Deep Seismic Sounding Data Research Articles

New Ideas about the Structure and Nature of the Crust of the Western Bay of Bengal, Obtained Taking into Account Deep Seismic Sounding Data

New Ideas about the Structure and Nature of the Crust of the Western Bay of Bengal, Obtained Taking into Account Deep Seismic Sounding Data

Contrasting lithospheric geophysical structure of the Black Sea subbasins: Relevance to testing geotectonic models for this mega depression

We present an integrated analysis of the geophysical parameters of the lithosphere of Black Sea basin obtained as a result of the interpretation of magnetic, gravity, thermal, deep seismic sounding and seismic-tomographic data. It first demonstrates inherent significant differences in geophysical parameters of lithosphere in the Western and Eastern Black Sea subbasins existing from the prerift stage. The set of reviewed parameters responsible for formation of the present-day lithosphere includes types of crust, depths to acoustic basement, configuration of subbasins, depths to Moho, heat flow and relief of the thermal-asthenosphere boundary (LAB), trends of main deep faults of the crystalline crust, their kinematic types, occurring linear magnetic anomalies, velocity pattern of subcrustal mantle. The above-mentioned parameters of the lithosphere are indicators of the age, geodynamics and driving mechanisms for opening of a subbasins. Oblique trends of the subbasins and the topography of Moho discontinuity in the west and east domains, oblique striking of pre-rifting Istanbul zone and the Shatsky Ridge and different trends of deep faults in the crystalline crust suggest distinct lithospheric structure existing from pre-opening of the Black Sea subbasins and different geodynamical conditions of its formation. The Odesa-Sinop-Ordu deep fault zone as a direct continuation of the Golovanivskaya suture zone of the Ukrainian shield and its slope, probably is of the Precambrian age. So it could be the tectonic boundary between two segments of pre-rift continental crust and between future subbasins. The examples illustrate how indicated parameters of the lithosphere can geophysically confirm the basic ideas of available models for geodynamics of the Black Sea.

SEISMIC INHOMOGENEITIES IN THE UPPER CRUST OF THE ALDAN-STANOVOY SHIELD AND IN ITS TOTAL CRUST (PROFILE 3-DV)

In accordance with the detailed first-arrival data, the near-surface part of the Earth’s crust consists of three layers. The upper discontinuous layer is represented by the Mesocenozoic deposits in local basins. The two inhomogenous layers decrease in thickness towards the north from 1.5 km to total thinning in the Stanovoy block and from 4 km in the Chulman basin to 1 km in the Aldan. It is implied that the nature of thinning lies in weathering and disintegration of crystalline rocks. Their underlying boundary at a depth of 1–4 km with a longitudinal wave velocity of 6.0–6.2 km/s can be assigned to the unchanged Early Precambrian basement surface.The deep seismic sounding data show that the two-layer crust thickness of the Stanovoy block reaches 40 km, and the three-layer crust in the Aldan block is as thick as 50 km. These blocks are separated by the vertical zone beneath the Stanovoy ridge with contrast inhomogeneities in the crust and step-like increase in the Moho depth beneath the Chulman basin.There was discovered an isostatic imbalance of the lithosphere, correlated with seismicity of the Chulman basin.

Characterizing the shallow structure with the multimodal dispersion curves and the body wave refraction traveltimes from deep seismic sounding data

Deep seismic sounding (DSS) profiles are one of the most powerful tools for detecting crustal structures, and they have been deployed worldwide. Generally, the analysis of DSS data mainly focuses on body waves, while the surface waves are considered noise. We suggest that the surface waves in DSS data can be used to constrain subsurface structures. In this study, we use a DSS profile in the Piedmont and Atlantic Coastal Plain as an example to present the usage of the DSS surface wave. Multimodal dispersion curves were extracted from the DSS data with the Frequency-Bessel transform method, and were used in Monte Carlo joint inversions with body wave refraction traveltimes to constrain the shallow structures. Through the inversion, a horizontal stratum on the surface was identified in the Piedmont, and a two-layer sedimentary structure was identified in the Atlantic Coastal Plain. Comparisons with existing studies verified the accuracy of the shallow structures obtained in this study, demonstrating that the shallow velocity structure could be well constrained with the additional constraints provided by the multimodal dispersion curves. Thus, we believe that further research on the surface waves recorded in DSS surveys is warranted.

