable to explain the observations. We propose a model in which the Oslo Graben is initially triggered by far field stresses in the latest Carboniferous and earliest Permian, which infers opening due to passive rifting. These far field stresses are suggested to be linked to the Hercynian Orogeny, active to the south in central Europe in Carboniferous times and the reorganisation of the Pangea supercontinent. In Early Permian times, the stress regime caused radial extension, indicative for uplift and a change to an active mode of rifting. Simultaneously with the radial extension, large volumes of magma are emplaced at near surface levels. Despite all alternative models, an anomalous thermal gradient is necessary to create these large volumes of magmatic material. We propose therefore that shortly after the onset of rifting the dominant rifting mode becomes active (plume-related), although far field stresses might still be present.

Anorogenic granites, magmatic underplating and the origin of intracratonic basins in a non-extensional setting

Abstract We present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating. A case study of a deeply eroded continental rift belt in south Finland shows close association of anorogenic intrusives and remnants of cratonic basins. Lower-crustal thinning took place during anorogenic magmatism, whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies. Combined analyses of field data, petrological constraints, and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning. This model involves the formation of an incipient crustal dome by asthenospheric upwelling, associated with basaltic underplating, and migration of induced lower-crustal partial melts to upper-crustal levels. In this model, light lower-crustal “diorite” is replaced by a denser mafic layer. The consequences for the post-magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification. Subsidence occurs in the absence of crustal extension, suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin. Subsidence curves for this model show an incipient, nearly linear fast downwarp related to crystallization of gabbro at the Moho, followed by slow contraction induced by solid state cooling. The predicted subsidence is similar to the pattern of the vertical movement induced by passive extension. In the proposed model of continental rifting, the effects of extension and underplating largely cancel out. Concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion. The model provides a consistent explanation for a number of key aspects of cratonic basins, including their circular shape, crustal configuration and evolution, and their spatial distribution on continents.

Thermomechanical lithospheric structure of the central Fennoscandian Shield

Abstract The deep seismic sounding (DSS) profiles BALTIC, including its southern continuation, the Sovetsk–Kohtla–Jarve (SKJ) profile, SVEKA, the northern part of BABEL, POLAR, FENNIA and Pechenga–Kovdor–Kostomuksha, were used in studying the present-day thermomechanical structure of the central Fennoscandian Shield. These profiles are located in different tectonic units, which represent different stages in Precambrian crustal and lithospheric growth. First, present-day geotherms were constructed for several points along the DSS profiles. Successively, strength envelopes were calculated using the obtained geotherms and rheological flow laws. Variations in strain rate were also considered in the computations of the strength envelopes. The integrated crustal and lithospheric strengths, the thicknesses of the mechanically strong crust (MSC) and mechanically strong lithosphere (MSL), and the rheological thickness of the lithosphere were derived from these strength envelopes. The obtained mechanical structures for different regions were analysed and compared with other geophysical data; e.g., seismicity-depth and isotherm-depth distributions. The rheological results show lateral variations in the lithospheric strength reflecting the geometry of the lithosphere and following roughly the same trend as the geochronological development of the Fennoscandian Shield. The mechanical structure shows distinct decoupling of the weak lower crust and the strong upper mantle, particularly with a wet rheology. This decoupling interrupts the transmission of the differential stress from the brittle upper crust to the ductile lower crust and through it to the mantle lithosphere. The weak lower crustal layer is also detected with a dry rheology in the Svecofennian area, whereas in the Archaean side, it is not distinct. The assumed frictional transition temperature of 350°C varies between the depths of 25 and 44 km with an average value of 35 km. This is in good agreement with the observed focal depth limit of 31 km. Consequently, it seems that the velocity weakening/velocity strengthening explains best the real lower boundary of seismicity.

Late Carboniferous-Permian of NW Europe: an introduction to a new regional map

Abstract The Carboniferous-Permian evolution of NW Europe has recently been the focus of an EC-funded Training and Mobility of Researchers (TMR) project ‘Permo-Carboniferous-Rifting in Europe’ (PCR). One of the main goals of this project was to produce a new map for this time period showing the distribution of Late Carboniferous-Early Permian (Lower Rotliegend) volcanics, dykes and sills, and the extent of the tectonic structures of the Early-Late Permian (Upper Rotliegend) sedimentary basins (better known as the Southern and Northern Permian Basins). In order to produce this map, an overview of all the available literature was made. The new map was completed based on our own interpretations from seismic and borehole data. Unpublished data were available through industrial partners associated with the PCR project.

