Geotektonika

The interaction of mobile lithospheric plates and mantle plumes approaching the surface, which have periodic supply of magmatic matter, leads to changes in geological and geophysical characteristics and the appearance of chains and compact groups of volcanoes of different ages in the oceans. Data on the relief, Bouguer anomalies and dating of seamount rocks along the hot spot tracks in the Atlantic, Pacific and Indian Oceans show the presence of stable temporary ~1.5, ~3.7, ~4.5‒7.5 and 10‒12 Ma periods of magmatism powered by different super-plumes. 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 channels. Analysis of the times of extremes in the tracks also indicates the compatibility of the periodicity of magmatism in phase. Groups of underwater magmatic 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 impulses of magmatism superimposed on the basement from 20 to 60 million years.

This and other facts indicate a fixed position of the supply channels 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 feeding plume. The pulses of magmatism that are currently continuing in various parts of the Atlantic were preceded by a pause in magmatism from 20 to 60 million years. 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 matter with simultaneous horizontal movement of plates.

The article discusses a plate-tectonic model of the formation of the Eurasian Basin based on the results of magnetostratigraphic analysis of linear magnetic anomalies covering the entire basin. According to this model, the Eurasian Basin is underlain by spreading oceanic crust, including wide marginal zones of the Nansen and Amundsen basins. A new reconstruction of formation stages of the Eurasian Basin is proposed, developed by us on the basis of the integrated multichannel seismic reflection data and wide-angle reflection/refraction data with involvement of geological data and materials of the island and coastal framework. The presented data show the superimposed nature of the Gakkel Ridge, which arose as a result of spreading, on the previously existing structural plan of the Eurasian Basin and the tracing of seismic horizons in the sedimentary cover of the basin that are more ancient than the Cretaceous. The stratigraphic reference of the identified complexes was made, the environment of their sedimentation and velocity parameters were clarified, and data on the relief of the unconformity surface were obtained. Based on the results of seismic reflection data, the deposits of the Jurassic‒Early Cretaceous complex of Taimyr Island and the Barents-Kara margin were traced in the sedimentary cover of the Nansen Basin. Based on the results of seismic observations of the CMP, the deposits of the Jurassic‒Early Cretaceous complex of Taimyr Island and the Barents-Kara margin were traced in the sedimentary cover of the Nansen Basin. The development of the Eurasian Basin before the opening of the Fram Strait is noted, and, accordingly, there is no connection with the spreading processes in the North Atlantic.

The article considers the segment of the Southwest Indian Ridge located between the Du Toit–Andrew Bain–Prince Edward fault zone system and the Bouvet triple junction. Two areas are distinguished within its boundaries, which differ in the structure of the seafloor topography and in their development. In the eastern area (from 9° E to 25° E) there are no transform faults and significant thermal anomalies in the mantle. The western section of the studied part of the ridge (from the Bouvet Triple Junction to 9° E) is dissected by several large transform faults and develops under the noticeable influence of the thermal anomaly of the Bouvet plume. Such a relationship between the segmentation of the seafloor topography and thermal anomalies of the mantle is atypical for areas of slow and ultra-slow spreading. Here the ridges are cut by transform faults, in areas with a noticeable thermal influence of mantle thermal anomalies these faults disappear. We carried out physical modeling and analysis of temperature field profiles in the constructed model to assess the influence of the melt accumulation depth on the segmentation of the Southwest Indian Ridge. We found that the melt accumulation depth has a noticeable effect on the segmentation of the mid-ocean ridge (MOR) bathymetry, but this is not the only main influencing factor. The segmentation of the MOR can be affected by the serpentinization process as well. A decrease in the spreading rate is accompanied by an increase in the depth of the magma chamber, or the area of the focused mantle upwelling. This leads to widespread serpentinization at the extension axis due to relatively low-intensity magmatism and high fracturing of rocks and, as a consequence, to the reorganization of the structural segmentation of the ridge due to the disappearance of transform faults with a decrease in the lithosphere strength. The combined effect of the depth of the melt accumulation and serpentinization on the section of the Southwest Indian Ridge, where there was no significant thermal anomaly in the mantle, could lead not only to the disappearance of transform faults, but also to maintain this state for a long period of time. In the western part of the study area of the Southwest Indian Ridge, which is under the influence of the mantle thermal anomaly, the conditions for serpentinization were less suitable, so transform faults are well developed here.