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Oblique Divergence Zones |
1 Department of Geology and Applied Geology, University of Natal, Durban 4041, South Africa
2 Hans Ramberg Tectonic Laboratory, Institute of Earth Sciences, Uppsala University, Villaväggen 16, S-752 36 Uppsala, Sweden
The southeastern margin of Africa is marked by a zone of complex faulting associated with crustal extension related to the Mesozoic break-up of Gondwana. The oldest age of faulting (180–175 Ma) displays differences in style in regions with Archaean as opposed to Proterozoic basement. Faulting in the Archaean region shows extension normal to the proto-plate boundary whereas transtension prevailed in the Proterozoic region. The north-south trending normal faulting in the Archaean craton, which cuts across basement trends, displays an asymmetry indicating simple shear extension of the lithosphere. In contrast, transtension occurred in the region of Proterozoic basement by ENE strike-slip movement of blocks along terrane boundaries. As these blocks moved, normal faults developed behind them as well as within them at high angles to the terrane boundaries, to accommodate extension. Analogue modelling reproduced the observed different fault patterns. It is proposed that sinistral movement along the interconnected Gastre Fault System and Falkland-Agulhas Fracture Zone, together with the presence of a releasing bend, are the reasons for extension in the Lebombo region. This movement may have been caused by plate edge events between the Patagonia block and the proto-Pacific Ocean.
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