Tectonics and sedimentation of a Late Proterozoic Damaran convergant continental margin, Khomas Hochland, Central Namibia

Abstract

Abstract provided by author:

The Late Proterozoic Damara Orogen (750-450 Ma) in Namibia forms part of the Pan-African mobile belt system which dissects southern Africa. The Khomas Trough in the inland branch of the orogen comprises thick multiply deformed metagreywackes and pelites of the Kuiseb Formation. Minor lithologies are graphite schists, calc-silicates and scapolite schists. The Matchless Amphibolite occurs structurally emplaced within the sequence and contains tholeiitic, mid-ocean ridge type metavolcanic rocks, including pillow lavas and breccias, as well as ultramafic lithologies and metagabbroic lenses. Pelagic graphite schists, pelitic schists and a marble unit are intercalated within the metabasic rocks. Original sedimentary structures, turbidite facies, the vertical facies distribution of progradational and retrogradational cycles, and the lateral extent of major sedimentary units indicate that psammitic parts of the sedimentary protoliths have been deposited as turbidites on an elongate submarine fan. Geochemical signatures suggest that an active continental margin source to the north-east of the present-day Khomas Trough supplied the vast amounts of clastic Sediment. The regional structural pattern is characterized by five phases of deformation with folds verging consistently to the south-east. The structural regime is markedly heterogeneous and is associated with thrusting which developed elongate thrust slices traceable laterally for at least 150 kilometres. Metamorphic index minerals indicate lower to upper amphibolite facies conditions but partial melting occurred in some areas along the northern margin of the Khomas Trough

These features are explained in a tectono-sedimentary model which involves the evolution of a Late Proterozoic accretionary prism, here named the Khomas Hochland accretionary prism, within a convergent continental margin setting. Rift initiation took place along old tectonic weakness zones between the Congo and Kalahari Cratons. This was followed by the formation of oceanic crust in the Khomas Sea which was an appreciably sized oceanic basin with a width of hundreds of kilometres minimum. Subsequent convergence and north-west directed subduction of oceanic crust beneath the Congo Craton produced an oceanic trench which contained the westward prograding elongate submarine fan. Pelagic sedimentation occurred contemporaneously within the basin. Basalts including a high proportion of pillow lavas and associated massive sulphide ore deposits formed at the mid-ocean ridge. The accretionary prism evolved through the offscraping of the trench sediments together with some pelagics from the oceanic plate and the accretion against the Congo cratonic margin. Early folding and thrusting occurred within the prism during this accretionary stage. Continental collision of the Congo and Kalahari Cratons resulted in the formation of a northerly steepening, thrust pile with an overall imbricate fan geometry. Subsequent phases of compressional deformation involved initially south-eastward directed thrusting and folding which was followed at a late stage by right-lateral strike-slip dislocation along the Okahandja Lineament. The Matchless Amphibolite has most probably been emplaced during the obduction of the accretionary prism onto the Kalahari Craton. Thermal relaxation subsequent to subduction led to a prolonged amphibolite facies regional metamorphism with the peak occurring late in the deformational history. Heterogeneous geothermal gradients in the Khomas Trough are interpreted to be due to differential uplift. The post-tectonic Donkerhoek Granite intruded along the Okahandja Lineament which is interpreted to represent the former backstop of the accretionary prism

Lithologies, structural styles, time sequence of events and the size of the Khomas Hochland accretionary prism may be compared with the Palaeozoic Southern Uplands accretionary prism of Scotland, the Cretaceous Chugach terrain in Alaska and the Shumagin region of the modern Aleutian Trench. Several aspects such as the inferred width of the Khomas Sea, the scarcity of arc magmatics to the north, the lack of recognition of fore-arc basin deposits, early compressional stresses, extensive sediment loading, and the intercalation of metabasalts, turbidites, pelagics and massive sulphide deposits all point to the shallow subduction of young, buoyant and hot oceanic crust. A comparison to the southern Peru - Chile Trench may be drawn