Abstract Inconsistencies between observations from long and short period seismic waves and geochemical data mean craton formation and evolution remains enigmatic. Specifically, internal layering and radial anisotropy are poorly constrained. Here, we show that these inconsistencies can be reconciled by inverting cratonic Rayleigh and Love surface wave dispersion curves for shear‐wave velocity and radial anisotropy using a flexible Bayesian scheme. This approach requires no explicit vertical smoothing and only adds anisotropy to layers where required by the data. We show that all cratonic lithospheres are comprised of a positively radially anisotropic upper layer, best explained by Archean underplating, and an isotropic layer beneath, indicative of two‐stage formation. Within the positively radially anisotropic upper layer, we find a variable amplitude low velocity zone within 9 of 12 cratons studied, that is well correlated with observed Mid‐Lithospheric Discontinuities (MLDs). The MLD is best explained by metasomatism after craton formation.

Our understanding of the present‐day state and evolution of the Canadian and Alaskan mantle is hindered by a lack of absolute P‐wavespeed constraints that provide complementary sensitivity to composition in conjunction with existing S‐wavespeed models. Consequently, cratonic modification, orogenic history of western North America and complexities within the Alaskan Proto‐Pacific subduction system remain enigmatic. One challenge concerns the difficulties in extracting absolute arrival‐time measurements from often‐noisy data recorded by temporary seismograph networks required to fill gaps in continental and global databases. Using the Absolute Arrival‐time Recovery Method (AARM), we extract >180,000 new absolute arrival‐time residuals from seismograph stations across Canada and Alaska and combine these data with USArray and global arrival‐time data from the contiguous US and Alaska. We develop a new absolute P‐wavespeed tomographic model, CAP22, spanning North America that significantly improves resolution in Canada and Alaska over previous models. Slow wavespeeds below the Canadian Cordillera sharply abut fast wavespeeds of the continental interior at the Rocky Mountain Trench in southwest Canada. Slow wavespeeds below the Mackenzie Mountains continue farther inland in northwest Canada, indicating Proterozoic‐Archean metasomatism of the Slave craton. Inherited tectonic lineaments colocated with this north‐south wavespeed boundary suggest that both the crust and mantle may control Cordilleran orogenic processes. In Alaska, fast upper mantle wavespeeds below the Wrangell Volcanic Field favor a conventional subduction related mechanism for volcanism. Finally, seismic evidence for the subducted Kula and Yukon slabs indicate tectonic reconstructions of western North America may require revision.

Ethiopia's Cenozoic flood basalt magmatism, uplift, and rifting have been attributed to one or more mantle plumes. The Nubian plate, however, has drifted 500–1,000 km north since initial magmatism at ∼45 Ma, having developed above mantle that now underlies the northern Tanzania craton and the low‐lying Turkana Depression. Unfortunately, our knowledge of mantle wavespeed structure and mantle transition zone (MTZ) topography below these regions is poorest, due to a historical lack of seismograph stations. The same data gap means we lack constraints on lithospheric structure in and around the NW–SE trending Mesozoic Anza rift. We exploit data from new seismograph networks in the Turkana Depression and neighboring northern Uganda to develop AFRP22, a new African absolute P‐wavespeed tomographic model that resolves whole mantle structure along the entire East African rift system. We also map MTZ thickness using Ps receiver functions. East Africa's thinnest MTZ (∼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 ∼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.

