Scythian Plate Research Articles

Diverse Continental Subduction Scenarios Along the Arabia‐Eurasia Collision Zone

AbstractIt is generally accepted that convergence of the Arabian and African plates is a subduction‐dominated process. However, the subduction scenarios are still debatable. Here we present a 3‐D model of the mantle to a depth of 700 km, based on a joint interpretation of the seismic tomography, residual topography, residual mantle gravity field, and seismicity. At the northwestern edge of the collision zone, we only observe partial underthrust of the Eurasian plate under the West Greater Caucasus. We suggest that this is the initial stage of subduction polarity reversal after the break off of the plate formerly subducted northward. To the southeast, counteracting subduction zones are found beneath northwest Zagros in the south and beneath the East Greater Caucasus and Alborz in the north. This scenario is likely the result of highly buoyant and weak blocks of the Lesser Caucasus, Alborz, and northwest Zagros, which are underlain in the south by the Arabian plate and the Scythian plate and South Caspian from the opposite side. Further to the southeast, a delaminated lithospheric slab is observed under southeast Zagros, while the Arabian and Eurasian plates only partially underthrust East Zagros and Kopet Dagh, respectively. In the southern part of the collision zone under Makran, only remnants of the formerly subducted slabs are found below 200 km.

Geometry and Cenozoic evolution of the Crimean fold-thrust belt from cross-section balancing and kinematic forward modeling

The Crimean fold-thrust belt comprises the onshore Crimean-dome fold-thrust belt and the offshore Sorokin accretionary wedge, a sub-surface imbricate stack with high oil and gas potential. We combine geomorphological and balanced cross-section analyses with published low-temperature thermo¬chronology and offshore seismic data to constrain both its present geometry and Cenozoic structural evolution. We interpret the Crimean dome as a map-scale, ramp-related antiform above the Main Crimean thrust, the basal detachment to the onshore part of the fold-thrust belt. The Main Crimean thrust separates the continental Scythian plate from the transitional to oceanic Eastern Black-Sea basin and likely reactivates an Upper Triassic–Lower Jurassic passive continental margin structure. The Crimean fold-thrust belt has accommodated ~24 km shortening since the Eocene, with ~12 km contraction each in the thick-skinned Crimean dome and the thin-skinned Sorokin accretionary wedge. The intermediate geometries in the kinematic evolution traced by the kinematic forward model are testable by future hydrocarbon exploration and thermochronologic studies.

Basalts of various genesis in the zone of the interconnection of the western Black Sea cavity, the Mountain Crimea and the Scythian plate

Basalts of various genesis in the zone of the interconnection of the western Black Sea cavity, the Mountain Crimea and the Scythian plate

The nature of inclined seismic boundary in the Earth crust of the Scythian plate along the DOBRE-5 Profile

Geological and tectonic interpretation of high-speed seismic model along the profile DOBRE-5 elaborated in 2011 by DSS method (wide-angle refraction and reflection (WARR)) with the main aim of clarifying the structure of the lithosphere and geodynamic features of the southwestern edge of the East-European platform (Scythian plate) has been conducted. Amplifying volumetric seismic tectonic model of the Scythian plate has been plotted; structural-high-speed element — sloping seismic boundary along the DOBRE-5 profile has been revealed, its location has been defined more precisely and possible mechanisms of formation have been proposed: it corresponds to the northern border of the Early Permian active over-thrust or thrust edge of southern dip and of formation of front-rank molassa sag, the so-called Euxinian orogenic belt or the North Crimean suture formed by the closure of the Paleo-Tethys Ocean in the late Paleozoic—Triassic, subduction of the East-European platform (EEP) under the Scythian plate (SP) southward and their collision. On the surface Pre-Cretaceuos basement of the North Crimean suture can be traced as Golitsyn and Azov fault zones usually considered as the border between EEP and SP. Dipping seismic border is a listric surface that falls to the south-southeast, with angles in the longitudinal intersection, α = 17°, and in sections perpendicular to the EEP and SP, α = 22°to 38°. This inclination of the DSB is close to the dip of the South-Crimean suture zone that was previously assumed by Yudin V.V.

