The comparison of segment lengths, relief, and gravity signature along the very slow spreading Southwest Indian Ridge (SWIR) between 49 ¿E and 69 ¿E suggests that the marked change in segmentation style that occurs across the Melville transform (60 ¿45'E) reflects a change in the modes of formation of the axial topography. We propose that the axial relief east of Melville is largely due to volcanic constructions that load the axial lithosphere from above. By contrast, the axial relief in segments west of the Melville fracture zone appears to be primarily due, as proposed for segments of the faster spreading Mid-Atlantic Ridge, to along-axis changes in the depth of the axial valley, and to partial compensation of negative 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-axis crustal thickness variations may be greater east of Melville than in other 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 underlain by cold mantle. A simple model of mantle melting and regional isostatic compensation suggests that differences in mantle temperature and in melt thickness between this deep eastern ridge region, and the shallower region west of the Gallieni transform (52 ¿20'E), are of the order of 80 ¿C and 4 km, respectively. ¿ 1999 American Geophysical Union