Search

The paper presents new data on the Doldrums megatransform fault located in the northern Equatorial Atlantic (6.5 degrees-9 degrees N), which consists of four transform faults with large offsets: Doldrums, Vernadsky, Pushcharovsky, and Bogdanov. Several types of spreading segments of the Mid-Atlantic Ridge (MAR) displaced by transform faults were identified by analyzing seafloor bathymetric survey results from a multibeam echo sounder on cruise 45 of the R/V Akademik Nikolaj Strakhov in 2019. Depending on the depth of the axial spreading zone, the structure of segments varies from typical for mid-oceanic ridges to intratransform spreading centers. The shallowest spreading segments south of the Bogdanov FZ and north of the Vernadsky FZ correspond to spreading cells with small neovolcanic rises and abyssal hills in combination with oceanic core complexes on the flanks of the MAR. The deepest intra-transform spreading center formed between two branches of the double Pushcharovsky FZ. This center is characterized by the largest neovolcanic rise, a nonstationary position, and origination of new spreading centers in areas of pull-apart basins, which form under the influence of extension directed towards the intra-transform domain of the Pushcharovsky FZ. Segments intermediate in depth and structure form between the Vernadsky, Pushcharovsky, and Bogdanov Fracture zones. The segments are characterized by large neovolcanic rises in the axial zone and asymmetrical structure of the flanks. The eastern flank exhibits oval rises, which are mostly composed of serpentinized ultramafic rocks; the western flank shows rare rift ridges (former neovolcanic rises). We analyzed the oblique structures with respect to the spreading direction, which are abundant in the studied region, and considered possible reasons for their origination in Equatorial Atlantic.

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 article concerns the original materials obtained in the 33-rd cruise of the R/V Akademik Nikolai Strakhov in the eastern part of the North Atlantic while studying the system of AtlantisMeteor seamounts. The system is a single volcanic uplift formed on the Canary Abyssal Plate, and is one of the key objects for understanding the geological history of this segment of the Atlantic Ocean. The article discusses basalts, tefrites, and organogenic-terrigenous lagoon-marine sediments dredged from the Atlantis, Plateau, and Cruiser seamounts. The petrographic properties and compositions of the basalts of the seamounts Atlantis and Cruiser reflect significant differences in the environments of their effusions. In the first case, well-crystallized, not showing signs of vesicularity, olivine basalts poured out in deep-water conditions. Glassy highly vesicular basalts from the Cruiser seamount are characteristic of shallow subaerial effusions. Evidence has been obtained in favor of the subaerial settings of the accumulation of tefrites from the Plato seamount. The regularities of lithogenetic transformations of organogenic-terrigenous deposits of the Cruiser seamount, exposed to high-temperature effects of subaerial lava flows, are revealed. In the course of volcanogenic-sedimentary lithogenesis, the lignite-like material lost its primary structure, turning into natural anisotropic coke with a wide development of fusinite and pyrofusinite. The authors of the article associate the studied volcanic manifestations with the final (Late MiocenePliocene) stage of volcanism in the seamount system, which preceded the disintegration of the system, its progressive immersion and the transformation of islands into guyots.

The Barents Sea offers a suitable location for documenting advection of warm and saline Atlantic Water (AW) into the Arctic and its impact on deglaciation and postglacial conditions. We investigate the timing, succession, and mechanisms of the transition from proximal glaciomarine to marine environment in the northwestern Barents Sea. Two studied sediment cores demonstrate diachronous retreat of the grounded ice sheet from the Kvitøya Trough (core S2528) to Erik Eriksen Trough (core S2519). Oxygen isotope records from core S2528 depict a two-step pattern, with lower δ18O values prior to the Younger Dryas (YD), and higher values afterward because of advection of the more saline,18O-enriched AW. At this location, subsurface AW penetration increased during the Allerød and YD/Preboreal transition. In the study area, foraminiferal and dinocyst data from the YD interval suggest cold conditions, extensive sea-ice cover, and brine formation, along with the flow of chilled AW at subsurface and the development of a high-productivity polynya in the Erik Eriksen Trough. Dense winter sea-ice cover with seasonal productivity persisted in the Kvitøya Trough area during the early Holocene, whereas surface warming seems to have occurred during the middle Holocene interval.