Late Carboniferous-Permian tectonics and magmatic activity in the Skagerrak, Kattegat and the North Sea

Abstract This study focuses on Late Carboniferous-Permian tectonics and related magmatic activity in NW Europe, and specifically in the Skagerrak, Kattegat and North Sea areas. Special attention is paid to the distribution of intrusives and extrusives in relation to rift-wrench geometries. A large database consisting of seismic and well data has been assembled and analysed to constrain these objectives. The continuation of the Oslo Graben into the Skagerrak has been a starting point for this regional study. Rift structures (with characteristic half-graben geometries) and the distribution of magmatic rocks (intrusives and extrusives) were mapped using integrated analyses of seismic and potential field data. For the analysis of the Sorgenfrei-Tornquist Zone and the North Sea, seismic and well data were used. The rift structures in the Skagerrak can be linked with extensional structures in the Sorgenfrei-Tornquist Zone in which similar fault geometries have been observed. Both in the Skagerrak and in the Kattegat, lava sequences were erupted that generally parallel the underlying Lower Palaeozoic strata. This volcanic episode, therefore, pre-dates main fault movements and the development of half-grabens filled with Permian volcaniclastic material. Upper Carboniferous-Lower Permian extrusives and intrusives have also been found in wells in the Kattegat, Jutland and the North Sea (Horn and Central grabens). Especially in the latter area, the dense seismic and well coverage has allowed us to map out similar Upper Palaeozoic geometries, although the presence of salt often conceals the seismic image of the underlying strata and structures. From the results, it is assumed that the pre-Jurassic structures below large parts of the Norwegian-Danish Basin and northwards into the Stord Basin on the Horda Platform belong to the same tectonic system.

Middle Proterozoic–early Palaeozoic evolution of central Baltoscandian intracratonic basins: evidence for asthenospheric diapirs

Abstract The three intracratonic sedimentary basins located in central Baltoscandinavia, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tectonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crustal extension was minor or did not take place during the Proterozoic basin evolution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a craton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fact, combined with the evidence provided by mantle-derived flood basalt magmatism, points to a moving asthenospheric diapir as the cause for basin development. Asthenospheric upwelling was probably also responsible for the second, Late Proterozoic, basin evolution phase, as evidenced by the lack of crustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at about the position of the intracratonic basins, is compatible with indications from glaciations. As the ages of Late Proterozoic intracratonic basins also decrease from east to west across the craton, the location of asthenospheric diapirism during this time interval was also moving. For the Fennoscandian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the Fennoscandian Shield. The spacing and size of the Proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh–Taylor instabilities in the sublithospheric mantle. Emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of early Palaeozoic sediments in the Bothnian Sea basin, taking into account post-Ordovician vertical and lateral erosion of the basin fill.

New constraints on the timing of late Carboniferous-early Permian volcanism in the central North Sea

Abstract The Permo-Carboniferous evolution of the central North Sea is characterized by three main geological events: (1) the development of the West European Carboniferous Basin; (2) a period of basaltic volcanism during the Lower Rotliegend (latest Carboniferous-early Permian); and (3) the development of the Northern and Southern Permian Basins in late Permian times. The timing of the late Carboniferous-Permian basaltic volcanism in the North Sea is poorly constrained, as is the timing of extensional tectonic activity following the main phase of inversion during the Westphalian, due to the diachronous propagation of the Variscan deformation front. Results of high precision Ar-Ar dating on basalt samples taken from a core from exploration well 39/2-4 (Amerada Hess) in the UK sector of the central North Sea suggests that basaltic volcanism was active in the late Carboniferous, at c. 299 Ma. The presence of volcanics below the dated horizon suggests that the onset of Permo-Carboniferous volcanism in the central North Sea commenced earlier, probably at c. 310 Ma (Westphalian C). This is contemporaneous with other observations of tholeiitic volcanism in other parts of NW Europe, including the Oslo Graben, the NE German Basin, southern Sweden and Scotland. Interpretations of available seismic data show that main extensional faulting occurred after the volcanic activity, but the exact age of the fault activity is difficult to constrain with the data available.