Sergey Yu. Sokolov

Seismoacoustic profiles along the western passive margin of the Chain transform fault made it possible to define anomalies in the wavefield within the upper sedimentary layers: abrupt loss of coherency, box folds, “flat” and “bright” acoustic spots, and thrust-type faults. These features are interpreted as signatures of the ascending fluid flow. The anomalies are localized in the transition zone between the Romanche and Chain transform faults, where negative Bouguer gravity anomalies are indicative of a reduced density in the upper mantle to a depth of several tens of kilometers. It is important to note that the sedimentary cover is less than 1000 m thick in these areas. Hence, the mud volcanism can be ruled out as a process for deep fluid ascent. Based on the total array of field data and theoretical prerequisites, the origin of the fluids is attributed to serpentinization. This process depends on the presence and activation of deep-seated fault systems within the intraplate setting. This fact explains the spatial correlation of the anomalies with the transform trough.

The structure and geodynamics of the Atlantic‒Arctic Rift System (AARS) are influenced by two main factors: the spreading rate and heated state of the lithosphere and sublithospheric space. It is shown that the geodynamic and structural properties of the lithosphere can be characterized by the following parameters: spreading rate (i), Bouguer anomalies (ii), the depth of the Moho boundary (iii), depth of the base of the lithosphere (iv), and the slice of seismic S-wave velocity variations at a sublithospheric depth of 150 km (v). Cluster analysis of these parameters divides the AARS lithosphere into nine stable combinations of these parameters, whereas visual analysis does not allow their identification. These combinations have a clear geological and geophysical interpretation and are segmented by demarcation faults, which determine the differences in the thermal evolution of the AARS segments. The structure of the AARS lithosphere exhibits superimposed variations relative to the smooth theoretical “cooling” trends of parameters from the axis of the Mid-Atlantic Ridge to the ocean–continent boundary. These variations are represented by manifestations of intraplate magmatism and zones of kinematic reorganization of AARS structure formation during the Mesozoic and Cenozoic. Pseudosynchronous and antiphase changes in the depths of the crustal base and lithosphere are observed, spatially associated with slowing of the spreading rate to ultraslow values. Theoretical trends are also disrupted by the superposition of plume heads with lithospheric plates. In the young segments of the AARS in its northern part, the shift of intervals with slower spreading rates west of undulations in the geometry of lithospheric boundaries may indicate a westward drift of the American plates relative to key sublithospheric rheological inhomogeneities. The resulting distribution of lithospheric parameters reveals a bimodal distribution of spreading rates, mantle thermal state, and their causal relationship with other parameters.

The basement structure of the Newfoundland–Iberian segment of the Atlantic–Arctic rift system bounded by the 1st order demarcation faults, within which the demarcation boundary of the 2nd order is distinguished, fundamentally differs to the north and south of it according to the relief and gravity anomalies. The “aspect” attribute calculated from the free air gravity is a morphometric attribute that reflects as plausibly as possible the basement structures beneath the sedimentary cover near the continental margins of the segment and in the Mid-Atlantic Ridge area free of sedimentary cover. Evaluation of methods for calculating the quantitative characteristics of this attribute in test areas with the predominance of typical mutually orthogonal seafloor structural elements showed that the 2D spectra of the analyzed field in the local areas, in addition to the orientation of structures, also demonstrate their amplitude in the initial field and their spatial periodicity, and adequately describe the structural style of the basement in the present-day coordinates, varying from the basin periphery to the mid-ocean ridge. Evolution leads to the formation of basement with a variable geometry, which involves seafoor rotation by ∼10°–15° and deviations due to superimposed atypical structures.

