Abstract:

The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggests that a large portion of crust at ultraslow spreading ridges is formed mainly by tectonic processes, with little or absence of magmatism.  Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment at 50.47°E, we discovered an extremely magmatic accretion of the oceanic crust. Our results reveal a low velocity anomaly (-0.6 km/s) at 4-8 km depth beneath the seafloor in the lower crust suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km), the thickest ever observed along mid-ocean ridges. We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30-50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow towards the center of the segment, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.