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The geological and geophysical data obtained during the 50th cruise of R/V Akademik Nikolaj Strakhov on the Charlie Gibbs megatransform system structure in the North Atlantic are presented. The structure of the Charlie Gibbs Fracture Zone was examined in detail, considering previously published data. It has been shown that the northern and southern segments of the Mid-Atlantic Ridge, separated by the Charlie Gibbs transform fault, are entirely different in morphology, and hence in terms of formation. The dredged rocks are represented by an entire spectrum from the mantle to upper crustal varieties and allow us to determine the origin of the main structures of the Charlie Gibbs megatransform system considering detailed data on the morphology.

The geological and geophysical data acquired during cruise 45 of R/V Akademik Nikolaj Strakhov regarding the structure of the Doldrums megatransform system in the Central Atlantic are presented. Taking into account the data of the previous expeditions, the structure of this region is examined in detail. The bathymetric data confirm the complex morphology, consisting of five dextral transform faults, separated by four active intra-transform rift segments and indicates the variability of tectonic conditions in this region. The dredged rocks are represented by the entire spectrum from the mantle to the upper crustal varieties and, together with the detailed topographic data, make it possible to identify the origin of the key structures in the Doldrums megatransform system.

The U–Pb age and oxygen and hafnium isotope ratios of zircon from rocks of oceanic core complexes along the Mid-Atlantic Ridge have been studied using SHRIMP and MC-LA-ICP-MS techniques. The U–Pb dating revealed four groups of zircon: (1) 0.6–1.7 Ma, (2) 6.7–11.2 Ma, (3) 12.9–17.6 Ma, and (4) ≥ 200 to 2044 Ma. The δ18O values range from 4.74 to 7.2‰ and are distinct for zircon grains of different ages. The hafnium isotope ratio for zircon with an age from 0.6 to 17.6 Ma corresponds or is close to that of the MORB from the Central Atlantic. The oxygen and hafnium isotope compositions of zircon older than 280 Ma correspond to those of the sialic continental crust. The hypothesis of involvement of the ancient “pre-Atlantic” sialic (≥280 Ma) and “old Atlantic” (7–17 Ma) crusts in the generation of the contemporary (“juvenile”) oceanic crust during the formation of the slow-spreading Mid-Atlantic Ridge has been proposed.

This paper considers the compositions of ferromanganese crusts and nodules (FMCNs) growing on bedrock of Mendeleev Rise, collected in 2016 using a research submarine. The concentrations of Fe, Mn, Co, Ni, Zn, Cu, Cr, and Pb are measured in the collected FMCNs. Analysis of the variations in the contents of ore elements in the studied FMCNs shows that these changes are determined by the following factors: (1) the relative heights of submarine relief (higher relief is characterized by higher concentrations of Co and Ni); (2) substrate character (if it is a tuff, then its alteration provides additional Mn and Zn enrichment of FMCNs); (3) participation of bottom ooze (in the case of extensive contact of substrate fragments with ooze under the influence of pore waters, FMCNs become enriched in elements such as Mn, Co, and Ni); (4) chemical properties of elements (Mn hydroxides concentrate Co and Ni, whereas Fe hydroxides host Cu and Zn).

The results of geochronological (U–Pb), isotope–geochemical (Lu–Hf), and geochemical (REEs) studies of young (MZ, KZ) and xenogenic (AR, PR) zircons from magmatic rocks of the Central Arctic rises of the Arctic Ocean (AO) and the crest zone of the Mid-Atlantic Ridge (MAR) are presented. The data obtained show that the depleted mantle could be a source of young (KZ) zircons of the MAR, whereas young (MZ) zircons of the MAR and all xenogenic (AR, PR) zircons of the AO and MAR are from crustal rocks of the continental lithosphere.