The Charlie Gibbs offsetting by ~340 km the Mid Atlantic Ridge (MAR) axis between 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced to the south by the Azores and to the north by the Iceland mantle plume, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ‘70 and ‘80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. In this contribution we present preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.

Herein we provide information on the integrated geological, geophysical, sedimentological, paleoceanographic, hydrophysical and biological investigations in the Central Atlantic during the cruise 45 of the R/V “Akademik Nikolaj Strakhov” in October-November 2019. The preliminary scientific results are discussed.

The Doldrums transform system offsets the Equatorial Mid Atlantic Ridge by similar to 630 km at 7-8 degrees N. This transform system consists of four intra-transform spreading centers (ITRs) bounded by five transform faults. The northern-most ITR is linked to the MAR axis by a similar to 180 km-long transform. Here, during two R/V A. N. Strakhov expeditions (S06 and S09), mantle peridotites were dredged along the transverse and median ridge of the transform, across the western flank of the ITR valley. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites showing evidence for melt-rock interaction characterize the ITR domain. Petrological and geochemical observations reinforced by geochemical modelling are used to define the behaviour of trace elements during melt extraction and melt-rock reaction in our rocks. Results suggest that residual peridotites derive from mantle rocks that have undergone a degree of partial melting up to 12%, with melting likely starting at the transition of garnet-spinel stability fields, whereas peridotites which suffered melt-rock reactions have been divided into two types: (i) pl-impregnated peridotites, formed by migration of melts at high porosity and high melt-rock ratio; and (ii) refertilized peridotites, generated at reduced porosity, when small fractions of the same percolating melt crystallized clinopyroxene and minor plagioclase. We suggest that the refertilizing agent was a melt highly depleted in incompatible trace elements, in turn produced by an ultra-depleted mantle source. This mantle experienced previous degrees of melt extraction at the ridge axis, before being transposed laterally along the transform where it melted a second time during the opening of the intra-transform spreading segment. (C) 2020 Elsevier B.V. All rights reserved.

The paper considers Cretaceous magmatism at the continental margin of the Arctic Region. It is shown that Cretaceous igneous rocks of this region are rather heterogeneous in age, composition, and geodynamic formation setting. This differentiates them from rocks of typical large igneous provinces (LIPs). Local areas of magmatic activity, their substantial remoteness them from one another, and significant distinctions in age, composition of rocks, and formation conditions prevent us from unreservedly combining all occurrences of Cretaceous magmatism at the continental margin of the Arctic Region into a common igneous province. The stage of tholeiitic magmatism in the Svalbard Archipelago, Franz Josef Land, Arctic Canada, and the Alpha–Mendeleev Rise, which can be considered an LIP, began in the Early Cretaceous and continued for a long time, at least until the Campanian. The magmatism apparently had a plume source and was caused by extension during opening of the Canada Basin. Tholeiitic magmatism gave way to the alkaline magmatism stage from the Campanian to the onset of the Paleocene, related to continental rifting at the initial stage of formation of Eurasian Basin in the Arctic Region. No convincing evidence for a genetic link between Early Cretaceous tholeiitic and Late Cretaceous alkaline magmatism is known at present, nor for the alkaline magmatism belonging to a plume source.

The 33rd cruise of the R/V “Akademik Nikolaj Strakhov” was in the Atlantic Ocean and the Baltic Sea, and included a complex of geological, hydrological and hydrochemical works. According to the results of a multibeam echo sounder survey of the bottom, digital models of bottom landscapes were constructed. Using the dredging method, new geological samples were obtained, and the values of geothermal flows in the Atlantic Ocean were measured.

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.

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle.

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.

