GD10.2 | Constraining mantle convection models with Earth’s observations
Constraining mantle convection models with Earth’s observations
Co-organized by EMRP2/SM6
Convener: Franck Latallerie | Co-conveners: Thomas Duvernay, James Ward, Emma Chambers, James Panton, Menno Fraters, Sarah Jane Fowler
Orals
| Thu, 18 Apr, 16:15–18:00 (CEST)
 
Room D2
Posters on site
| Attendance Fri, 19 Apr, 10:45–12:30 (CEST) | Display Fri, 19 Apr, 08:30–12:30
 
Hall X2
Posters virtual
| Attendance Fri, 19 Apr, 14:00–15:45 (CEST) | Display Fri, 19 Apr, 08:30–18:00
 
vHall X2
Orals |
Thu, 16:15
Fri, 10:45
Fri, 14:00
Mantle circulation simulations are now capable of a high level of precision and complexity that allows the creation of numerous "Earth-like" models. Likewise, advances in observation resources and methods have improved the quantity and quality of data on the Earth's interior. Combining these developments presents a unique opportunity to enhance our understanding of mantle dynamics and evolution over geological time scales. However, the exact physics leading to Earth-like simulations remains debated (e.g. the existence of a primordial layer, the core-mantle-boundary temperature, etc...). Furthermore, constraining geodynamical simulations or assessing their predictions with observational data can be challenging, for example, due to data noise, issues related to inverse methods, or uncertainty propagation.

This session aims to explore how observational data can be used to constrain or assess geodynamical simulations and advance our knowledge of the physical processes that govern the Earth's mantle. We invite submissions from various fields, including seismology, geochemistry, mineral physics or geomagnetism where observations have the potential to constrain geodynamical simulations or assess their predictions. The nature of these studies can be purely observational, exploring the inversion of data to possible Earth models or proposing metrics to assess how Earth-like a model is.

This session also aims to compare these observations and address their potential to constrain or assess geodynamical simulations, with the ultimate goal of better understanding which parameters may cause models to be more or less Earth-like.

Orals: Thu, 18 Apr | Room D2

Chairpersons: Franck Latallerie, Thomas Duvernay, Emma Chambers
16:15–16:16
16:16–16:26
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EGU24-6130
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ECS
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On-site presentation
Scott Akhtar-Lewis

The effects of pressure and temperature on the phase transformation of olivine to wadsleyite and then ringwoodite within the mantle is well understood. However, the extent to which stress affects this phase transformation is not clear. Understanding how stress influences the kinetics of the olivine to spinel phase transformation and the mechanism in which it does so at grain scale, will have broader implications for mantle dynamics. Deformation experiments using Mg2GeO4 have been used as an approximate analogue for fayalite as it transforms from olivine to ringwoodite at lower pressures and temperatures rather than the conditions found at d410 (Vaughan, 1981). This enables the use of larger samples than possible for the silicate system, and allows for extensive microstructural investigations. This session aims to discuss high pressure deformation experiments on Mg2GeO4 (olivine) during the transformation to ringwoodite using a Griggs-type, solid medium, deformation apparatus. These experiments expand on (Vaughan 1984) which linked kinetics of the reaction in a model that matches other stressed reactions in the mantle (Wheeler, 2020). Experiments were conducted at a range of confining pressures 0.8 - 1.2 GPa at a fixed temperature of 900 °C and a strain rate of 10-6 /s. The four samples were deformed to finite strains ranging from 10 to 45 %. The aim of the conditions chosen was to apply varying amounts of differential stress and therefore differing the σ1 stress on the sample as a whole. Samples were characterised down to the level of individual interfaces using Electron Backscatter Diffraction (EBSD) to understand the physical mechanism of the reaction and the kinetics that govern it.

How to cite: Akhtar-Lewis, S.: Effects of Stress on the Olivine–Spinel Phase Transformation in the Mantle: Griggs-Type Deformation Experiments Using Mg2GeO4 as an Analogue., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6130, https://doi.org/10.5194/egusphere-egu24-6130, 2024.

16:26–16:36
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EGU24-2139
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ECS
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solicited
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On-site presentation
Isabel Papanagnou, Bernhard S. A. Schuberth, Christine Thomas, and Hans-Peter Bunge

A main objective in geodynamics is to create models that provide quantitative information to other Earth science disciplines. In order to assess the validity of the underlying assumptions and chosen input parameters related to different geodynamic hypotheses, it is crucial to test these models against observations. In this, thermodynamic models of mantle mineralogy represent an essential tool. On the one hand, they enable the linking of temperature fields from mantle circulation models (MCMs) to seismic observations. On the other hand, they provide critical information on material behaviour in response to changing temperature and pressure conditions that occur over time within such mantle convection simulations. Some of the most interesting aspects in this context relate to mineral phase transitions and associated dynamic effects on mantle flow.

