| Abstrakt: |
Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X‐discontinuity (X) at 230–350 km depth: the coesite‐stishovite phase transition, the enstatite to clinoenstatite phase transition, and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5 Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, such as common conversion‐point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233–340 km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings. Plain Language Summary: Local variations in the mineral chemistry of the upper mantle results in sharp changes in velocity and density. Seismic waves that convert from compressional‐to‐shear wave propagation (P‐to‐S) in the upper mantle are sensitive to these jumps in velocity and density, revealing variations in mineral chemistry. One such jump in velocity and density, the X‐discontinuity, has several proposed explanations and detecting its presence across a range of mantle conditions allows us to test these possible hypotheses. We search for observations of the X‐discontinuity, where P‐to‐S conversion occurs between 230 and 350 km depth, beneath the continent of Africa. Our observations are found beneath many regions of surface magmatism across Africa, suggesting that the X‐discontinuity has multiple origins. Key Points: P wave receiver functions reveal the X‐discontinuity beneath the East African Rift, Morocco, Cameroon, Hoggar, and several ocean islandsObservations are collocated with recent surface magmatism suggesting widely distributed chemical heterogeneity below AfricaNo relationships exist between depth and amplitudes of observed X and estimated temperatures, suggesting multiple causal mechanisms [ABSTRACT FROM AUTHOR] |
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