Abstract

Near 7° S. latitude, the Ascension 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 tectonic trend 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 American 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, followed by a reorientation of the southern ridge axis that proceeded from south to north and has not been completed. The hypothesis accounts for most observations except the heat-flow pattern, the absence of epicenters on the southernmost ridge crest, and some small structural features.

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