The geochemistry and composition of peridotite rock-forming minerals from blocks in the serpentinite melange of Karaginskii Island have been studied. The composition features of the rock-forming minerals are indicative of the fact that they represent abyssal peridotites of the mid-oceanic ridges that did not undergo remelting under suprasubduction conditions. According to the geochemical data, these rocks were subject to metasomatic alterations under mantle conditions in the suprasubduction setting, which were caused by metasomatizing melts and/or fluids generated in the subduction zone.

This work presents new data on the composition of volcanics, developed within the Alazeya Plateau of the Kolyma–Indigirka fold area (Northeast Russia), which indicate essential differences in their composition and, accordingly, different geodynamic settings of the formation of rocks. The studied igneous rocks are subdivided into two groups. Volcanics of the first group of the Late Cretaceous age, which are represented by differentiated volcanic rock series (from andesitobasalts to dacites and rhyolites), were formed under island arc conditions in the continent–ocean transition zone. Volcanics of the second group are ascribed to the tholeiitic series and were formed under the other geodynamic setting, which is associated with the regime of extension and riftogenesis, manifested in the studied area probably at the later stage.

This paper reports original data on the composition of volcanic rocks in the western part of the Andrew Bain Fault of the South-West Indian Ridge obtained in the 23rd voyage of R/V Akademik Nikolai Strakhov. In accordance with high La/Th and low Nb/U ratios, the basalt compositions of stations S2317, S2318, and S2330 could result from melting of the DM-type source with HIMU traces. Meanwhile, the enriched samples of station S2326 correspond to a mantle source with a considerable contribution of recycled sediments (EM). Sample S2326/35, which is composed of a melt almost completely depleted in EM material, corresponds to the volcanic rocks of the Marion and Prince Edward islands. The obtained and available data on the SWIR segment from Bouvet Island to Andrew Bain Fault are indicative of small mantle heterogeneities in this region. Two possible variants of their origin are considered: either preservation of the enriched material fragments in the depleted mantle during the split of Gondwana or “contamination” of the mantle with plume material with the formation of vein irregularities before opening of the ocean in this region. In the latter case, the plume material could cover a huge area not constrained by the young plume magmatism regions on Bouvet, Marion, and Prince Edward islands.

This communication considers the ideas about the geological structure of the southwestern Mendeleev Rise belonging to the East Arctic rises of the Arctic Ocean. These ideas have resulted from analyzing the data obtained from bathymetric surveys, visual observations, and bottom coring using the technical tools of a research submarine. We distinguished the lower sequence of quartzite sandstones and dolomites, which has a visible thickness of about 230 m and occurs in the lowermost visible section, at depths between 1500 and 1270 m. This sequence is superimposed with stratigraphic and angular unconformity by the upper sequence of limestones and sandstones having a visible thickness of 40 m. The lower sequence is pierced by subvolcanic rocks of basaltic to andesitic composition, and in the lowermost part of the slope, a tuffaceous sequence having a visible thickness of 50 m adjoins it.

The petrology, geochemistry, and isotope ratios of volcanics dredged during the 43rd cruise of R/V Academik Ioffe on the Bathymetrists Seamounts in the eastern equatorial Atlantic have been studied. These are alkaline volcanics of basic and ultramafic compositions. Spider diagrams of the trace elements of volcanic rocks demonstrate strong fractionation, indicating formation of their primary melts from an enriched mantle source at garnet depth facies. Considering the isotope ratio values of 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr and the character of their variations, the volcanic mantle source was chemically heterogeneous: for various volcanic rocks it was a mixture of the mantle components HIMU with EM–1 or EM–2. Limestones dredged together with the volcanics yielded microfossils suggesting a Middle Eocene age of their formation in a carbonate platform environment.

