Geodynamic Settings of the Seafloor Relief Formation in the Madagascar Basin from Data of the 29th Cruise of R/V Akademik Nikolai Strakhov

Abstract

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.