Evidence of crustal thinning beneath the Limpopo Belt and Lebombo monocline of southern Africa based on regional gravity studies and implications for the reconstruction of Gondwana

Abstract The 600-km-long by 250-km-wide Limpopo Belt forms a wedge-shaped structure between the Zimbabwe and Kaapvaal cratons in southern Africa. The results of a gravity survey which included the part of the Limpopo Belt within Zimbabwe are incorporated with other data and presented as a regional gravity map. Three gravity profiles crossing the Limpopo Belt and one within Mozambique have been interpreted in terms of lateral changes in crustal structure. These interpretations are controlled by density measurements and seismic refraction data across the northern and central parts of the Belt. The Zimbabwe craton appears from gravity modelling to be characterised by a horizontal Moho at 34 km depth. The Limpopo Belt is parallel to the ENE-trending gravity contours and in the east has a southwest-northeast line of gravity highs coincident with the Nuanetsi-Sabi volcanic rocks. This study has shown that the regional gravity high associated with these features is consistent with crustal thinning probably caused by the breakup of Gondwana. The crust thins in a south-southeast direction to about 30 km in the Central Limpopo Belt with a dip on the Moho which varies from about 4° to 7°. The short-wavelength positive gravity anomalies occurring on the Nuanetsi-Sabi regional gravity high are due to late Karoo igneous intrusions and volcanics. A north-south similar line of gravity highs exists along the Lebombo monocline on the Mozambique/South Africa border. Beyond the Lebombo monocline to the east the gravity anomaly suggests a thickness of 6 km or more of sediments that could represent a paleo-deltaic environment prograding over the continental margin. The Lebombo-Nuanetsi-Sabi gravity anomaly that dominates the gravity response in the eastern part of the study area is likely to be a failed-arm triple junction and could imply a much tighter fit of Gondwana than has been previously suggested. As such the geometry of such a margin may provide a major constraint on the geometric fit of East Antarctica with Africa.