The inner structure and kinematics of the Zvizdal-Zalisk and Brusyliv fault zones of the Ukrainian shield by the results of tectonophysical, magnetometrical data

In 2021, the tectonophysical measurements of cracking, structural-textural elements of rocks near Malyn, Radomyshl, Pohrebyshche, Buky, Kashperivka and magnetometric field studies of Zvizdal-Zalisk dyke were carried out. And also the deep seismic sounding data along II and VI geotravers were analyzed in order to purpose to finding out of the internal structure and kinematics of Zvizdal-Zalisk and Brusyliv fault zones of the Ukrainian Shield.
 According to our tectonophysical data, the studied rocks were formed under landslide deformation processes with alternating sublatitudinal and submeridional compression regimes, which correspond to the youngest Subbottsi-Moshoryn stage of faulting formation within the Ukrainian Shield (1.80—1.77 Ga), with subhorizontal axes of compression σ1 — 315° and tensile σ3 — 45°. Cracks and bands of gneisses of moderate dip are also widely developed in the study area. Established, that it has deformation throw regime with σ1 — 100/85°, σ3 — 280/05°, σ2 — 10/02°. Authors of article associate this deformation with formation of the main breeds Zvizdal-Zalisk dikes. Deformations of the Nemyriv faulting stage (1.99 Ga) were fixed at the intersection node of the Zvizdal-Zalisk faulting zone with Nemyriv fault zone (1.99 Ga). Stress field is σ1 — 301°, σ3 — 31° at the intersection node the Brusyliv fault zone with Nemyriv fault zone and it repeat main deformation strains of the Novograd-Volyn and Uman massifs (~2.05 Ga).
 By the data of the deep seismic sounding along II and VI Geotraverses, the Zvizdal-Zalisk fault zone enters the mantle all along, in the north and in the south. The crust structure in the study area has two layers of upper crust (VР=5,9–6,4 km/s),with intermittent low- (5,8—6,2 km/s) and high-velocity (6,3—6,9 km/s) horizons, the middle crust (6,4—6,9 km/s), and lower crust (6,9—7,0 km/s) that has a wavy dome-like shape with high differentiated velocity picture differing on both sides of the Central faulting area. The Bug megablock has a dome-like structure along all VI Geotraverse and is divided in half by the Zvizdal-Zalisk fault zone at the Moho discontinuity according to the data of the tomographic inversion along the velocity change gradient.
 By the magnetometric data, the Zvizdal-Zalisk fault zone lies almost fully within the regional minimum of the magnetic field (∆В)а,reg, which encompasses areas of the crust with minimum values of magnetization. The dyke was formed at the same time of the Korosten pluton formation and at activation of the Zvizdal-Zalisk fault zone. Its small depth confirms its crust origin; the variable sharp incline of the dyke’s parts is evidence of its subvertical falling which corresponds to the vertical position of the Zvizdal-Zalisk fault zone.
 The data on the chemical-mineralogical composition and position of mineral deposits suggest that the main regional structures which have influenced the metallogenic specialization of the region are the Zvizdal-Zaliska, Brusyliv, Nemyriv, Khmelniksk, Central, and Sarnensko-Varvarivka fault zones.

