Damaran basement-cored fold nappes incorporating pre-collisional basins, Kaoko Belt, Namibia, and controls on Mesozoic supercontinental break-up

Abstract

Deformation styles due to the collisional convention of Gondwana that ended ~500 Ma are reassessed in the Kaoko fold-thrust belt of the NE Damaran orogen. Downward facing sequences on the overturned limb of a major recumbent fold nappe are exposed in the Hoanib valley beneath pre-Damara basement, initially identified in a D2/D3 antiformal refold core, constituting the Obias River Window. Basement paragneisses, psammitic and pelitic protoliths, were infolded into the core of a major recumbent anticlinal Di fold nappe structure (dimensions 55 km across × 150 km+ along tectonic strike), comparable in dimensions to other major fold nappes (e.g. the Loch Tay nappe of the Scottish Highlands). D2/D 3 refolding produced large-scale polyphase fold traces later affected by a D4 phase of open cross folding. The nappes detached along flat lying sole thrusts (Eastern Zone), soled by the Sesfontein master thrust, and are backed by a structurally steep root zone (Central Zone). Farther west ramped-up syn-orogenic granitoids intruded into the Damara sediments (Western Zone). Some thrust zones mark major fades changes and imply a pre-compressional identity, with most complete stratigraphic sequences immediately to their western side. The latter Damaran (750 to 600 Ma) sequences progressively on-lap westwards onto basement, to exclude successively lowest stratigraphic units. Prior to collision half-graben sub-basin fills were accommodated by extensional fault systems. Subsequent Damaran compression reversed extensional faults as thrust zones. Intra-Pangean extension and break-up later reactivated Damaran thrusts, expressed as rift basins that developed during the latest Palaeozoic and culminated during the Early Cretaceous with the opening of the South Atlantic. Karoo to post-Karoo sediments and volcanics were accommodated in the resulting half-grabens. Indeed the Damaran continental suture probably inverted as the southern South Atlantic breakaway zone between Africa and South America. Atlantic shelf sediments now cover the suture apart from in southernmost Namibia, where it is marked by the Damaran thrust-floored Oranjemund Complex of the Gariep Belt. The Oranjemund Complex incorporates an exotic Adamastor Oceanic volcanic prominence thrust onto the proto-African foreland.