An application of the response function technique to profiles of bathymetry and gravity from the Gulf of Aden

The relationship between bathymetry and gravity in the eastern, and the western Gulf of Aden, is analysed by computing response functions. The form of the response functions indicates that the short-wavelength topography is isostatically compensated. The most appropriate mechanism of compensation is one in which the topography is not locally compensated, but regionally compensated by flexure of the lithosphere. Though the sets of profiles from the eastern and the western Gulf of Aden are separated by only 500–750 km, the observed response functions are very different. The lithosphere in the eastern Gulf of Aden can be modelled by an elastic plate 15 ± 3 km thick, whereas an elastic plate 3 ± 2km thick is probably an appropriate model for the lithosphere in the western Gulf of Aden.

A different mechanism of isostasy (the Pratt) operates in the compensation of the long-wavelength topography of the mid-ocean ridge in the Gulf of Aden. A larger negative broad Bouguer gravity anomaly across the western Gulf of Aden than across the east, indicates that upper mantle temperatures increase from east to west. The long-wavelength shallowing of the Gulf of Aden and increasing elevation of the adjacent continents from east to west are therefore probably mainly the result of thermal uplift. The subdued nature of the median valley and (consequently) other short-wave-length topography in the west, can be explained by hot, low viscosity, asthenosphere, and the inferred thin lithosphere in the west also suggests high temperature mantle. Finally, a magnetic anomaly superimposed on the axial anomaly, which is probably related to the depth of the Curie temperature, is more common in the western Gulf of Aden than in the east, and this is also consistent with upper mantle temperatures increasing to the west.

There are a number of similarities between the mid-ocean ridge in the western Gulf of Aden and the Reykjanes ridge. It is suggested that the western Gulf of Aden is anomalous because of its proximity to the Afar ‘hot spot’ in the same way that the Reykjanes ridge is often considered to be anomalous because of its proximity to the Iceland ‘hot spot'.