The whole-rock geochemistry and isotopic composition of the basalts dredged from the Mid-Atlantic Ridge (MAR) segment between the Mercury and Vema fracture zones during cruise 32 of the R/V Professor Logachev were studied. In addition to typical basalts with moderate petrochemical parameters, there are high-Ca and low-Na types of basalts representing higher degree melts and high-Fe varieties generated at great depths. The basalts derived by high degrees of melting exhibit a close spatial association with the central portions of three on-axis rises on the rift valley, which reach a height of 300–500 m above the floor of the valley and represent the loci of the subaxial mantle upwelling. The position of the Northern rise at the northern intersect of the Mercury transform fault is inconsistent with focused upwelling. The basalts with a more radiogenic Nd, Sr, and Pb composition are identified above the loci of the subaxial upwelling. They are compositionally similar to basalts, which have HIMU-like affinities and are interpreted to be derived under the influence of the 14° N plume between the Marathon and Cape Verde fracture zones. The relatively high isotopic ratios of these basalts anti-correlate with their low (La/Sm) n values typical of the depleted N-MORB-type basalts. The results show that enriched melts contributing to basalts from the central portions of on-axis rises are associated with one of the flows of plume material channeled from the 14° N anomaly to these rises. The partial melting of the rising plume material causes the formation of localized accumulation of magmas at structural barriers and creates new centers of upwelling, which, being enhanced by the regular upwelling, lead to increased magma production and changes in the composition of basalts.

This study presents Sr, Nd, and Pb isotopic data for basalts collected from the MAR axis between the Arkhangelsk transform fault and 5° N. The isotopic data for basalts from the MAR segment between the Arkhangelsk and Vernadsky faults indicate the presence of N-MORB depleted tholeiites, which are similar to some basalts from the MAR segment between the Cape Verde and Marathon faults, at the location of the 14° N geochemical anomaly. Broad positive along-axis gradients in isotopic values (and negative for 143Nd/144Nd) are observed from south to north, superimposed by isotopic variations in the 9.6° N anomalous basalts. A north-south increase in (La/Sm) n suggests a smaller degree of partial melting. The range of isotopic variations in basalts south of the Bogdanov fault is similar to that of basalts recovered at the more southerly 1.7° N anomaly. P-MORB compositions with a large proportion of the HIMU-like component are widely distributed in the vicinity of the 5.4° N rise. The isotopic ratios decrease gradually (with increasing 143Nd/144Nd) farther to the north toward the Bogdanov transform fault. A similar trend is observed for (La/Sm) n . Therefore, the basalts recovered to the south and north of Bogdanov fault have different sources of radiogenic isotope enrichment: 5.4° N and 9.6° N rises. Basalts recovered to the north of the Vernadsky transform fault have higher 143Nd/144Nd and lower 206Pb/204Pb at similar 87Sr/86Sr ratios. We conclude that the localized manifestations of enriched basalts with isotopic characteristics close to the HIMU-like plume-derived component, including the 14° N and 1.7° N geochemical anomalies, may have been formed by microplumes entrained at the base of the upper mantle by asthenospheric mantle flows arranged into a system of cylindrical convective cells, 450–500 km in diameter. Northward of the Bogdanov transform, the upper mantle is contaminated with lower continental crust material, which was entrained during tectonic erosion of the lithosphere beneath the equatorial segment of Gondwanaland during its late stage opening.

The results of study of Fe–Mn crusts from the Mendeleev Ridge in the Arctic Ocean sampled with manipulators from a submarine are presented. In almost all the samples, the ore phase is significantly enriched in some valuable trace elements (Ni, Co, Cu, etc.), the contents of which exceed those in ores from the pelagic zones of other oceans. The high ore potential of the Arctic pelagic zone is stated and substantiated.

Micropalentological studies of limestones and isotopic-geochronological determinations of the age of zircons found in vulcanites dredged from the region of the northern chain of the group of Bahia seamounts in the 28th cruise of the R/V Akademik Vavilov were undertaken. The recognized age difference of the northern chain of the Bahia seamounts of northwestern strike from 75 to 84 Ma from the southeast to the northwest indicates the fact that this chain was formed as a result of hot spot activity. The age of vulcanites is in good correlation with the Maastricht and Coniacian-Santonian age of limestones occurring on the slopes of the seamounts. The similarity of the seamounts age and the age of the oceanic bottom indicates the fact that the hot spot was localized near the spreading axial zone of the MAR. Along the strike of the chain, the centers of volcanic activity are correlated to the zones of transform faults, which are important in the hot spot activation. The formed chain is a weak zone in the lithosphere and the focus of younger stages of volcanic activity; there were at least two of stages within the northern chain of the Bahia seamounts, and these occurred about 75 and 59 Ma ago.

