The EUROBRIDGE deep seismic sounding (DSS) pro®le is a key component of a EUROPROBE project to ex... more The EUROBRIDGE deep seismic sounding (DSS) pro®le is a key component of a EUROPROBE project to examine Palaeoproterozoic processes of continental collision and crustal accretion. Its purpose is to establish the deep lithospheric structure of the East European Craton between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields. In 1994 a DSS experiment was recorded across the Baltic Sea from Va Èstervik (Sweden) to Shventoji (Lithuania). We report on EUROBRIDGE'95, the ®rst onshore stage of the seismic pro®le. It is 280 km long, recorded from NW to SE on the Lithuanian part of the East European Platform, traversing the Proterozoic West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (EL) terranes. Explosive shots of up to 1000 kg TNT were detonated at 10 shotpoints (SP01±SP10) at intervals of about 30 km. Arrivals were recorded at 76 3-component seismograph stations with an average station spacing of 3.5 km, providing high quality records. A 11th shot of 3000 kg (SP00) was ®red in the Baltic Sea close to Gotland. Raytracing analysis of refracted and re¯ected P-waves has been used to determine a 2-dimensional seismic velocity model for the crust and uppermost mantle below EUROBRIDGE'95. The thickness of the Phanerozoic sedimentary cover decreases from 2.2 km in the north±west near the Baltic Sea coast to 0.4 km at the south±east end of the pro®le near the Lithuania/Belarus border. Crust in the north±west and central part of pro®le consists of two major layers with a thickness of about 44 km, increasing to 50 km and three layers in the south±east. Crystalline upper crust is about 20 km thick, thinning in the south±east, with P-wave velocities of 6.0±6.3 km/s. A very weak low velocity zone, with a velocity contrast of 0.1±0.2 km/s, occurs at 8±13 km depth below the north±west and central part of the pro®le only. Lower crust exhibits velocities of commonly 6.5±6.9 km/s, and thickens to the south±east with P-wave velocities up to 7.0 km/s in the deepest parts. Crystalline crust is characterised by low velocity gradients and small velocity contrasts at most seismic boundaries. Major lateral changes in crustal velocity structure at Tectonophysics 339 (2001) 153±175
The Donbas Foldbelt (DF) is the uplifted and deformed part of the up to 20-km-thick Dniepr-Donets... more The Donbas Foldbelt (DF) is the uplifted and deformed part of the up to 20-km-thick Dniepr-Donets Basin (DDB) that formed as the result of rifting of the East European Craton (EEC) in the Late Devonian. Uplift, especially of the southern margin of the basin, occurred in Early Permian times, in a (trans)tensional tectonic stress regime while folding and reverse faulting mainly occurred later-mainly during the Late Cretaceous. A seismic refraction/wide-angle reflection survey was carried out in 1999 (DOBREfraction'99) to complement existing Deep Seismic Sounding (DSS) data from the area that did not record significant Pn phase arrivals because of insufficient maximum offset. DOBREfraction'99 comprised some 245 recording stations along a line of 360 km length with 11 in-line shot points as well as a 100 km away, parallel 190 km long subsidiary fan profile. The main profile runs between the shores of the Azov Sea in the south to the Ukraine-Russia border in the north, across the Azov Massif (Ukrainian Shield), the Foldbelt, and the Voronezh Massif. Particular scientific targets include the nature of the crust-mantle transition and the geometry of crustal-upper mantle structures related to rifting and subsequent basin inversion. Tomographic inversion as well as ray-trace based velocity modelling has been carried out. The velocity signature of the sedimentary basin itself is well resolved, indicating an asymmetric form, with a steeper basement surface in the south than in the north, and a total thickness of about 20 km. A thick (>10 km) high velocity (>6.9 km/s) lower crustal body lies beneath the rift basin itself, offset slightly to the north compared to the main basin depocentre, likely related to the rifting processes. Velocities in the crust below the Ukrainian Shield, south of the
We study the azimuthal velocity variation of Pg waves in the Bohemian Massif using data collected... more We study the azimuthal velocity variation of Pg waves in the Bohemian Massif using data collected during Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. We analyze travel times of waves generated by 28 shots and recorded by 256 portable and 19 permanent seismic stations deployed on the territory of the Czech Republic and in adjacent areas. We use recording offset ranging from 30 to 190 km with azimuths covering the whole interval of angles. The observed travel times are inverted for parameters of a velocity model formed by an isotropic low-velocity subsurface layer with a varying depth lying on a homogeneous transversely isotropic half-space with a horizontal axis of symmetry. The recovered velocity displays a systematic azimuthal variation indicating a regional-scale intrinsic or effective anisotropy in the Bohemian Massif. The mean, minimum and maximum values of the velocity are v mean = 6.03 km/s, v min = 5.98 km/s, v max = 6.10 km/s, respectively, indicating an anisotropy of 1.5-2.5%. The direction of the maximum propagation velocity is $N35°E being approximately perpendicular to the present maximum compression in the Earth crust in central Europe. The observed anisotropy cannot be induced by stress-aligned cracks in the crust, because the crack models predict azimuthal velocity variations completely inconsistent with the observed one. Therefore we suggest the crustal anisotropy to be induced by a preferred orientation of rock-forming minerals and large-scale intrusion fabrics developed during a tectonic evolution of the Bohemian Massif.
