The Alpha Ridge is not a spreading centre

A sediment-corrected mean water depth profile across the Alpha Ridge is obtained using water depths from four submarine echograms and an assumed sediment distribution. The sediment-corrected relief of the Alpha Ridge is 2900 ± 500 m. This relief is of the same order as that observed for 70 - 80 Ma old oceanic crust about currently active spreading ridges in other oceans. Lack of present seismic activity along Alpha Ridge indicates that it is not a centre of active sea-floor spreading. If it represents a former spreading ridge that has been inactive over the past 40 - 70 Ma, then simple thermal cooling models predict that the relief of the ridge should have decayed by 2200 - 2900 m since that time. That is, the Alpha Ridge should display little present relief (500 m or less). Active accretion along the ridge within the past few million years is also unlikely judging from the presence of marine Maastrichtian (65 - 70 Ma) fossils near the ridge crest and the 380 - 1200 m thicknesses of pelagic sediments present over the ridge. It is therefore concluded that the spreading ridge hypothesis cannot explain the origin of the Alpha Ridge.