Crustal Structure of the Baltic Shield Along the Pechenga–Kostomuksha–Lovisa Geotraverse

Results of geologic and geophysical modeling are presented, based on detailed seismic studies along two profiles—Pechenga-Kostomuksha and Lieksa-Lovisa. Density, geothermal, magnetic, and geoelectric models were obtained from the interpretations of various geophysical fields and correlated with the reference seismic sections. All the models were combined in order to compile a geologic-geophysical crustal section. The crustal thickness along the Pechenga-Kostomuksha-Lovisa geotraverse varies from 38 to 65 km. Two anomalous structures have been observed that are referred to as the Belomorian-Karelian and Ladoga-Bothnian zones. These zones are characterized by enhanced values of magnetic fields, presence of seismic foci and wave attenuation, and variation of the depth and magnitude of modern crustal movements. These zones are distinguished by the discontinuity M reconstruction, an increase in transitional layer thickness (to 25 km) at the base of the crust, and an increase in depth down to the discontinuity M (50 to 65 km). On average, the crust is thinner (40 km) in the ancient part of the shield than in the younger Svecofennian province (45 km). The velocity differences also are important: for example, the crust of the ancient shield is characterized by lower velocities and the transitional high-velocity layer is absent or thinner. The Karelian granite-greenstone area (a fragment of the Archean craton) has the most simple and balanced deep structure. Within the Karelian area, the layers are nearly horizontal and their thickness is rather constant. The northeastern part of a fragment of the Murmansk block has similar crustal characteristics within the Kola area, where it has undergone Early Proterozoic deformation. Geological and geophysical data for the Pechenga-Varzuga zone suggests that there was intracontinental rifting and a subsequent construction regime during the Svecofennian orogeny that involved a considerable part of the shield. The deep-crustal structure is more complicated to the south. An increase in volume of material with the properties of granulites and basic rocks is observed in the upper crust. The rocks form an inclined alternation of high-density and high-velocity plates and lenses. The packet of tectonic clustering of supracrustal rocks is most conspicuous in the Lapland-Kolvitsa granulite belt. The packet thickness does not exceed 13 km.     

Deep Structure and Geodynamics of the Black Sea–Caspian Region

Geotectonics - New data on the crust structure of the Black Sea?Caspian region, including the Scythian and Anatolian plate margins, the Caucasus, Black Sea and Southern Caspian structures are...     

Structure and Composition of the Holocene–Pleistocene Sediments in the Northern Barents Sea

Lithology and Mineral Resources - Signs of extensive migration of gas-bearing fluids from the sedimentary cover into the water column were identified in the northern Barents Sea during Cruises 25...     

Structure of the Khatanga–Lomonosov Fracture Zone according to Seismic Data

Doklady Earth Sciences - Based on the results of geophysical data interpretation, the structural features of the Khatanga–Lomonosov fracture zone (KhLZ), adapted to the junction zone between...     

Giant Quasi-Ring Mantle Structure in the African–Arabian Junction: Results Derived from the Geological–Geophysical Data Integration

Geotectonics - The tectonic–geodynamic characteristics of the North African–Arabian region are complicated by the interaction of numerous factors. To study this interaction, we...     

Structure of the magnetic field at altitudes of 1–1.15 solar radii

An analysis of the characteristics of unipolar structures detected at latitudes from ?40? to +40?, longitudes of 0??360?, and altitudes of 1–1.15 solar radii during the period from May 1996 (the 23rd solar minimum) to October 2000 (the 23rd solar maximum) has been carried out. Synoptic maps of the solar radial magnetic field calculated in a potential approximation are used. The boundaries between unipolar structures with opposite magnetic polarities (“+/?” and “?/+” polarities) form chains extending along meridians at all the considered latitudes and altitudes. Depending on the latitude, the single-peaked distributions of the number of structures found at the lowest altitudes are replaced by double-peaked distributions at higher altitudes. The time variations of the total number of structures are non-monotonic. The growth in the number of unipolar structures begins before the growth in the Wolf number. This indicates that new unipolar structures already appear together with flocculi, preceding the formation of sunspots. It is found that structures with positive field have larger mean sizes that do structures with negative field. The polar field in the northern hemisphere penetrates to middle latitudes of the southern hemisphere. The existence of sets of structures with typical sizes is shown. The sizes of the smallest structures vary little with latitude, but increase slightly with altitude.     

