Tamara Yegorova

The evolution of the southern margin of Eastern Europe (Eastern European and Scythian platforms) from the latest Precambrian-Early Palaeozoic to the Early Cretaceous

Abstract The southern part of the Eastern European continental landmass consists mainly of a thick platform of Vendian and younger sediments overlying Precambrian basement, referred to as the East European and Scythian platforms (EEP and SP). Some specific geological features, such as the Late Devonian Pripyat-Dniepr-Donets rift basin, the Karpinsky Swell, the Permo(?)-Triassic troughs of the SP, and the deformed belt running from Dobrogea to Crimea and the Greater Caucasus, in which rocks as old as Palaeozoic crop out, form a record of the geodynamic processes affecting this part of the European lithosphere. Hard constraints on the Palaeozoic history of the SP are very sparse. The conventional view has been that the SP is a Late Palaeozoic orogenic belt. However, it is shown that the few available data are also consistent with an alternative interpretation in which it is the thinned margin of the Precambrian continent, reworked by Late Palaeozoic-Early Mesozoic rifting events. The geodynamic setting of the margin is classically reported as one of active convergence throughout the Late Palaeozoic and Early Mesozoic, with subduction of the Palaeotethys Ocean beneath Europe. Actually, there are no direct observations constraining the polarity of Palaeotethys subduction in this area although indirect evidence is not inconsistent with the conventional model. In such a case, the sedimentary-tectonic record of the SP suggests that convergence during the Permo-Triassic(?) and certainly during the Early and Mid-Jurassic was oblique. An Eo-Cimmerian (Late Triassic-Early Jurassic) event is widespread and implies a tectonic compressional regime with systematic inversion of most sedimentary basins. There is also a widespread unconformity at the end of the Mid-Jurassic and in the Late Jurassic. These can be interpreted as indicators of compressional tectonics; however, nowhere is there evidence of intense shortening or other orogenic processes. A revised tectonic model is proposed for the area but, given the degree of uncertainty characterizing the geology of this area, it is best considered as a basis for further discussion.

3-D gravity analysis of the Dniepr–Donets Basin and Donbas Foldbelt, Ukraine

Abstract The Dniepr–Donets Basin (DDB) is a linear, NW–SE trending, late Palaeozoic and younger sedimentary basin on the East European Platform separating the Ukrainian Shield from the Voronezh Massif. Its northwestern (Dniepr) segment has the characteristics of a typical rift basin. To the southeast, through a transition zone of approximately 200 km length, the DDB is progressively uplifted and compressionally deformed into the correlatable Donbas Foldbelt (DF). Along the axis of the Dniepr segment a series of gravity highs has been previously explained by high-density crystalline crust beneath the axis of the basin caused by intrusion of mafic and ultramafic rocks. In this paper, the results of a 3-D gravity analysis, using a gravity backstripping technique, is described that investigates the crustal and upper mantle structure in the region of the DDB–DF transition zone and DF. A residual gravity field I, obtained by subtracting the gravity influence of the sedimentary succession of the DDB from the observed field, reveals a distinct positive anomaly along the axis of the rift basin increasing in amplitude to the southeast in concert with increasing sedimentary thickness. A residual gravity field II, derived by removing the gravity effects of a modelled homogeneous crystalline crust from residual field I, reaches 200 and 100 mGal amplitude in the DF for two respective Moho models based on different interpretations of the published crust and upper mantle seismic velocity models. The first of these (model A) assumes crustal thickening beneath the transition zone and DF (to a Moho depth up to 50 km) whereas the second (model B) assumes a Moho shallowing (to depths in the range 35–37 km) along the whole basin axis. For each residual anomaly II, the best-fitting 3-D distribution of average density in the crystalline crust has been computed. Both models indicate the existence of a high-density body in the crystalline crust along the DDB axis, increasing in density from the Dniepr segment to the DF, with higher average crustal density required in the case of Moho model A (3.17×103 kg m−3 versus 3.06×103 kg m−3 for model B). The preferred interpretation of the density models is one in which the denser crystalline crust underlying the DDB–DF transition zone and DF is explained by intrusion of mafic and ultramafic rocks during late Palaeozoic rifting processes. Invocation of processes related to the uplift and inversion in the DF are not required to explain the observed gravity field although reworking of the crust during Permian and younger tectonism in the DF cannot be ruled out.

