Crustal structure of the mid-ocean ridges: 2. Computed model from gravity and seismic refraction data

Abstract

Seismic results have demonstrated that the crust under the mid-ocean ridges is not thicker than under the ocean basins. Seismic results also show that the velocity in the mantle immediately under the ridge flanks is normal but that it is low under the axial zones, indicating an ‘anomalous mantle.’ The free-air gravity anomaly over the entire ridge is close to 0 (within ±50 mgal), and consequently the gradients in the Bouguer anomaly are steep (sometimes approaching 1 mgal/km). In order for gravity and seismic results to agree, the anomalous mantle, which provides compensation in the axial zone of the ridge, is postulated to extend to the flanks below the normal mantle observed by seismic refraction measurements. A model computed on the basis of continuous gravity measurements over the mid-Atlantic ridge and the seismic refraction results given in part 1 is presented. Alternative models that provide compensation at greater depth under the ridge flanks are ruled out by the steep gradients in the Bouguer anomaly. It is shown that a similar model can explain the seismic and gravity data obtained over the east Pacific rise but that a deeper compensation is not impossible because of the smaller gradients in the topography and Bouguer anomaly curves. A lack of correlation between the sharp east Pacific rise heat-flow anomalies and the magnetic and gravity anomalies is also noted.