The mantle transition zone (TZ) in particular is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions as well as an increase in viscosity with depth. Still, neither its seismic structure nor the associated dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) have classically been related to phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. The resulting topography of these discontinuities is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

Here, we present hypothetical realizations of TZ seismic structure and major discontinuities based on the 3-D temperature field of a published MCM for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (< 2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660 —with average deflections of 14 and 6 km, respectively— due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.

How to cite: Papanagnou, I., Schuberth, B. S. A., Thomas, C., and Bunge, H.-P.: Geodynamic-mineralogical predictions of mantle transition zone seismic structure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2139, https://doi.org/10.5194/egusphere-egu24-2139, 2024.

16:36–16:46
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EGU24-11790
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On-site presentation
Magdala Tesauro, Mikhail Kaban, and Mohammad Youssof

Xenolith data reveal lateral and vertical compositional variations of the upper mantle of the Precambrian cratons, indicating a different degree of refertilization with respect to the most depleted mantle in iron components, characterizing the oldest Archean cratons. The South African cratonic region is composed of the Kaapvaal and Zimbabwe craton, both of Archean age, having deep and fast lithospheric roots, which are likely depleted in heavy constituents. In contrast, there exist regions, such as the Limpopo belt, a terrane that was trapped between the Kaapvaal and Zimbabwe cratons during their collision (2.6–2.7 Ga), and Bushveld Complex, an area characterized by intraplate magmatism occurred 2.05 Ga, whose negative velocity anomalies in the upper mantle, indicate a more fertile composition due to metasomatism. To unravel the origin of these anomalies and link them to the tectonic history of the area, we apply an integrative technique based on a joint interpretation of the seismic tomography and gravity data, which can discern temperature and compositional variations. To this aim, we combine the global surface seismic tomography model [1] with the embedded regional model [2], derived from teleseismic tomographic inversion of the S-body wave dataset recorded by the Southern African Seismic Experiment. The combined seismic model is inverted for temperature, assuming an initial composition, representative of a refertilized upper mantle [3], using a mineral physics approach [4]. The composition and temperature of the upper mantle are iteratively changed, increasing progressively the amount of iron depletion, to fit the residual density, obtained from the joint inversion of the residual gravity and residual topography. The great advantage of using both the gravity field and residual topography lies in their different dependence on the distribution of density heterogeneities (depth and size). In a second type of inversion we included the GOCE gravity gradient [5]. The obtained results show that the most depleted lithosphere is confined at depth lower than 100 km, generating a temperature higher than ~200, with respect to that of a refertilized lithosphere. The Southeastern Terrane of the Kaapval craton are characterized by thicker and more depleted cratonic roots than the Zimbawe craton. The presence of a depleted mantle below the cratonic crust may indicate that the crust and mantle have been connected since the craton formation. These results, related to the different structures and properties of the upper mantle, improve our understanding of the evolution of the South African cratonic lithosphere.

References

[1] Schaeffer and Lebedev, 2013. https://doi.org/10.1093/gji/ggt095

[2] Youssof et al., 2015. http://dx.doi.org/10.1016/j.epsl.2015.01.034

[3] Griffin et al., 2004. doi:10.1016/j.chemgeo.2004.04.007

[4] Conolly, 2005. doi:10.1016/j.epsl.2005.04.033

[5] Kaban et al., 2022. doi.org/10.1007/s00024-021-02925-6

How to cite: Tesauro, M., Kaban, M., and Youssof, M.: Thermo-compositional model of the South African cratonic mantle obtained from seismic and gravity data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11790, https://doi.org/10.5194/egusphere-egu24-11790, 2024.

16:46–16:56
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EGU24-7341
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Highlight
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On-site presentation
Romain Tilhac, Carlos Garrido, Stephan König, and María Isabel Varas-Reus

The presence of lithologies derived from recycled oceanic lithosphere in the convective mantle is an expected consequence of subduction. Geochemical studies have provided compelling evidence of the contribution of recycled eclogite and pyroxenite in the mantle source of oceanic basalts, particularly ocean island basalts (OIB). However, identifying their signatures in mid-ocean ridge basalts (MORB) is challenging due to more intricate melting and mixing processes. Furthermore, the use of elemental and isotopic proxies of different geochemical affinities provides contrasting pictures on their source heterogeneity. Understanding the role of pyroxenite and eclogite during partial melting bears critical information regarding the fate of recycled lithospheric material, the dynamics and timescales of mantle convection and the thermal regime of mid-ocean ridges.