Nature of the Kergelen Plateau and Its Place in the Structural Plan of the Southern Sector of the Indian Ocean

— The morphostructural characteristics of the main taxa of southern Indian Ocean, such as the Del Cano-Crozet, Conrad, and Kerguelen plateaus, are given based on structural-tectonic analysis. A depth map of the acoustic basement of the eastern part of the Afro-Antarctic megadepression is first built using the digitized NGDS database on the thickness of the sedimentary cover of the Indian Ocean, which is available on the Internet. A critical analysis of deep seismic sounding (DSS) data on the Kerguelen plateu hs been performed. A geochemical analysis of more than 100 samples of basalts, acidic rocks, and rocks of sedimentary origin, collected on the islands of the Kerguelen archipelago and raised by dredging and deep-sea drilling from the Kerguelen and Conrad plateaus, is carried out. The study has shown that the plateaus resemble the Madagascar Ridge in terms of morphostructure and a number of other features. It is concluded that these flat-topped horst massifs were the part of the “structural bridge” that connected Madagascar and the eastern part of mainland Antarctica no later than the Paleogene. A proposed alternative version of the geological interpretation of the deep seismic data states that the plateau has a relatively homogeneous structure along the strike, including a thick (at least 15-km) layer of continental crust. An analysis of the depth map of the acoustic basement made it possible to identify the depocenters of the relatively isolated Enderby, Shackleton and Labuan basins framing the Kerguelen Plateau on the southern side. The paragenetic connections of the Kerguelen Plateau with the basins indicate that the isolation of the Kerguelen Plateau from East Antarctica and the formation of the modern structure of the region began in the Late Cretaceous and ended with the last phase of tectonic activation in the Late Miocene–Pliocene. The results of geochemical analysis show that the geochemical and petrological anomalies of the plateau and islands are associated with the crust, and not with the mantle contamination, as well as with the continental nature of the Kerguelen Plateau.

Technique for the imaging crystalline basement according to the DSS data

The method of deep seismic sounding (DSS), the observation systems in which are characterized by an irregular arrangement of both sources and receivers along the profile, a significant step between receivers, as well as maximum source-receiver distances exceeding several hundred kilometers, makes it possible to obtain an image of the crystalline basement using seismic migration fields of reflected/refracted waves. The main part of the existing migration methods, the use of which makes it possible to form an image of the deep structure of the study area in the dynamic characteristics of the recorded wave field, is focused on processing seismic data obtained by the method of reflected waves with multiple overlap observation systems (MOV—CDP). And, as a rule, these migration methods are designed for a smooth change in speed with depth. At the same time, at the boundary of the crystalline basement, the speed changes very sharply, which must be taken into account when processing data using migration. The proposed method for constructing an image of the crystalline basement is based on the use of finite-difference migration of the field of reflected/refracted waves, which was developed at the Institute of Geophysics named after S. I. Subbotin National Academy of Sciences of Ukraine. This migration method is designed to isolate supercritically reflected and refracted waves recorded from the basement in the far zone of the source and takes into account the full trajectory of waves passing through a two-layer medium, at the boundary of which there is a significant jump in velocity. Thus, the migration of the field of reflected/refracted waves makes it possible to obtain a correct image of the structure of the refractive layer of the crystalline basement. The article describes in detail the algorithm of the technique for constructing an image of the crystalline basement using finite-difference migration of the field of reflected/refracted waves and its difference from similar methods of migration. The advantages and disadvantages of the proposed method are shown when solving problems of regional seismic research. Explained and illustrated the features of constructing the image of violations on the border of the foundation. The effectiveness of the technique is demonstrated on a model example and real seismic data observed by the DSS method on the territory of Ukraine.

Basin architecture and lithosphere structures of Western Central Asia – Uzbekistan: from geophysical studies

This paper presents an integrated geophysical study of the Western Central Asia (WCA) in the Uzbekistan area. It presents new interpretations of deep seismic sounding (DSS) data and new analyses of potential fields data. An integrated model of the physical properties and lithosphere structures displays distinct features that are related to tectonic history of the study WCA-Darius area. Task-oriented complex geological and geophysical research revealed in the lithosphere of western Uzbekistan series of crust blocks with abnormal petrophysical characteristics and established some correlation dependences between the distribution and placement of these objects (in plan) with deposits of minerals.

