Free-air gravity anomalies in the world's oceans and their relationship to residual elevation

Abstract

Summary Surface‐ship gravity measurements were used to obtain 5 times 5° average free–air gravity anomalies over much of the world's oceans. Comparison of the surface data with the recent GEM 6 ’combination’ field shows the combination solution to be in reasonable agreement with the surface data in most areas in the general location and amplitude of major features of the Earth's gravity field. There is, however, significant disagreement on the location of extrema and on the exact values at specific points. It is thus necessary to use surface data to make detailed studies of the relationship of the gravity field to surface features at wavelengths of less than 4000 km. Mid‐ocean ridges are consistently characterized by a gravity maximum. The gravity anomaly across mid‐ocean ridges can be described by an empirical 1°× 1° average gravity–age relationship which shows a positive gravity anomaly of about 25 mgal falling off to zero by about 40 my. The observed gravity–age relationship is generally compatible with current thermal models of the lithosphere. The empirical gravity–age curve was removed from the free–air anomalies to produce residual gravity anomalies which can be compared to similarly obtained residual depth anomalies. A correlation between the two is found over some features. The most prominent of these are intermediate wavelength positive features associated with areas of extensive off‐ridge volcanism. This applies not only to recently active areas but also to inactive areas such as Bermuda and aseismic ridges. On the other hand, areas of unusual elevation and volcanic activity located at a mid‐ocean ridge crest such as Iceland and the Azores do not have a corresponding gravity amomaly at intermediate wavelengths. The long‐wavelength component of the Earth's gravity field is characterized by large areas in which significant anomalies of constant sign are found. There is no apparent relationship between the long‐wavelength gravity anomalies and oceanic depths. If the gravity anomalies have their origin within the asthenosphere, the lithosphere must in effect be decoupled from the main body of the asthenosphere. This could result from the effects of a strongly depth‐dependent viscosity on the convection pattern under the elastic plates.

Models of mantle convection incorporating plate tectonics: The Australian region since the Cretaceous

“…In addition, we correct for sediment loading using the procedure in Schroeder [1984]. When corrected for normal subsidence and sediment loading, the resulting residual depth is characterized by a prominent linear north-south trending depression from the GAB through to the AAD (Cochran and Talwani, 1977] and down to nearly the Antarctic margin with depths exceeding 1000 meters (Fig. 3C).…”

Mantle plume-midocean ridge interaction : geophysical observations and mantle dynamics

“…Such variations in gravity and bathymetry indicate segment-scale changes in crustal thickness and/or mantle density and thus may reflect anomalies in along-axis mantle temperatures. Near hot spots, however, the extent of along-axis variation in density structure is broader than individual ridge segments, indicating a larger scale influence by mantle plumes Cochran andTa/wani, 1977: Bell andBuck, 1992]. The influence of mantle plumes on crustal composition is also evident by enrichments of trace elements and isotopes along the Reykjanes Ridge near the Iceland hot spot.…”

Anomalous mantle at 45°N Mid‐Atlantic Ridge?

“…The filter computed at 0.5 ø intervals the mean of values of the input gridded values within a 50 km radius (100 km full width). The 100 km window largely removes the effect of short-wavelength uncompensated topographic relief [Cochran and Talwani, 1977] and also averages the topography associated with the median valley.…”