Structure of the Earth’s crust in the northern part of the Barents–Kara region along the 4-AR DSS profile

The 1370 km long 4-AR reference profile crosses the North Barents Basin, the northern end of the Novaya Zemlya Rise, and the North Kara Basin. Integrated geophysical studies including common deep point (CDP) survey and deep seismic sounding (DSS) were carried out along the profiles. The DSS was performed using autonomous bottom seismic stations (ABSS) spaced 10–20 km apart and a powerful air gun producing seismic signals with a step size of 250 m. As a result, detailed P- and S-wave velocity structures of the crust and upper mantle were studied. The basic method was ray-tracing modeling. The Earth’s crust along the entire profile is typically continental with compressional wave velocities of 5.8–7.2 km/s in the consolidated part. Crustal thickness increases from 30 km near the islands of Franz Josef Land to 35 km beneath the North Barents Basin, 50 km beneath the Novaya Zemlya Rise, and 40 km beneath the North Kara Basin. The North Barents Basin 15 km deep is characterized by unusually low velocities in the consolidated crust: The upper crust layer with velocities of 5.8–6.4 km/s has a thickness of about 15 km beneath the basin (usually, this layer wedges beneath deep sedimentary basins). Another special property of the crust in the North Barents Basin is the destroyed structure of the Moho.     

Structure and Variability of the Meridional Overturning Circulation in the North Atlantic Subpolar Gyre, 2007–2017

Doklady Earth Sciences - The vertical structure and interannual and long-term variability of the meridional overturning circulation in the North Atlantic Subpolar Gyre is analyzed. A close...     

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 (m_b ≤ 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.     

Specific Features in the Deep Structure of the Naryn Basin–Baibichetoo Ridge–Atbashi Basin System: Evidence from the Complex of Geological and Geophysical Data

New magnetotelluric data were obtained for the Karabuk profile crossing the Naryn basin–Baibichetoo Ridge–Atbashi basin geodynamic system (Central Tien-Shan). The complex geological–geophysical cross section along the profile provides a good agreement between the surface tectonic structures and the deep geoelectric model. The electric conductivity anomalies revealed as subvertical conductors striking along the flanks of basins may be explained by the zones of dynamic influence of faults and cataclasis of granite.     

Structure of the Soresat Metamorphic Complex,North Sanandaj–Sirjan Zone,northwest Iran

The metasedimentary and granitoid rocks of the Soresat Metamorphic Complex occur along the northern margin of the Sanandaj–Sirjan Zone in northwest Iran. Four different deformational events (D1–D4) are recorded in the Soresat Metamorphic Complex. The D1 and D2 progressive deformation events resulted from north-northeast–south-southwest regional horizontal shortening due to the subduction of Neo-Tethys oceanic lithosphere beneath the Sanandaj–Sirjan Zone. Post-suturing convergence between Arabia and Iran, which resulted in a right lateral-reverse displacement along the suture caused the north-northwest–south-southeast horizontal shortening of D3. D4 is recorded by normal faulting. Andalusite, cordierite and sillimanite (fibrolite) record the thermal peak (with a geothermal gradient >30°C/km). Field and microscopic studies of intruded granitoid rocks in the Soresat Metamorphic Complex divide them into three major groups: (i) syn-deformation (syn-D2) granitic gneiss; (ii) late- to post-deformation (late- to post-D2) granites and granodiorites; and (iii) post-deformation (post-D2) alkali granites.     

The Electrical Conductivity Structure of the Lanping–Simao Basin and its Implications for Mineralization

正Objective The Lanping–Simao Basin in western Yunnan,located in the southeastern margin of the Tibetan Plateau,is tectonically in the transition zone between the Gondwana and Eurasia tectonic domains.It is also the frontier zone of northeastern extrusion of the Indochina Plate towards the     

The migmatite–gneiss complex of the Chuya–Kendyktas sialic massif (Southern Kazakhstan): Structure and age

A migmatite–gneiss complex made up of paraand orthogneisses and crystalline schists with bodies of ultrametagenic tonalites is distinguished in the basement of the northwestern part of the Chuya–Kendyktas Massif. ID-TIMS dating of accessory zircon from orthogneisses and ultrametagenic tonalites in combination with LA-ICP-MS analyses of detrital zircons from garnet–biotite paragneisses showed that the migmatite–gneiss complex was formed after protoliths with an age within 800–770 Ma and completed its evolution in the first half of the Neoproterozoic.     

Redetermination of the Crystal Structure of Vesignieite

Vesignieite, Cu_3Ba(VO_4)_2(OH)_2, crystallizes in space group C_2 / m with a°=1.0270(2), b°=0.5911(1),c°=0.7711(2)nm and β=116.42(3)°. The intensity data were collected with the RIGAKU RASA-IISsingle-crystal four-circle diffractometer. The structure was determined by Patterson and Fourier methods andrefined by the least-square technique to a final R index of 0.051 for 614 independent diffraction points with|F? |>3σ|F?|. The crystal structure analysis shows that vesignieite has a layer structure parallel to (001). The powder diffraction lines were reindexed.     