In the autumn of 1989 a cooperative experiment involving 12 research institutions in northwestern... more In the autumn of 1989 a cooperative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. The 121 litre airgun array used for this profiling was also recorded by 62 multicomponent land stations to provide coincident refraction surveys, fan-spreads, and 3-D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high-resolution impedance contrast images as well as more regional 3-D velocity models in both Pand S-waves. In the Bothnian Bay a south-dipping, non-reflective zone coincides with the conductive Archaean-Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9 Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic-arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide-angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Aland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non-reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70-1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. The c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near-and far-offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.
The POLONAISE'97 (POlish Lithospheric ONset-An International Seismic Experiment, 1997) seismic ex... more The POLONAISE'97 (POlish Lithospheric ONset-An International Seismic Experiment, 1997) seismic experiment in Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: f 29 km below the Palaeozoic Platform; 35 -47 km at the crustal keel at the Teisseyre -Tornquist Zone (TTZ), slightly displaced to the northeast of the geologic inversion zone; and f 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the depth down to the consolidated basement is 6 -14 km, as characterised by a velocity of 5.8 -5.9 km/s. The low basement velocities, less than 6.0 km/s, extend to a depth of 16 -22 km. In the middle crust, with a thickness of ca. 4 -14 km, the velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8 -7.0 km/s at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0 -7.2 km/s at a depth of 30 km. In the lowermost crust, a body with a velocity of 7.20 -7.25 km/s was found above Moho at a depth of 33 -45 km in the central part of the profile. Sub-Moho velocities are 8.2 -8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at f 45-km depth beneath the Palaeozoic Platform and f 55-km depth beneath the TTZ. The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be
Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea,... more Multidisciplinary studies of geotransects across the North European Plain and Southern North Sea, and geological reexamination of the Variscides of the North Bohemian Massif, permit a new 3-D reassessment of the relationships between the principal crustal blocks abutting Baltica along the Trans-European Suture Zone (TESZ). Accretion was in three stages: Cambrian accretion of the Bruno-Silesian, Lysogory and Malopolska terranes; end-Ordovician/early Silurian accretion of Avalonia; and early Carboniferous accretion of the Armorican Terrane Assemblage (ATA). Palaeozoic plume-influenced metabasite geochemistry in the Bohemian Massif explains the progressive rifting away of peri-Gondwanan crustal blocks before their accretion to Baltica. Geophysical data, faunal and provenance information from boreholes, and dated small inliers and cores confirm that Avalonian crust extends beyond the Anglo-Brabant Deformation Belt eastwards to northwest Poland. The location and dip of reflectors along the TESZ and beneath the North European Plain suggest that Avalonian crust overrode the Baltica passive margin, marked by a high-velocity lower crustal layer, on shallowly southwest-dipping thrust planes forming the Heligoland-Pomerania Deformation Belt. The ''Variscan orocline'' of southwest Poland masks two junctions between the Armorican Terrane Assemblage (ATA) and previously accreted crustal blocks. To the east is a dextrally transpressive contact
⎯For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies th... more ⎯For studying the structure of the lithosphere in southern Ukraine, wide-angle seismic studies that recorded the reflected and refracted waves were carried out under the DOBRE-4 project. The field works were conducted in October 2009. Thirteen chemical shot points spaced 35-50 km apart from each other were implemented with a charge weight varying from 600 to 1000 kg. Overall 230 recording stations with an interval of 2.5 km between them were used. The high quality of the obtained data allowed us to model the velocity section along the profile for P-and S-waves. Seismic modeling was carried out by two methods. Initially, trial-and-error ray tracing using the arrival times of the main reflected and refracted P-and S-phases was conducted. Next, the amplitudes of the recorded phases were analyzed by the finite-difference full waveform method. The resulting velocity model demonstrates a fairly homogeneous structure from the middle to lower crust both in the vertical and horizontal directions. A drastically different situation is observed in the upper crust, where the Vp velocities decrease upwards along the section from 6.35 km/s at a depth of 15-20 km to 5.9-5.8 km/s on the surface of the crystalline basement; in the Neoproterozoic and Paleozoic deposits, it diminishes from 5.15 to 3.80 km/s, and in the Mesozoic layers, it decreases from 2.70 to 2.30 km/s. The subcrustal Vp gradually increases downwards from 6.50 to 6.7-6.8 km/s at the crustal base, which complicates the problem of separating the middle and lower crust. The Vp velocities above 6.80 km/s have not been revealed even in the lowermost part of the crust, in contrast to the similar profiles in the East European Platform. The Moho is clearly delineated by the velocity contrast of 1.3-1.7 km/s. The alternating pattern of the changes in the Moho depths corresponding to Moho undulations with a wavelength of about 150 km and the amplitude reaching 8 to 17 km is a peculiarity of the velocity model.
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Papers by H. Thybo