Seismic crustal structure of the ultraslow spreading Southwest Indian Ridge segment at 50º28’E

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 is formed mainly by tectonic processes, with little or absence of magmatism. Although the thermodynamic models predict a largely reduced melt supply at ultraslow ridges, the strong along-axis variations indicate that melt focusing should exist to form large volcanic centers. In this thesis, I study a magmatically robust segment of the ultraslow spreading Southwest Indian Ridge at 50?28’E (segment 27) with a set of active-source ocean bottom seismometer data. A three-dimensional (3-D) travel time tomography at the shallow segment 27 reveals a round-shaped (diameter of ~7 km) low velocity anomaly (-0.6 km/s) in the lower crust beneath the segment center, extending from 4 km depth below seafloor to the Moho boundary. It is accompanied by an unusually thick crust (~9.5 km), the thickest ever observed along ’normal’ mid-ocean ridges, indicating an extremely magmatic accretion. The resolution of the LVA is further improved by a 2-D full waveform inversion of an across-axis profile running over the segment center, which shows a strong velocity contrast at its top boundary, suggesting the correspondence to an axial magma chamber (AMC) and the accumulation of melt at the roof. I also perform a 2-D travel time tomography of an along-axis profile that extends from segment 27 to segment 28 and non-transform discontinuities (NTDs) on the west side. The two-segment tomographic image shows a significant along-axis variation in the crustal thickness from ~9.5 km at the center of segment 27 to less than 4 km beneath the neighboring NTD, within a 30-50 km distance, requiring a highly focused melt delivery from the mantle. The crustal thickness is rapidly increased to ~7 km beneath the center of segment 28 to form a narrow (10-15 km) crustal root, which is closely surrounded by the cold and fractured lithosphere beneath the NTDs. While the NTD between segments 27 and 28 exhibits a typical structure composed of a thin, intensely fractured volcanic crust overlying a partially serpentinized mantle, it may have been fed by the AMC at segment 27 through shallow processes, such as lateral dyke propagation. The evidence of such processes is found in the 3-D tomography results as a small low-velocity anomaly at the mid-crustal depth on the west side of the AMC.