Seismic structure and magmatic construction of crust at the ultraslow-spreading Southwest Indian Ridge at 50°28'E

International audienceWe present a three-dimensional crustal structure of a magmatically robust segment of the ultraslow-spreading Southwest Indian Ridge at 50°28'E based on tomographic inversions of an ocean bottom seismometer data set. Our results show an upper crustal low-velocity band in the axial zone, which is attributed to increased porosities due to active extensions, leading to anisotropy in the upper crust with a fast direction subperpendicular to the spreading direction. In the lower crust, the results reveal a round-shaped low-velocity anomaly at the segment center, indicative of high temperatures and/or a small amount of melt, suggestive of the presence of an axial magma chamber. At the midcrustal depth, an along-axis asymmetry is observed with respect to the segment center. While a small low-velocity anomaly indicates lateral magma redistribution toward the western segment end, the deep-penetrating low velocities and high velocity gradients toward the eastern end suggest that the crust is colder and contains a thicker fractured layer. This asymmetry occurs very close to the axial magma chamber (