The Sierra Leone Ridge is the equatorial portion of the Mid-Atlantic Ridge (MAR) between the St. Paul (0-1 degrees N) and Doldrums (7-9 degrees N) Fracture Zones. At similar to 80 Ma, a submarine plateau -the Sierra Leone Rise which is now located off-ridge on the African plate and the Ceara Rise on the South American plate- formed due to excess magma production. Incompatible element-enriched MORB with high Sr-Pb but low Nd isotope ratios suggests that the high magma production could result from the interaction of a mantle plume with the Sierra Leone Ridge, the so-called Sierra Leone mantle plume that may now be centered at similar to 1.7 degrees N along the MAR. In order to define the nature of the mantle source beneath the Sierra Leone Ridge, we present major-trace element concentrations and radiogenic isotope ratios from abyssal peridotites and MORB from 4 to 7 degrees N along the MAR. High Hf isotope ratios are preserved in clinopyroxenes from abyssal peridotites (epsilon(Hf) = 12-54), indicating that the mantle beneath the Sierra Leone Ridge underwent extensive melting several 10(8)-10(9) years before remelting under the present MAR. Most peridotites have high epsilon(Hf), but low epsilon(Nd) similar to MORBs, although some extend to unusually low epsilon(Nd) of similar to-6. We argue that these peridotites not only melted, but were also re-enriched in compatible elements concurrent to melting in the past, and that such incompatible element re-enriched peridotites are now the main component of the sub-ridge mantle in the Sierra Leone area. Extensive remelting of ancient, incompatible element re-enriched peridotite that is compositionally buoyant, in addition to minor amounts of recycled crust, may therefore have caused the abundant magmatism that characterizes this portion of the MAR since formation of the Sierra Leone and Ceara Rise similar to 80 Ma ago, and accounts for its elevated topography.

The paper, based on geological and geophysical data obtained during 55-th expedition of the research vessel “Akademik Nikolaj Strakhov”, examines the structure of the King’s Trough and its immediate surroundings (King’s mesostructural cluster), located on the eastern flank of the Mid-Atlantic Ridge in the North Atlantic. Six provinces were identified within the southeastern part of the King’s Trough, based on the results of bathymetric mapping, each of which has its own morphostructural appearance, which was formed as a result of multistage tectonic and volcanic processes, alternating and conjugate with each other in time. According to seismoacoustic profiling data, three main types of seismic faces have been identified: a) pelagic complexes; b) deposits of turbidite flows; c) chaotic facies of gravitational genesis. It is shown that the anomalous magnetic field of the study area is a superposition of linear and isometric anomalies. The first were formed during the generation of oceanic crust in the axial spreading zone. The second are associated with volcanic massifs formed in intraplate conditions. The obtained data confirm the assumption that the formation of the King’s Trough was preceded by the formation of an extended arched uplift, which became the scene of intense intraplate volcanism, the intensity of which increased from the southeast to the northwest. This stage was followed by subsidence of the axial part of the uplift with the formation of the King’s Trough and the Peak and Freen Troughs.

Petrogeochemical data on basalts (lithophile elements and Sr–Nd–Pb isotopes, liquidus olivine and spinel compositions) from the transition zone of the Southwest Indian Ridge (SWIR) in the Du Toit–Andrew Bain fault zone indicate significant differences in their composition. Tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are developed within the rift valley adjacent to the faults. Deep-type basalts (TOR-1) are present in the western wall of the Andrew Bain Fault. The outpouring of these magma types reflects a possible change in geodynamic setting during formation of this zone: from deep and high temperature to shallower magma generation conditions (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the composition of liquidus olivine. The olivines from rift valley are similar to the typical Na-TOR olivines with Mg number of Fo88-87, low Ni and elevated Mn contents. On the contrary, olivines in tholeiite from the Andrew Bain Fault Zone are enriched in Ni and depleted in Mn, which may indicate the involvement of pyroxenite in melt generation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which were subsequently involved in melting during the spreading axes jumping. A similar process is typical of the Bouvet Triple Junction, where the trace-element composition of olivine shows significant heterogeneity. The radiogenic composition of Pb and Sr of the Andrew Bain Fault tholeiites are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but differ from those of the Conrad and Af. Nikitin rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), which likely contains an admixture of EM-II component (with elevated Rb/Sr).

