Interpretation of African gravity and its implications for the breakup of the continents.

To obtain a more quantitative interpretation of the 'great negative Bouguer anomaly' over Africa, a study is made of all the gravity data for the whole of the African continent. Histograms from 1^o x 1^o means are presented for heights, free air, and Bouguer anomalies. The modal values are found to be 400m, + 70 +- 5.1 gu (gravity unit; equal to 0.1 mGal or 10^-6 m/z²) and -480 gu, respectively. These are considered to be the best estimates for Africa away from the regions of rifting and are used to construct a 'standard African lithosphere' with an assumed thickness of 100 km. The large-scale Bouguer anomalies are then interpreted as being due to variations in the thickness of this assumed standard lithosphere rather than to the traditional compensation at the base of the crust. On this assumption, the relative density contrast at the lithosphere-asthenosphere boundary is found to be -0.05. After corrections for the effects of light volcanics and sediments, a massive replacement of the lithosphere is required beneath much of the east African rift system. -Authors