Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Kuiseb Formation is a flysch-like quartz-plagioclase-mica schist succession with quartz-rich and mica-rich layers and forms the upper unit of the Swakop Group in the Damara Orogen. It is the only stratigraphic unit in most of the Okahandja Lineament Zone and the structurally asymmetric highpressure-low-temperature Southern Zone; in these two zones, it occupies, together with intrusive granite, a volume of about 160 000 km3 . The average composition in the Okahandja Lineament and Southern Zones lies between averages for shale and grcywacke. An excess of K20 over Na20 distinguishes the succession from most greywackes and implies a granitic/rhyolitic source area. Three groups of samples are distinguished on the basis of maturity index; northern and southern groups increase in maturity southwards. The central group is associated with a 350 km-long linear belt of ocean-floor basic volcanic rocks and is distinctly less mature. The Kuiseb Formation sediments appear to have been derived from several sources. The central and southern groups of samples formed during a spreading phase in a relatively narrow Damaran ocean and are associated with passive margin sediments. The northern group of samples appears to overlie the central and southern groups, was derived from a rhyolite-rich magmatic arc to the north that is now represented by the granite-rich Central Zone, and was deposited in a forearc basin bordering an active continental margin during closure of the Damaran ocean and northwestward subduction of the Kalahari Craton beneath the Congo Craton. The contact between spreading phase Kuiseb and the overlying fore-arc deposits has been obliterated by two intense transposition cleavages that formed during two phases of isoclinal folding and culminated in extensive thrusting along the leading edge of the subducting continent, the Southern Margin Zone. The relationship between Southern Zone and Southern Margin Zone bears similarities to that between a fore-arc basin and subduction complex. These fore-arc deposits in the Okahandja Lineament and Southern Zones must be younger than lithostratigraphic equivalents in the Northern and Central Zones.