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 230-350km 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 >0.5Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, like 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 233-340km 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.

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.

Previous studies of the East African upper mantle have invoked one or more mantle upwellings with varying thermochemical nature to underly the distribution of surface volcanism. For example, Boyce and Cottaar (2021) suggest that a hot, chemically distinct upwelling beneath the southern East African Rift (EAR) is sourced from the African Large Low Velocity Province (LLVP), while magmatism in Ethiopia may lie above an additional purely thermal upwelling. 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, two causal mechanisms are possible to explain the X-discontinuity (X; 230-350km): the coesite-stishovite phase transition and/or carbonate silicate melting, both of which require entrainment of basalt from the lower mantle. Intriguingly, carbonate silicate melt was invoked by Rooney et al., (2012) to explain the discrepancy in upper mantle temperature anomalies predicted by seismic wavespeed and petrological estimates beneath East Africa. Further, active carbonatite magmatism occurs along the edge of the Tanzanian craton (Muirhead et al., 2020). Several recent regional to continental receiver function (RF) studies have identified potential observations of the X in East Africa. These studies are not focused on the presence of these upper mantle phases or lack the spatial sampling needed to robustly identify the X and its causal mechanism. Targeted high-resolution observations of the X are required to confirm the presence of exotic converted phases in the East African upper mantle and their relationship to mantle upwellings. We capitalise on the new TRAILS dataset from the Turkana depression (Bastow, 2019; Ebinger, 2018) and an adjacent network in neighbouring Uganda (Nyblade, 2017), to supplement our existing RF database and characterise the X across active continental rift setting in unprecedented detail. The prevalence of the X is mapped beneath East Africa, and subsequently compared to other areas of the African continent.

Mapping absolute P-wavespeeds in the Canadian and Alaskan mantle will further our understanding of its present-day state and evolution. S-wavespeeds are relatively well constrained, especially across Canada, but are primarily sensitive to temperature while complimentary P-wavespeed constraints provide better sensitivity to compositional variations. One technical issue concerns the difficulties in extracting absolute arrival-time measurements from often-noisy data recorded by temporary seismograph networks. Such processing is required to ensure that regional Canadian datasets are compatible with supplementary continental and global datasets provided by global pick databases. To address this, we utilize the Absolute Arrival-time Recovery Method (Boyce et al., 2017). We extract over 180,000 new absolute arrival-time residuals from seismograph stations across Canada and Alaska that include both land and ocean bottom seismometers. We combine these data with the latest USArray P-wave arrival-time data from the contiguous US and Alaska. Using an adaptively parameterised least-squares tomographic inversion we develop a new absolute P-wavespeed model, with focus on Canada and Alaska (CAP21). Initial results suggest fast wavespeeds characterise the upper mantle beneath eastern and northern Canada. A sharp transition between the slow wavespeeds below the North American Cordillera and the fast wavespeeds of the stable continental interior appears to follow the Cordilleran Deformation Front (CDF) in southwest Canada. Slow wavespeeds below the Mackenzie Mountains may extend further inland of the CDF in northwest Canada. In Alaska, CAP21 illuminates both lithospheric structure and the along strike morphology of the subducting slab. The newly compiled data may also improve resolution of subducted slab remnants in the mid-mantle below the North American continent, crucial to help constrain the formation of the Alaskan peninsular at ≥50Ma.