Diverging Margins – Review of the Rift Concept

Standard
Fig27 Edwarded

Strike-slip fault deformation pattern in the Pleistocene sediments of Lake Edward. Line drawing after a seismic profile published in the Journal of Sedimentary Research, McGlue et al., 2006. Green dotted lines are showing the original fault interpretation, red lines are evidencing our interpretation.

The East African Rift

The Eastern African Rift System is given in many places as the best example for continental rifting; hence it is worth considering its structural elements from a kinematic point of view.

The Turkana Corridor is found in eastern Africa, at the intersection of two different fault sets, and forms a geomorphological corridor between the Ethiopian and Kenyan highlands. The axis of the Turkana Corridor corresponds to the Y-shear of the Cretaceous Panglobal Fracture System.

The Turkana Rift (Fig. 30) is a minor N-S trending rift, which is situated in the continuation of the Kenya and Main Ethiopian Rifts, inside the Turkana Corridor. It is actually a younger rift that is superposing an older NE-SW trending rift, the Anza Rift, which is a Neocomian transtensional basin, a lateral ramification of the Cretaceous Panglobal Fracture System, born as a major tension fracture. During Late Cretaceous, the Anza Basin has undergone a phase of major fault activity, when older basins were partly cannibalized (Bosworth and Morley, 1994). Towards the Chalbi Desert area, the oldest known deposits are Cenomanian lacustrine carbonates (Morley et al., 1999a). On the western side of Lake Turkana Oligocene half-grabens were also outlined (Vétel, 2004).

Tectonic inversion has affected several basins in eastern Africa; it is present in the Rukwa and Anza Basins, but the strongest inversion among all of them occurred in the Turkana Basin, twice, in the Paleogene and the Plio-Pleistocene (Morley et al., 1999b).

In the case of the North Malawi Basin (Fig. 32), Mortimer links basin-orientation to the underlying Pan-African foliation, however he notes that opinions are differing about the timing and the effect of strike-slip kinematics on the basin development of the Malawi Rift (Mortimer, 2007). Based on multifold seismic profiles Wheeler states that some aspects of the Livingstone Basin (North Malawi or Nyasa Rift) resemble pull-apart, extensional duplex, and extensional imbricate fan fault geometries, as expected in an incipient strike-slip basin (Wheeler and Rosendahl, 1994). Fault striations are evidencing both normal and strike-slip displacement (Delvaux et al., 1992).

McGlue has studied the depositional systems of Lake Edward because, unlike the other larger rift basins, it shows only one undisturbed depositional system from a seismic stratigraphic point of view. However, it seems that rifting preconceptions may have biased at least the structural interpretation of seismic profiles. It is more likely the basin was deformed by a strike-slip fault system instead of classic normal faults (McGlue, 2006).

In our interpretation, the East African Rift is originating in the internal simple shear stress field of the African Plate, which has been generated by the velocity contrast of the different plate segments during their eastward movement. The main (Y) shear direction is roughly parallel to the Equator, and the principal displacement was materialized in the Cretaceous Panglobal Fracture Zone.

Other important principal displacement zones of Africa, showing similar Y-shear directions, can be identified at the Congo – Tanganyika, Tanganyika– Malawi, Malawi – Okavango microplate boundaries.

Cyclicity of the observed extensional and inversion periods can be translated into periods of transtension and transpression involving large separation regional strike-slip faults.

Congo Basin

The Congo Basin is one of the largest intracratonic basins with an almost complete Neoproterozoic to Recent sedimentary sequence. It is underlain by a ~200km thick lithosphere showing current seismic activity and tectonic dislocations (Kadima et al., 2011). Various mechanisms such as a downward dynamic force linked to a high-density lithospheric object (Downey and Gurnis, 2009) or downwelling mantle plume (Hartley and Allen, 1994) are invoked to explain subsidence observed in the Congo Basin.

According to Fig. 33, the last major simple shear stress related structural event of the Busira Subbasin happened in the early Paleozoic involving the Neoproterozoic – earliest Paleozoic deposits, as suggested by the onlapping sedimentary sequence (above the green unconformity). Strike-slip faults are reaching to the surface, indicating that the Paleozoic fault system has experienced some minor reactivations recently.

In our view, the structural style of the Congo Basin is perfectly fitting into the GSST model, because it is mapping the current stress field and it is compatible with that observed in the East African Rift area, despite the more reduced degree of deformation. Hence, subsidence history of the Congo Microplate should be interpreted in the common, codependent history of the Central African kinematic chain.

In conclusion, in any of the reviewed African ‘rift basins’ strike-slip tectonics is unequivocally present, and the basin opening and later deformations are related to the movement of the equatorial kinematic chains. Due to the irregularity of microplates local stress fields are variable along microplate boundaries. Thus, rift zones in the context of GSST are assimilated with simple shear tension fractures, and consequently, the East African basin evolution is guided by the movement of microplates and by their inherent low hierarchy plate fragments. The takeaway for the GSST approach it is that we should avoid applying the rift term for transtensional basins and incipient tension fractures, unless oceanic sea floor spreading can be proved.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.