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

We present a three-dimensional crustal structure of a magmatically robust segment of the ultraslow- spreading Southwest Indian Ridge at 50 degrees 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 (< 5 km) and seems to be related to the fact that the oblique-spreading domain at the eastern end offsets the ridge axis by a larger distance than that at the western end. We suggest that an along-axis deep-penetrating hydrothermal circulation develops on the east side of the axial magma chamber, in response to the rapid change from orthogonal-to oblique-spreading domains and cools the crust.