An Erratum to this paper has been published: https://doi.org/10.1134/S1028334X23060016

The structure of King’s Trough and its surroundings (King’s mesostructural cluster), located on the eastern flank of the Mid-Atlantic Ridge in the North Atlantic, is described in this paper. This work is based on geological and geophysical data obtained during the 55th expedition of the R/V Akademik Nikolaj Strakhov. Six provinces were identified within the southeastern part of King’s Trough, based on the results of bathymetric survey. Each province has its own morphostructural feature resulting from multistage tectonic and volcanic processes, which alternate and conjugate with each other in time. According to seismoacoustic profiling data, three main types of seismic facies have been identified: (a) pelagic complexes; (b) deposits of turbidite flows; and (c) chaotic facies of gravity origin. It is shown that the anomalous magnetic field of the study area is the superposition of linear and isometric anomalies. The first were formed during the formation of the oceanic crust in the axial zone of spreading. The second are associated with volcanic massifs formed under intraplate conditions. The obtained data confirm the assumption that the formation of King’s Trough was preceded by the formation of an elongated arched rise, which became a scene of intense intraplate volcanism that increased from southeast to northwest. This stage was followed by subsidence of the axial part of the rise with the formation of King’s Trough and the Peake and Freen Troughs.

Seafloor anomalies along mid-ocean ridges with exceptionally thick and compositionally distinct basaltic crust, for example, at Iceland, suggest that the underlying mantle is hotter and chemically different from the adjacent subridge mantle. Here we present hafnium and neodymium isotope ratios of peridotites from the Charlie Gibbs Transform Zone, which is located at the southern end of the Reykjanes Ridge south-west of Iceland. These peridotites are strongly depleted in incompatible elements with extremely high hafnium isotope ratios, suggesting that they had already melted to a large extent before being incorporated into the plume, at least 1 billion years ago, and thereby also became less dense. We argue that seismic velocity anomalies, geodynamic models and geochemical affinities of ridge basalts connect the peridotites from the Charlie Gibbs Transform Zone to the ‘Iceland plume’. The thermochemical buoyancy of the moderately hot Iceland plume, but also that of other plumes worldwide, may therefore be strongly influenced by composition. Variable peridotite depletion along the rising Iceland plume could also cause the transient, density-driven pulses in plume flux, which have formed the V-shaped Reykjanes Ridge south of Iceland. Overall, expansion of a ridge-centred plume along adjacent ridges and melting of heterogeneous plume material explains the topographic swell, the seismic anomaly and the formation of V-shaped ridges, as well as the regional distribution of basalts with Icelandic affinity. The upwelling mantle beneath Iceland underwent melt depletion at least 1 billion years ago and is therefore compositionally buoyant, according to a study of neodymium and hafnium isotope ratios in peridotites from the Charlie Gibbs Transform Zone.

This article presents new data on the structure and relationship of tectonic and magmatic processes during the formation of the Mid-Atlantic Ridge between the Charlie Gibbs and Maxwell Fracture Zones in the North Atlantic. It is shown that this region is characterized by significant reduction in volcanism, which leads to the uplift of deep-seated rocks (ultramafic rocks and compositionally diverse gabbroids) to the seafloor surface. Both separate oceanic core complexes of the most varied configurations and extended sublatitudinal ridges composed of plutonic rocks are formed. Our analysis showed that this geodynamic regime exists during at least 14–16 Ma. The formation of most oceanic core complexes is associated not only with tectonic factors, but also with the serpentinization of peridotites, which leads to a decrease in density, an increase in volume, and, as a result, to the ascent of large ultramafic massifs, including disintegrated blocks of gabbroids, dolerites, and basalts. Numerous zones of sliding, crushing, abrasion, and deformation of rocks are evidence of tectonic movements. The area of study is characterized by numerous nontransform displacements of different amplitudes, which resulted due to relative displacements of oceanic lithosphere segments in wide areas under shear and extension conditions. The morphology of the forming tectonomagmatic structures is determined by tectonic factors. The exceptions are cases where the volumes of basalt melt that come to the seafloor surface for a short period of time are significantly higher than the average ones for a certain segment of the rift valley. The analysis we performed shows the presence of sources of heterogeneous magnetic anomalies both of volcanic origin and those associated with superimposed tectonic processes.

