The geophysical mapping of Mesozoic dyke swarms in southern Africa and their origin in the disruption of Gondwana

Mafic dykes usually give rise to magnetic anomalies that are prominent on t he aeromagnetic anomaly maps that now approach universal coverage for the l and areas of the world. A database of over 14,000 dykes mapped in this way for southern Africa allows the geometry of dyke swarms there to be studied with new completeness. The multitudinous dykes of Jurassic age are interpre ted as products of successive stages in the interaction of the tectonic pla tes and intraplate fragments that underwent relative (modest) movement duri ng the disruption of Gondwana. These stages commenced with east-west riftin g off the eastern margin of the present Kaapvaal Craton, followed by southw ard drifting of East Gondwana against Africa. The separation of Antarctica- Australia from India-Madagascar subsequently led to a clockwise migration o f (particularly) Antarctica around the southern margin of Africa, eventuall y with dextral strike-slip on the Agulhas Fault and the opening of the Sout h Atlantic. All these events left a record of dyke emplacement in southern Africa. The geometry of the dyke swarms so created are interpreted here in support of this geodynamic model. A systematic programme of modern radiomet ric dating of representative dykes selected from these swarms would serve t o time-calibrate these events and test this tectonic history. (C) 2000 Else vier Science Limited. Ail rights reserved.