The lack of high heatflow values at hotspots has been interpreted as showing that the mechanism forming the associated swells is not reheating of the lower half of oceanic lithosphere. Alternatively, it has recently been proposed that the hotspot surface heatflow signature is obscured by fluid circulation. We re-examine closely spaced heatflow measurements near the Hawaii, Réunion, Crozet, Cape Verde, and Bermuda hotspots. We conclude that hydrothermal circulation may redistribute heat near the swell axes, but it does not mask a large and spatially broad heatflow anomaly. There may, however, be heatflow perturbations associated with the cooling of igneous intrusions emplaced during hotspot formation. Although such effects may raise heatflow at a few sites, the small heatflow anomalies indicate that the mechanisms producing hotspots do not significantly perturb the thermal state of the lithosphere.

Abstract Near 7° S. latitude, the A\scension fracture zone offsets the Mid-Atlantic Ridge right- laterally over 230 km. North of the fracture zone, which trends about N. 80° E.;The ridge crest is perpendicular to its trend, but to the south, near 8° S., the initially perpendicular trend changes to nearly northerly, Ascension Island lies approximately 50 km south of the fracture on magnetic anomaly 4. with an inferred age of 7 m.y. It is not on any major tectonics rend and there is no evidence that it is part of a volcanic chain. Spreading rates in the region increase from north to south, proportional to the distance from the pole of rotation of the African and South Aroerrcan plates, and may be slightly different on the east and west sides of the ridge. Normal to subnormal heat-flow-values prevail except for one high value east of the northern ridge axis. The Ascension-fracture valley is wide and filled with thick sediments implying an anomalously high age. Earthquake epicenters are aligned along the ridge crest. but near the fracture zone they define an activity belt south of it and more nearly east-west trending. The data suggest a shift of the fracture zone to an east-west trend about. 10 m.y. ago. iollowed by a reorientation of the southern ridge axis that proceeded from south to north and has not been completed. The hvpothesis accounts for most observations except the heat-flow pattern, the absence of epicenters on the southernmost ridge crest, and some small structural features.