This article presents results of the structural and morphological analysis of the fracture zones which are part of Doldrums Megatransform System (MTS), located in the northern part of the Equatorial Atlantic (6.5°–9° N) that include Vernadskiy and Bogdanov transform faults and the Doldrums and Pushcharovskiy megatransforms. Bathymetric map, based on the multibeam echo sounding data, collected during 45 cruise of the R/V Akademik Nikolaj Strakhov was used for this analysis. It was established that large-scale variations in the width of fracture zone valleys are determined by the distribution of stresses perpendicular to the fracture zone. In the areas with compressive stresses, the fracture zone valleys are narrower, and the in extension areas are wider. The difference in geodynamic settings within the MTS is due to the difference in spreading directions, which change from \(\perp \)89° to \(\perp \)93° when moving from south to north. The depth of fracture zone valleys consistently increases from the periphery of the MTS (Bogdanov and Doldrums faults) to the center (Pushcharovskiy fracture zone) in accordance with a decrease in the upper mantle temperature. In each fracture zone, the valley depth decreases from the rift- fracture zone intersections towards the center of the active part to a certain background depth. It is assumed that this phenomenon is the result of the uplift of the valley bottom, which occurred due to the decompaction of the lithosphere, caused by the serpentinization of ultramafic rocks. The violation of the revealed variations in the width and depth of fracture zone valley patterns occurs as a result of various ridges and uplifts formation in the fracture zone. In the axial zones of the active parts of the fracture zone valleys median ridges are widespread, extending parallel to the fracture zone and representing serpentinite diapirs squeezed out above the bottom surface. Transversal ridges which were formed 10‒11 million years ago as a result of the lithospheric plate edge flexural bending under extensional conditions are now located in the western passive parts on the southern sides of the of Doldrums and Pushcharovskiy fracture zone valleys. The transverse ridge on the northern side of the Vernadskiy fracture zone, which includes Mount Peyve, was formed between 3.65‒2.4 Ma. Due to the frequent jumps of the spreading axis in this region, it was divided into three segments. There are interfracture zone ridges in megatransforms, which in the active part consist of two fracture zone valleys. Time of their formation: in Pushcharovskiy megatransform ‒ 30‒32 million years ago and in Doldrums megatransform ‒ about 4 million years ago. Due to the curvilinearity of the outlines and under the pressure of moving lithospheric plates, the interfracture zone ridges experience longitudinal (along the fault) compressive and tensile stresses, which are compensated by vertical uplifts of their separate blocks and the formation of depressions, pull apart depressions, and spreading centers (the latter are only in Pushcharovskiy megatransform). Structure-forming processes that determine pattern and morphology of the fracture zones as a part of the MTS are related by their origin to the spreading and transform geodynamic systems.

