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The segmentation of mid-ocean ridge axes and its variation with spreading rate provide constraints on models of mantle upwelling and the supply of basaltic melt to the crust. We present seismic refraction results from three segments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variations in maximum crustal thickness between segments, we infer that melt generation and/or delivery is focused at segment midpoints and varies between segments. Anomalously thin crust—2.0–2.5 km beneath the nontransform segment boundaries and 3.5–6.0 km beneath the segment mid-points—is caused by restricted melt generation resulting from conductive heat loss from the upwelling mantle under the spreading center.