The crustal structure of the transition from the East Black Sea Basin to the Shatsky Ridge from the reinterpretation of deep seismic sounding data on profiles 14-15-16

The paper represents the results of velocity modeling (ray-tracing modeling) performed for three deep seismic sounding (DSS) profiles 14, 15 and 16 acquired in the eastern part of the Black Sea more than 40 years ago. These profiles represent a system of radial profiles diverging from one common shot point in the East Black Sea Basin (EBSB) and crossing the Shatsky Ridge. The performed modeling showed that thin (~10 km) crystalline EBSB crust, with velocities increasing from 6,5 km/s in the basement to 7,0 km/s on the Moho (20—22 km), is overlain by sediments as thick as ~10 km. The continental crust of the Shatsky Ridge of ~30 km thickness comprises two layers — the upper crust (Vp=6,0÷6,5 km/s) as thick as 15 km and the 10 km thick lower crust (Vp =6,5÷7,0 km/s). The transition from thin EBSB suboceanic crust to the Shatsky Ridge continental crust occurs rather sharply, over an interval of ~25 km, where changes are observed in all crustal layers — from sediments to the Moho. The transition of the two types of the crust is ascribed by a lineament, parallel to the coastline of the eastern part of the Black Sea, and is associated with a collinear Alushta-Batumi magnetic anomaly of the same (NW) strike. These features may testify in favor of the tectonic nature of the transition zone, the formation and activation of which took place during the main stages of evolution of the study region — at the closure of Mesozoic Tethys ocean, during the riftogenic opening of the EBSB in Cretaceous, and during the Alpine orogeny in the compression setting. The wedge-like shape of the EBSB, expanding southeastward up to 160—180 km width, is consistent with the concept of riftogenic opening of the EBSB in the Early Cretaceous as a result of clockwise rotation of the Mid Black Sea Ridge.

Selection of deep, energetically weak waves in seismic DSS records

To solve direct problems of seismic, namely, to construct velocity DSS models we need to have a maximal number of hodographs of reflected waves from geological borders. The strongest reflections occur at a short distance from the explosion point. The more is the distance, the smaller the amplitude of vibrations of seismic reflected waves and more difficult their selection from the wave field. The object of the paper is a proposal of the ways for increasing information value of DSS data by selection of small amplitude reflected waves and their hodographs from the borders which occur at deep depths. The basis of the method is a difference method of spatial selection of hodographs of the target waves by the calculated or arbitrarily chosen direction. As a result the initial wave field is subdivided into the field with noise-waves of different origin and the wave field of efficient signals.Working capacity of the method is demonstrated on the model temporal seismic sections obtained with the help of the program of integral-wave modeling Tesseral 2D. As a field material the data of deep seismic sounding (DSS) were used, obtained during a fulfillment of the marine part of DOBRE-2 project. For the Black Sea the hodographs of the waves reflected from the basement were selected overlapped by various noise-waves from the upper part of the seismic section. For the Sea of Azov an area was processed situated at the distance of 130-150 km from the source of seismic vibrations. Attenuated wave reflected from the Moho border up to the depth of 45 km was hidden by the noise-waves, and so for selection of the target reflected wave calculated direction of the hodograph was used. Application of the proposed method allows an increase of the amount of hodographs applied for construction of velocity models that in its turn increases the significance of solving direct and inverse problems.

