IT is commonly accepted that as oceanic lithosphere cools, it slides away from the spreading centre under its om weight(1). It has been suggested(2-5) that even at the spreading axis this process controls the dynamics, causing asthenospheric mantle material to rise passively before being dragged away from the ridge by the overlying lithosphere. Alternatively, the upwelling of the asthenosphere at the ridge might be ’active’, caused by the buoyancy of partial melt(6-10) or of the less dense peridotite left behind by such melting(11,12). At slow-spreading ridges, ’bull’s-eye’ gravity anomalies(13,14) have been interpreted as suggesting focused accretion, and hence active mantle diapirs. At fast-spreading ridges, however, these anomalies are weaker and less clearly three-dimensional(15), which may reflect passive mantle upwelling(16). Here we show that structural data from the Oman ophiolite, which is thought to have been created at a fast-spreading ridge(17,18) are consistent with active mantle upwelling beneath the ridge, and not with models of passive flow. The absence of well marked bull’s-eye anomalies at fast spreading ridges may be explained by their relatively constant crustal thickness and near-isostatic equilibrium.