Structure of the Earth's crust and upper mantle of the West- and East-Black Sea Basins revealed from geophysical data and its tectonic implications

Abstract A back-arc Black Sea Basin consists of two deep sub-basins – the West-Black Sea (WBS) and the East-Black Sea (EBS) – filled with thick sediments (up to 12–14 km), which are separated by the mid–Black Sea Ridge (MBSR) – a NW trending basement uplift structure. For a better understanding of the lithosphere structure of these two sub-basins, the authors made a comprehensive analysis of the available geological and geophysical data, including carrying out a three-dimensional (3D) gravity back-stripping analysis, a reinterpretation of a number of seismic refraction profiles as well as the re-evaluation of seismological data and local seismic tomography. Inferred differences in the basin architecture and lithosphere structure of the WBS and EBS can be explained by different affinities of the underlying crustal domains and by the peculiarities of their (Cretaceous and younger) rift and post-rift history. Rifting that led to oceanic crust formation in the WBS occurred within the continental crust of the Moesian Platform along Mesozoic sutures with adjoining accreted terranes. The EBS, most probably, formed within the Transcaucasus continental domain due to strike–slip movements along the MBSR. Underthrusting of the EBS oceanic lithosphere beneath the continental domain of the Scythian Platform led to the formation offshore of the Crimean orogen of accretional wedge of Sorokin Trough.

The crustal structure of the Black Sea from the reinterpretation of deep seismic sounding data acquired in the 1960s

Abstract A ray-tracing modelling of seismic refraction data acquired in the 1960s has been undertaken on two north–south lines – Profile 25 in the western part of the Black Sea and Profile 28/29 crossing the Azov Sea and central part of the Black Sea. The velocity model along Profile 25 shows two domains interpreted as thin (5 km) high-velocity (sub-)oceanic crust below the deep-water part of the Western Black Sea (WBS) basin, covered by 12–13 km of Cretaceous and younger sediments, and a 39 km thick continental domain of the Scythian Platform and southernmost part of the East European Platform. They are separated by a high-amplitude normal fault, interpreted as being related to the opening of the WBS during Late Cretaceous rifting. The velocity model on Profile 28/29 shows what is interpreted as oceanic crust on the northwestern extremity of the Eastern Black Sea Basin (EBS) and thinned continental crust (Moho depths at 29 km) underlying the mid-Black Sea Ridge (MBSR) that separates the EBS and WBS. The basement of the MBSR comprises three units, which in an en echelon-like manner elevate southwards from a depth of 10–11 km beneath the Andrusov Ridge to 6 km on Arkhangelsky Ridge. An inclined seismic boundary at the Moho interface may be related to oblique rifting setting during the initial formation of the EBS.

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

Abstract 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.

Key problems of stratigraphy in the Eastern Crimea Peninsula: some insights from new dating and structural data

Abstract The tectonic evolution of the Eastern Black Sea Basin has previously been explained based on offshore and onshore data, some of the latter from the Crimean Mountains (CM). However, changes in the stratigraphy of the CM have recently been proposed: the Late Triassic–Early Jurassic Tauric Group was assigned as younger (Albian). To clarify the stratigraphy and the tectonic evolution of this area, we sampled the eastern CM for micropalaeontological datings (nannoplankton). The results demonstrate an Early Cretaceous age for the Tauric Group in the eastern CM. The samples contained substantial amounts of volcanic ash, indicating a period of magmatic activity along all the eastern CM. Our field observations allowed us to propose a new structural map and cross-sections, using which three main tectonic units were distinguished. We define a phase of extension during the Early Cretaceous and one of shortening during the Paleocene–Early Eocene, before the main Middle Eocene limestone unconformity. These two phases are related to: (1) the opening of the Eastern Black Sea Basin along NNW–SSE-trending normal faults and the associated magmatism; and (2) north–south shortening that could be comparable with the inversion in Dobrogea and/or with north–south shortening linked to the collision of continental blocks in the Pontides and Taurides domains.

