THIN CRUST ON THE FLANKS OF THE SLOW-SPREADING SOUTHWEST INDIAN RIDGE

The Southwest Indian Ridge marks an end-member of mid-ocean ridge spre ading rates, with a current full rate of only 13-15 mm a(-1). Most the rmal models of mid-ocean ridges suggest a decrease in crustal thicknes s 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 form ed at very slow-spreading ridges are sparse. We have reanalysed data f rom a two-ship, split spread seismic refraction experiment conducted i n 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variatio ns, which were carefully recorded by the original investigators. 1- an d 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity o f similar to 6.0 km s(-1) near the top of the crust, increasing to sim ilar 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 t he crust, which is intermediate between values for gabbros and serpent inized upper mantle. The crustal thickness is consistent with a passiv e mantle upwelling model of melt generation at mid-ocean ridges. The u nusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.