One-Dimensional Velocity Model, Station Correction and Earthquake Relocation of Local Earthquakes in the Koyna–Warna Region, India

In this paper we investigated the feasibility of using the higher-frequency waveforms to characterize the seismicity in the Koyna–Warna region, globally well-known for its reservoir-triggered earthquakes. The local seismicity and source parameters have been extensively explored by many researchers using various types of data, e.g. analog data, broadband seismological data, and deep seismic sounding data. However, here we have utilized the seismological data recorded by 97 temporary 4.5-Hz geophones to locate the local seismicity in the region. We first investigated an appropriate 1-D P-wave velocity model through the inversion of P- and S-times. In order to compensate for the thick and laterally heterogeneous basalt, we incorporated the station corrections in the 1-D velocity inversion process. The station correction varies from + 0.09 to − 0.20 s for the P-waves, with clear demarcations of positive and negative station corrections corresponding to the northern and southern regions, respectively. The patterns of positive and negative station corrections suggested that the northern part is characterized by lower velocity, while slighter higher velocities present in the southern part of the study area are relative to the estimated 1-D velocity model. The earthquake locations are further improved by using the double-difference (hypoDD) approach, which enabled us to accurately map shallow subsurface seismogenic faults in the region. The data comprises 10,062 P-wave and 9049 S-wave catalog differential times from 348 earthquakes (1.0 ≤ ML ≤ 4.5). The precisely relocated earthquakes (ERH < 0.05 km ERZ < 0.5 km and rms < 0.02 s) form the three clusters of earthquakes in the NW–SE direction along the lineaments close to Warna Reservoir. The observed seismicity trends on the surface could be due to the vertical manifestations of faults or fractures. Their alignments are confirmed by the analysis of focal mechanism which exhibits a predominant normal type of faults with one nodal plane consistent with the main alignment of the seismic events.

Crustal structure of the Gunsan Basin in the SE Yellow Sea from ocean bottom seismometer (OBS) data and its linkage to the South China Block

Two major tectonic units in NE Asia are the Sino-Korean (or North China) and South China Blocks that collided in the Permo-Triassic periods. The South China Block is suggested to extend eastward to the Korean Peninsula across the southern Yellow Sea. The Gunsan Basin is a well-defined, fault-bounded sedimentary basin in the SE Yellow Sea and is regarded as a structural link of the South China Block to the Korean Peninsula. We collected deep seismic sounding data recorded on ocean bottom seismometers to investigate how the crustal structure and nature of the South China Block vary in the Gunsan Basin toward the middle part of the Korean Peninsula. The main part of the Gunsan Basin is underlain by relatively thin (∼10 km thick) upper and much thicker (∼20 km thick) lower crustal layers, indicating a close affinity to the South China Block; whereas it shows distinction from the Korean Peninsula underlain by upper and lower crustal layers with equal (∼15 km) thickness. A change in crustal structure is recognized under the eastern margin of the Gunsan Basin toward the Korean Peninsula, that features a transition to the Korean Peninsula. The thick lower crustal layer in the Gunsan Basin appears to be inherited from the evolution of the South China Block including crustal shortening associated with the assembly of Rodinia in the Neoproterozoic before the collision between the Sino-Korean and South China Blocks.

Spatial Distribution of Crustal Earthquakes in the Caucasus

Previous results on the location of crustal earthquake foci in the Caucasus clearly show that the methods used in seismic service practice to determine the hypocenters of earthquakes do not allow accurate localization of events and can hardly be used to forecast earthquakes or study the structure of the Earth’s crust and upper mantle in detail. The arrival times of seismic waves often have large errors that prevent qualitative processing of observations and, therefore, is is difficult to obtain reliable results. In addition, the use of only one velocity curve or one set of travel-time curves for all stations of the network hinders accurate determination of the location of earthquake hypocenters. In order to localize earthquake foci with high precision, it is also necessary to have a sufficiently detailed representation of the structure of the Earth’s crust and upper mantle in the studied region. It is also desirable to take information about the structure of the region not from earthquake data, but based on other observations, e.g., deep seismic sounding (DSS) data. Using seismological bulletins for 1970–2015, we have redetermined the hypocentral coordinates of crustal earthquakes in the Caucasus. The new distribution of crustal earthquake hypocenters in the Caucasus differs significantly from the old distribution obtained from the catalog. This may become the basis for refining ideas about the regional geodynamics.

