Continental rift asymmetry and segmentation – contributions from the African plate

Abstract

Prior to the mid-1980s, continental rifts were generally depicted as roughly symmetrical graben, with little complexity along the strike of the basin. In reality, most continental rifts originate as asymmetric, large-scale half graben that are strongly segmented along-strike. This basic architecture does not usually persist unaltered to the rift-to-drift transition but can commonly be observed in both failed continental rift systems and along the passive margins of oceanic plates where rifting succeeded. These geometric aspects of continental rifting have fundamental implications for both hydrocarbon exploration and understanding the causes and processes of continental rupture. Contributions to our understanding of rift asymmetry and segmentation have spanned the past seven decades and come from many settings around the world. However, Africa has played a key role in this undertaking, due to both the presence of well-exposed, active extensional basins in the Afro-Arabian rift system and numerous buried Mesozoic and early Cenozoic rift systems.The first well-documented examples of both rift asymmetry and along-strike segmentation occurred in the Gulf of Suez, northern Egypt, in the 1970s. These early ideas were greatly enhanced and expanded by later studies in both the Western and Eastern branches of the East Africa Rift, including field and refraction/reflection seismic work in Burundi, the Democratic Republic of the Congo, Ethiopia, Kenya, Malawi, Tanzania, and Uganda during the 1980s and 1990s. Asymmetry and segmentation were recognized to be fundamental characteristics of both magmatic and amagmatic rifts, although important differences regarding scale and temporal evolution are evident. At smaller scales, hard-linkage of growing and interacting extensional faults was another key component of subsurface interpretation in the Gulf of Suez by the mid-1970s. Later, outcrop studies in East Africa, supported by subsurface seismic and well data, led to greatly improved models of syn-tectonic sedimentation in step with the advances in understanding of structural geometries. Most recently, concepts derived from the continental margins of other plate settings have been incorporated with the earlier African observations to produce new models of the tectonic evolution of the Red Sea and Gulf of Aden, which in turn has then influenced our overall understanding of the progression from continental to oceanic rifting. This is an ongoing topic of research and discussion that will certainly proceed for many years and require the collaboration of both the international scientific community and local African earth science expertise.