Micropaleontological and isotope-geochronological investigations of calcareous sedimentary rocks and volcanites dredged out from the central portion of the submarine Vitoria-Trindade Ridge during the 24th cruise of R/V Akademik Vavilov have been conducted. It has been established based on micropaleontological analysis, which included determination of the species composition of foraminifera and nannoplankton, that the sequence of sedimentary rocks having a pelagic nature formed on the slopes of the volcanic seamounts in the central portion of the Vitoria-Trindade Ridge from the Early to Mid-Miocene to the Holocene; a good correlation between the degree of lithification of these rocks and their age is observed. It has also been established that the carbonate platforms on the abraded tops of the Davis Seamount and the Dogaressa Bank, which are located in the east-central portion of the Vitoria-Trindade Ridge, started forming in the Early Miocene (19–24 Ma). It has been determined using local U-Pb dating of zircon grains with a SHRIMP-II high resolution secondary ion mass spectrometer that the volcanites forming the upper portion of the volcanic rock sequence of the Jaseur Seamount (29.8 ± 6.6 Ma) located in the west-central portion of the Vitoria-Trindade Ridge date to the Oligocene. The investigations conducted have confirmed the opinion that the Vitoria-Trindade Ridge formed in general because of the activity of the hot spot located under the volcanic Trindade and Martin Vaz Islands. However, separate extended lenticular segments of this ridge existed for a long time as single structures, within which the age of the seamounts was not linearly dependent on the distance from the location of the hot spot. Lenses of hot mantle matter that form at the sublithospheric level as a result of impulses of plume activity and move along with the lithospheric plate play a defining role in the development of individual segments forming the Vitoria-Trindade Ridge.

During the 28th trip of the R/V Akademik Sergei Vavilov undertaken in 2009, exploration was carried out using a Parasound acoustic profilograph that permitted examining the upper portion of the sedimentary cover and getting transverse bathymetric profiles through a number of seamounts within the northern range of the Bahia group of seamounts, which were also sampled by dredging. The studies performed indicated that this linear structure consists of paleovolcanoes and formed under impulse conditions of volcanic activity. A number of seamount types have been distinguished, the morphostructural settings and the positions of which reflect the intensity of volcanic impulses and the association with deep permeable zones. The strongest impulse led to the formation of a northwest striking arched uplift, on which there are several central type paleovolcanoes of different heights. The largest of these paleovolcanoes, including the Stocks and Groll Seamounts, have flattened tops formed as a result of marine abrasion. Isolated seamounts have formed during shorter and lower power impulses of volcanic activity. The northern range of the Bahia seamounts, which have a major northwesterly strike, is an ensemble of structures with northwesterly, nearly east-west, and nearly north-south strikes, which indicates the presence of relevant deep permeable zones for the mantle matter; that mantle matter is the source of the melts, eruptions of which led to the formation of the volcanic edifices. The prevailing zone of weakness has a northwesterly strike. Volcanic activity in the Bahia group of seamounts ended in the Paleocene, but tectonic movements continued there up until the Holocene. Neotectonic structures are represented by horsts, diapirlike piercement structures, and folded zones. The paleovolcanoes are areas where slope processes of transport of sedimentary material occurred.

