Mid-Atlantic Ridge from 47° to 51° North
An extensive survey of the Mid-Atlantic Ridge from 47° to 51° N. has recovered a large quantity of detailed bathymetric data from the North Atlantic sea floor. Supplem ented by seismic reflectiotrand magnetic profiles, these data reveal some surprising characteristics of sea-floor spreading on the Mid-Atlantic Ridge. Closely spaced transform faults, generated about 60 m.y. B.P., extend to crust of about 20 m.y. B.P. age. Significantly, however, demonstrable transform faults are nearly absent on ocean crust younger than about 10 to 20 m.y. B.P. Possibly in response to recent increased spreading rates or increased asthenosphere flow below the spreading axis, the ridge axis has replaced the transform pattern within about the last 20 m.y. B.P. with alternating normal andoblique spreading axes. The normal-spreading-segments are generating a basement level 0.5 to 1 km higher th and that produced by the oblique segments, probably because of greater-viscous head losses for magma rising within oblique rifts. The late Tertiary spreading pattern of normal and oblique rift segments has not been stationary with respect to the transform direction but has m igrated southward at 1 or 2 mm per yr, possibly a measure of slow southward subaxial m otion in the asthenosphere. A problem with this hypothesis is th a t the rift-valley floor and the regional bathym etty seem to be shoaling very gradually (1 :1,000),toward the Azores, which suggests a northw ard-directed pressure gradient in the asthenosphere. Although the top of the oceanic basement in the northern Atlantic deepens w ith increasing age there are large depth anomalies with respect to crustal subsidence of the eastern Pacific. Crust of ages 60 to 50 and 10 to t) m.y. B.P. is elevated about 1 km above coeval Pacific crust, w ith lesser anomalies occurring elsewhere. Because the highest basement levels seem to correlate with probable times of most intense plume Convection, it is inferred that a lithosphere of lower density or thicker crust is formed at such times.
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.