Evidence from the asymmetry of fast-spreading ridges that the axial topographic high is due to extensional stresses

Along fast-spreading mid-ocean ridges such as the East Pacific Rise, there is an axial topographic high, 5–20 km wide, which stands 200–400 m above the background slope caused by thermally induced seafloor subsidence. There are also smaller topographic lows flanking the axial high along most of the East Pacific Rise from 20° S to 15° N. The existence of these lows is predicted by models of the origin of the axial high. One model postulates that the axial high is created by buoyant uplift from a narrow zone ofconcentrated partial melt extending tens of kilometres down into the mantle1,2,3,4. Another model requires no such buoyant zone, suggesting instead that the axial high is generated by dynamic, extensional stresses in the lithosphere and shallow asthenosphere5. Here we show that the observed asymmetry of the flanking lows can be used to distinguish between these two proposed mechanisms. Although either model can be adapted to match the asymmetry on individual profiles, the along-axis variation in degree of symmetry favours the model of dynamic, extensional stresses for the origin of the axial high and its flanking lows.