Mechanisms of isostatic compensation of the Zimbabwe and Kaapvaal cratons, the Limpopo Belt and the Mozambique basin

Abstract

SUMMARY The Bouguer gravity anomaly map of the Zimbabwe and Kaapvaal cratons, the Limpopo Belt and the Mozambique basin shows a regional gravity high in the southeast. Superimposed on this gravity high is a line of elongate gravity highs that is coincident with the Lebombo-Nuanetsi-Sabi volcanics. Gwavava et al. ( 1992) argue that the regional anomaly over this region is the effect of Mesozoic crustal thinning and igneous activity during the breakup of Gondwana. Lithospheric extension resulted in crustal thinning by at least 4 km beneath the central Limpopo Belt. 2-D Fourier transforms of Bouguer gravity and topography data have been used to obtain the admittance and coherence throughout the region. Assuming a regionally compensated isostatic model comprising two layers with statistically independent surface and subsurface loading, the predicted coherence was compared to the observed coherence to obtain the best estimate of the effective elastic thickness of the lithospheric plate. This coherence analysis reveals that the Archaean cratons have a minimum effective elastic thickness of about 56 km, whereas that of the area beneath the Mozambique basin is only about 22 km, a value similar to that obtained in other East African rift basins. Hence, the stable cratonic areas are more rigid than the area beneath the Mozambique basin, which was subjected to Mesozoic crustal extension and thinning. An isostatic anomaly map has been computed by filtering the Bouguer gravity in the wavenumber domain with the square root of the predicted coherence function of the whole area generated from the best-fitting two-layer model with an effective elastic thickness of 52 km. The isostatic anomaly map is characterized by (1) a zone of positive isostatic anomalies over the Lebombo-Nuanetsi-Sabi area, which are partly due to crustal thinning and partly to basic plutons emplaced in the upper crust, both resulting from an extensional episode associated with Gondwana breakup, (2) positive isostatic anomalies over the Southern Marginal Zone of the Limpopo Belt, which we interpret as being due to dense granulites within the upper crust, (3) negative isostatic anomalies over the Northern Marginal Zone of the Limpopo Belt, which may have resulted from recent rapid erosion, and (4) negative isostatic anomalies over the Mozambique basin, which are due to the gravity effect of low-density Cretaceous and Tertiary sediments.