The Glueckstadt Graben of the North-German Basin: new insights into the structure from 3D and 2D gravity analyses

The structure of the Glueckstadt Graben has been investigated by use of 3D gravity backstripping technique and by 2D gravity and magnetic modelling. Subtracting the gravity effects of the Meso-Cenozoic sediments together with Permian salt reveals a positive residual anomaly within the Glueckstadt Graben. This anomaly includes two local maxima over the Westholstein and Eastholstein Troughs. The 2D gravity models point to the presence of a high-density body within the lower crust of the Glueckstadt Graben. In addition, the results of 2D magnetic modelling indicate that the central part of the high-density body is overlain by an area with high susceptibility. Most probable, the formation of this high-density body is a result of complex poly-phase tectonic history of the study area. Finally, the results of gravity modelling indicate that Permian salt is not homogeneous. 3D gravity analysis and, especially, 2D gravity modelling have distinguished the differences in degree of salt saturation in salt-rich bodies, and elucidate the proportion of Rotliegend salt.     

Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10⁻⁵ ms⁻² amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10⁻⁵ ms⁻² magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.     

The lithosphere structure and geodynamics of the West and East Black Sea Basins

The velocity structure of the Black Sea lithosphere has been studied using the local seismic tomography method based on the Backus-Gilbert approach and applied for a quite large amount of seismological data. As seismic sources, we used the earthquakes that occurred within the Black Sea and adjoining regions and generated seismic waves recorded by seismic stations around the Black Sea. This yielded information on the 3D distribution of P-waves within the most heterogeneous and poorly investigated uppermost layer of the Black Sea region (down to a depth of 60 km). The interpretation of these results, together with new data on the velocity structure of the crust of the Black Sea led to the conclusion about the different lithosphere beneath the West and the East Black Sea Basins. This can be explained by origination of the depressions in the western and eastern parts of the Black Sea at different microplates and by specific features in their development at rifting and post-rifting stages.     

Large-scale 3-D Gravity Analysis of the Inhomogeneities in the European-Mediterranean Upper Mantle

—Methods and the results of estimating the anomalies characterising the density inhomo geneities in the European-Mediterranean upper mantle are described. These anomalies were obtained by subtracting the gravity effect of a crustal density model derived from seismic velocities from the observed gravity field averaging over an area of 1°× 1°. The 3-D density model of the study region comprises two regional layers of varying thickness with lateral variation of average density the sedimentary cover and the crystalline crust. The average densities for model layers were evaluated by using a velocity/density conversion function and taking into account sediment consolidation with depth. Clear correlation between residual gravity anomalies and both velocity heterogeneities and thermal regime data of the upper mantle has been revealed. An agreement of positive anomalies over the Alps, the Adriatic plate and the Calabrian Arc with high velocity domains in the upper mantle and reduced temperatures at the subcrustal layer are caused by lithospheric "roots" and thickened lithosphere below these structures. Gravity residual lows, revealed over the Western Mediterranean Basin and Pannonian Basin, are in correspondence with both low velocities and high temperatures in the upper mantle. These anomalies are the result of the presence of asthenosphere in shallow near-Moho depths below these basins.     

Collision processes at the northern margin of the Black Sea

Extended along the Crimea–Caucasus coast of the Black Sea, the Crimean Seismic Zone (CSZ) is an evidence of active tectonic processes at the junction of the Scythian Plate and Black Sea Microplate. A relocation procedure applied to weak earthquakes (m_b ≤ 3) recorded by ten local stations during 1970–2013 helped to determine more accurately the parameters of hypocenters in the CSZ. The Kerch–Taman, Sudak, Yuzhnoberezhnaya (South Coast), and Sevastopol subzones have also been recognized. Generalization of the focal mechanisms of 31 strong earthquakes during 1927–2013 has demonstrated the predominance of reverse and reverse–normal-faulting deformation regimes. This ongoing tectonic process occurs under the settings of compression and transpression. The earthquake foci with strike-slip component mechanisms concentrate in the west of the CSZ. Comparison of deformation modes in the western and eastern Crimean Mountains according to tectonophysical data has demonstrated that the western part is dominated by strike-slip and normal- faulting, while in the eastern part, reverse-fault and strike-slip deformation regimes prevail. Comparison of the seismicity and gravity field and modes of deformation suggests underthusting of the East Black Sea Microplate with thin suboceanic crust under the Scythian Plate. In the Yuzhnoberezhnaya Subzone, this process is complicated by the East Black Sea Microplate frontal part wedging into the marginal part of the Scythian Plate crust. The indentation mechanism explains the strong gravity anomaly in the Crimean Mountains and their uplift.     