During the 28th cruise of R/V Akademik Sergei Vavilov in 2009, five different mountains belonging to the northern chain of the Baia Seamounts, located in the Brazil Basin, were dredged. Igneous rocks, limestones, and Fe-Mn crusts were collected. Igneous rocks are greatly altered resulting from halmyrolithic and/or low-temperature hydrothermal processes; the main secondary minerals are smectite, iron hydroxide, and phillipsite. Igneous rocks are subdivided into two groups, namely, trachybasalts and trachyandesites. Trachybasalts are aphyric rocks, consisting of basal plagioclase microlites, Fe-Ti ore mineral, olivine, and clinopyroxene. Trachyandesites are rarely porphyre rocks. Inclusions in trachyandesite are represented by acidic plagioclase, olivine, biotite, and zircon. The main mass is formed by acidic plagioclase and a small quantity of clinopyroxene.

In 2008, during cruise 24 of the R/V Akademik Vavilov, much of our research work was focused on the central segment (Jaseur and Davis seamounts, Dogaressa Bank) of the Vitoria-Trindade seamount chain (west of the Brazil basin) extending along 20.5° S. Work was conducted to survey the upper part of the sedimentary cover and to perform subbottom profiling. The samples dredged on the seamount slopes are represented by volcanites and Fe-Mn crusts.

In volcanic tuffs, dredged during Cruise 23 of the R/V Akademik Nikolaj Strakhov, accessory zircon was found; except for the mineral-forming components, there were ∼2% of ThO2 and 0.75% of Ce2O3 in zircon. During rapid uplift of magmatic masses to the ocean bottom surface, admixture elements isolated into specific minerals. As a result, destruction structures were formed in the rim parts of primary zircon crystals, and the new-formed association of zircon + thorite + cerite + thoriante + baddeleyite appeared.

Neotectonic deformations of the oceanic bottom are widespread in the Central Atlantic (1) developing in the MidAtlantic Ridge, deep oceanic basins, and even its marginal areas (2). The problems of the origin, scales, and mechanisms of these tectonic movements remain debatable. In this connection, of great interest are new data on tectonic deformations developed in the upper sedimentary layers of the western Brazil Basin. They were obtained during the acoustic study of the oceanic bottom with the profilograph Parasound in Cruise 24 of the R/V Akademik Sergey Vavilov car� ried out in 2008. The bottom structure of the western Brazil Basin is

This study presents new data on the geochemistry and mineral chemistry of ultramafic and mafic rocks in ophiolits from the base of the Kumroch segment of the Achayvayam-Valagin paleoarc. The new data enabled us to consider peridotites and the associated diabases and gabbros enclosed as separated blocks into a serpentinite melange as a single ophiolite complex formed in a supra-subduction setting and subsequently disintegrated as a result of nappe formation. The variations identified in the geochemistry and compositions of rock-forming minerals are shown to be characteristic of the other study ophiolite complexes of Eastern Kamchatka. This is suggested to reflect spatial-temporal heterogeneity of partial melting during evolution of the Achayvayam-Valagin island arc.

In 2007, the Geological Institute of the Russian Academy of Sciences (GIN RAS) carried out investigations in the North Atlantic, in the southern part of the Knipovich Ridge extending for 600 km from the Mohns spreading ridge to the Molloy fault zone (chief of the cruise A.V. Zaionchek). The investigations were conducted according to the Program of the RAS Presidium entitled “Basic Problems of Oceanology: World Ocean Physics, Geology, Biology, Ecology” (Project “Regularities of the Structure and Formation of the Oceanic Crust in Characteristic Regions of the Atlantic Ocean: Tectonics, Magmatism, Composition and Genesis of Fe‐Mn Deposits,” supervisor Academician Yu.M. Pushcharovskii). The problem facing the expedition was to study the geological structure of the Knipovich‐Mohns junction. With the help of the R/V Akademik Nikolaj Strakhov (Cruise 25), there were conducted complex areal, medium-scale, regional geological study of the selected object, which involved echo-sounding with SeaBat 7150 multibeam sounder, continuous seismic profiling (CSP), high-frequency sounding with the Edgetech 3300 profiler, and bottom dredging (Fig. 1). Within the region of 74 ° N, south of the Greenland Fault Zone (FZ) extending southeastward, the Mohns spreading ridge passes into the north‐south-trending Knipovich Ridge. The peculiarity of both ridges is that these are unified extensive spreading structures not broken into segments by transform faults. The ridges differ in the time and conditions of formation. From the beginning of formation, regular and steady growth of the oceanic crust in the rift zone was characteristic of the Mohns Ridge, which is marked by the symmetrical and natural position of linear magnetic anomalies relative to the rift valley axis [1, 2]. The Knipovich Ridge began forming under unsteady geodynamic conditions, which was reflected in the disordered position and fragmentation of magnetic anomalies. The region of the Mohns and Knipovich junction attracts the attention of researchers in that this is a unique area where one spreading ridge passes into another with rift valley structures gradually bending by 40 ° without apparent transform faults serving as accommodation zones for stresses generated in the course of plate motions. Hence, the geodynamics of structures in this key region has been the subject of investigations.

