Abstract
A comparison of velocity functions from 15 regions within the conjugate margins of the Norwegian-Greenland Sea, Southeast Indian Ocean, North Atlantic, and South Atlantic Ocean shows that velocity functions are not significantly different across most conjugate margins. One major exception is the segment from the Blake Plateau to New England in the western North Atlantic, and its counterpart in northwest Africa. Here the velocity gradients are consistently steeper on the African side compared with those on the North American side. The reason for this unexpected difference was sought in the geologic literature of the two margins. Published studies of these two margins show that although total subsidence and sediment thickness of each of the margins are about equal, the older beds (pre-Cenozoic) are much thicker on the African side. This fundamental difference in depositional history seems to be the major cause of the difference in velocity profiles. Possible differences in the distribution of salt along the margins of Africa and North America are not considered a likely source for the trans-Atlantic differences in the velocity functions. The North Atlantic data therefore illustrate that velocity increases more rapidly with depth in sections biased toward greater proportions of older beds; that is, sedimentation rates decrease as the basin ages. These studies also reveal that sound-velocity profiles are sensitive to the early conditions of deposition on passive margins.
