Geophysics and the Deep Structure of the Barberton Belt: A Review

The Barberton Greenstone Belt has received world-wide attention as one of the classical areas for studying early Archaean rocks and their environments. However, the Barberton Belt has received little attention from the geophysical community. In 1970 Burley and others made the first attempt to estimate the depth extent of the belt using the gravity anomalies in Swaziland. In a somewhat oversimplified analysis they deduced that the gravity anomaly over the belt can be produced by a flat, steep-sided body, outcropping at the surface and extending to a maximum depth of 3.2 km. In 1975 Darracott compiled a Bouguer gravity anomaly map of the area, which showed that the mafic and ultramafic rocks comprising the Onverwacht Group are associated with a positive gravity anomaly of 20 to 30 mgal. Interpretation of this anomaly indicated that the Barberton Greenstone Belt has a probable depth extent of 3 to 4 km, with the possibility of depth-extents reaching 6 km beneath the deeply infolded sediments of the Fig Tree and Moodies Groups in the vicinity of Barberton. In 1988 De Beer, Stettler and others in a study using the DC resistivity and gravity methods showed that the rocks in the Barberton Belt and the surrounding granitoid terrain have distinctive resistivity and density properties. The models based on the integrated data set established that the depth extent of the greenstone belt does not exceed 8 km. The maximum depth extent is always more than 4 km on all profiles modelled. The authors also found large areas of granitic terrain underlain by greenstone material. The three independent studies basically reached the same conclusions. With better coverage and larger data sets, the estimates for the maximum depth extent increased. The results are in agreement with similar studies in other parts of the world. Invariably the geophysically determined depth extent is much less than the geologically determined stratigraphic thickness.