The actual problems of the study of the recent platform structures (on the example of the East European platform and the adjacent partof the Scythian plate)

The actual problems of the study of recent tectonics of the East European Platform and the Scythian Plate: development of cover formations of neotectonic structures, discontinuous distribution and bad of safety of correlative deposits, different views on the type of structural forms - block or bending, the nature and type of boundaries between structural forms, a change in the structural plans for the latest phase, the conditions and sources of stress in the formation of new structures.

Геоэлектрическая модель зон сочленения Скифской плиты с Восточно-Европейской платформой и Горным Крымом

Геоэлектрическая модель зон сочленения Скифской плиты с Восточно-Европейской платформой и Горным Крымом

The structure and evolution of lithosphere of the central part of the northwestern shelf of the Black Sea

Density modeling has been conducted that made possible to specify the structure of lithosphere of the central part of northwestern shelf of the Black Sea.In the section of the profile the boundary between the East European platform and the Scythian plate has been identified, which is the fault inclined southward and located in the upper part of the section about 30km northward of the Golitsyn fault. On the base of the plotted paleostructural section evolution of Scythian plate in the crossec-tion of the profile since Paleozoic has been traced. The basement of northwestern shelf of the Black Sea and its central part was broken by numerous disjunctives and fault zones, many of them can be traced within sedimentary cover and stretch to the lower levels of crystal crust.

The recent tectonics of the Caspian Sea region

Recent uplifts and depressions complicated by lineaments, many of which are faults, were found within the Caspian depression using complex structural and geomorphological studies, which included visual interpretation of different-scale topographic maps, as well as an analysis and computer processing of radar satellite images. In many respects, the latest dislocations correlate with the structures and sedimentary cover of the basement and are also determined by salt tectonics. Their formation is substantially influenced by the Ural Orogen and the Scythian plate is involved in the entire uplift at the latest stage.

The collision model of the Crimean orogen - the experience of the finite element method

Finite elements method is attracted for numerical modeling of crust deformation of Crimea Mountains within hypothesis of its collisional formation. Elastic and elastoplastic (Coulomb) ap-proaches are used. As a result next basic positions of collisional model are proved: 1) localized indenter compression of margin of Scythian plate from side of East Black Sea basin; 2) lateral extru-sion of south-west part of orogen; 3) ramp-obductional mechanism of interacting crust microplate with continental margin. Seismicity is explained as localization of plastic deformation in set of restricted subduction of mantle lithosphere and owerthrusting crust plate. Gravitational maximum within south-west margin of Crimea Mountains is connected to extensional zone of extrusive block, Alushta-Batun zone of magnetic anomalies is regarded as manifestation of crustal indenter moving responsible for collisional setting in Crimea.

A gravity model of the deep structure of South Caspian Basin along submeridional profile Alborz–Absheron Sill

The South-Caspian Basin (SCB) underlies the southern part of the Caspian Sea, between the ranges of the eastern Greater Caucasus, Talysh, Alborz, and Kopet Dagh. A 2-D regional gravity model along a profile from the Alborz Mountains to the Absheron Ridge has been constructed, constrained by deep (20s TWT) seismic reflection data. The deep structure model has been evaluated in terms of earthquake focal mechanisms and GPS velocity data to elucidate active tectonic processes and the geodynamic evolution of the SCB.We believe that the rapid increase in the thickness of Mesozoic sediments along the profile from ~8km in the middle part of the profile up to ~15km in the area of the Absheron-Ridge can be explained by inherent basin geometry created by thermal subsidence followed by sediment loading as well as additional effect of tectonic related shortening of sedimentary succession. Near the boundary of oceanic and continental crust in the northern SCB, flexure of oceanic crust is inferred from the observed seismic data and gravity modeling, most probably connected to ongoing subduction of lithosphere of the South Caspian underneath the Scythian Plate of the Mid-Caspian. Subduction beneath the Absheron Ridge is accompanied by the delamination of sediments from the oceanic crust (“basaltic” layer) and creation of accretionary wedge in the overlaying sedimentary succession. The focal mechanisms of the larger earthquakes (M>6) occurring along the northern boundary of the SCB show steep normal-type faulting above the bend of the downgoing slab while, along the southern boundary, thrust faults are inferred. Some thrust-type earthquakes near the northern boundary occur in the lower crust or uppermost mantle and may be associated with compression in the lower part of the brittle lithosphere due to plate flexure. Displacements measured along the coastline of the Caspian Sea by GPS are consistent with the direction of potential oblique subduction of oceanic crust of the SCB beneath the Mid-Caspian. It is speculated that subducting oceanic crust beneath the Absheron–Pribakhan Ridge will eventually be consumed and fold-thrust belt tectonics similar to Eastern Caucasus will commence.