Geomorphological analysis and age correlation of the Late Pliocene–Quaternary regional stages identified in the ARC1407A seismic section by onlapping the oceanic basement, with the age specified based on calculations of the position of theoretical linear magnetic anomalies, were carried out. Interpretation of the ARC1407A seismic time section makes it possible to use the previous seismostratigraphic correlations of glaciomarine deposits for the western Barents Sea and the northeastern part of the adjacent deep-water basin. Based on the geomorphological analysis, extended canyon systems were identified in the Nansen Basin and eastern Amundsen Basin. Significant amounts of sedimentary cover in the Nansen Basin are glaciomarine sediments deposited since the end-Late Pliocene. In the central Nansen Basin, glaciomarine sediments were deposited simultaneously from two closely located troughs (Saint Anna and Voronin). Glaciomarine sediments, transported to the central Nansen Basin cyclically, have a mixed character (underwater landslides and turbidite flows). In the Amundsen and Podvodnikov basins, glaciomarine deposits began to form in the second half of the Middle Neopleistocene.

The interaction of mobile lithospheric plates and mantle plumes approaching the surface, which have a periodic supply of magmatic material, leads to changes in geological and geophysical characteristics and the appearance of chains and compact groups of volcanoes of different ages in oceans. Data on the relief, Bouguer anomalies, and dating of seamount rocks along hot spot tracks in the Atlantic, Pacific, and Indian oceans show the presence of stable temporal—~1.5, ~3.7, ~4.5‒7.5, and 10‒12 Ma—periods of magmatism powered by different superplumes. These values correspond to the periods of maxima of the spectral density of sea level fluctuations. The same frequency set of these phenomena indicates a single mechanism and time modulation of activity in magma-conveying conduits. Analysis of the times of extrema in the tracks also indicates the compatibility of the periodicity of magmatism in phase. Groups of underwater igneous structures without plate movement tracks in the coordinates of the age of the basement and the analytical age of the rocks form compact but geographically separated groups in this reference system, in the range of all ages of the basement of the Atlantic Ocean, and have a duration of pulses of magmatism overprinted on the basement from 20 to 60 Ma. This and other facts indicate a fixed position of the supply conduits relative to the African Plate on the eastern flank of the Mid-Atlantic Ridge during the Cenozoic. They substantiate the assumption of the general western drift of the lithospheric plates and their displacement from the feeder plume. The pulses of magmatism that are currently continuing in different parts of the Atlantic were preceded by a pause in magmatism from 20 to 60 Ma. Analysis of seismic tomography data allows us to explain the discrete spatiotemporal distribution of magmatic pulses by a combination of a variable regime of vertical supply of heated material with simultaneous horizontal movement of plates.

Based on the results of complex geological and geophysical works carried out during the 38th, 52nd and 56th cruises of the R/V “Akademik Nikolaj Strakhov” and the 51st cruise of the R/V “Akademik Boris Petrov”, the fluidogenic relief was studied in detail at five key polygons in the southern and northeastern parts of the Barents Sea shelf. Digital elevation models of sea bottom obtained as a result of multibeam bathymetric survey allowed identifying of 2218 pockmarks. Their diameter varies from the first dozens of meters to 250 m, and the depth from 0,2 to 7,8 m. The morphometric analysis of these landforms provided for studying the relationship between the area, depth and shape of the pockmarks cross-sectional profile and physical geographic conditions (tectonic structure, lithology of Quaternary sediments, background topography, hydrological conditions). The distribution of pockmarks and other manifestations of degassing is traceable associated with the fault network. It acts as a pathway for fluid uplift from the gas-saturated sediments of predominantly Mesozoic age to the surface, which proves the deep origin of fluids. The tectonic structure of the area is also reflected in the mutual location of pockmarks within the polygons: the areas of increased density of landforms are near fault zones and in depressions, which are probably of tectonic origin. It was found that the morphology of pockmarks is primarily determined by the lithology of the Quaternary sediments. Large landforms with a more gentle profile are more common in the areas with sandy loams and sands. Small landforms with a V-shaped cross-sectional profile are within the areas with denser clay sediments. It was suggested that the deepest pockmarks occur in the areas covered by glaciers in the recent past. It may be related to active degassing of sediments after removal of the glacial load. Modern exogenous subaquatic processes have a significant influence on the change of pockmark morphology. The slope processes make the cross-sectional profile of pockmarks more complex, i. e. asymmetric and micro-stepping, while the influence of bottom currents leads to landforms elongation.

