Interpretation of African gravity and its implication for the breakup of the continents

Abstract

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°×1° means are presented for heights, free air, and Bouguer anomalies. The modal values are found to be 400 m, +70±5.1 gu (gravity unit; equal to 0.1 mGal or 10⁻⁶ m/s²) 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. The interpretation requires: (1) a wide variation in lithospheric structure beneath different parts of the rift system, (2) that the lithosphere be thinnest beneath the eastern rift, and (3) that the thinning increases from south to north. The ‘lesser negative Bouguer anomaly’ associated with the Adrar‐Hoggar‐Tibesti‐Jebel Marra chain of volcanoes is treated in a similar way, and the replacement of the lithosphere is shown to be correspondingly less. Possible implications for the mechanism of rifting and break up of the continents are given.