We present a numerical approach based on the thermodynamically constrained Mixed-Source Melting model (MSM3), enabling a coherent assessment of the role of recycled lithologies. Within a comprehensive plate tectonic cycle, the MSM3 model simulates the two-stage recycling of eclogites derived from subducted oceanic crust in a marble-cake mantle.

  • Stage 1 corresponds to the formation of secondary pyroxenite from the hybridization of high-degree eclogite-derived melts interacting at high pressure with peridotite in the convective mantle.
  • Stage 2 corresponds to the formation of MORB in a triangular melting regime from the adiabatic decompression melting of a 3-lithology source of peridotite, pyroxenite and residual eclogite obtained from stage 1.

To tackle the diversity of geochemical proxies applied to oceanic basalts, MSM3 recovers melt and residual compositions in terms of major elements and sulfur, as well as any lithophile and chalcophile trace elements and isotope systems. This is achieved thanks to the integration of melting models with pMELTS calculations constrained by a thermodynamic parametrization specific to pyroxene-rich lithologies (Melt-PX), calculations of sulfur concentration at sulfide saturation (SCSS), and composition-dependent partition coefficients. To take into account the inherent variability of most parameters (e.g., potential temperature, source proportions, sulfur contents) and avoid arbitrary choices, we use a stochastic approach by running the MSM3 model as an inversion based on adaptative Monte Carlo simulations.

We here demonstrate the flexibility of this approach, even for systems controlled by sulfides. We show that, over potential temperatures ranging between 1280 and 1420 ºC, the generation of 0-10% of pyroxenitic heterogeneities from subducted eclogite, and the contribution of both eclogite and pyroxenite in the melting regime of MORB produce 20-95 % of the melts aggregated at the ridge. Such proportions correspond to up to 30 times the proportion of these lithologies in the mantle. This over-contribution is controlled by the melting regime properties and is enhanced or attenuated by the mass balance specific to the elements and isotope systems considered (concentrations, partitioning behavior, modal evolution of the main host minerals in the different lithologies). In other words, the more-fusible pyroxenite and eclogite act as chocolate in the marble-cake mantle, giving the dominant flavor to its melting products, although different geochemical proxies may "taste" it differently.

How to cite: Tilhac, R., Garrido, C., König, S., and Varas-Reus, M. I.: Chocolate in the marble cake: the fate of eclogite and pyroxenite during mantle convection and melting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7341, https://doi.org/10.5194/egusphere-egu24-7341, 2024.

16:56–17:06
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EGU24-557
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ECS
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On-site presentation
Callum Pearman, Nicky White, John Maclennan, and Chia-Yu Tien and the IODP Expedition 395 science party

The Icelandic mantle plume is regarded as one of the most significant mantle upwellings on Earth, however the dynamics of its interaction with the surrounding asthenosphere and mid-oceanic ridge systems in the North Atlantic are poorly understood. The clearest manifestation of this plume-ridge interaction are the Reykjanes V-shaped ridges and V-shaped troughs (VSRs and VSTs) that straddle the Reykjanes Ridge axis south of Iceland. These time-transgressive linear features are particularly well exposed by short-wavelength gravity data and are thought to represent the progressive sampling of thermal asthenospheric pulses that horizontally advect away from the Icelandic mantle plume conduit. The Reykjanes Ridge therefore acts as a ‘window-sampler’ into the temporal and spatial dynamics of plume outflow. International Ocean Discovery Program (IODP) Expedition 395C drilled into two VSR and VST pairs along a plate-spreading flow line approximately 600 km south of Iceland in summer 2021. Over 400 m of basalt was recovered, which represents a magmatic record over 15 Ma of plate spreading at a fixed distance from the mantle plume conduit. We present Nd isotopic analysis of recovered whole-rock that reveals a linear isotopic evolution from ƐNd of 7.5 to 10.5 over 14 Ma (n = 50), which implies that the ‘plume-like’ enriched component of the mantle source has been progressively diluted by mixing with depleted upper mantle material. This evolution occurred synchronously with the entire timeframe of VSR formation as defined by free-air gravity anomalies, and a long-wavelength increase in crustal thickness implied by wide-angle seismic experiments. It is therefore apparent that the dynamics of plume-ridge interaction are directly interlinked with changes in magmatism, structural tectonics and crustal production. