Crustal and Upper Mantle Velocity Model along the DOBRE-4 Profile from North Dobruja to the Central Region of the Ukrainian Shield : 2. Geotectonic Interpretation

Peer reviewe

Lithospheric structure along wide-angle seismic profile GEORIFT 2013 in Pripyat–Dnieper–Donets Basin (Belarus and Ukraine)

The GEORIFT 2013 (GR'13) WARR (wide-angle reflection and refraction) experiment was carried out in 2013 in the territory of Belarus and Ukraine with broad international co-operation. The aim of the work is to study basin architecture and deep structure of the Pripyat-Dnieper-Donets Basin (PDDB), which is the deepest and best studied Palaeozoic rift basin in Europe. The PDDB is located in the southern part of the East European Craton (EEC) and crosses Sarmatia-one of the three segments of the EEC. The PDDB was formed by Late Devonian rifting associated with domal basement uplift and magmatism. The GR'13 extends in NW SE direction along the PDDB strike and crosses the Pripyat Trough (PT) and Dnieper Graben (DG) separated by the Bragin Uplift (BU) of the basement. The field acquisition along the GR'13 (of 670 km total length) involved 14 shots and recorders deployed every similar to 2.2 km for several shot points. The good quality of the data, with first arrivals visible up to 670 km for several shot points, allowed for construction of a velocity model extending to 80 km depth using ray-tracing modelling. The thickness of the sediments (Vp <6.0 km s(-1)) varies from 1-4 km in the PT, to 5 km in the NW part of the DG, to 10-13 km in the SE part of the profile. Below the DG, at similar to 330-530 km distance, we observed an upwarping of the lower crust (with Vp of similar to 7.1 km s(-1)) to 25 km depth that represents a rift pillow or mantle underplate. The Moho shallows southeastwards from similar to 47 km in the PT to 40-38 km in the DG with mantle velocities of 8.35 and similar to 8.25 km s(-1) in the PT and DG, respectively. A near-horizontal mantle discontinuity was found beneath BU (a transition zone from the PT to the DG) at the depth of 50-47 km. It dips to the depth of similar to 60 km at distances of 360-405 km, similar to the intersecting EUROBRIDGE'97 profile. The crust and upper mantle structure on the GR'13 may reflect varying intensity of rifting in the PDDB from a passive stage in the PT to active rifting in the DG. The absence of Moho uplift and relatively thick crystalline crust under the PT is explained by its tectonic position as a closing unit of the PDDB, with a gradual attenuation of rifting from the southeast to the northwest. The most active stage of rifting is evidenced in the DG by a shallower Moho and by a presence of a rift pillow caused by mafic and ultramafic intrusions during the active phase. The junction of the PT and the DG (the BU) locates just at its intersection with the NS regional tectonic zone Odessa-Gomel. Most likely, the 'blocking' effect of this zone did not allow for further propagation of active rifting to the NW.Peer reviewe

Crustal and Upper Mantle Velocity Model along the DOBRE-4 Profile from North Dobruja to the Central Region of the Ukrainian Shield : 1. Seismic Data

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 sub-crustal 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.Peer reviewe

Moho depth across the Trans-European Suture Zone from P-and S-receiver functions

The Mohorovicic discontinuity, Moho for short, which marks the boundary between crust and mantle, is the main first-order structure within the lithosphere. Geodynamics and tectonic evolution determine its depth level and properties. Here, we present a map of the Moho in central Europe across the Teisseyre-Tornquist Zone, a region for which a number of previous studies are available. Our results are based on homogeneous and consistent processing of P- and S-receiver functions for the largest passive seismological data set in this region yet, consisting of more than 40 000 receiver functions from almost 500 station. Besides, we also provide new results for the crustal Vp/Vs ratio for the whole area. Our results are in good agreement with previous, more localized receiver function studies, as well as with the interpretation of seismic profiles, while at the same time resolving a higher level of detail than previous maps covering the area, for example regarding the Eifel Plume region, Rhine Graben and northern Alps. The close correspondence with the seismic data regarding crustal structure also increases confidence in use of the data in crustal corrections and the imaging of deeper structure, for which no independent seismic information is available. In addition to the pronounced, stepwise transition from crustal thicknesses of 30km in Phanerozoic Europe to more than 45 beneath the East European Craton, we can distinguish other terrane boundaries based on Moho depth as well as average crustal Vp/Vsratio and Moho phase amplitudes. The terranes with distinct crustal properties span a wide range of ages, from Palaeoproterozoic in Lithuania to Cenozoic in the Alps, reflecting the complex tectonic history of Europe. Crustal thickness and properties in the study area are also markedly influenced by tectonic overprinting, for example the formation of the Central European Basin System, and the European Cenozoic Rift System. In the areas affected by Cenozoic rifting and volcanism, thinning of the crust corresponds to lithospheric updoming reported in recent surface wave and S-receiver function studies, as expected for thermally induced deformation. The same correlation applies for crustal thickening, not only across the Trans-European Suture Zone, but also within the southern part of the Bohemian Massif. A high Poisson’s ratio of 0.27 is obtained for the craton, which is consistent with a thick mafic lower crust. In contrast, we typically find Poisson’s ratios around 0.25 for Phanerozoic Europe outside of deep sedimentary basins. Mapping of the thickness of the shallowest crustal layer, that is low-velocity sediments or weathered rock, indicates values in excess of 6km for the most pronounced basins in the study area, while thicknesses of less than 4km are found within the craton, central Germany and most of the Czech Republic.Peer reviewe

Lithospheric structure of the TESZ in Poland based on modern seismic experiments

This paper presents the results of seismic investigations on the structure of the lithosphere in the area of the Trans-European Suture Zone (TESZ) in Poland that is located between the southwestern margin of the East European Craton (EEC) to the north-east, the West and Central European Palaeozoic Platform (PP) to the south-west and the Carpathians to the south. Based on results of the modern POLONAISE’97 and CELEBRATION 2000 projects, as well as older profiles, models are presented for the configuration and extent of different crustal types. In the investigated area, the EEC has a relatively uniform crustal thickness of 40 to 50 km with its three-layered crystalline crust displaying P-wave velocities of 6.1–6.4, 6.5–6.8 and 6.9–7.2 km/s in the upper, middle and lower parts, respectively. The Variscan consolidated crust is covered by 1–2 km thick sediments and consists of two layers with velocities of 5.6–6.3 and 6.5–6.65 km/s. In the Carpathians, sediments reaching to depths of some 20 km and are characterized by velocities of <5.6–5.8 km/s, whilst the underlying two-layered crystalline crust displays velocities of 6.0–6.2 and 6.5–6.9 km/s. The crust of the TESZ can be divided into the Małopolska, Kuiavia and Pomerania blocks that are overlain by up to 9–12 km thick sediments having velocities <5.4 km/s. In the area of the TESZ, the upper part of the consolidated crust has to depths of 15–20 kmrelatively low velocities of <6.0 km/s and is commonly regarded as consisting of deformed and slightlymetamorphosed Early Palaeozoic sedimentary and volcanic series. In this area the middle and lower crust are characterized by velocities in the range of 6.3–6.6 km/s and 6.8–7.2 km/s, respectively, that are comparable to the EEC. Based on the dense network of seismic profiles the map of the depth toMoho is given for the area of Poland. Uppermost mantle reflectors occur about 10 to 15 km below the Moho whereas the deepest reflectors are recorded at depths of 90 km. Future investigations ought to aim at an integrated geological-geophysical program, including deep near-verical reflection-seismic profiling and ultimately the drilling of deep calibration boreholes

Crustal structure of the Trans-European Suture Zone in Central Poland - reinterpretation of the LT-2, LT-4 and LT-5 deep seismic sounding profiles

The paper gives a reinterpretation of data from three deep seismic sounding profiles-LT-2, LT-4 and LT-5 - acquired in 1974-1979 between the Precambrian East European Craton (EEC) and the Palaeozoic Platform (PP) in Central Poland. Good quality seismic records in the distance interval from 50-90 to 200-280 km were the input data for the modelling of the crustal and uppermost mantle structure. Clear first arrivals and later phases of waves reflected/refracted from the crustal and the Moho boundaries were interpreted using a two-dimensional (2-D) ray tracing technique. In general, the crustal thickness along the three profiles varies from 30-35 km in the Palaeozoic platform area, to 42-44 km in the Polish part of the EEC, being 35-40 km in the transition zone between the PP and the EEC. In the transition area, the P-wave velocity is very low (Vp < 6.0 km/s) down to depths of 15-18 km, indicating that a very thick succession of sedimentary, metamorphosed or volcanic origin rocks is present there. All three 2-D models of the crust are discussed together with results obtained 20-30 years ago, particularly taking into account the difference in interpretation methods and new computation possibilities. Jointly with recent seismic studies along the profiles LT-7 and TTZ, as well as the POLONAISE'97 profiles P1-P4, the reinterpreted old profiles provide a collection of crustal models of the TESZ in Poland