Crustal and upper mantle structures in oceanic regions

Abstract

Bathymetric data from the North Atlantic and other oceans reveal that, in addition to the well-documented variations of structure which occur at right angles to the ocean ridge crests, variations of structure also exist parallel to the strikes of the ridges. A thermal model of sea-floor spreading, together with data concerning possible mineralogical phase changes, is used to generate synthetic ocean ridge topography. Comparison of this with observed bathymetric data indicates that a peridotitic composition including water is favoured for at least the top half of the lithosphere, and allows a picture of the variations of structure in the upper mantle to be built up. It is found that lateral inhomogenities in mantle temperatures are able to explain both the variation of ocean ridge dimensions and the uplift of different parts of the ocean basins relative to sea level which are evidenced by observed data. An empirical relationship between the calculated thicknesses of the lithosphere and the depths of the ocean basins is suggested. A major area of raised temperatures may underly the North Atlantic, stretching from the Azores to a focus at Iceland. There is evidence for the existence of other regional anomalies in mantle temperatures, but none of the magnitude of that suggested to lie beneath the northern North Atlantic. The time taken for thicker lithospheres to cool to equilibrium following their formation at the ocean ridges imposes a fundamental limitation on the capabilities of the model, and makes undisturbed bathymetric profiles essential if reliable measurements are to be made in areas of lower mantle temperatures. The results support suggestions that large scale mantle convection, not necessarily related geographically to the ridge crests, may occur, and indicate that active replenishment of the thermal anomalies may be required to sustain them over geologically significant periods of time.