Structure and development of the eastern part of northwestern shelf of the Black Sea (according to the results of geophysical data interpretation along II profile of CMRW)

Complicated block structure of the eastern part of northwestern shelf of the Black Sea has been found as a result of two-dimensional gravity modeling and plotting paleostructural section (according to the MVS CMP data). Scythian plate consists of sub-continental crust from 31.5 km thick under the Marginal bench to 43.5 km thick on the border with East-European platform (EEP). Formation of the Marginal bench, limited from the both sides by deep faults of post-maikopian age is related to dipping of the West Black Sea lowland. In the core of the Calamitian swell presence of a body with density, specific to the rocks of tavrian series has been found. The structure of the swell has been formed as a result of dipping the margins of Triassic-Jurassic basin during the Cretaceous. Crystal basement of the Carkinite sag consists of alternation of blocks with normal and increased density values caused by implantation of the mantle material during riftogenesis. On the northern side of the sag a lens of Middle-Upper-Jurassic sedimentary-volcanogenic rocks has been revealed by modeling under Cretaceous sediments. According to the results of studies the border of the Scythian plate and EEP is found to be much northward from the Golitsyn’s structure. The velocities of sediments accumulation, determined on the basis of data on the thicknesses of sedimentary complexes along the profile allowed to specify the dynamics of tectonic movements, which showed that all the structures of the studied area had their own development history during Meso-Cenozoic.

Deep structure and development of the Scythian plate in the Mezokainozoic in the section of the profile of MOV-OGT No. 598506

Deep structure and development of the Scythian plate in the Mezokainozoic in the section of the profile of MOV-OGT No. 598506

Recent tectonics of the West (Sub-Dobrogea) segment of the Scythian plate and adjacent areas

Recent tectonics of the West (Sub-Dobrogea) segment of the Scythian plate and adjacent areas

Fold-thrust dislocations in sedimentary cover of the Sea of Azov

As follows from geological and geophysical data, tectonic deformations formed under lateral compression are widespread in the sedimentary cover at the bottom of the Sea of Azov. Fold-thrust dislocations are established in the North Azov Trough on the southern slope of the Ukrainian Shield, in the Azov Swell of the young Scythian Plate, and in the Indol-Kuban Foredeep. The transregional Main Azov Thrust Fault and smaller thrust faults of listric morphology tens of kilometers in extent are known. Asymmetric anticlines are related to their fronts. In plan view, the crests of these anticlines are displaced down the dip of the controlling faults. Folds and thrusts developed in pulsatory manner in the regime of periodically acting tangential compression are recorded in the thickness of the plate complex and stratigraphic and angular unconformities. Some of these dislocations are active and accompanied by anomalously high formation pressure, temperature and hydrochemical anomalies, tectonic brecciation, and mud volcanoes.

Some results of applying methods of natural electromagnetic and seismic fields in the North Caucasus

A comprehensive analysis by the methods of natural electromagnetic fields, earthquake converted waves, and deep seismic sounding provided detailed constraints on the section of the crust and sedimentary cover in Ciscaucasia and revealed areas of the maximum crustal permeability on the Scythian plate and suture and underthrusting zones in the North Caucasus fold system. Based on seismic and electric evidence, the connected porosity is estimated at 1.5–3%, with the background value being smaller than 0.1%. These junction zones of the Scythian plate and fold block structures of the Main Caucasus Ridge are characterized by a decrease in the electrical resistivity by more than an order of magnitude, a more than fivefold drop in converted wave amplitudes, and a decrease in the converted and longitudinal wave velocities by up to 6%.