Underground hydrosphere and geofluids of the Earth’s crust and upper mantle

The physicochemical state of the water at various temperature and pressure conditions of the Earth’s interior has been characterized. Two groups of geofluidic systems for water have been established: subcritical ( T 374 ¸ 450 °C), which together with the hydrogeological system of structured waters constitute hydrophysical, hydro- and fluid-geological lithosphere zonation. Based on the systematization of data on the decrepitating of gas-liquid inclusions in minerals of rocks of various facies of metamorphism, as well as fluid manifestations in zones of global rifting, a high content of mineral substances in the composition of dense supercritical fluids and a high electrical conductivity of such fluids is established. Recognition of high “salt content” and high electrical conductivity of supercritical fluids opens new possibilities in interpreting local high-speed anomalies and anomalies of electrical conductivity in the Earth’s crust, revealed from deep seismic and magnetotelluric sounding data. It is shown that with the water condensate — a kind of liquid-phase “restite” from the sphere of supercritical fluids separation — the genesis of highly mineralized waters and brines is connected in the crystalline basement of ancient platforms. On the example of the lithosphere of Belarus, for the first time, deep boundaries of localization of both structured water and supercritical geofluids have been determined. The hydro- and fluid-geological zonation of the lithosphere and upper mantle has been established as a basis for solving the scientific and practical problems of tectonophysics, mineralogy, hydrogeology, genesis and distribution of different type’s minerals, including hydrogenogenic minerals. A thermophysical method for determining the depth and thickness of a zone of underground structured waters is proposed. The method is based on comparison of seismic and thermal characteristics of rocks with sub- and supercritical temperatures for water.

The Crustal Structure of the North–South Earthquake Belt in China Revealed from Deep Seismic Soundings and Gravity Data

The North–South earthquake belt (NSEB) is one of the major earthquake regions in China. The studies of crustal structure play a great role in understanding tectonic evolution and in evaluating earthquake hazards in this region. However, some fundamental crustal parameters, especially crustal interface structure, are not clear in this region. In this paper, we reconstructed the crustal interface structure around the NSEB based on both the deep seismic sounding (DSS) data and the gravity data. We firstly reconstructed the crustal structure of crystalline basement (interface G), interface between upper and lower crusts (interface C) and Moho in the study area by compiling the results of 38 DSS profiles published previously. Then, we forwardly calculated the gravity anomalies caused by the interfaces G and C, and then subtracted them from the complete Bouguer gravity anomalies, yielding the regional gravity anomalies mainly due to the Moho interface. We then utilized a lateral-variable density interface inversion technique with constraints of the DSS data to invert the regional anomalies for the Moho depth model in the study area. The reliability of our Moho depth model was evaluated by comparing with other Moho depth models derived from other gravity inversion technique and receiver function analysis. Based on our Moho depth model, we mapped the crustal apparent density distribution in the study area for better understanding the geodynamics around the NSEB.

Geoelectric section of the crust and upper mantle of the northern Sikhote-Alin from magnetotelluric sounding data

