A top to bottom lithospheric study of Africa and Arabia
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East African Rift
Indian Shield
Passive margin
The present state of knowledge of the East African Rift System is described, with particular reference to the Gregory Rift, which is characterised by the widest diversity of volcanic formations; its detailed study over the past ten years has provided the most comprehensive data on the structural and volcanic evolution of any part of the Rift System.
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The East African Rift System (EARS) transecting the high-elevation East African plateau is one of the most outstanding rift systems on earth. Rifting was caused by a huge uprising mantle plume under East Africa. Two distinct rift branches are distinguished: an older, volcanically very active Eastern Branch and a younger, much less volcanic Western Branch. The Eastern Branch is generally characterized by high elevation, whereas the Western Branch comprises a number of deep rift lakes (e.g., Lake Tanganyika, Lake Malaŵi). These differences reflect different plate strengths, the latter of which are largely governed by differences in how the mantle plume interacted with the East African lithosphere. Much of the topography forming the East African plateau has been caused by the uprising mantle plume. The onset of topographic uplift in the EARS is poorly dated but preceded graben development, the latter of which commenced at ~24 Ma in the Ethiopian Rift, at ~12 Ma in Kenya, and at ~10 Ma in the Western Branch. Increased uplift of the East African plateau since ~15–10 Ma might be connected to climate change in East Africa and human evolution. East Africa experienced cooling starting at 15.5–12.5 Ma that heralded profound faunal changes at 8–5 Ma, when the hominin lineage split from the chimpanzee lineage. The Pliocene is characterized by warm and wet climate between 5.3 and 3.3 Ma transitioning into a period of cooler and more arid conditions after ~3 Ma. The climate in the EARS is controlled by westerly monsoonal flow over equatorial West Africa and easterly monsoonal flow over the Indian Ocean. The uplifting East African plateau intercepted those winds and contributed to the increased aridification of East Africa.
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The seismicity of East Africa has been investigated using the Durham University seismometer army at Kaptagat. Events recorded by this station originate from five main seismic regions. Three of the five regions, the Kavirondo Rift, the Siria Fault and the area from Entebbe to the Ruwenzori Mountains, form east-west bands of activity. The activity In the remaining regions, the Western Rift, In particular the Ruwenzori Mountains, and the Gregory Rift shows a north-south distribution. Earthquakes from the Western Rift are associated with the boundary faults. In the Kavirondo Rift the events are associated with the eastern end of the graben and have been used to Infer an easterly extension of the faults. Within the central Gregory Rift the shocks are associated with the axis of the rift. Where the Gregory Rift passes Into the North Tanzania Divergence and the Turkwel Depression the events are distributed across the width of the Rift. From the slope, b, of the cummulative frequency-magnitude curve It Is suggested that the Western Rift can be divided Into a northern and southern section. The former section Is the younger and Is tectonically similar to the Kavirondo graben. The Eyasi and Gregory Rifts have shown the same value for 'b' and seem to be tectonically similar. However, they differ In the upper cut off limit on the magnitude of the earthquakes. The pattern of seismicity In the Gregory Rift and the travel time, compared to normal shield structure, across the rift have been Interpreted to give a model for the crystal structure within the rift. The model consists of a continuous velocity Increase with depth Interrupted by a drop In velocity. The anomalous material which the model represents also has a low Q value of approximately 100.
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The seismically and volcanically active East African rift system is a system of faultbounded sedimentary basins and uplifted flanks spanning the African continent from the Horn of Africa to southern Africa. The East African rift transects a broad zone of uplift associated with the African Superplume province, with large tracts of the rift experiencing pre- and syn-rift magmatism. We review the rifting processes creating the broad plateaux, the uplifted rift flanks and fault-bounded rift valleys, outline the complex interplay between tectonically induced vertical crustal movements, climate, erosion, and sedimentation in rift zones, characterize spatial and temporal patterns of rift architecture from rift inception in Botswana to rupture within the Afar rift, and summarize the sedimentary framework of late Cenozoic basins in the East African rift system. Consistent spatial and temporal patterns document the feedbacks between faulting and sedimentation, and enable the development of predictive models for rift basin structure and stratigraphy, including the role of magmatism prior to and during rifting.
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Abstract Close relationships between deformation and volcanism are well documented in relatively late evolutionary stages of continental rifting, whereas these are poorly constrained in less mature rifting stages. To investigate the control of inherited structures on faulting and volcanism, we present a statistical analysis of volcanic features, faults and pre‐rift fabric in the Tanzania Divergence, where volcanic features occur extensively in in‐rift and off‐rift areas. Our results show that in mature rift sectors (Natron), magma uprising is mostly controlled by fractures/faults responding to the far‐field stress, whereas the distribution of volcanism during initial rifting (Eyasi) is controlled by inherited structures oblique to the regional extension direction. Off‐rift sectors show a marked control of pre‐rift structures on magma emplacement, which may not respond to the regional stress field. Thus, the use of off‐rift magmatic features as stress indicators should take into account the role of pre‐existing structures.
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