Development of the Lebombo rifted volcanic margin of southeast Africa

Abstract

The north-south-trending Lebombo region of southeast Africa has many of the elements of a classic rifted volcanic margin, including a monocline, seaward-dipping basalts, and the mid-ocean ridge basalt-like Rooi Rand sheeted dike swarm. At the northern end it also has the appearance of a classic triple junction, the Sabi monocline being the other successful rift arm and the Okavango dike swarm being the failed arm. Consequently the region has been interpreted as developing due to the impact of a plume into a stable region with the simultaneous development of the three arms. However analysis of the geology reveals that this is not the case. There is evidence for tectonic activity at least 70 m.y. before the arrival of the plume, dextral transtension taking place in the Limpopo Belt basement during deposition of Permian-Triassic Karoo sediments. The Jurassic Karoo dolerite dike swarms display a number of trends, some of which are influenced by preexisting structures. These dikes intruded during different but overlapping times ca. 183 Ma in a linked sequence that does not correspond to a synchronous development of the classic triple junction shape. The initial 183 Ma nephelinite and picrite volcanism and dikes occurred at the northern end of the Lebombo, along or adjacent to the east-northeast-trending Limpopo Belt. It was prevented from spilling southward due to the presence of a long-lived paleohigh on the northeast Kaapvaal craton. With the onset of low MgO basaltic volcanism, dikes injected along a west-northwest trend to form the Okavango dike swarm, which extends from the northern end of the Lebombo. Dilation of this swarm caused older fractures to open and resulted in the intrusion of the northeast-trending Olifants River swarm. Dilation also resulted in movement along the Agulhas-Falklands Fracture Zone that caused the coastal faulting in KwaZulu-Natal at the southern end of the Lebombo. This induced east-west extension across the Lebombo, which then completed the sequence of fracture events by linking back to the site of initial volcanism at the northern end. Subsequent thermal doming of the Kaapvaal craton enabled both north-south and east-west dikes to intrude at the same time. Further extension on the Lebombo involved the intrusion of the Rooi Rand sheeted dike swarm, followed by extrusion of rhyolites produced by partial melting of underplated basaltic material. During this event, the main monoclinal flexing and faulting took place, which affected earlier normal faults. There was no further significant east-west extension along the Lebombo even during the actual breakup of Gondwana, which occurred 40 m.y. later with the opening of the South Atlantic and the development of Cretaceous volcanism east of the Lebombo. In this breakup event the Falkland Plateau was pulled out along the southeast coast of Africa so that the extension direction was virtually north-south.