Composition of Fluids Responsible for Gold Mineralization in the Pechenga Structure of the Pechenga–Imandra–Varzuga Greenstone Belt,Kola Peninsula,Russia

This study presents the first fluid inclusion data from quartz of albite–carbonate–quartz altered rocks and metasomatic quartzite hosting gold mineralization in the Pechenga structure of the Pechenga–Imandra–Varzuga greenstone belt. A temperature of 275–370°C, pressure of 1.2–4.5 kbar, and the fluid composition of gold-bearing fluid are estimated by microthermometry, Raman spectroscopy, and LA-ICP-MS of individual fluid inclusions, as well as by bulk chemical analyses of fluid inclusions. In particular, the Au and Ag concentrations have been determined in fluid inclusions. It is shown that albite–carbonate–quartz altered rocks and metasomatic quartzite interacted with fluids of similar chemical composition but under different physicochemical conditions. It is concluded that the gold-bearing fluid in the Pechenga structure is similar to that of orogenic gold deposits.     

Structure and Evolution of the Belomorian–Severodvinsk Shear Zone in the Late Proterozoic and Phanerozoic,East European Platform

Geotectonics - The tectonics, morphological features, and development stages of the Belomorian‒Severodvinsk shear zone (northwestern part) found in the East European Platform are considered....     

Cretaceous sedimentary–volcanogenic complexes of the Kamchatka Isthmus: Structure,composition and geodynamic formation conditions

New data on the chemical and rare-element composition and age of the rocks referred earlier to the Iruney suite of the Kamchatka Isthmus are received. In the recent structure these rocks compose the structural–strata complexes of the nappe-folded Lesnovsky Rise. Radiolarian analysis data substantiate that the deposits belonging to the Ening series and the middle and upper parts of the Iruney suite were formed in a single sedimentation basin in the Campanian time. The discovery of a new occurrence of Prunobrachidae representatives on the Kamchatka Peninsula allows us to draw wide interregional correlations and reconstruct the sedimentation conditions. The studied volcanites relate to different igneous series and were formed in geodynamic conditions of the marginal sea and the volcanic arc. The igneous rocks of the Ening stratum are similar to the N-MOR and OI basalts that were formed within the marginal sea (Iruney Marginal Sea) basin. The Upper Cretaceous formations of the eastern slope of the Sredinny Range were formed within the volcanic rise with the island-arc type of volcanism. The younger Eocene igneous rocks of the neo-autochthon (granites and granodiorites) and the volcanic rocks of the Kinkil suite mark a new orogenic stage of development of the Kamchatka margin.     

Structure and tectonic evolution of the Late Jurassic–Early Cretaceous Wandashan accretionary complex,NE China

The Late Jurassic–Early Cretaceous Wandashan accretionary complex (AC) in NE China is a key region for constraining the subduction and accretion of the Palaeo-Pacific Ocean; however, the protoliths and structure of the region remain poorly understood, resulting in debates regarding crustal growth mechanisms and subduction-related accretionary processes in Northeast China. In this contribution, we integrate detailed field observations, ocean plate stratigraphy (OPS) reconstruction, and associated geological data to determine the structure and tectonic evolution of the Wandashan AC. The Wandashan AC formed through the progressive incorporation of OPS units along an oceanic trench. The observed OPS comprises, in ascending order, Permian basalt and limestone, Middle Triassic–Early Jurassic chert, Middle Jurassic siliceous shale and mudstone, and Late Jurassic–Early Cretaceous turbidite. Numerous NNE–SSW-striking thrust faults have segmented the OPS into a series of bedding-parallel tectonic slices that were successively thrust over the Jiamusi massif along a basal thrust (the Yuejinshan Fault), producing a large-scale imbricate thrust system. The Wandashan AC underwent oceanward accretion via multiple deformational processes. The OPS units were detached and rearranged along or within a decollement through offscraping, underplating, thrusting, and duplexing. The units were then emplaced over the Jiamusi massif along the basal thrust. The timing of accretion and thrusting is constrained to the latest Middle Jurassic to earliest Early Cretaceous (ca. 167–131 Ma). Reconstructed accretion-related structural lines within the Wandashan AC trend dominantly NE–SW, close to the direction of Jurassic extension at the eastern Asian continental margin. Large-scale left-lateral strike-slip movement on the Dunmi Fault during the late Early Cretaceous resulted in the folding of structural lines within the Wandashan AC, producing their present-day westward-convex orientation.     

Modeling the Seismicity of the Altai–Sayan–Baikal Region

Doklady Earth Sciences - Numerical modeling of the dynamics of the block structure and the resulting seismicity of the Altai–Sayan–Baikal region has been carried out. The earthquake...