Wichura et al. 2011; Geological Society of London Special Publication

THE FORMATION AND EVOLUTION OF AFRICA: A SYNOPSIS OF 3.8 GA OF EARTH HISTORY

Describtion
The African continent preserves a long geological record that covers almost 75% of Earth’s history. The Pan-African orogeny (c. 600–500 Ma) brought together old continental kernels (West Africa, Congo, Kalahari and Tanzania) to form Gondwana and subsequently the supercontinent Pangaea by the late Palaeozoic. The break-up of Pangaea since the Jurassic and Cretaceous, primarily through opening of the Central Atlantic, Indian, and South Atlantic oceans, in combination with the complicated subduction history to the north, gradually shaped the African continent.
This volume contains 18 contributions that discuss the geology of Africa from the Archaean to the present day. It celebrates African geology in two ways: first, it highlights multidisciplinary Earth science research by viewing the formation and evolution of Africa from 18 different angles; second, it celebrates the work of Kevin Burke and Lewis Ashwal and portrays the wide range of interests and research angles that have characterized these two scientists throughout their careers, working in Africa, and studying African geology.


CHAPTER 14:
THE MID-MIOCENE EAST AFRICAN PLATEAU: A PRE-RIFT TOPOGRAPHIC MODEL INFERRED FROM THE EMPLACEMENT OF THE PHONOLITIC YATTA LAVA FLOW, KENYA

Henry Wichura, Romain Bousquet, Roland Oberhänsli, Manfred R. Strecker, Martin H. Trauth

Abstract
High topography in the realm of the rifted East African Plateau is commonly explained by two different mechanisms: (1) rift-flank uplift resulting from mechanical and/or isostatic relaxation and (2) lithospheric uplift due to the impingement of a mantle plume. High topography in East Africa has far-reaching effects on atmospheric circulation systems and the amount and distribution of rainfall in this region. While the climatic and palaeoenvironmental influences of high topography in East Africa are widely accepted, the timing, the magnitude and this spatiotemporal characteristic of changes in topography have remained unclear. This dilemma stems from the lack of datable, geomorphically meaningful reference horizons that could unambiguously record surface uplift. Here, we report on the formation of high topography in East Africa prior to Cenozoic rifting. We infer topographic uplift of the East African Plateau based on the emplacement characteristics of the c. 300 km long and 13.5 Ma Yatta phonolitic lava flow along a former river valley that drained high topography, centred at the present-day rift. The lava flow followed an old riverbed that once routed runoff away from the eastern flank of the plateau. Using a compositional and temperature-dependent viscosity model with subsequent cooling and adjusting for the Yatta lava-flow dimensions and the covered palaeotopography (slope angle), we use the flow as a ‘palaeo-tiltmeter’. Based on these observations and our modelling results, we determine a palaeoslope of the Kenya dome of at least 0.28 prior to rifting and deduce a minimum plateau elevation of 1400 m. We propose that this high topography was caused by thermal expansion of the lithosphere interacting with a heat source generated by a mantle plume. Interestingly, the inferred Mid-Miocene uplift coincides with fundamental palaeoecological changes including the two-step expansion of grasslands in East Africa as well as important radiation and speciation events in tropical Africa.

Copyright © 2011 The Geological Society of London. All rights reserved.


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