were analyzed for major and trace elements at the chemical laboratories of the Geological Institute (Moscow) and the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences (table). All volcanic rocks dredged from Carter Seamount are strongly altered. The losses of ignition vary from 10 to 14%. In terms of major element composition, the volcanic rocks of group B1 are ascribed to olivine melilitites, while B2 are classed with nephelinites. Olivine melilitites contain 3‐5% clay pseudomorphs (0.5‐ 5 mm in size) after olivine phenocrysts. Microphenocrysts (up to 0.1‐0.3 mm) of clinopyroxene (augite and Ti-augite) and melilite account for 2‐3%. The matrix is made up of lathlike microlites of melilite (20‐30%) and clinopyroxene (augite) (20‐30%), as well as equant grains of Ti-magnetite (about 5%) embedded in a flaky aggregate of secondary minerals (possibly chlorite and zeolite). About 5‐8% is occupied by 0.5‐3 mm pores filled with various zeolites. Nephelinites contain about 2‐3% microphenocrysts of greenish egirine‐augite 0.2‐1 mm in size. The groundmass consists of microlites of egirine‐augite (15‐20%) and nepheline (20‐30%) and grains of Ti-magnetite (about 5%) set in a flaky matrix of secondary minerals. In the oxide‐MgO variation diagrams, the composi

The problem of typification of tectonically disintegrated ophiolite complexes in the accretionary framing of the northwestern Pacific remains a matter of debate. In addition to detailed geological, mineralogical, and geochemical data, the isotopic characteristics of the rocks of ophiolite complexes play an important role in solving this problem. We studied ophiolites of the Eastern peninsulas of Kamchatka (Kamchatskii Mys and Kronotskii peninsulas), which are the fragments of the Late Cretaceous‐Eocene Kronotskii paleoarc (Fig. 1). In order to obtain the isotopic characteristics of the ophiolite complexes, we studied Pb and Nd isotopic ratios in the least altered rocks. This paper presents new data on the isotopic composition of ophiolites of the Kronotskii paleoarc. The 143 Nd/ 144 Nd, 208 Pb/ 204 Pb, 206 Pb/ 204 Pb, and 207 Pb/ 204 Pb isotopic ratios were analyzed on a Triton TI solid phase multichannel mass spectrometer at the Center of Isotopic Research (Karpinskii All-Russia Research Institute of Geology, St. Petersburg) using the standard technique. The average reproducibility of analyses during determination of the Nd isotopic composition was no worse than 0.005% (2 σ ) for the 143 Nd/ 144 Nd ratio. The isotopic composition of the Pb standard measured during the analytical works was as follows: 143 Nd/ 144 Nd = 0.512112 ± 0.000002 for JNdi-1 standard; 206 Pb/ 204 Pb = 16.937 ± 0.011, 207 Pb/ 204 Pb = 15.492 ± 0.017, 208 Pb/ 204 Pb = 36.722 ± 0.017 for NIST-981 standard. We united ophiolite fragments in the Kamchatskii Mys Peninsula into three ophiolite complexes developed in the Afrikan block [1]: (1) Aptian‐Cenomanian oceanic complex represented by tholeiitic basalts, jaspers, and limestones of the Afrikan Complex; (2) Late Cretaceous suprasubduction complex including ultrabasic rocks of Mt. Soldatskaya and Ol’khovsk gabbro‐ plagiogranite massif; (3) Paleocene‐Eocene intra-arc complex including basalts, jaspers, and mudstones of the Kamensk Complex and gabbroids of the Olenegorsk Complex. The Stolbovskii block of the Kamchatskii Mys Peninsula contains Maastrichtian islandarc tholeiites of the Tarkhovka Subformation (Stolbovskaya Group), which are likely complementary to suprasubduction ophiolites of Mt. Soldatskaya. Similar island-arc volcanic rocks occur as tectonic blocks in the