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.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]The Mantle Seismic Structure below Canada and Alaska Constrained by a New Absolute P-wavespeed Tomographic ModelAuthorsMitchLiddelliDAlistairBoyceiDStephenPughiDJoeBrowniDErinMcMurchieiDAmberParsonsClémentEstèveiDScottBurdickFionaDarbyshireiDSanneCottaariDIanBastowiDAndrewSchaefferPascalAudetiDDerekSchuttiDRichardAsteriDSee all authors Mitch LiddelliDCorresponding Author• Submitting AuthorUniversity of Quebec at Montreal UQAMiDhttps://orcid.org/0000-0002-0646-8144view email addressThe email was not providedcopy email addressAlistair BoyceiDUniversity of CambridgeiDhttps://orcid.org/0000-0002-2666-9517view email addressThe email was not providedcopy email addressStephen PughiDUniversity of CambridgeiDhttps://orcid.org/0000-0001-5997-9004view email addressThe email was not providedcopy email addressJoe BrowniDUniversity of CambridgeiDhttps://orcid.org/0000-0002-9377-9038view email addressThe email was not providedcopy email addressErin McMurchieiDUniversity of CambridgeiDhttps://orcid.org/0000-0001-7387-3222view email addressThe email was not providedcopy email addressAmber ParsonsUniversity of Cambridgeview email addressThe email was not providedcopy email addressClément EstèveiDMcGill UniversityiDhttps://orcid.org/0000-0003-2580-4833view email addressThe email was not providedcopy email addressScott BurdickWayne State Universityview email addressThe email was not providedcopy email addressFiona DarbyshireiDUniversity of Quebec at Montreal UQAMiDhttps://orcid.org/0000-0003-0967-7248view email addressThe email was not providedcopy email addressSanne CottaariDUniversity of CambridgeiDhttps://orcid.org/0000-0003-0493-6570view email addressThe email was not providedcopy email addressIan BastowiDImperial College LondoniDhttps://orcid.org/0000-0003-1468-9278view email addressThe email was not providedcopy email addressAndrew SchaefferSidney Subdivision Natural Resources Canadaview email addressThe email was not providedcopy email addressPascal AudetiDUniversity of OttawaiDhttps://orcid.org/0000-0003-2364-9454view email addressThe email was not providedcopy email addressDerek SchuttiDColorado State UniversityiDhttps://orcid.org/0000-0002-4504-5662view email addressThe email was not providedcopy email addressRichard AsteriDColorado State UniversityiDhttps://orcid.org/0000-0002-0821-4906view email addressThe email was not providedcopy email address

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]The X-discontinuity as a Tracer for Chemical Heterogeneity: Observations from East AfricaAuthorsStephenPughiDAlistairBoyceiDIanBastowiDCynthiaEbingeriDSanneCottaariDSee all authors Stephen PughiDCorresponding Author• Submitting AuthorUniversity of CambridgeiDhttps://orcid.org/0000-0001-5997-9004view email addressThe email was not providedcopy email addressAlistair BoyceiDUniversity of CambridgeiDhttps://orcid.org/0000-0002-2666-9517view email addressThe email was not providedcopy email addressIan BastowiDImperial College LondoniDhttps://orcid.org/0000-0003-1468-9278view email addressThe email was not providedcopy email addressCynthia EbingeriDTulane University of LouisianaiDhttps://orcid.org/0000-0002-6211-3399view email addressThe email was not providedcopy email addressSanne CottaariDUniversity of CambridgeiDhttps://orcid.org/0000-0003-0493-6570view email addressThe email was not providedcopy email address

The distribution of chemical heterogeneity beneath hotspots provides important constraints on the source of magmatism and mantle convection. Chemical heterogeneities in the upper mantle produce discontinuities that can be interrogated seismically. One such discontinuity, the X-discontinuity (230350 km depth), is observed intermittently across the globe, with multiple possible causal mechanisms. However, to better understand the cause of the X-discontinuity and its relationship to mantle upwellings, we require global short wavelength observations, previously unresolved by reflected phases with broad upper mantle Fresnel zones. Discontinuities resulting from seismically observable impedance contrasts can be targeted using high frequency P wave receiver functions (RFs) possessing short wavelength lateral sensitivity (similar to 100 km) to structures almost directly beneath the station. Thus, below well-instrumented hotspots we can investigate the properties of the X-discontinuity and constrain the causal mechanisms in these locations. Using P-to-s converted teleseismic phases between 30-90 degrees, we stack RFs in the depth and time-slowness domains at 28 hotspots and 6 cratons globally. This reveals 15 robust X-discontinuity observations beneath hotspots between 244-344 km depth. A further 10 potential observations are present beneath hotspots, and 6 null observations beneath cratons. Two causal mechanisms remain possible at the elevated mantle temperatures expected beneath hotspots. The coesite-stishovite phase transition occurs in eclogite and/or carbonated silicate melt may form under certain conditions. For either mechanism to provide seismically observable impedance contrasts, the upper mantle must be locally enriched in basalt. Assessing the X-discontinuity global distribution reveals a correlation with the Large Low Velocity Provinces, suggesting that mantle plumes may entrain recycled basalt from the lowermost mantle. (C) 2021 Elsevier B.V. All rights reserved.