Mid ocean ridge basalts are partial melts of mantle rocks that have become incompatible element depleted prior to recent sub‐ridge melting. This prior extent of melt extraction and concomitant incompatible element depletion is often inferred from the radiogenic isotope ratios of ridge basalts, such as hafnium and neodymium. However, hafnium isotope ratios by far exceeding those in ridge basalts are observed sporadically in exposed mantle rocks, abyssal peridotites. At least locally, therefore, Earth's mantle has evolved with much larger extents of incompatible element depletion over geologic time periods (10 8 –10 9 years) than inferred from ridge basalts. Here we show that such “ultra‐depleted” hafnium isotope signatures also characterize peridotites exposed at the Doldrums Fracture Zone in the equatorial Mid Atlantic. These peridotites are closely associated with peridotites characterized by hafnium‐neodymium isotope ratios similar to ridge basalts, but extensive incompatible element depletion. Our results demonstrate that Earth mantle's isotopic record of prior melt extraction is dampened by ubiquitous past and recent reaction with migrating melts. Hence, a better understanding of chemically reactive flow is required for deducing the mantle's rate of incompatible element depletion by recurring processing through melting regions in the shallow mantle, which is the driving force for silicate Earth evolution. Ultimately, better constraining the rate of incompatible element depletion from Earth's mantle is therefore of first‐order importance for understanding our planet's basic mode of operation.

— This article presents results of the structural and morphological analysis of the fracture zones that are part of Doldrums Megatransform System (DMS), located in the northern part of the Equatorial Atlantic (6.5°–9° N) that include Vernadskiy and Bogdanov transform faults and the Doldrums and Pushcharovskiy megatransforms. Bathymetric map, based on the multibeam echo sounding data, collected during the 45th cruise of the R/V Akademik Nikolaj Strakhov was used for this analysis. It was established that large-scale variations in the width of fracture zone valleys are determined by the distribution of stresses perpendicular to the fracture zone. In the areas with compressive stresses, the fracture zone valleys are narrower and the extension areas are wider. The difference in geodynamic settings within the DMS is due to the difference in spreading directions, which change from ⊥ 89° to ⊥ 93° when moving from south to north. The depth of fracture zone valleys consistently increases from the periphery of the DMS (Bogdanov and Doldrums faults) to the center (Pushcharovskiy fracture zone) in accordance with a decrease in the upper mantle temperature. In each fracture zone, the valley depth decreases from the rift- fracture zone intersections towards the center of the active part to a certain background depth. It is assumed that this phenomenon is the result of the uplift of the valley bottom, which occurred due to the decompaction of the lithosphere, caused by the serpentinization of ultramafic rocks. The violation of the revealed variations in the width and depth of fracture zone valley patterns occurs as a result of various ridges and uplifts formation in the fracture zone. In the axial zones of the active parts of the fracture zone valleys median ridges are widespread, extending parallel to the fracture zone and representing serpentinite diapirs squeezed out above the bottom surface. Transverse ridges that were formed 10‒11 million years ago as a result of the lithospheric plate edge flexural bending under extensional conditions are now located in the western passive parts on the southern sides of the of Doldrums and Pushcharovskiy fracture zone valleys. The transverse ridge on the northern side of the Vernadskiy fracture zone, which includes Mount Peyve, was formed between 3.65‒2.4 Ma. Due to the frequent jumps of the spreading axis in this region, it was divided into three segments. There are interfracture zone ridges in megatransforms, which in the active part consist of two fracture zone valleys. The times of their formation were in the Pushcharovskiy megatransform, 30‒32 million years ago and in the Doldrums megatransform, about 4 million years ago. Due to the curvilinearity of the outlines and under the pressure of moving lithospheric plates, the interfracture zone ridges experience longitudinal (along the fault) compressive and tensile stresses, which are compensated by vertical uplifts of their separate blocks and the formation of depressions, pull apart depressions, and spreading centers (the latter are only in Pushcharovskiy megatransform). The structure-forming processes that determine the patterns and morphology of the fracture zones as a part of the DMS are related in their origin to the spreading and transform geodynamic systems.

