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 1.80–1.74-Ga gabbro–anorthosite–rapakivi Korosten Pluton in the Ukrainian Shield: a 3-D geophysical reconstruction of deep structure

The deep structure of the gabbro–anorthosite–rapakivi granite (“AMCG-type”) Korosten Pluton (KP) in the northwestern Ukrainian Shield was studied by 3-D modelling of the gravity and magnetic fields together with previous seismic data. The KP occupies an area of ca. 12,500 km² and comprises several layered gabbro-anorthositic intrusions enveloped by large volumes of rapakivi-type granitoids. Between 1.80 and 1.74 Ga, the emplacement of mafic and associated granitoid melts took place in several pulses. The 3-D geophysical reconstruction included: (a) modelling of the density distribution in the crust using the observed Bouguer anomaly field constrained by seismic data on Moho depth, and (b) modelling of the magnetic anomaly field in order to outline rock domains of various magnetisation, size and shape in the upper and lower crust. The density modelling was referred to three depth levels of 0 to 5, 5 to 18, and 18 km to Moho, respectively. The 3-D reconstruction demonstrates close links between the subsurface geology of the KP and the structure of the lower crust. The existence of a non-magnetic body with anomalously high seismic velocity and density is documented. Most plausibly, it represents a gabbroic stock (a parent magma chamber) with a vertical extent of ca. 20 km, penetrating the entire lower crust. This stock has a half-cylindrical shape and a diameter of ca. 90 km. It appears to be connected with a crust–mantle transitional lens previously discovered by EUROBRIDGE seismic profiling. The position of the stock relative to the subsurface outlines of the KP is somewhat asymmetric. This may be due to a connection between the magmatism and sets of opposite-dipping faults initially developed during late Palaeoproterozoic collisional deformation in the Sarmatian crustal segment. Continuing movements and disturbances of the upper mantle and the lower crust during post-collisional tectonic events between 1.80 and 1.74 Ga may account for the long-lived, recurrent AMCG magmatism.     

3-D paleotemperature modeling of the geothermal regime of sedimentary basins: Example of the Lunskaya depression,Sakhalin Island

The task of 3-D modeling of the thermal field of a sedimentary basin during sedimentation is considered. The aim of the modeling is to determine the temperature at any point of the basin at a given moment of geological time. The mathematical model is based on a system of equations of thermal conductivity for a heterogeneous layered medium with dynamic boundaries. The conditions of the continuous temperature and thermal flow are given at the boundaries of the adjacent layers. The temperature values, which are determined by the values of the secular course of the earth temperature, are given at the upper boundary coinciding with the sedimentation surface. The thermal flow value is considered to be given at the lower boundary. The medium is approximated using a vertical triangle prism, which is accepted in algorithms of interpretation of the gravitation field and characterized by random upper and lower basements and given values of the thermal physical parameters. The equations of thermal conductivity are solved on the basis of potential theory. The precision of this algorithm is demonstrated by calculation of a test example. The thermal evolution of the sedimentary complexes and dynamics of the major zone of oil formation are reconstructed and possible errors of paleotemperature interpretations caused by ignored 3-D modeling medium are determined on the example of the sedimentary basin of the Lunskaya depression of Sakhalin.     

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.     

Finite-difference migration of the field of refracted waves in studies of the deep structure of the Earth’s crust and the upper mantle based on the DSS (on the example of the <Emphasis Type="Italic">DOBRE</Emphasis> profile)

The main results of deep seismic sounding (DSS) are usually presented in the form of high-velocity models of the medium. Some model examples and the international DOBRE profile have shown that the informativeness of the data obtained can be significantly enhanced by the construction of wave images of the Earth’s crust, based on the migration of refracted and wide-angle reflected waves. The Donets Basin Refraction/Reflection Experiment (DOBRE) profile crosses the Dnieper-Donets paleorift zone in the Donbas region. Along the profile, refracted waves from the basement and the upper mantle and the reflections from the crust basement (the M boundary) are reliably traced. This wave migration has been used to construct a wave image of the structure of the Earth’s crust. As a result, a clear seismic image of the basement surface, whose depth changes along the profile from 0 to 20 km, was obtained. In near-slope parts of the basin, several major faults were identified that had not been identified previously during standard kinematic data processing. It is shown that the crust-upper mantle transition zone is a clearly reflective horizon only within the crystalline massifs; under a depression, it is represented by a lens-shaped highly-heterogeneous area. As shown in the model examples, the images obtained using such a migration accurately reflect the structural features of the medium, in spite of its complicated structure.