Geology and evolution of the Natal belt, South Africa

Abstract

This paper outlines our current understanding of the geology and evolution of the Natal belt, the eastern sector of the ∼400 km wide Mesoproterozoic Namaqua–Natal Tectonic Province of southern Africa. The Natal belt lies adjacent to the southeastern margin of the Archaean Kaapvaal craton and is divisible, from north to south, into the Tugela, Mzumbe and Margate terranes. The Margate terrane is the structurally highest panel and is characterised by granulite facies rocks. The underlying Mzumbe terrane comprises gneisses at upper amphibolite and locally granulite facies and the Tugela terrane rocks at amphibolite and greenschist facies. The boundaries between the three terranes are everywhere tectonic. New precise U–Pb zircon geochronology indicates that the supracrustal gneisses, granitoid gneisses and intrusive rocks that characterise the belt formed between 1235 Ma and 1025 Ma. The Natal belt is modelled as a composite arc terrane that developed in response to Mesoproterozoic oceanic-arc tectonics in a basin to the south (present co-ordinates) of the Kaapvaal craton, followed by accretion of the resultant arcs to the continental margin of the Kaapvaal craton during closure of the “Tugela” ocean basin. Two accretion events have been recognised. The older event occurred between ∼1208 Ma and ∼1155 Ma and resulted in the deformation and metamorphism of the supracrustal and plutonic rocks in the Tugela terrane. An early phase of deformation and metamorphism occurred in the Mzumbe terrane in a similar time frame. The regional foliation in the Tugela terrane is deformed into northeast-vergent asymmetrical folds and transposed along southward dipping, geometrically related, thrust faults and thrust-sense ductile shear zones. These structures are interpreted as a response to syn- to post-accretion uplift that accommodated emplacement of the Tugela terrane onto the southeastern margin of the Kaapvaal craton between 1155 Ma and 1140 Ma. The younger accretion event is only recognised from the Mzumbe and Margate terranes. It occurred between ∼1091 and ∼1070 Ma and is recorded by the M 1 granulite facies regional metamorphism in the Margate terrane. Post-accretion history of the belt is dominated by the intrusion in the Mzumbe and Margate terranes of A-type rapakivi-textured granitoids and charnockite forming the Oribi Gorge Suite. This magmatism took place in two distinct pulses at around 1070 Ma and 1030 Ma. During intrusion of the Oribi Gorge Suite the kinematic pattern of the Natal belt changed from north-vergent fold and thrust tectonics to ENE trending sinistral strike-slip tectonics. The resultant kilometre scale shear zones are interpreted to be a response to continuing NE–SW-directed convergence along the east–west-trending margin of the Kaapvaal craton.