Deep Mantle Contributions to African Volcanism Revealed by Absolute P-wave Tomography and Transition-Zone Receiver Functions


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African Cenozoic volcanism is attributed to mantle upwellings at a variety of scales. Within East Africa, much of the along-rift volcanism has been linked to the African Superplume and underlying large low velocity province (LLVP) on the core mantle boundary. Elsewhere, upper mantle convective instabilities may dominate. The thermochemical properties of African mantle upwellings can be constrained by combining both wavespeed and converted wave observations. However, previous tomographic models possess depth and/or resolution limitations, while comparisons between regional transition-zone studies are hindered by the varying methods used to measure seismic discontinuity depth. To address this, we develop a new continental P-wave tomographic model facilitating resolution of upper-to-lower mantle structure using the Absolute Arrival-time Recovery Method (Boyce et al., 2017) to extract >87,000 residuals from all freely-available broadband African seismograph deployments active from 1990-2019. Using identical station distribution, we utilise this new tomographic model within common conversion point (CCP) stacks of >28,000 newly compiled P-to-s (Pds, PPds, and PKPds) receiver functions to interrogate mantle transition-zone discontinuities below Africa for thermochemical signatures. The broad heterogeneous African Superplume reaches the upper mantle below the southern East African rift. Here, depression of both transition-zone discontinuities underlain by additional seismic conversions at ~1050km depth strongly support a thermochemical upwelling from the African LLVP. Further north, below Ethiopia/Afar, we image a second whole-mantle slow wavespeed anomaly rooted at the core-mantle boundary outside the African LLVP. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicative of a predominantly thermal anomaly at transition-zone depths. Our results indicate multiple whole-mantle plumes, with variable thermochemical properties, sourced disparately at the core-mantle boundary contribute to East African rift magmatism. Elsewhere, slow wavespeeds and a thinned mantle transition-zone below Cameroon and Madagascar hint at a contribution to volcanism from below the upper mantle.