Possible Linear Plume Under Southernmost Africa

Abstract

A large negative isostatic gravity anomaly exists in southeastern South Africa, between the Great Escarpment and the coast. It reaches a minimum of −80 mgal and is elongated along an east-west axis (Fig. 1). In reporting this anomaly1 Hales and I suggested that it was caused by a relict root, or thickening of the crust which has provided Airy type compensation for a mountain range since eroded away. A mountain range which would just be supported by the root would be 80 km wide and 1.4 km high. Large stresses are necessarily associated with isostatic gravity anomalies of as large magnitude and area as here involved. The magnitude of the stresses depends on their distribution and so on the strength of the sub-crustal mantle. If the negative load is carried by both crust and mantle, stress differences are least and reach 230 bar (ref. 1) (1 bar=105 N m−2). If a strong crust floats on a weak mantle the stress may reach 2,200 bar in the crust1,2. It was shown that a sequence of failures could be predicted1, leading finally to uplift near the anomaly axis and reduction of the isostatic anomaly and the stresses. The absence of any large anomaly in the western half of the coastal strip, it was suggested, could be explained if the uplift process were much more advanced there. Large vertical throw had been observed on faults in the western region3,4 but not in the east, where the isostatic anomaly lies.