RESIDUAL GEOID ANOMALIES IN ATLANTIC OCEAN-BASIN - RELATIONSHIP TO MANTLE PLUMES

The relations of geoid data from radar altimeter measurements (GEOS 3) to water depth and geochemical variations along the Mid-Atlantic Ridge are investigated. Eight geophysical lines across the ridge in the Atlantic Ocean were constructed approximately normal to the local ridge trend. Slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography, which has better symmetry between the two flanks. Subtracting a regional field, based on previous work, produces residual geoid anomalies whose source must be in the earth's outer 600 km. The residual geoid anomalies of the North Atlantic show striking correlation with the axis of the Mid-Atlantic Ridge, but those in the South Atlantic do not. Instead, major positive residual geoid anomalies appear over the Rio Grande and Walvis ridges. Correlation of residual geoid anomalies with depth, geochemical variations along the length of the Mid-Atlantic Ridge, and the chemistry of the Rio Grande Rise and Walvis Ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects.