Constraints on the Mantle Wavespeed and Discontinuity Structure below
the Turkana Depression, East Africa: Insights into Topographic
Development and Ethiopian Flood Basalt Volcanism
Abstract
The subdued topography of the Turkana Depression separates the elevated
Ethiopian and Kenyan Plateaus in East Africa. Mechanisms to explain its
topography are debated because constraints on upper mantle structure and
dynamics are lacking. Attempts to understand the role of the mantle
below Turkana in the evolution of rifting between the Main Ethiopian and
Southern East African rifts and the onset of Ethiopian Flood Basalt
volcanism are also hindered by limited data availability. Here, recently
deployed seismic networks in Turkana and neighboring Uganda enable us to
develop a new absolute P-wavespeed tomographic model (AFRP21) to image
mantle structure below the Turkana depression. Additionally, we use
P-to-s receiver functions to map the mantle transition zone (MTZ)
discontinuity structure. In the shallow mantle, broadly distributed slow
wavespeeds reside below the Main Ethiopian rift. To the south, slow
wavespeeds occur in a focused zone below the East African rift, but
beneath the northern Turkana depression these are cross-cut by a narrow
E-W band of fast wavespeeds. At upper MTZ depths slow wavespeeds are
broadly continuous below the East African rift but begin to separate
into two distinct anomalies at the base of the MTZ. While receiver
functions reveal a broadly thinned MTZ below Cenozoic rift-related
magmatism in East Africa, the thinnest transition zone exists below the
Turkana Depression. Slow wavespeeds and a thinned MTZ below the Turkana
Depression indicate hot upwelling material, thus its low-lying nature is
not due to the lack of underlying dynamic support. Instead, the
depressed topography may be better explained by Mesozoic-Cenozoic E-W
rifting associated with the imaged shallow fast wavespeed band.
Furthermore, the main eruptive phase of Ethiopian Flood basalt volcanism
may be associated with the African plate’s position over the anomalously
thinned MTZ in Turkana at ~30Ma.