Caracterização e implicações tectônicas do Enxame de Diques da Costa do Esqueleto da região noroeste da Namíbia.

Abstract

The Early Cretaceous, Skeleton Coast Dyke Swarm (SCDS) was intruded into Permo-Carboniferous sediments, Pan-African granites, Neoproterozoic metasediments, and Archaen to Mesoproterozoic basement of NW Namibia. Aeromagnetic data and satellite imagery has been used to map the distribution of the generally coast parallel mafic dykes and a significant number of dykes that cut across Pan-African structures of the Kaoko Belt. Dykes of the SCDS were mapped up to 200km from the coast and across 2000km of NW Namibia with likely continuation into Angola. Field relationships indicate that the majority of the dykes are related to the Etendeka volcanics of Namibia. 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 (119-139 Ma). Geochemical analysis of about 30 dykes show that the majority of the dykes are low-Ti quartz tholeiites of two types: 1) qtz-tholeiites enriched in LREE [(La/Yb)N - 3.43 to 6.46]; & 2) more 'primitive' qtz-tholeiites with REE patterns similar to E-MORB [(La/Yb)N < 3.4]. The former dykes typically strike N15W whilst the latter strike N70E / N85E. By applying the principals 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 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) ENE-WSW dykes often with a sinistral component indicating NW-SE extension; & 3) WNW-ESE to NW-SE trending dykes with sinistral or dextral components related to NNE-SSW extension. The SDCS has been compared with the contemparary Florianopolis dyke swarm (FDS), by restoring South America to its pre-break-up position, showing that the principal direction of both dyke swarms is equivalent. Multiple generations of dykes are also recognised in the FDS, but we interpret that the majority of the dykes of the SCDS & FDS were intruded during an early phase of Gondwana break-up characterised by ENE-WSW extension.