Characterisation and tectonic implications of the Early Cretaceous, Skeleton Coast Dyke Swarm, NW Namibia. (February 2019)

• Record Type: Journal Article
• Authors: McMaster, M.; Almeida, J.; Heilbron, M.; Guedes, E.; Mane, M.A.; Linus, J.H.
• Is Part Of: Journal of African earth sciences. Volume 150(2019)
• Page Start: 319
• Page End: 336
• Publication Date: 2019-02
• Subjects: Kinematic analysis -- Paraná-Etendeka -- West Gondwana -- Conjugate margins; Earth sciences -- Africa -- Periodicals; Earth sciences -- Middle East -- Periodicals; Geology -- Africa -- Periodicals; Geology -- Middle East -- Periodicals; Sciences de la terre -- Afrique -- Périodiques; Sciences de la terre -- Moyen-Orient -- Périodiques; Géologie -- Afrique -- Périodiques; Géologie -- Moyen-Orient -- Périodiques; Earth sciences; Geology; Africa; Middle East; Periodicals; Electronic journals; 556.05
• DOI: 10.1016/j.jafrearsci.2018.11.010
• Languages: English
• ISSNs: 1464-343X

Abstract

Abstract : The Early Cretaceous, Skeleton Coast Dyke Swarm (SCDS) was intruded into Permo-Carboniferous sediments of the Karoo Supergroup, Pan-African granites, Neoproterozoic metasediments, and Archean to Mesoproterozoic gneisses of the Kaoko Belt, NW Namibia. Aeromagnetic data and satellite imagery has been used to map the distribution of the generally coast parallel mafic dykes along with a significant number of dykes that cut across Pan-African structures of the Kaoko Belt. Dykes of the SCDS were mapped for some 500 km along the Skeleton Coast of NW Namibia and up to 200 km inland, and with a likely continuation into Angola. The geochemistry and published geochronological data suggest that the majority of the dykes are related to the Etendeka volcanics. These volcanics, together with the basalt and rhyolite lava flows and associated mafic dyke swarms of S-SE Brazil, form the Paraná-Etendeka magmatic province of Early Cretaceous age. Geochemical analysis of 26 dykes show that the majority of the dykes are Low-TiO2 quartz tholeiites of two types: 1) qtz-tholeiites enriched in LREE [(La/Yb)N - 3.43 to 6.46]; & 2) qtz-tholeiites with REE patterns similar to E-MORB [(La/Yb)N < 3.4]. The former typically strike N15W whilst the latter strike N70E/N85E. By applying the principles of kinematic analysis and through the identification of asymmetrical features such as zigzagging, branching and en échelon dykes, and bridges between dyke segments, we were able to identify that in addition to extension perpendicular to dyke walls a significant number of dykes display lateral displacements of their walls consistent with oblique extension. This information was used to determine the shear sense and estimate the direction of maximum and minimum stress of the regional stress field active during dyke intrusion. Three dyke generations have been identified within the SCDS: 1) NNW-SSE dykes associated with normal ENE-WSW extension; 2) WNW-ESE to NW-SE trending dykes with sinistral or dextral components related to NNE-SSW extension; & 3) ENE-WSW dykes often with a sinistral component indicating NW-SE extension. When South America is restored to its pre-break-up position, the principal direction of the Skeleton Coast and the Florianopolis dyke swarms are equivalent. Multiple generations of dykes are also recognised in the Florianopolis dyke swarm, but we interpret that the majority of the dykes of the SCDS & FDS were intruded during initial rifting that was characterised by ENE-WSW extension. Graphical abstract: Image 1 Highlights: Geophysical, structural and geochemical characterisation of the Skeleton Coast Dyke Swarm. Cinematic evolution of dyke emplacement related to the opening of the South Atlantic Ocean. Skeleton Coast Dyke Swarm correlated with the Florianopolis Dyke Swarm across the South Atlantic.