Ascension Fracture Zone, Ascension Island, and the Mid-Atlantic Ridge
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.
Figures & Tables
Edited by Peter A. Roma and published in 1976, Mid-Atlantic Ridge contains a collection of related articles reprinted from other Geological Society of America publications as well as a brief review of exploration of the Mid-Atlantic Ridge from 1960 to 1975.