SUBDUCTION, MEGA-SHEAR SYSTEMS AND LATE PALEOZOIC BASIN DEVELOPMENT IN THE AFRICAN SEGMENT OF GONDWANA

Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambri an basement in the interior and mid-Palaeozoic strata along the palaeo -Pacific margin. Small sea-board Pacific basins form an exception in h aving a Carboniferous to Early Permian fill overlying Devonian metased iments and intrusives. The Late Palaeozoic geographic and tectonic cha nges in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian t o Early Carboniferous period (up to 330 Ma) accretion of microplates a long the Patagonian margin of Gondwana resulted in the evolution of th e Pacific basins. Thermal uplift of the Gondwana crust and extensive e rosion causing a break in the stratigraphic record characterised the p eriod between 300 and 330 Ma. At the end of this period the Gondwana I ce Sheet was well established over the uplands. The period 260-300 Ma evidenced the release of the Gondwana heat and thermal subsidence caus ed widespread basin formation. Late Carboniferous transpressive strike -slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellswor th-Central Transantarctic Mountains) in which thick glacial deposits a ccumulated, formed inboard of the palaeo-Pacific margin. In the contin ental interior the formation of Zambesi-type rift and extensional stri ke-slip basins were controlled by large mega-shear systems, whereas ra re intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to n orthwest-directed subduction along the palaeo-Pacific margin. The orog enic cycle between 240 and 260 Ma resulted in the formation of the Gon dwana fold belt and overall north-south crustal shortening with strike -slip motions and regional uplift within the interior. The Gondwana fo ld belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dex tral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-T ethys shear) split the supercontinent in the Permo-Triassic into a Wes t and an East Gondwana. By a slight: clockwise rotation of East Gondwa na a supradetachment basin formed along the Tethyan margin and northwa rd displacement of Madagascar, West Falkland and the Gondwana fold bel t occurred relative to a southward motion of Africa.