Rift basin evolution in Africa: the influence of reactivated steep basement shear zones

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Abstract
Summary: Phanerozoic rift basin evolution in Africa has been strongly influenced by the pre-existing basement structure inherited from Precambrian tectonics. Two modes of basement reactivation may be identified; the largely dip-slip reactivation of gently dipping shear zone and thrust structures and the largely strike-slip reactivation of steep shear zones and faults. In particular, crustal scale steep basement shear zones have exercised a profound control on rift basin formation on a regional scale. Such shear zone structures are seen to govern the location, trend and structural style of rifts and appear to constrain the orientation of displacements generating the rifts. The shear zones are relatively narrow, high strain zones formed during ductile deformation. They occur as steep, crustal scale ramps that define a pronounced mechanical anisotropy within the African continental crust. These structures are difficult to reactivate by dip-slip movement and thus tend to act as major strike-slip transfer zones during later extensional events. This paper discusses examples of the steep shear zone-rift relationship from three of the rift systems of Africa: the Upper Palaeozoic (Karoo) rifts of Central Africa, the Mesozoic rifts of the Sudan and the Cenozoic rifts of East Africa. These discussions indicate that steep shear zones are sites of rift nucleation and play a fundamental role in the mechanisms of rift propagation and subsequent continental breakup.