Implications for Lithospheric Reheating beneath the African Superswell from P nl Wave Propagation in Central and Southern Africa,
PENNSYLVANIA STATE UNIV UNIVERSITY PARK DEPT OF GEOSCIENCES
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The African superswell is an extensive topographic anomaly comprising the eastern and southern African plateaus, and an area of elevated bathymetry in the southeastern Atlantic Ocean basin. By comparing the P-wave velocity structure of unrifted Proterozoic lithosphere beneath e.g., southern Africa and away from e.g., central Africa the African superswell, we investigate the hypothesis that uplift within the superswell distal to the Cenozoic East African rift valleys has been caused by lithospheric reheating. If lithospheric reheating is the primary cause of uplift within the superswell, then P-wave velocity gradients beneath the southern Africa should be about 0.001 s-1 lower than beneath central Africa. Lithospheric P-wave velocity structure is obtained by forward modeling of P sub nl waveforms from four moderate sized earthquakes. Results suggest that upper mantle beneath central and southern Africa is similar, characterized by a lid structure with a constant velocity. There appears to be little evidence for 1 mantle velocity gradients beneath southern Africa that are lower than beneath central Africa, 2 a low velocity zone as shallow as 120 km beneath central Africa, and 3 the existence of a pronounced, shallow or 80 km depth low velocity zone beneath southern Africa. Additionally, modeling results yield an average thickness of 45 km for Proterozoic crust beneath central Africa and 40 km beneath southern Africa, and somewhat slower crustal velocities beneath central Africa compared to southern Africa. The similarity in upper mantle structure between central and southern Africa suggests that uplift away from the Cenozoic East African rifts is not caused entirely by reheating of the lithosphere, but the possibility that a small amount of lithospheric reheating may have occurred cannot be ruled out.
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