Thin Crust On the Flanks of the Slow-Spreading Southwest Indian Ridge Free

The Southwest Indian Ridge marks an end-member of mid-ocean ridge spreading rates, with a current full rate of only 13–15 mm a⁻¹. Most thermal models of mid-ocean ridges suggest a decrease in crustal thickness at such slow spreading rates, because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression. Seismic measurements of the thickness of crust formed at very slow-spreading ridges are sparse. We have reanalysed data from a two-ship, split spread seismic refraction experiment conducted in 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variations, which were carefully recorded by the original investigators. 1- and 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity of ≈6.0 km s⁻¹ near the top of the crust, increasing to ≈7.0 km s⁻¹ just above the Moho, and a crustal thickness of 5 km. Converted shear waves yield a Poisson's ratio of 0.30±0.01 in the crust, which is intermediate between values for gabbros and serpentinized upper mantle. The crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid-ocean ridges. The unusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.