Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives

Abstract

The westernmost part of the Central Zone of the Limpopo Belt in Botswana is commonly interpreted as a frontal ramp of a westward extruding block or nappe sheet, related to a late Archæan Limpopo Orogeny. In this paper the structural observations from this area are reviewed and new geochronological results presented, in order to test the existing tectonic models. Two tectonometamorphic events can be distinguished. Neoarchæan granulite metamorphism (M 1) is associated with the generation of voluminous granitic bodies at ca 2.6 Ga. The nature of the Archæan tectonometamorphism is difficult to interpret because of a major high-grade metamorphic overprint at 2.0 Ga, which is characterised by a complicated succession of ductile deformational phases. The structural patterns indicate that during the 2.0 Ga evolution of the Limpopo Belt convergence directions changed from north to northwest and west. During the same period metamorphic conditions gradually decreased from upper amphibolite-facies to greenschist-facies. The structural features in the western part of the Central Zone are not compatible with a frontal ramp geometry. Models proposing a single Neoarchæan Limpopo Orogeny do not account for the polyphase tectonometamorphic evolution in the Central Zone and are also rejected. This study suggests that a Proterozoic orogeny involving transpression best explains the geometries encountered in the western Limpopo Belt.