Abstract The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P‐to‐s receiver functions (RFs; using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990–2019. We exploit a recent high‐resolution African continental P‐wavespeed model to migrate our RFs to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (∼100–150 K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ∼1,025 km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar.

Abstract Africa's Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large‐scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S‐wavespeed tomographic images of Africa are legion, but higher frequency P‐wavespeed whole‐mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental‐scale P‐wave tomographic model capable of resolving structure from upper‐to‐lower mantle depths using a recently developed technique to extract absolute arrival‐times from noisy, temporary African seismograph deployments. Shallow‐mantle wavespeeds are δVP ≈ −4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub‐vertical slow wavespeed anomaly rooted near the core‐mantle boundary outside the African LLVP, meaning multiple disparately sourced whole‐mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135 km depth. When interpreted alongside Lower Eocene on‐craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan‐African orogeny.

Cratons, the ancient cores of the continents, have survived thermal and mechanical erosion over multiple Wilson cycles, but the ability of their margins to withstand modification during continental convergence is debated. The Proterozoic Grenville orogeny operated for >= 300 Myr along the eastern edge of the proto-North American continent Laurentia, whose age varied north-to-south from similar to 1.5-0.25 Gyr at the time of collision. The preserved Grenville Province, west of the Appalachian terranes, has remained largely tectonically quiescent since its formation. Thick, cool, mantle lithosphere that underlies these Proterozoic regions is typically identified by elevated seismic velocities but lithospheric modification by fluid/melt-derived metasomatic enrichment above a subduction zone, can lead to a reduction in V-P with little effect on V-S and density. Absolute P-wavespeed constraints are therefore a vital complement to existing S-wave tomographic models of North America to investigate craton edge modification mechanisms in the Grenville orogen. New P-wave tomographic imaging of the North American continent, which benefits from recent developments in arrival-time processing of regional network deployments from the Canadian shield, reveals along strike wavespeed variation in the Grenville orogen. In the north, high seismic wavespeeds (to depths of 250 km) extend eastwards, from the Archean core of North America to beneath the Canadian Grenville Province. In contrast, below the southern U.S., high lithospheric wavespeeds are restricted to west of the Grenville Province, in particular at depths less than 150 km. We argue that subduction-derived metasomatism beneath eastern Laurentia modified the southern Grenville, prior to thermal stabilization and perhaps mantle keel formation. Beneath the northern Grenville, the thick, depleted Laurentian lithosphere resisted extensive metasomatism. Along strike age differences in Grenvillian terranes and their resulting metasomatic modification histories suggests that at least 250 Myr is required for Proterozoic lithosphere to gain resistance to modification. (C) 2019 Elsevier B.V. All rights reserved.

High frequency converted seismic phases can detect seismic velocity discontinuities and sometime map their topography. The discontinuities across the upper mantle and transition zone are generally related to mantle phase transitions, and contain information about the mantle plume dynamics and composition.

Dense, short-term deployments of seismograph networks are frequently used to study upper-mantle structure. However, recordings of variably emergent teleseismic waveforms are often of lower signal-to-noise ratio (SNR) than those recorded at permanent observatory sites. Therefore, waveform coherency across a network is frequently utilized to calculate relative arrival times between recorded traces, but these measurements cannot easily be combined or reported directly to global absolute arrival-time databases. These datasets are thus a valuable but untapped resource with which to fill spatial gaps in global absolute-wavespeed tomographic models. We developed an absolute arrival-time recovery method (AARM) to retrieve absolute time picks from relative-arrival-time datasets, working synchronously with filtered and unfiltered data. We also include a relative estimate of uncertainty for potential use in data weighting during subsequent tomographic inversion. Filtered waveforms are first aligned via multichannel cross correlation. These time shifts are applied to unfiltered waveforms to generate a phase-weighted stack. Cross correlation with the primary stack or the SNR of each trace is used to weight a second-higher SNR stack. The first arrival on the final stack is picked manually to recover absolute arrival times for the aligned waveforms. We test AARM on a recently published dataset from southeast Canada (similar to 10; 000 picks). When compared with the available equivalent earthquake-station pairs on the International Seismological Centre (ISC) database,similar to 83% of-AARM-picks agree to within +/- 0: 5 s. Tests using synthetic P-wave data indicate thatAARMproduces absolute arrival-time picks to accuracies of better than 0.25 s, akin to uncertainties in ISC bulletins.