Gravity Anomalies, Regional Elevation, and the Deep Structure of the North Atlantic
The pattern of depth and gravity anomalies in the North Atlantic Ocean was examined by using 1° × 1° and 5° × 5° averages. The gravity field is dominated by two features: a broad high in the northern and central portion of the ocean and a large area of very negative anomalies in the western basin. The negative anomaly in the western North Atlantic has no expression in residual depth anomalies and does not appear to be related to surface features. The North Atlantic Gravity High is bounded on the south by an important and distinct boundary near 30°N which is present in both depth and gravity anomalies. North of this latitude, residual gravity anomalies (corrected for the ‘ridge anomaly’) take the form of a very broad high of about 20 mGal with the values along isochrons nearly constant from 30°N to at least as far north as 75°N. The depths within this region are consistently shallow with 5° × 5° average residual depth anomalies which vary greatly from a few hundred meters near the Charlie Gibbs fracture zone to a few thousand meters near Iceland and the Azores. We used three-dimensional spherical earth models to investigate simple compensated mass distributions which could explain the observed depth and gravity anomalies. The acceptable family of models has the common characteristics that within the anomalous region north of the 30°N boundary a portion of the compensation must be distributed to depths of several hundred kilometers and that this deep mass deficiency must be nearly uniform throughout the area. However, the compensation for more local features within this region must be shallow (within the lithosphere). Thus the topographic highs surrounding Iceland and the Azores are compensated within the lithosphere, but the overall elevation is maintained at some greater depth by a mass deficiency in the asthenosphere. This mass deficiency can be explained by an increase in temperature of about 75°C. Thus the gravity data enable us to establish the presence of a very large upper mantle hot spot that might be associated with the broad overall compensation of the entire North Atlantic north of 30°N. Whether this upper mantle hot spot is associated with a deep mantle plume is uncertain, since the gravity effect of a plume is undetectable. However, its presence suggests that the immediate source of the unusual amount of material erupted at Iceland and the Azores is within the upper mantle.
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Also Published In
- Journal of Geophysical Research