Geodynamic condition of the bottom topography formation at the Madagascar Basin from data of 29th cruise of R/V “Akademik Nikolaj Strakhov”

Madagascar basin bottom morphology from Mauritius Isl. to South-West Indian Ridge (SWIR) is represented by a ridge-echeloned topography type of the spreading basement, the azimuth of which 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 bottom topography shape is given, which is formed when the existing basement breaks up and the crust accretion begins orthogonal to the azimuth that existed before the rupture. The formation of the wedge in the eastern part of the SWIR began ~41 Ma ago and is expressed 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 a ~24° change in the azimuth of lithospheric block spreading 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 a slowdown in the spreading rate by almost 3 times when the kinematics of the plates changed. The high-amplitude ridge relief in the ultra-slow segment of the SWIR with signs of nontransform displacement is combined with the maxima and minima of the Bouguer anomalies, in the localization of which, according to literary data, serpentinized peridotites and basalts are obtained, indicating the presence of detachments with the exposure of ultramafic rocks and minimal magmatic output. The Bouguer anomalies along the regional profile fully reflect the deep density inhomogeneities, which for intraplate volcanic structures have a much greater effect of upper mantle density loss than for the active interplate boundary of the SWIR. The absence of a deep upwelling under the newly formed SWIR segment and the presence of a “cold” gap in the “hot” lenses of mantle according to seismotomography data indicates the impact of tangential forces in the lithosphere that are not associated with general mantle convection. The formation of a new orthogonal rift system with ultra-slow rates is an adaptation to variations in the kinematics parameters of adjacent lithospheric plates.