Magnetotelluric soundings (MTS) were conducted in a broad frequency range of 10 kHz to 0.001 Hz at a total of fifty-seven sounding sites of the profile spaced 5 km apart and intersecting the northern Sikhote-Alin across the strike. The analysis of the obtained magnetotelluric parameters has been made which shows three-dimensional geoelectric nonuniformities in the lower crust and upper mantle. The MTS curve interpretation was carried out in the framework of a three-dimensional model. As a result of the inverse problem solution, the geoelectric section has been constructed down to 150 km depth. The section distinguishes the crust with a resistivity higher than 1000 Ohm m and variable thickness between 30 and 40 km which is consistent with deep seismic sounding (DSS) data. The crust is subdivided into four blocks by deep faults, and each block is characterized by a set of parameters. The data support the existence of the Vostochny deep fault in the study area, whereas, on the contrary, the deep roots for the Central Sikhote-Alin fault have not been established. The upper mantle structure is nonuniform; three low-resistivity zones are identified that coincide with the boundaries of crustal blocks. In the revealed zones, an increase in the resistivity is noted from the continent to the Tatar Strait coast. A high-resistivity layer of 300–400 Ohm m was observed in the coastal area, which was steeply dipping from the crustal base down to 120 km depth and extended beneath the continent. Based on a set of geological and geophysical data, the ancient subducting plate is suggested in this area, and the evolutionary model of the region is proposed starting from the Late Cretaceous. The most probable mechanism of conductivity within the upper mantle is determined from petrological and petrophysical data. The low resistivity values are linked to dry peridotite mantle melting.

Seismically Derived Gondwana and Proterozoic Sediments East of Cuddapah Basin, South Indian Shield and Its Possible Geotectonic Implications

The study area situated east of the intracratonic Proterozoic Cuddapah basin, that covers Nellore Schist belt, Ongole domain of the Eastern Ghats Belt and adjacent East Coast Terrain, forms a geodynamically important segment of the Dharwar craton. This region came into focus due to proposition that during Paleoproterozoic to Mesoproterozoic era, it underwent persistent oceanic subduction, continent–continent collision and multi-stage accretional growth. In view of this, we have reprocessed in detail the deep seismic sounding data over this region and integrated the obtained results with other geophysical studies to decipher the deep subsurface crustal structure underneath. The derived shallow seismic section across this region showed the presence of a thin layer (~250 m) of Gondwana sediments with a distinct velocity of 4.20 km/s and a lateral dimension of about 40 km around Vinjamuru region of the Nellore Schist Belt. Another important finding has been the depiction of 5.3 and 5.5 km/s velocity sediments (comparable to upper and lower Cuddapah sediments) between Nellore Schist Belt and the east coast, indicating the possible presence of an additional Proterozoic sedimentary terrain (named here as East Coast Sedimentary Basin), overlying the concealed Archean crystalline basement. The presence of this sedimentary terrain, which is separated from the Cuddapah basin by an exhumed horst-like feature, situated below Nellore Schist Belt, would indicate Proterozoic sedimentation in a much larger area during Columbia and Rodinia supercontinental assembly periods than hitherto known. Importantly, no seismic trace of the Eastern Ghats Belt has been found between Nellore Schist Belt and the east coast. It appears that during the Gondwana period, entire eastern segment of the east Dharwar craton has been persistently rifting, uplifting and eroding together with other Gondwana grabens in India, thereby leaving behind only a thin veneer of Gondwana sediments at many places. In view of the presence of number of Gondwana occurrences all along the east coast, it is felt that the rifting during the Gondwana period may have been a prelude to India–Antarctica breakup, with final separation between the two taking place much later during early Cretaceous.

Subcrustal structure of the black sea basin from seismological data

The P-wave travel time data from the earthquakes offshore and onshore around the Black Sea are used for the tomographic reconstruction of the three-dimensional (3D) velocity distribution in the lithosphere of the region. The preliminary refinement of the foci parameters (the coordinates and origin time) has reduced the random errors in the travel-time data. The earthquake data were supplemented by the previous deep seismic sounding (DSS) data on the profiles in Crimea and offshore off the Black Sea. The dataset included more than 4000 travel times overall. In order to eliminate the crustal effect, the travel times were reduced to a surface at a depth of 35 km corresponding to the mean Moho depth in the region. The improved crustal model was used for removing the contribution of the crust from the initial data. The new tomography method, which was recently developed by one of the authors and which relies on the assumption of smoothness of the lateral velocity variations, was applied for reconstructing the velocity structure of the upper mantle beneath the Black Sea up to a depth of 95 km. The lateral velocity variation maps at different depths and the vertical velocity distributions along the meridional and sublatitudinal cross sections across the Black Sea were constructed. High velocities were revealed in the subcrustal lithosphere, and the structural difference below two subbasins—the West Black Sea (WBS) and the East Black Sea (EBS) ones—was established. It shows that the high-velocity body below the WBS is located deeper than below the EBS and is distinguished by higher velocities. Based on these results, it is concluded that the lithosphere beneath the Black Sea has a continental origin.

