Heat flow in the oceanic crust bounding Western Africa

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Over four hundred widely distributed heat-flow measurements have been made in the eastern Atlantic Ocean between the Azores—Gibraltar Ridge and the Walvis Ridge, permitting a study of the correlation of heat flow with crustal age and tectonic province. The mean heat flow over crust less than 80 m.y. old is less than that predicted by cooling-lithosphere models. Near the ridge axis the standard deviation of the heat-flow values is as large as the mean, but it decreases rapidly near the 20 m.y. isochron to about half of the mean and then again near 70–80 m.y. to about one third of the mean. The heat-flow values observed over crust older than 80 m.y. are the same as predicted by a cooling-lithosphere model and show little scatter. In the younger ocean crust, water circulation between the crust and ocean bottom water is the dominant mechanism of heat transfer from the crust, but after 80 m.y., heat is transferred primarily by conduction. The age dependence of the mean heat flow and standard deviation north and south of the equator were examined independently and were found to be nearly identical. This implies that heat-transfer processes in the oceanic crust evolve in a similar manner in the two portions of the ocean. Over the older oceanic crust, regions delineated by their mean heat flow and the scatter in the heat-flow values correspond roughly with tectonic province. Volcanic regions have a higher and more variable heat flow than a normal oceanic basin. The scatter in the heat-flow values may be caused by water circulation between the crust and the bottom water. In the equatorial region, the Guinea and Sierra Leone basins have a uniform and relatively high heat flow, averaging 1.4 HFU (10 −6 cal cm −2 sec −1). This may mean that the equatorial eastern Atlantic Ocean is underlain by an anomalously thin lithosphere, possibly caused by upwelling asthenosphere and higher shear-stress heating at the base of the lithosphere.