Analysis of the chemical composition of mud (Table 1) by the X-ray fluorescence method revealed that highly water-saturated intervals are enriched in Na and Cl. These samples were omitted from consideration. Variations of petrogenic elements are insignificant in other samples. Data on the mineral and chemical compositions were used for estimating the quantitative proportions of minerals in the mud: quartz 30%, plagioclase 10%, mica 25%, kaolinite 15%, chlorite 5%, and smectite 15%. The micropaleontological analysis revealed that all the samples contain foraminifers, while only some of them enclose nannofossils. These finds indicate, in particular, that polynyas appeared or the thick ice cover was periodically absent here from the Late Glacial period up to the present-day time. The sharp increase in abundance of calcareous microorganisms at 14 cm and its growth upward through the section record the transition from the last glaciation to the Holocene. The maximal concentration of nannofossils Emiliania huxleyi

Neotectonic uplifts of the Vostochnyi (Eastern) mountain range of Kamchatka are composed of Cretaceous‐Paleogene complexes with fold-nappe and imbricate structures that represent fragments of the southern Ozernov‐Valagin segment of the Achaivayam‐Valagin paleoarc [1]. The composition of island-arc volcanic rocks developed within the uplifts has been studied to a variable extent. Among the best studied are the volcanic rocks of the Ozernoi Peninsula and Tumrok Ridge [2‐4]. We obtained new data on the composition of the least studied volcanic rocks of the Kumroch Ridge, as well as on the Tumrok and Valagin ridges, which allowed us to refine the geodynamic conditions of the volcanic sequences of this segment of the paleoarc. The sampling localities of the studied volcanic rocks are shown in Fig. 1 and Table 1. In the Kumroch Ridge, the Late Campanian‐Danian island-arc rocks compose the tectonic nappes of an autochthon (rocks of the Khapitsk sequence) overlain by a serpentinite melange nappe. The nappe includes blocks of Albian‐Cenomanian and Santonian‐Early Campanian siliceous rocks, basalts, diabases, gabbros, and serpentinized peridotites. Volcanic rocks of the Khapitsk sequence are mainly composed of clinopyroxene‐plagioclase porphyritic basalts and basaltic andesites. In the P 2 O 5 ‐TiO 2 diagram [5], these volcanic rocks are plotted in the island-arc rock field. In terms of REE distribution patterns (Fig. 2), the studied samples are ascribed to the calc-alkaline and tholeiitic series. The location of volcanic rocks in the section suggests the simultaneous formation of these two rock groups. In the REE patterns (Fig. 2), the samples of calc-alkaline series demonstrate LREE enrichment relative to HREE, while LREE and MREE contents in tholeiitic rocks show a similar (almost chondrite-type) pattern, with an insignificantly elevated LREE content. The spidergrams of the studied volcanics have island-arc signatures: deep negative anomalies of HFSE (Nb, Ta, Zr, Ti) and REE enrichment relative to LILE (Ba, U, Rb) (Fig. 3). The calc-alkaline rocks have higher contents of Nb, Ta, Ba, U, and Rb. The two rock groups also differ in bulk composition. Relative to the tholeiitic basalts, the calc-alkaline rocks have a more potassic composition. In general, they are depleted in Ti and enriched in P (Table 1).