Seismic imaging across the Eastern Ghats Belt-Cuddapah Basin collisional zone, southern Indian Shield and possible geodynamic implications

The evolutionary nature of the area encompassing Cuddapah basin, Nellore Schist Belt and the Ongole domain of Eastern Ghats Belt (south India), which contains Paleo-Neoproterozoic imprints of thick sedimentation, intense magmatism, subduction/accretion and possible collision between eastern Dharwar craton and east Antarctica, is still being debated. An attempt has been made here to decipher the deep crustal structure and tectonics of this region, by reprocessing Deep Seismic Sounding data acquired earlier through the modeling of first arrival refraction and wide angle reflection travel times. The derived crustal seismic velocity structure reveals that the intracratonic Proterozoic Cuddapah basin containing only 4km thick sediment is bounded by two major faults. The Moho reaching fault detected on its eastern boundary, demarcates Cuddapah basin from the Nellore Schist Belt. Although a deep normal fault is mapped within the Nallamalai Fold Belt, but no thrusting signatures are apparent. Almost all the crustal layers beneath Nellore Schist Belt and the Ongole domain of Eastern Ghats Belt, show distinct eastward dipping trend conforming to an upthrusted feature, suggesting possible presence of a collisional suture. Besides, the areas lying east of Cuddapah Basin appear to be an accreted orogenic terrain, beneath which lower crust has upwarped substantially. The entire stretch of the studied region is underplated by unprecedently thick (∼20km) high velocity (7.0-7.4km/s) magma layer above the Moho, indicating strong crust-mantle thermal perturbation and massive subcrustal erosion. Further, an expression of a deep seated mantle thermal anomaly has also been found below the Parnapalle region of the SW Cuddapah basin beneath which deeper crustal layers have exhumed.

Geodynamic features of joint zone of the Eurasian plate and the Alpine-Himalayan belt within the limits of Ukraine and adjacent areas

Based on the analysis of geophysical and geological data of recent 10—15 years the boundaries and inner structure of the transition zone from the Eurasian plate (EAP) to the Alpine- Himalayan folded belt (AHB) extending from the Eastern Carpathians to the Great Caucasus were defined here. DSS (deep seismic sounding) data of the International projects EUROBRIDGE, DOBRE, CELEBRATION 2000 and the results of reinterpretation of earlier DSS profiles along the Black Sea region were used. Results of seismic tomography studies, fulfilled by the Institute of Geophysics of NAS of Ukraine within the Eurasian part of the study area, were geologically reinterpreted. Paleogeodynamic studies and paleogeographic reconstructions, performed by the Western, Ukrainian and Russian scientists, were also involved. It is shown that velocity and density characteristics of the EAP and AHB lithosphere, that is their buoyancy, do not contradict the possibility of subduction of lithospheric plates under the EAP as well as under the AHB. In the upper mantle the sloping layers 150—200 km thick and 200—750 km long were revealed, corresponding to the areas of lithospheric plates (slabs), being submerged into the asthenosphere during different time, since the Permian-Triassic period. Almost everywhere the beginning of these slabs at the surface coincides with known and supposed, according to geological data, suture zones. Due to divergent and convergent processes, the junction zone of the EAP and AHB in the south changed its position within the band (on modern geographical basis) from the Northern Donbas to the middle part of the Black Sea basin. It has been geophysically proved that the Scythian and Rava-Russian epiorogenic zones were formed as parts of the EAP, though later they were separated from it.