Data on the composition of rocks in linear tectono-magmatic rises in the Brazil Basin indicate that volcanic rocks in the Vitoria—Trindade seamount chain were derived from a mantle reservoir unevenly enriched in phosphorus under the effect of melts close to subalkaline picrobasalt. These melts contained much of the EM I mantle component because the plume material was contaminated with continental lithospheric component. A long-lived isotopic homogeneity of the source is typical of the whole structure, including the Trindade and Martin Vaz Islands and the Abrolhos Plateau. The analogous isotopic ratios of rocks at the Fernando de Noronha Islands are reportedly explained by a similar mechanism of melt derivation and the similar evolution of the mantle plume material, which was originally situated beneath the South American continent. Compared to the melts of volcanic rocks of all other seamounts discussed herein, the parental melts of volcanics at the Victoria—Trindade Seamounts were derived at lower degrees of melting of enriched source material at a greater depth. The overwhelming majority of volcanic rocks at the northern chain of the Bahia Seamounts were produced by melts generated with the involvement of material of the HIMU type. At the same time, one of our rock samples was derived from a source of composition close to DM with a certain admixture of enriched material like EM I. The mantle source of rocks of the Pernambuco Seamounts consisted of a mixture of DM and HIMU material with a certain admixture of EM I (or, perhaps, EM II). The 10°–11° S Seamounts were formed near the MAR axial zone at the decompressional melting of chemically homogeneous mantle source that consisted of DM material with an admixture of EM I (or, perhaps, EM II) component.

The paper considers various aspects of the formation of the Central Atlantic Igneous Province. Generation and eruption of enormous bodies of basic magmas over a short time interval (a few million years) are explained by the accumulation of thermal energy beneath the continental lithosphere. The thermal energy was transferred by heated matter or fluid flows from the lower mantle along separate channels or permeable zones and accumulated beneath the continental lithosphere of Pangea over a long period of time (tens of millions of years) over a vast area. The large thickness ofthe lithosphere hindered melting. A change in the geodynamic regime with the onset of the breakdown of Pangea resulted in extension, formation of linear permeable zones, local decompression, and, as a consequence, in generation and ascent of huge magma bodies along extended linear tectonically weakened zones. The homogeneity of igneous rocks is explained by the short time interval favorable for magma generation, when all the stored thermal energy had been exhausted.

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

The Mesozoic and Cenozoic seamounts and submarine ridges in the east of the South Atlantic are considered and compared with the coeval tectonomagmatic structures of West Africa. The conclusion is drawn that within-plate magmatism of the Atlantic is a waning process related to the ascent of several large plumes beneath West Africa beginning from the Triassic and subsequent lateral spreading of their material. It is shown that the heated plume material can spread beneath the lithosphere for a great distance, mixing in various proportions with asthenospheric matter, forming melts variable in geochemistry and isotopic characteristics. Cooling of the material takes many tens of years with retention of small magma sources episodically supplying melts to the surface. Localization of permeable zones in the lithosphere, along which the melts ascend, is determined by global stress fields responsible for the formation of long-lived linear tectonic elements on continents, inherited by young oceanic tectonic lines.

Based on the investigation of samples recovered during Cruise 25 of the R/V “Akademik Nikolai Strakhov,” the character of magmatism was determined in the flank parts of the rift zone at the 74°05′N and 73°50′N region, where the direction of the rift valley changes from the north-northwest in the Knipovich Ridge to the northeast-trending structures of the Mohns Ridge. It was shown that the tholeiitic magmas of this region shows all the geochemical characteristics of TOR-2, which is typical of the Mohns Ridge and most oceanic rift zones worldwide, and differ from the basalts of the Knipovich Ridge, which are assigned to a shallower type of tholeiitic magmatism (Na-TOR). The persistent depletion of the magmas in terms of lithophile element contents and radiogenic isotope ratios of Sr, Nd, and Pb reflects the conditions of their formation during the ascent of the depleted oceanic mantle, which has occurred without significant complications since the early stages of the formation of the Mohns Ridge.

