Segmentation and melt supply at the Southwest Indian Ridge

The segmentation of mid-ocean ridge axes and its variation with spreading r ate provide constraints on models of mantle upwelling and the supply of bas altic melt to the crust. We present seismic refraction results from three s egments of the very slow spreading Southwest Indian Ridge. From the absence of oceanic layer 3 beneath segment boundaries, and from significant variat ions in maximum crustal thickness between segments, we infer that melt gene ration and/or delivery is focused at segment midpoints and varies between s egments. 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 rest ricted melt generation resulting from conductive heat loss from the upwelli ng mantle under the spreading center.