The regions of conjugation of continental rise with abyssal oceanic basins at the margins of the Atlantic— a transitional zone between continental and oceanic lithospheres—are still poorly studied in geological terms. In the course of expeditions conducted by the Geological Institute (Moscow), the structure of this zone was studied at the continental slope of Africa, south of the Cape Verde Islands. In this area, the continental rise widens sharply making up a near-latitudinal promontory that divides the abyssal Cape Verde Basin in the south and the Canary Basin in the north. The study area is situated in the pinchout area of the system of transform fracture zones (TFZ) located south of the Fifteen Twenty TFZ [1, 2]. The near-latitudinal linear ridges and troughs on the bottom of the Cape Verde Basin are the eastern flanks of the Vema, Doldrums, Arkhangelsky, and Vernadsky TFZs of the Mid-Atlantic Ridge. Near the continental slope of Africa, these TFZs are cut off by the WNW-trending escarpment (Fig. 1). The bathymetric survey of a local area in the deepwater Cape Verde Basin that adjoins the southern margin of the Cape Verde Seamount (Figs. 1, 3) was carried out during Cruise 22 of the R/V Akademik Nikolai Strakhov in 2000. We have established an azimuthal unconformity between near-latitudinal depressions and ridges that extend from MAR, on the one hand, and the WNW-trending transversal Cabo Verde Escarpment, on the other hand [3]. The seafloor in the studied test area is complicated by volcanic edifices and by the anomalously deep (>6000 m) Strakhov Basin trending in the NW direction discordantly relative to other structural units. The previously unknown Neva deepwater channel was also found (Fig. 3). This paper has been prepared on the basis of the data collected during Cruise 16 of the R/V Akademik Ioffe in 2004. During this cruise, the sedimentary cover was studied with continuous seismic profiling (CSP). The structure of the upper part of the sedimentary cover and the bottom topography was investigated with a Parasound acoustic profilograph along a profile between 11.52 ° N × 22.67 ° W and 10.13 ° N × 24.07 ° W (Fig. 1). The structure of the upper part of the sedimentary cover in the Neva Channel and the Strakhov Basin was studied with the same method. The bedrock samples and cores of bottom sediments were recovered in the same place. Structure of the sedimentary cover (based on CSP data). The CSP profile across the junction of continen

We have studied major and trace element (including REE) geochemistries of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5 -7 10 0 N). The links of basalts of various compositions with particular ocean- floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwest flank of the MAR crestal zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of a deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are dierent for depleted tholeiites (44-55 km, 1320-1370 C) and enriched tholeiites (45-78 km, 1330-1450 C). Mantle plume impact is shown to aect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in the melts parental to the rift-related basalts occurring near the plume. T- and P-MORBs are also developed near the areas where mantle plumes are localized. The high-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov FZ) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. The likely causes of the temporal and spatial instability of mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In the sulfide-bearing rift morphostructures (the so-called "Ore area" and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. The functioning of these chambers is a combined eect of a singular geodynamic regime and plume activity. In these chambers, melts undergo deep dierentiation leading to progressively increasing concentration of the sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

Acoustically stratified seismic images are widely used in studies of sedimentation mechanisms producing thick sedimentary aprons on continental rises. However, insuffcient vertical resolution of seismic records commonly restricts a more detailed interpretation of individual reflectors within accumulations to reconstruct specifc short-term sedimentological events. (Reflectors correspond to boundaries between sedimentary layers with different acoustic properties and are imaged in seismic records as continuous lines.) Very high resolution (VHR) seismic profiling conducted during Cruise 33 of the Russian research vessel Akademik Ioffe (2011) provided correlation of acoustic reflectors, distanced from one another by less than 50 centimeters, with thin sedimentary layers recovered by gravity cores.