New petrological and geochemical data were obtained for basalts recovered during cruise 24 of the R/V “Akademik Nikolay Strakhov” in 2006. These results significantly contributed to the understanding of the formation of tholeiitic magmatism at the northern end of the Knipovich Ridge of the Polar Atlantic. Dredging was performed for the first time both in the rift valley and on the flanks of the ridge. It showed that the conditions of magmatism have not changed since at least 10 Ma. The basalts correspond to slightly enriched tholeiites, whose primary melts were derived at the shallowest levels and were enriched in Na and depleted in Fe (Na-TOR type). The most enriched basalts are typical of the earlier stages of the opening and were found on the flanks of the ridge in its northernmost part. Variations in the ratios of Sr, Nd, and Pb isotopes and lithophile elements allowed us to conclude that the primary melts generated beneath the spreading zone of the Knipovich Ridge were modified by the addition of the enriched component that was present both in the Neogene and Quaternary basalts of Spitsbergen Island. Compared with the primitive mantle, the extruding magmas were characterized by positive Nb and Zr anomalies and a negative Th anomaly. The formation of primary melts involved melting of the metasomatized depleted mantle reservoir that appeared during the early stages of opening of the Norwegian-Greenland Basin and transformation of the paleo-Spitsbergen Fault into the Knipovich spreading ridge, which was accompanied by magmatism in western Spitsbergen during its separation from the northern part of Greenland.

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.

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

The magmatic and tectonic activity of eastern South America and the western South Atlantic shows that extension of the continental crust is the determinant factor of magmatism. Heating of the upper mantle is a necessary condition of its manifestation. Ascending plume material is a source of additional heat. In the Early Mesozoic, Eastern Brazil was situated above a large, ascending and probably ramifying plume, which has supplied heat and material since the Triassic, creating favorable conditions for continental magmatism. Magmatic activity continued, gradually waning, until the Neogene as evidence for long-term retention of heat energy beneath the continental lithosphere after the plume ascent. It has been shown that heated mantle material can be displaced from the continent to the ocean for a significant distance beneath the lithosphere with the formation of linear tectonomagmatic rises of the oceanic crust. The structural elements inherited certain directions on the continent and in the ocean, beginning from the Neoproterozoic. These directions were reactivated and continued to control the younger structural grain and magmatic activity. In Southeastern Brazil, these were the structural units striking in the southeastern (about 120° SE) and northeastern directions parallel to the continent-ocean boundary. In Northeastern Brazil, the W-E- and N—S-trending structural units are predominant. All these directions are manifested in oceanic structural units (Rio Grande, Vitoria-Trindadi, Fernando de Noronha, Pernambuco rises, etc.).

New data are obtained on the structure, evolution, and origin of zones of nontransform offsets of adjacent segments in the Mid-Atlantic Ridge (MAR), which, in contrast to transform fracture zones, so far are studied insufficiently. The effects of deep mantle plumes developing off the crest of the MAR on the processes occurring in the spreading zone are revealed. These results are obtained from the geological investigation of the crest of the MAR between 19.8 ° and 21° S, where bottom sampling, bathymetric survey, and magnetic measurements have been carried out previously. Two segments of the rift valley displaced by 10 km relative to each other along a nontransform offset are revealed. A volcanic center of a spreading cell, which has been active over the last 2 Ma, is located in the northern part of the southern segment and distinguished by a decreased depth of the rift valley and increased thickness of the crust. Magnesian, slightly evolved basalts of the N-MORB type are detected in this center, whereas evolved and high-Fe basalts are found beyond it. The variation in the composition of the basalts indicates that the volcanic center is related to the upwelling of the asthenospheric mantle, which spread along and across the spreading ridge. In the lithosphere, the melt migrated off the volcanic center along the rift valley. In the northern segment, a vigorous volcanic center arose 2.5 Ma ago near its southern end; at present, the volcanic activity has ceased. As a result of the volcanic activity, an oval rise composed of enriched T-MORB-type basalts was formed at the western flank of the crest zone. The isotopic signatures show that the primary melts are derivatives of the chemically heterogeneous mantle. The mixing of material of the depleted mantle with the mantle material pertaining either to the Saint Helena or the Tristan da Cunha plumes is suggested; the mixture of all three sources cannot be ruled out. The conclusion is drawn that the mantle material of the Saint Helena plume was supplied to the melting zone beneath the axial rift near the oval rise along a linear permeable zone in the mantle extending at an azimuth of 225° SW. The blocks of mantle material that got to the convecting mantle from the Tristan da Cunha plume at the stage of supercontinent breakup were involved in melting as well. The nontransform offset between the two segments arose on the place of a previously existing transform fracture zone about 5 Ma ago. The nontransform offset developed in the regime of oblique spreading at the progressive propagation of the southern segment to the north. The zone of nontransform offset is characterized by recent volcanic activity. Over the last 2 Ma, spreading of the studied MAR segment was asymmetric, faster in the western direction. The rates of westward and eastward half-spreading in the northern segment are estimated at 1.88 and 1.60 cm/yr, respectively.

