The morphology of the seafloor of the Madagascar Basin from Mauritius Island to the Southwest Indian Ridge (SWIR) is represented by a ridge-echeloned relief of the spreading basement. The azimuth of the relief differs by 90° for the basin north of the SWIR and its wedge-shaped sublatitudinal rift system, separated by an abyssal escarpment. A genetic definition of this seafloor relief shape is given. This shape is formed when the ancient basement breaks up and accretion of the crust orthogonal to the azimuth, which existed before the rupture, begins. The formation of a wedge in the eastern part of the SWIR began about 41 Ma ago and is manifested by higher (±1100 m) amplitudes of relief variations than at the basement before the rupture (±250 m). The change in morphology is also associated with the change in the spreading azimuth of the lithospheric block by about 24° north of the SWIR, which opened up a new space for accretion. The morphology of the relief in the wedge and beyond shows the relationship of its parameters with slowdown in the spreading rate by almost three times when the kinematics of the plates have changed. The high-amplitude ridge-echeloned relief in the ultraslow segment of the SWIR with signs of nontransform displacement is combined with the maxima and minima of Bouguer anomalies. According to the published data, serpentized peridotites and basalts are obtained in the localization of the anomalies. These rocks indicate the presence of detachments with the exposure of ultramafic rocks and minimal magmatic output. Bouguer anomalies along the regional profile perfectly reflect deep density inhomogeneties. For intraplate volcanic edifices, they have a much greater deconsolidation in the upper mantle than near the active interplate boundary of the SWIR. According to the seismic tomography data, the absence of a deep upwelling under the newly formed SWIR segment and the presence of a “cold” gap in the “hot” lenses of the mantle indicates the action of tangential forces in the lithosphere that are not associated with general mantle convection. The formation of the new orthogonal rift system with ultraslow rates is an adaptation to variations in the kinematics parameters of the adjacent lithospheric plates.

The thickness of the Atlantic Equatorial segment sedimentary cover decreases with the distance from the continental source of mass removal and increases slightly with the distance from the Mid-Atlantic Ridge (MAR). Background sedimentation makes a small contribution to the total sedimentary volume near the continents and the major contribution in basins with the formation of sedimentary bodies with a thickness of no more than 500 m. Sedimentary bodies with a thickness up to 1000 m near MAR are the results of unloading of bottom currents near structural barriers in topography and in fault troughs. The total thickness tends to increase from north to south on the western flank of the MAR. The multidirectional spatial migration of the basement depressions filled with sediments is revealed. To the west of the MAR, contrasting acoustic stratification and tectonic deformations of sediments were found. To the east of the MAR, deformations are rare and expressed by piercing structures and normal faults. Anomalies of the "bright spot" type associated with local manifestations of magmatism forming aligned high-amplitude reflectors are revealed. Normal faulting in the passive parts of the transforms zones indicates the presence of local stretching, which is part of the simple shear paragenesis. In the troughs, sediment deposition from the bottom currents, which forms channel drifts, is revealed.

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 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.

Приведены сведения о комплексе геолого-геофизических, литолого-палеоокеанологических, гидрофизических и биологических исследований в Центральной Атлантике в 45-м рейсе НИС “Академик Николай Страхов” в октябре–ноябре 2019 г. Обсуждаются предварительные результаты экспедиции.