Structure of the lithosphere below the southern margin of the East European Craton (Ukraine and Russia) from gravity and seismic data

The present study was undertaken with the objective of deriving constraints from available geological and geophysical data for understanding the tectonic setting and processes controlling the evolution of the southern margin of the East European Craton (EEC). The study area includes the inverted southernmost part of the intracratonic Dnieper-Donets Basin (DDB)–Donbas Foldbelt (DF), its southeastern prolongation along the margin of the EEC–the sedimentary succession of the Karpinsky Swell (KS), the southwestern part of the Peri-Caspian Basin (PCB), and the Scythian Plate (SP). These structures are adjacent to a zone, along which the crust was reworked and/or accreted to the EEC since the late Palaeozoic. In the Bouguer gravity field, the southern margin of the EEC is marked by an arc of gravity highs, correlating with uplifted Palaeozoic rocks covered by thin Mesozoic and younger sediments. A three-dimensional (3D) gravity analysis has been carried out to investigate further the crustal structure of this area. The sedimentary succession has been modelled as two heterogeneous layers—Mesozoic–Cenozoic and Palaeozoic—in the analysis. The base of the sedimentary succession (top of the crystalline Precambrian basement) lies at a depth up to 22 km in the PCB and DF–KS areas. The residual gravity field, obtained by subtracting the gravitational effect of the sedimentary succession from the observed gravity field, reveals a distinct elongate zone of positive anomalies along the axis of the DF–KS with amplitudes of 100–140 mGal and an anomaly of 180 mGal in the PCB. These anomalies are interpreted to reflect a heterogeneous lithosphere structure below the supracrustal, sedimentary layers: i.e., Moho topography and/or the existence of high-density material in the crystalline crust and uppermost mantle. Previously published data support the existence of a high-density body in the crystalline crust along the DDB axis, including the DF, caused by an intrusion of mafic and ultramafic rocks during Late Palaeozoic rifting. A reinterpretation of existing Deep Seismic Sounding (DSS) data on a profile crossing the central KS suggests that the nature of a high-velocity/density layer in the lower crust (crust–mantle transition zone) is not the same as that of below the DF. Rather than being a prolongation of the DDB–DF intracratonic rift zone, the present analysis suggests that the KS comprises, at least in part, an accretionary zone between the EEC and the SP formed after the Palaeozoic.     

Deep Structure and Geodynamics of the Black Sea–Caspian Region

Geotectonics - New data on the crust structure of the Black Sea?Caspian region, including the Scythian and Anatolian plate margins, the Caucasus, Black Sea and Southern Caspian structures are...     

Lithosphere structure of the Ukrainian Shield and Pripyat Trough in the region of EUROBRIDGE-97 (Ukraine and Belarus) from gravity modelling

The southern segment of the seismic profile EUROBRIDGE—EUROBRIDGE-97 (EB'97)—located in Belarus and Ukraine, crosses the suture zone between two main segments of the East European Craton—Fennoscandia and Sarmatia—as well as Sarmatia itself. At the initial stage of our study, a 3-D density model has been constructed for the crust of the study region, including the major part of the Osnitsa–Mikashevichi Igneous Belt (OMIB) superimposed by sediments of the Pripyat Trough (PT), and three domains in the Ukrainian Shield—the Volhyn Domain (VD) with the anorthosite–rapakivi Korosten Pluton (KP), the Podolian Domain (PD), and the Ros–Tikich Domain (RTD). The model comprises three layers—sediments with maximum thickness (6 km) in the PT and two heterogeneous layers in the crystalline crust separated at a depth of 15 km. 3-D calculations show the main features of the observed gravity field are caused by density heterogeneities in the upper crust. Allocation of density domains deeper than 15 km is influenced by Moho topography. Fitting the densities here reveals an increase (up to 2960 kg m⁻³) in the modelled bodies accompanied by a Moho deepening to 50 km. In contrast, a Moho uplift to a level of 35–37 km below the KP and major part of the PT is associated with domains of reduced densities. An important role for the deep Odessa–Gomel tectonic zone, dividing the crust into two regions one of basically Archean consolidation in the west (PD and RTD) and one of Proterozoic crust in the east (Kirovograd Domain)—was confirmed.2-D density modelling on the EB'97 profile shows that in the upper crust three main domains of different Precambrian evolution—the OMIB (with the superimposed PT), the VD with the KP, and the PD—can be distinguished. Deeper, in the middle and lower crust, layered structures having no connection to the surface geology are dominant features of the models. Least thickness of the crust was obtained below the KP. Greatest crustal thickness (more than 50 km) was found below the PD, characterised also by maximum deviation of velocity/density relation in the rocks from a standard one. The velocity and density models along the EB'97 profile have been interpreted together with inferred V_p/V_s ratios to estimate crustal composition in terms of SiO₂ content. In the course of the modelling, the status of the PD as a centre of Archean granulitic consolidation has been confirmed. The crustal structure of the anorthosite–rapakivi KP is complex. For the first time, a complicated structure for the lower crust and lower crust–upper mantle transition zone beneath the KP has been determined. The peculiarities of the crustal structure of the KP are quite well explained in terms of formation of rapakivi–anorthosite massifs as originating from melt chambers in the upper mantle and lower crust. An important role for the South Pripyat Fault (SPF), repeatedly activated during Proterozoic–Palaeozoic times, has been ascertained. At the subplatform stage of crustal evolution the SPF was, probably, a magma channel facilitating the granitic intrusions of the KP. In the Palaeozoic the fault was reactivated during rifting in the PT.