The structure of the ocean floor and the composition of basalts and dolerites of the MAR segment between the Maxwell and Charlie Gibbs FZs (North Atlantic) were studied based on the data obtained during the 53rd cruise of the R/V Akademik Nikolaj Strakhov organized and performed by the Geological Institute of the Russian Academy of Sciences over the period of July 7–August 15, 2022. In this segment, areas of greater and lesser magmatic productivity, which correspond to higher and lower bottom relief, alternate along the spreading axis. In high-relief areas, spreading cells form in the axial zone, and rises of tectonic and volcanic genesis dominate in the crest zone. In low-relief areas, the rift valley consists of deep rift basins; low ridges are developed on the flanks, which are separated by wide depressions. Oceanic N- and T-type tholeiites and E-MORB-type basalts and dolerites are distinguished among the studied volcanics. The N-tholeiites are widespread and were derived from mainly depleted mantle (source of DM). E-MORB-type basalts and dolerites are found in high-relief areas. Their mantle substrate is formed by a mixture of DM and EM-2 material with the subordinate role of HIMU. T-MORB volcanics are mainly localized on large volcanic rises in the southern part of the studied MAR segment and were melted from a substrate formed by a mixture of DM and HIMU material with the subordinate role of EM-2. Two types of mantle inhomogeneities involved in melting are reconstructed: passive and active. The former are represented by blocks of the transformed continental lithosphere that are similar in composition to the EM-2 mantle source. Active inhomogeneities associate with the uplift near Maxwell FZ of the microplume of the enriched mantle with a composition close to HIMU and with its subaxial flow in the north direction up to the Charlie Gibbs FZ.

Composition of basalts collected from the slopes of a single seamount (guyot) located south of the eastern flank of the Charlie Gibbs FZ in the North Atlantic, was studied. High-titanium and low-titanium groups related respectively to E-MORB and T-MORB oceanic tholeites, were singled out. Basalts of the first group are characterized by higher concentrations of TiO2, Na2O, K2O, P2O5, Fe2O3 and incoherent trace-elements. Basalts of both groups were melted from a mantle substrate of similar composition, which is a mixture of DM and HIMU mantle material and to a lesser extent – EM-2. Their difference in composition is due to the fact that high-titanic melts were formed deeper at a level between the spinel and garnet facies. Basalts similar in petro-geochemical and isotopic parameters were found in the axial zone of the Mid-Atlantic Ridge between the Charlie Gibbs and Maxwell FZ, where they compose large neovolcanic edifices formed with a sharp increase in the intensity of axial magmatism as a result of microplumes rise. Our studies give grounds to believe that the studied seamount was also formed as a large neovolcanic structure 64–67 million years ago under the influence of a microplume, which was branch of the Milne plume of the deep mantle.

The article provides information on geological and geophysical studies of the structure of the Mid-Atlantic Ridge between the Charlie Gibbs and Maxwell transform faults in the North Atlantic during cruise 53 of the R/V Akademik Nikolaj Strakhov, July–August 2022. The preliminary results are discussed.

This study investigates crustal accretion processes along the northern stretch of the Mid-Atlantic Ridge (MAR) between the Charlie Gibbs (52 & DEG;-53 & DEG;N) and Bight (57 & DEG;N) transforms. These long-lived transform systems, active for more than 40 Ma, bound a -550 km-long MAR segment influenced to the South by the Azores and to the North by the Iceland mantle plumes. The Bight transform is located at the tip of the Reykjanes Ridge, where the spreading direction, influenced by the southward propagation of the Iceland plume, changes from oblique (30 & DEG; to the axis) to perpendicular to the axis. Four hundred kilometres to the south, the MAR is offset by the Charlie Gibbs transform system consisting of two long-lived right-lateral transform faults linked by a short -40 km-long spreading segment. Previous expeditions surveyed large areas of these two transform systems, defining their main morphological features. Based on these bathymetric data, Expedition V53 of the R/V A.S. Vavilov carried out an intense dredging program coupled with magnetic surveys in an area spanning from 57 & DEG; to 52 & DEG;N, covering both the Bight and the Charlie Gibbs transform systems. We collected 1850 kg of rock samples including limestones, basalts, gabbros and mantle peridotites from 27 dredging sites, along with two 6-m long sedimentary cores. The sampled lithologies are globally in agreement with the con-trasting morphological features of the two transform faults. We discuss here and compare the geology of these two major transform systems and assess the influence of the Icelandic plume on seafloor morphology at the Bight Fracture Zone.

