Formation of the axial relief at the very slow spreading Southwest Indian Ridge (49 degrees to 69 degrees E)

The comparison of segment lengths, relief, and gravity signature along the very slow spreading Southwest Indian Ridge (SWIR) between 49 degrees E and 69 degrees E suggests that the marked change in segmentation style that occ urs across the Melville transform (60 degrees 45'E) reflects a change in th e modes of formation of the axial topography. We propose that the axial rel ief east of Melville is largely due to volcanic constructions that load the axial lithosphere from above. By contrast, the axial relief in segments we st of the Melville fracture zone appears to be primarily due, as proposed f or segments of the faster spreading Mid-Atlantic Ridge, to along-axis chang es in the depth of the axial valley, and to partial compensation of negativ e loads (thicker lower crust and/or lighter upper mantle) acting within the plate, or at the bottom of the plate. In terms of geology, this means that the contribution of the uppermost, effusive, part of the crust to along-ax is crustal thickness variations may be greater east of Melville than in oth er regions of the study area. Regional axial depths suggest that the ridge east of Melville is also characterized by a low melt supply and is underlai n by cold mantle. A simple model of mantle melting and regional isostatic c ompensation suggests that differences in mantle temperature and in melt thi ckness between this deep eastern ridge region, and the shallower region wes t of the Gallieni transform (52 degrees 20'E), are of the order of 80 degre es C and 4 km, respectively.