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.

The accretion of oceanic crust under conditions of oblique spreading is considered. It is shown that deviation of the normal to the strike of mid-ocean ridge from the extension direction results in the formation of echeloned basins and ranges in the rift valley, which are separated by normal and strike-slip faults oriented at an angle to the axis of the mid-ocean ridge. The orientation of spreading ranges is determined by initial breakup and divergence of plates, whereas the within-rift structural elements are local and shallow-seated; they are formed only in the tectonically mobile rift zone. As a rule, the mid-ocean ridges with oblique spreading are not displaced along transform fracture zones, and stresses are relaxed in accommodation zones without rupture of continuity of within-rift structural elements. The structural elements related to oblique spreading can be formed in both rift and megafault zones. At the initial breakup and divergence of continental or oceanic plates with increased crust thickness, the appearance of an extension component along with shear in megafault zones gives rise to the formation of embryonic accretionary structural elements. As opening and extension increase, oblique spreading zones are formed. Various destructive and accretionary structural elements (nearly parallel extension troughs; basin and range systems oriented obliquely relative to the strike of the fault zone and the extension axis; rhomb-shaped extension basins, etc.) can coexist in different segments of the fault zone and replace one another over time. The Andrew Bain Megafault Zone in the South Atlantic started to develop as a strike-slip fault zone that separated the African and Antarctic plates. Under extension in the oceanic domain, this zone was transformed into a system of strike-slip faults divided by accretionary structures. It is suggested that the De Geer Megafault Zone in the North Atlantic, which separated Greenland and Eurasia at the initial stage of extension that followed strike-slip offset, evolved in the same way.

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 paper presents materials on the composition and texture of weakly serpentinized ultrabasic rocks from the western and eastern walls of the Markov Deep (5°30.6′–5°32.4′N) in the rift valley of the Mid-Atlantic Ridge (MAR). The predominant harzburgites with protogranular and porphyroclastic textures contain two major generations of minerals: the first generation composes the bulk of the rocks and consists of Ol89.8–90.4 + En90.2–90.8 + Di91.8 + Chr (Cr#32.3–36.6, Mg#67.2–70.0), while the second generation composes very thin branching veinlets and consists of PlAn32–47 + Ol74.3–77.1 + Opx55.7–71.9 + Cpx67.5 + Amph53.7–74.2 + Ilm. The syndeformational olivine neoblasts in recrystallization zones are highly magnesian. The concentrations and covariations of major elements in the harzburgites indicate that these rocks are depleted mantle residues (the high Mg# of minerals and whole-rock samples and the low CaO, Al2O3, and TiO2 concentrations) that are significantly enriched in trace HFSE and REE (Zr, Hf, Y, LREE, and all REE). The mineralogy and geochemistry of the harzburgites were formed by the interaction of mantle residues with hydrous, strongly fractionated melts that impregnated them. The mineralogical composition of veinlets in the harzburgites and the mineralogical-geochemical characteristics of the related plagiogranites and gabbronorites suggest that these plagiogranites were produced by melts residual after the crystallization of gabbronorites. The modern characteristics of the harzburgites were shaped by the following processes: (i) the partial melting of mantle material simultaneously with its subsolidus deformations, (ii) brittle-plastic deformations associated with cataclastic flow and recrystallization, and (iii) melt percolation along zones of the maximum stress relief and the interaction of this melt with the magnesian mantle residue.

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.

