Mantle Wavespeed and Discontinuity Structure below East Africa: Implications for Cenozoic Hotspot Tectonism and the Development of the Turkana Depression

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Authors
Boyce, Alistair 
Kounoudis, Rita 
Bastow, Ian 
Cottaar, Sanne 
Ebinger, Cynthia 
Abstract

Ethiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500\add[A.B.]{--1000},km north since initial magmatism at $\sim45Ma,havingdevelopedabovemantlethatnowunderliesthenorthernTanzaniacratonandthelowlyingTurkanaDepression.Unfortunately,ourknowledgeofmantlewavespeedstructureandmantletransitionzone(MTZ)topographybelowtheseregionsispoorest,duetoahistoricallackofseismographstations.ThesamedatagapmeanswelackconstraintsonlithosphericstructureinandaroundtheNW−−SEtrendingMesozoicAnzarift.WeexploitdatafromnewseismographnetworksintheTurkanaDepressionandneighboringnorthernUgandatodevelopAFRP22,anewAfricanabsolutePwavespeedtomographicmodelthatresolveswholemantlestructurealongtheentireEastAfricanriftsystem.WealsomapMTZthicknessusingPsreceiverfunctions.EastAfricasthinnestMTZ(\sim$25,km thinning) underlies the northwest Turkana Depression. AFRP22 reveals a co-located, previously-unrecognized, slow wavespeed plume tail, extending from the MTZ, deep into the lower mantle. This plume may thus have contributed, along with the African Superplume, to the development of the 45--30,Ma flood basalt province that preceded extension. Pervasive sub-lithospheric slow wavespeeds imply that Turkana's present-day low elevation is explained best by Mesozoic and Cenozoic-age crustal thinning. At $\sim$100,km depth, AFRP22 illuminates a fast wavespeed SE Ethiopian plateau. In addition to governing the northernmost limit of Mesozoic Anza rifting, the refractory nature of this lithospheric block likely minimized Cenozoic flood basalt magmatism there.

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Keywords
body wave tomography, receiver functions, East African rift system, Turkana Depression, mantle plumes, Ethiopia-Yemen flood basalts
Journal Title
G3: Geochemistry, Geophysics, Geosystems: an electronic journal of the earth sciences
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Journal ISSN
1525-2027
1525-2027
Volume Title
Publisher
American Geophysical Union
Sponsorship
Natural Environment Research Council (NE/R010862/1)
European Research Council (804071)