Crustal structure of the Mid-Atlantic ridge crest at 37  N

A structural model of the Mid-Atlantic Ridge at 37° N is proposed on the basis of travel-time data and synthetic seismograms. At the ridge axis the crust is only 3 km thick and overlies material with an anomalously low ‘upper mantle’ velocity of 7.2 km s−1. Crustal thickening and the formation of layer 3 and a layer with velocity 7.2–7.3 km s−1 takes place within a few kilometres of the axis, producing a 6–7 km thick crust by less than 10 km from the axis. A normal upper mantle velocity of 8.1 km s−1 exists within 10 km of the axis. Shear waves propagate across the axis, thus precluding the existence of any sizeable magma chamber at shallow depth.