The evolution of the southern margin of the East European Craton based on seismic and potential field data

This paper presents an integrated geophysical study of the southern margin of the East European Craton (EEC) in the Karpinksy Swell-North Caucasus area. It presents new interpretations of deep refraction and wide-angle reflection “deep seismic sounding” (DSS) data as well as conventional seismic and CDP profiling and new analyses of potential field data, including three-dimensional gravity and magnetic modelling. An integrated model of the physical properties and structure of the Earth's crust and, partially, upper mantle displays distinct features that are related to tectonic history of the study area. The Voronezh Massif (VM), the Ukrainian Shield and Rostov Dome (RD) of the EEC as well as the Donbas Foldbelt (DF), Karpinsky Swell (KS), Scythian Plate (SP) and Precaspian Basin (PCB) constitute the geodynamic ensemble that developed on the southern margin of the continent Baltica. There proposed evolutionary model comprises a stage of rifting during the middle to late Devonian, post-rift extension and subsidence during Carboniferous–early Permian times (synchronous with and related to the southward displacement of the Rostov Dome and extension in a palaeo-Scythian back-arc basin), and subsequent Mesozoic and younger evolution. A pre-Ordovician, possibly Riphean (?), mafic magmatic complex is inferred on a near vertical reflection seismic cross-section through the western portion of the Astrakhan Dome in the southwest part of the Precaspian Basin. This complex combined with evidence of a subducting slab in the upper mantle imply the presence of pre-Ordovician (Riphean?) island arc, with synchronous extension in a Precaspian back-arc basin is suggested. A middle Palaeozoic back-arc basin ensemble in what is now the western Karpinsky Swell was more than 100 km to the south from its present location. The Stavropol High migrated northwards, dislocating and moving fragments of this back-arc basin sometime thereafter. Linear positive magnetic anomalies reflect the position of associated faults, which define the location of the eastern segment of the Karpinsky Swell. These faults, which dip northward, are recognised on crustal DSS profiles crossing the Donbas Foldbelt and Scythian Plate. They are interpreted in terms of compressional tectonics younger than the Hercynian stage of evolution (i.e., post-Palaeozoic).

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.

Helium isotopes, tectonics and heat flow in the Northern Caucasus

109 new measurements of 3He/ 4He ≡ R in subsurface fluids of the Northern Caucasus coupled with the data obtained previously allow regional regularities in the distribution of helium isotopic composition to be examined. Cis-Caucasian foredeeps show the lowest radiogenic R-values. The average R av-value is slightly higher in gases of the Scythian plate beyond the Stavropol arch. Within the arch, elevated R = (1.6–4.5) × 10 −7 indicates an input of mantle-derived helium. This input is even more evident to the south of Starvropol arch, in the Caucasian Mineral Water area, where the ≈8 Ma old laccolithes occur and R-values approach (5–11) × 10 −7. The highest R-values, up to (0.7–0.9) × 10 −5, are observed further to the south, in the central segment of the Greater Caucasus, where recent volcanism is manifested. Enhanced R-values do not correlate with the crustal thickness but reflect degassing of magmatic reservoirs including those yet unknown. According to the recent Sr-Nd-O data, the young volcanic rocks are of mantle affinity but they are contaminated by a crustal component. The average R av-values in fluids and 87Sr/ 86Sr ratios in host magmatic rocks show an inverse correlation suggesting mixing of crustal and mantle materials. R-values vary inversely with apatite fission-track ages of crystalline basement rocks. The ages increase westward of the Elbrus volcano, most likely recording the thermal degradation of the Greater Caucasus since the pre-Cainozoic magmatic activity. A direct correlation between R av-values and background conductive heat flow densities implies that discharge of the mantle melts into the crust is the common cause of the geochemical, geochronological and geothermal regularities observed. Elevated R-values are generally observed in CO 2-bearing fluids, low values are typical of CH 4 gases, a few N 2-rich gases display highly variable R. Relationships between the major gas constituents and noble gas isotopes are discussed. Fractionation, loss, and gain of these species are considered as the processes controlling the compositions of underground fluids.