Comparison of the geophysical data obtained along the Mid-Atlantic Ridge (MAR) segment between 10° and 25° N and seafloor massive sulfide (SMS) occurrences made it possible to reveal a number of regularities and relationships in their spatial distribution. SMSs are grouped in the areas with the following features: near-subvertical “microplumes” which a single axial anomaly of the MAR determined by the UU-P07 seismotomographic model is divided into; maxima of Bouguer anomalies between the Vema and Kane fracture zones corresponding to solid mantle volumes and minimum magmatic flow rate; pronounced alternation of spatiotemporal seismicity “voids” and “bands” with a step of ∼1.5°, comparable to the near-surface δVp minima; inversion of spreading rate asymmetry from the east of the MAR to the west within 20° and 24° N south of the Kane Fracture Zone with nontransform south-oriented displacements; and areas with a positive correlation of Bouguer anomalies and the terrain between 19° and 22° N. The relationship of SMSs with the specified geophysical features forms a basis for predicting this type of seabed minerals.

This research is based on the results of multibeam echo-sounding and high-frequency seismic profiling, carried out during the cruises of the R/V Akademik N

The Charlie–Gibbs Fracture Zone is the main transform system in the North Atlantic Ocean. It serves as a primary deep-water gateway for the Iceland–Scotland Overflow Water current. Using high-resolution seismo-acoustic profiling, three types of contourite drift were identified in the valleys of the inactive fractures and active transform of the Charlie–Gibbs Fracture Zone: (1) channel-related drifts; (2) confined drift; and (3) plastered drift. The Iceland–Scotland Overflow Water current is the main agent controlling drift formation in the northern valley and eastern part of the southern valley, while it is suggested that the Denmark Strait Overflow Water or Lower Deep Water dominate in the western part of the southern valley. Examination of two sediment cores shows that the contourite drifts at least at their upper parts consist of alternating muddy and silty contourites and pelagic/hemipelagic sediments. The contourite deposition corresponds to the present interglacial interval. Pelagic/hemipelagic sedimentation prevailed during a glacial interval (marine isotope stages 3–2) and the last glacial/interglacial transition, corresponding with high ice-rafted debris input. The position of the highly productive Subarctic Front over the study area induced the formation of diatom ooze beds at the beginning of marine isotope stage 1 and high-carbonate pelagites during some intervals of marine isotope stage 3.

Distribution with the fault network defined from seismic data, and kinematic characteristics of seismic activity spatial migration are obtained. It is shown that seismic events recorded by the Norwegian NORSAR regional network within the Russian part of the Barents Sea shelf are grouped into linear clusters along shear kinematics faults. The fault network displaces Mesozoic seismic complexes and reaches the bottom surface, displacing quaternary deposits, which clearly indicates the modern age of displacements along which linear clusters of weak seismicity are grouped. The calculation of the total seismic moment in the space-time dimension showed the presence of seismic activity migration along short fragments of faults on the shelf with velocities from 10.5 to 25.7 km/year. There has been a surge in general activity in the shelf area since 2012. A comparison of the temporal evolution of seismic activity on the shelf with fragments of the Atlantic-Arctic Rift system suggests that it is the effect of tectonic deformation waves triggered along the geodynamically active intraplate boundary and propagating to the shelf at a speed of 20–22 km/year. Migration rates with speeds up to 77 km/year are less likely. There is a possibility that the increase in the intensity of seismic activity on the shelf after 2012 is not an emission from the effects of a slow deformation wave, but the result of a direct trigger effect on the shelf from the structures of the Knipovich and Gakkel ridges.