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K2 O and TiOz (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K2O, TiO2, H2O. Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H2O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl2+ H2O; T= 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO2 and H2O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC12+ H2O + CO2; T= 200–310 °C; P= 900–1700 bar).

The Romanche is a long offset (∼950 km), slow slip (∼1.7 cm/yr) transform; thus a hot ridge axis should meet a ∼50-m.y.-old, thick and cold lithosphere at the ridge-transform intersection (RTI). A strong thermal/topographic “transform cold edge effect” is therefore predicted. A morphobathymetric, seismic reflection and petrologic study of the eastern Romanche RTI shows that as the Mid-Atlantic Ridge approaches the transform, a well-formed axial rift valley disappears about 80 km from the RTI and is substituted by short en echelon, poorly developed axial ridge segments; they too disappear about 30 km from the edge of the transform valley. The predicted gradual deepening of the ridge axis toward the transform was not observed. An active nodal deep and an “inside corner high” are also absent. These observations, and the distribution of earthquake epicenters, suggest a poorly developed, diffuse RTI. An inactive rift valley ∼80 km west of the present RTI suggests ridge jumping within the last ∼4 m.y. The present poorly developed RTI may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform. We infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle-derived serpentinized peridotites outcrop ubiquitously on the seafloor starting ∼30 km from the edge of the transform valley. The unusually deep (∼4 km below sea level) axial ridge segments, the lack of crust, and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region. Absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column. Mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform. These results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold RTIs and that ultracold RTIs are magma starved, short lived, and unstable in time and space.

The Romanch½ transform offsets the Mid-Atlantic Ridge (MAR) axis by about 950 km in the equatorial Atlantic. Multibeam and high-resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the RIV Akademik Strakhov as part of the Russian-Italian Mid-Atlantic Ridge Project (PRIMAR). Morphobathymetric data show the existence on the northern side of the transform of a major 800-kin-long aseismic valley oriented 10 o to 15 o from the active valley; it disappears about 150 km from the western MAR segment. The aseismic valley marks probably the former location of the Romanche transform ("PaleoRomanche") that was active up to roughly 8-10 Ma, when the transform boundary migrated to its present position. A temporary microplate developed during the migration and reorientation of the transform. This microplate changed its sense of motion as it was transferred from the South American to the African plate. A prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side, reaching its shallowest part (shallower by over 4 km than the predicted thermal contraction depth) in a zone opposite the eastern MAR axis/transform intersection (RTI). Flat-top peaks on the summit of the transverse ridge are capped by acoustically transparent, weakly stratified, shallow water platform/lagunal/reef limestones. This limestone unit is a few hundred meters thick and overlies igneous basement. Evaluation of the seismic reflection data as well as study of samples of carbonates, ventifact basaltic pebbles and gabbroic, peridofitic and basaltic rocks recovered at different sites on the transverse ridge, suggest that (1) the summit of the transverse ridge was above sea level at and before about 5 Ma; (2) the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate; its summit was flattened by erosion at sea level during subsidence; (3) the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature; and (4) transtensional and transpressional tectonics have affected the transverse ridge. Hypotheses on the origin of the Romanche transverse ridge include (1) lateral heat conduction across the RTI; (2) shear heating; (3) lithospheric flexure due to thermal stresses in the cooling lithosphere; (4) viscoelastic deformation of the lithosphere; (5) hydration/dehydration of mantle peridotites; and (6) longitudinal flow of melt and igneous activity across the RTI. These processes cannot by themselves explain the transverse ridge, although some of them could contribute to its formation to a small extent. Vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge/transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence. Uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former (PaleoRomanche) and the younger transform boundaries, before and during the transition to the present boundary. After migration of the transform boundary to its present position, transpression was replaced by transtension and by subsidence of the transverse ridge. An aseismic axial rift valley impacting against the transform valley about 80 km west of the present RTI suggests eastward ridge jumping that probably followed transform migration. Localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements, such as those that formed an ultradeep (>7800 m) pull-apart basin along the transform valley.

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.