Geological structure of the northern part of the Eastern Black Sea from regional seismic reflection data including the DOBRE-2 CDP profile

Abstract The margin of the northeastern Black Sea is formed by the Crimea and Kerch peninsulas, which separate it from the Azov Sea to the north. The age and architecture of the sedimentary successions in this area are described from exploration reflection seismic profiling acquired in the area, in addition to the regional DOBRE-2 CDP profile acquired in 2007. The sediments range in age from Mesozoic to Quaternary and can be divided into five seismo-stratigraphic complexes linked to the tectono-sedimentological evolution of the area. The present regional basin architecture consists of a series of basement structural highs separating a series of sedimentary depocentres and is mainly a consequence of the compressional tectonic regime affecting the area since the Eocene. This has focused shortening deformation and uplift along the axis of the Crimea–Caucasus Inversion Zone on the Kerch Peninsula and Kerch Shelf of the Black Sea. Two major sedimentary basins that mainly formed during this time – the Sorokin Trough in the Black Sea and the Indolo-Kuban Trough to the north of the Kerch Peninsula in the Azov Sea – formed as marginal troughs to the main inversion zone.

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 (mb ≤ 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.

Density Heterogeneities of the European Upper Mantle Inferred from 3-D Large-Scale Gravity Modelling

For Europe the 3-D density model of the Earth’s crust (in one degree averaging) has been constructed. The model is based on generalised velocity model and consists of two regional layers — the sedimentary cover and the crystalline crust with lateral variation of average parameters (velocity and density). Gravity residual anomalies, obtained by subtracting the calculated effect of the model from the observed gravity field, are caused mainly by density heterogeneities of the upper mantle. The mantle origin of the residual anomalies is confirmed by both the information on velocity structure of the upper mantle according to seismotomography study and geothermal data.

Crustal Structure of the Crimean Mountains along the Sevastopol–Kerch Profile from the Results of DSS and Local Seismic Tomography

The paper discusses the velocity structure of the crust beneath the Crimean Mountains from the results of active and passive seismic experiments. Based on a new interpretation of seismic data from the old Sevastopol–Kerch DSS profile by modern full-wave seismic modeling methods, a velocity model of the crust beneath the Crimean Mountains has been constructed for the first time. This model shows the significant differences in the structure of two crustal blocks: (1) one characterized by higher velocities and located in the western and central Crimean Mountains, and (2) the other characterized by lower velocities and located in the east, in the Feodosiya–Kerch zone, which are subdivided by a basement uplift (Starokrymskoe Uplift). The former block is characterized by a more complex structure, with the Moho traced at depths of 43 and 55 km, forming two Moho discontinuities: the upper one corresponds to the platform stage, and the lower one, formed presumably at the Alpine stage of tectogenesis as a result of underthrusting of the East Black Sea microplate beneath the southern margin of the Scythian Plate in Crimea. At depths of 7–11 km, velocity inversion zone has been identified, indicating horizontal layering of the crust. Local seismic tomography using the data on weak earthquakes (mb ≤ 3) recorded by the Crimean seismological network allowed us to obtain data on the crustal structure beneath the Crimean Mountains at depths of 10–30 km. The crustal structure at these depths is characterized by the presence of several high-velocity crustal bodies in the vicinity of cities Yalta, Alushta, and Sudak, with earthquake hypocenters clustered within these bodies. Comparison of this velocity model of the Crimean Mountains with the seismicity distribution and with the results from reconstruction of paleo- and present-day stress fields from field tectonophysical study and earthquake focal mechanisms allowed the conclusion that the Crimean Mountains were formed as a result of on mature crust at the southern margin of the East European Platform and Scythian Plate, resulting from processes during various phases of Cimmerian and Alpine tectogenesis in the compressional and transpressional geodynamic settings. The collisional process is ongoing at the present-day stage, as supported by high seismicity and uplift of the Crimean Mountains.