The Damara mobile belt in the south-Western Kaokoveld

• Main Author: Guj, P
• Format: Doctoral Thesis
• Language: English; English
• Published: University of Cape Town 2017
• Subjects: Geology Namibia.; Stratigraphic; Namibia.; Stratigraphic Geology; Geology
• Online Access: http://hdl.handle.net/11427/23739

After the establishment of the N-S Andib-Ganamub trough, the ubiquitous Lower Nosib subarkose was succeeded in the west by the Upper Nosib greywacke which indicates that orthogeosynclinal subsidence was already active in pre-Damara times. A pulse of NNE folding (F₁N) concluded this sedinentary cycle and was accompanied by amphibolite-grade metamorphism (M₁), occasional migmatisation (A₁), and later feldspar blastesis. This episode was most intense in the west, so that the overlying Damara rocks paraconformable in the east, are unconformable there. Martin's (1965) separation of the succeeding Damara basin into a western eugeosyncline and an eastern miogeosyncline applies very well to this area, though the separating hinge-line appears to have been located farther to the west (±80 Km) of the Kamanjab inlier, along the western flank of a N-S geanticlinal belt. The Lower Bakos semipelite and lenticular carbonate, thinning out against the eastern flank of this positive element controlling the deposition, may, prior to deformation, have created a transition between the western eugeosynclinal greywacke and the eastern miogeosynclinal Lower Otari carbonates. The fractured hinge-zone provided channels for the ascent of syndepositional intrusions and volcanics. Regional erosion following upwarping in the east caused a large influx of detritus into the Khomas furrow prior to the disconformable deposition of the Tillite Substage, which is the only reliable chronostratigraphic link between the base of the Upper Otavi and that of the Upper Bakos Series, the latter partially intertongueing with the succeeding Khomas Series. Gravitational creeping of the miogeosynclinal carbonates down the western slope of the upwarped Kamanjab inlier towards the sinking basin, produced the first folding (F₂O) in the east. Almost contemporaneously, buoyant uplifting forces, related to anatexis (A₂) along the axis of the eugeosyncline, created slopes sufficient to convey nappes (F₂S) of scarcely metamorphosed Damara schists towards the eastern foreland, overriding the miogeosyncline along the N-S Sesfontein Thrust. Eastern foredeeps were rapidly filled by the Mulden molasse. Steep slip folding (F₂S and F₄) in the cores and roots, and the formation of scattered domes at depth, controlled the successive structural evolution of the nappes. Large-scale axial undulations were later produced by orthogonally superposed F₃ folds striking east. The Damaran metamorphism (M₂ ), which outlasted deformation, has a Barrovian character and grades in the Damara sediments from amphibolite (sillimanite) grade in the west to greenschist (sericite- chlorite) grade in the east. In the previously dehydrated, deformed and metamorphosed, polymetamorphic (M₁/M₂ ) Nosib terrains, retrograde parageneses and transition granulites developed to the east and to the west of the Damaran hornblende-oligoclase isograd respectively. The Damaran tectono-thermal episode culminated in advanced migmatisation of the Khomas greywacke (possibly equivalent to the Salem granite-gneiss) and final anatexis (A₂ ) in the late-kinematic Ganias -Uhima (possibly equivalent to the Donkerhoek) granite, followed by widespread feldspar blastesis. The drop in rheomorphism in the deformed mass at the margin of the migmatite zone induced intense shearing and produced belts of mylonite and cataclastic gneiss. At a later stage, the Sesfontein Thrust was buffered against the Otavi dolomite relief and its speed of advancement reduced to match that of the erosion of its frontal toe, which now separates two completely different structural-stratigraphic domains: i) an eastern miogeosynclinal, feebly metamorphosed and volcanic-free domain (Damara System, Outjo Facies) deformed into non-basement-involving tectonites, overridden by ii) a western domain of suprastructural nappes and metamorphosed eugeosynclinal sediments (Damara System, Swakop Facies) deformed in response to basementinvolving tectonic processes promoted by deeply seated anatexis along the axis of the former eugeosyncline. The present model involves many stratigraphic revisions, one of the most important being that no rocks older than the Nosib Formation outcrop in the area and possibly along most of the Damara mobile belt in the western Kaokoveld.