Dredged glass from the southern Mid-Atlantic Ridge near the Bouvet Triple Junction has unique major element, trace element, and isotopic composition, distinct from typical mid-ocean ridge basalts, It is a high-Mg (Mg# 67.8), high-Ni (NiO 290 ppm) andesite depleted in highly incompatible and heavy rare-earth elements with an isotopic signature of ancient continental lithosphere (i.e., low (206)Pb/(204)Pb and Nd-143/Nd-144 and high Sr-87/Sr-86 and delta O-18). The origin of this glass is attributed to melting of a Precambrian garnet-bearing, mafic lithology, possibly related to lower crustal blocks stranded in the upper mantle during breakup of Gondwana and opening of the Atlantic. This composition can be used to explain anomalous geochemical features of oceanic rocks in the Southern Hemisphere.

Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°–15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a “cold” upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (∼8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites. Finally, the 15°20′ FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°–3°N and 14°–15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a “hot spot”-type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero-age topographic highs in the 2°–3°N and 14°–15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O-rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.

Composition of the deep-seated Inclusions of ultrabasic and basic rocks in kimberlites and basalts is correlated with the type of construction of the crust. We suggest that these inclusions reflect the composition of the lower horizons of the crust and the top of the mantle, and that ktmberlite was originally a gassaturated tectonite, developing on the boundary between mantle harzburgite and. the lower horizons of the crust (the Mohorovicic geophysical surface). This surface is probably one of the principal tectonic planes during the displacement of continents. Geological analysis suggests that the deep-seated inclusions in kimneriites are rocks from the base of the crust of the Precambrian metamorphic blocks. This once again makes it possible to the raise the question of the interpretation of the diamond occurrences in the rocks of the deep Precambrian. —.Authors.

(1973). The Paleozoides of Eurasia and certain aspects of the geosynclinal evolution. International Geology Review: Vol. 15, No. 8, pp. 869-882.

The South American, African, and Antarctic lithospheric plates meet in the Bouvet Triple Junction (BTJ) located in the South Atlantic near the island of Bouvet. Multibeam, magnetic, gravimetric, and seismic reflection data have been used to understand the evolution of the three accretionary/transform boundaries that converge in the BTJ. The easternmost segments of the American-Antarctic Ridge (AAR) have a spreading full rate of 19.5 mm/yr for the last 8 m.y. They are oriented N-S, except for some NE-SW segments, probably created by magma-poor extension. The southernmost portion of Mid-Atlantic Ridge (MAR) (spreading full rate of 30.5 mm/yr for the last 9 m.y.) has elevated topographic anomalies; it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation. The MAR axial valley bifurcates at its southern tip in two branches oriented 115 o and 180 o that are, or have been up to recently, loci of crustal accretion. The bifurcation represents a former ridge-ridge-ridge triple junction. The westernmost segments of the Southwest Indian Ridge (SWIR) are anomalously high. The segment adjacent to the island of Bouvet (spreading rate 14.5 mm/yr) is shallower than normal by almost 1 km due to the influence of the Bouvet hot spot. The westernmost SWIR segment (Spiess Ridge) consists of a "swollen" volcanic ridge that reaches 320 rn below sea level and has a deep caldera on its summit. Spiess Ridge narrows and deepens to the NW; V-shaped topographic and magnetic lineations suggest that it propagates NW at a rate of 40 to 50 mm/yr. The Spiess magmatic event started at roughly 1 Ma, when it caused deactivation of the 115 o spreading branch. Therefore the Antarctic, South American, and African plates meet presently not in a triple point but in a broad zone of diffuse deformation. An area of extensional deformation observed east of Spiess Ridge may be caused by excess crustal formation at Spiess Ridge that cannot be accommodated by motion of rigid plates. The evolution of the BTJ since 10 Ma involves stages of RRR, RFF and RRF configurations with highly variable geometry of the accretionary/transform boundaries. Topographic anomalies, anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread, strong thermal anomalies that have influenced the configuration of the BTJ, and determined indirectly intraplate deformation in wide areas of the BTJ region. The thermal anomaly that gave rise to the SWIR-Spiess excess magmatism is the prime cause of the recent disruption of a former RRR configuration, and of the imminent establishment of a new RRR Triple Junction to the north.