The paper presents new data on the Doldrums megatransform fault located in the northern Equatorial Atlantic (6.5°–9° 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.

We provide information on geological and geophysical investigations of the structure of the area between the Bight and Charlie Gibbs transform faults in the North Atlantic during cruise 53 of the R/V Akademik Sergey Vavilov in September–October 2021. The structure of the Eastern Tula Rise is also considered. The preliminary results of the expedition are discussed.

The prominent Charlie Gibbs right-lateral multi-transform system (52°-53°N) offsets the Mid Atlantic Ridge (MAR) by ~340 km. The transform system is formed by two distinct transform faults linked by a short ~40 km-long intra-transform spreading centre (ITR). The two adjacent MAR segments are influenced by both the Azores and the Iceland mantle plume. Recently, high resolution multibeam surveys and a dense sampling program of the entire transform system, including the adjacent southern and northern MAR segments, were carried out during expeditions of R/V Celtic Explorer (2015, 2016 and 2018) [1], R/V A.N. Strakhov (2020) and A.S. Vavilov (2021) [2]. The new surveys show widespread occurrence of large structures with corrugated surfaces and exhumed lower crust and mantle rocks on both sides of the intra-transform spreading axis. Morphological analyses of the intra-transform domain and magnetic data indicate that crustal accretion was driven by flip-flop detachment faulting [3], with minimal ridge melt supply and little axial volcanism. The tectonic spreading persisted for tens of millions of years. Along axis MORB chemistry shows that changes in seafloor accretion styles are mirrored by variations in melt supply, in turn dependent on mantle temperature and by a large-scale mantle heterogeneity. Charlie Gibbs is a key case study of how seafloor accretion modes at a spreading segment is critically dependent on mantle thermal state but also on its intrinsic compositional heterogeneity. [1] Georgiopoulou A. and CE18008 Scientific Party, 2018. Tectonic Ocean Spreading at the Charlie-Gibbs Fracture Zone (TOSCA): CE18008 Research Survey Report. Marine Institute of Ireland, Dublin, pp 1-24. [2] Skolotnev, S. et al., 2021. Seafloor Spreading and Tectonics at the Charlie Gibbs Transform System (52-53ºN, Mid Atlantic Ridge): Preliminary Results from R/V AN Strakhov Expedition S50. Ofioliti, 46(1). [3] Cannat, Met al., 2019. On spreading modes and magma supply at slow and ultraslow mid-ocean ridges. Earth and Planetary Science Letters, 519, 223-233.

Geological and geophysical data collected during the 53rd cruise of the R/V Akademik Sergey Vavilov are presented. It is shown that the lateral distribution of the Icelandic plume material to the south is not bounded by the Bight transform fault and continues further to the south. The Bight FZ affects the processes of crustal accretion and the formation of tectonic structures only in a spreading segment directly adjoining the Bight FZ from the south, i.e., the most distant from the Icelandic plume. The anomalous magnetic field studied made it possible to determine the half-spreading rate (11.7 mm/yr) in the westerly direction for the area between the Bight transform fault and the Modred nontransform fault of up to 3.6 Ma in age. It is shown that the flanks of the Mid-Atlantic Ridge represented a shallow-water area with some areas of land in the recent past.

The geological structure of a large volcanic edifice in the eastern flank of the large Charlie Gibbs Fracture Zone in the region of the Eastern Thule submarine rise (North Atlantic) is discussed. It is shown that the volcanic edifice was formed near the axial zone of the Mid-Atlantic Ridge in the interval of 64–67 Ma ago. Subsequently, its summit part was destroyed by wave abrasion, and then it sank along with the oceanic part of the plate to 2500 m deep. It was concluded that volcanism in the Rockall Rift Basin affected the region further to the south, which was already an oceanic basin and was separated from the rift basin with the Charlie Gibbs Fracture Zone.