Geophysical studies of the Limpopo Belt

Barker, Wendy Helen
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The multiply deformed, metamtlJphic Limpopo belt separates the granite- greenstone terranes of the Kaapvaal and Zimbabwe Cratons. The belt is divided into three zones, the Southern Marginal Zone (SMZ), the Central Zone (CZ) and the Northern Marainal Zone (NMZ), all consisting of high- grade metamorphic rocks, Previous geophysical investigations, which included gravity (Coward and Fairhead, 1980) and electrical metbods (Van Zijt, 1978) presented models in which the Kaapvaal Craton (KC) had been thrust over the Zimbabwe Craton (ZC). Recent geophysical studies (De Beer and Stettler. 1988) indicate a south dipping. contact between. the ZC and NMZ and a north dipping contact between the KC and SMZ. which suggests a pop-up structure (Van Reenen et al., 1990). This study includes seismic reflection and refractton work as well as gravity modelling. The seismic reflection section is typical of Archean sections, i.e. a reflective uppet crust overlying a seismically transparent lower crust. The SMZ and a block thought to be the NMZ are eharacterlsed by numerous short, dipping reflectors. The CZ is largely seismically transparent except for sedimentary rocks close to surface. The KC is characterised by sub-horizontal reflectors indicating thin skinned tectonics. The contact between the SMZ and KC is seen as a zone of discontinuous north dipping reflectors. No continuous reflector at mid-crustal levels is seen connecting the SMZ and NMZ which could represent the decollement as postulated by McCourt and Veamcornbe (1987), free-air gravity modelling results indicate that the positive anomaly over the SMZ is " result of dense rocks within the upper crust haying a maximum thickness of 7 km, The lower regional value over the CZ can (l)eaccommodated by increasing the crustal thickness beneath the CZ by between 2 and 5 km. Thls is in agreement with the refraction data. The 0.6 s slowdown seer. . the reduced travel-time refraction data near the boundory between the CZ and SMZ is modelled using a 3 to 4 km thick low velocity layer at a depth of 42 km rather than an increase in crusta) thickness.
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg for the Degree of Master of Science Johannesburg, 1991