Evolution of the North Caucasus foredeep: constraints based on the analysis of subsidence curves

Using a database of more than 130 wells the Alpine evolution of the North Caucasus foredeep can be described in three main periods. (1) The Early Jurassic to Middle–Late Cretaceous (including the Cenomanian) relates to the initial rifting phase and was characterised by succession of comparatively high rates of subsidence and uplift. During this stage, many events in the eastern and western parts were synchronous, whereas some of them appeared to be smoothed in the Stavropol high. The southern border of the area had a more complex behaviour often moving in the opposite direction relative to the rest of the area. The main peculiarities of evolution of the Great Caucasus region can be explained if we adopt the hypothesis of Stamply and Pillevuit (1993)and suppose that the Early Jurassic extension of the trough was accompanied by a strong left-lateral transform movement. As a result the central part of the Great Caucasus trough formed as a pull-apart basin. Analysis of the style of movements of the southern border of the Scythian plate as well as data on tectonics and volcanism in the Great and Lesser Caucasus showed that other regional compressional and extensional Mesozoic events could also have a transform component. Shear stresses within the lithosphere of the Great Caucasus can be due to oblique subduction or even transform movements at the plate boundary to the south of the Caucasus region ( Dercourt et al., 1993). (2) The period from the Late Cretaceous to the Middle Eocene (from the Turonian to the Bartonian) relates to the oceanic suture of the Lesser Caucasus and was characterised in the Great Caucasus area by alternating subsidence and uplift events of considerably lower amplitude (at least up to the Maastrichtian). Beginning in the Late Paleocene, subsidence curves for almost all the area reflect the same events, but in the western part and also in the north of the central part the rate of movements was considerably higher and since the Late Paleocene short term events in this area took place on the background of rather fast subsidence at nearly constant rate. We believe that this subsidence and formation of the East Black Sea depression have the same origin. We consider strong differences in evolution of the eastern, western and central parts during the second stage to be due to closure of the Lesser Caucasus oceanic basin and arrival of the Nakhichevan block. This led to changes of configuration of the plate boundary and resulted in reorganisation of stress and displacements within the Caucasus region. This reorganisation was a reason for the opening of the East Black Sea depression and rapid subsidence of the western and central parts of the area at the end of the second stage. (3) The period from the Middle Eocene to the present relates to the development of a foreland basin coeval with shortening and uplift in the adjacent Great Caucasus range. It was characterised by alternation of relatively short uplift and longer subsidence events. An important feature of this stage is that although the amplitude of movements varied from place to place, these events were synchronous in all parts of the foredeep (at the Stavropol region sediments preserved only for the beginning of this stage). Formation and evolution of the foredeep during the third stage can not be explained exclusively by elastic flexure. To do this we used the model of a small-scale convection within the asthenosphere ( Mikhailov et al. 1996, 1999). By the comparison of numerical results with the data on the evolution of the North Caucasus foredeep we concluded that there were four main stages of compression in the processes of formation of the Great Caucasus mountain belt. The first compression took place before the Maykopian (between 39.5 and 36.0 Ma). The other three were in the Tarkhanian (16.6–15.8 Ma), Konkian–Early Sarmatian (14.3–12.3 Ma) and Pontian (7.0–5.2 Ma). The different width of the Great Caucasus trough by the beginning of the compression, as well as variations in thickness of the lithosphere and a different thermal state can cause interruption of the formation of the foredeep at the Stavropol high.

A geological interpretation of the gravitational and magnetic fields of the northern and middle Caspian, based on estimates of their ratios

A geological interpretation of the gravitational and magnetic fields is a method based on Poisson's relation between the gravitational and magnetic potentials. Within the northern and middle Caspian there are parts of the crust with different types of spatial relation between the density and magnetic-action boundaries, corresponding to different structural-tectonic elements of platforms of different ages. The authors locate the southern boundary of the East European Platform; they distinguish the Scythian and Turanian plates: in the Paleozoic base of the Scythian plate they distinguish a graben of the axial zone and a limb of the Donets- Caspian fold system; they determine the position of the upper Paleozoic downwarps filled with volcanic-clastic strata at the edges of the Rostov -Kochubeyev zone of the pre-Permian basement and on the slopes of the middle Caspian massif. —Authors.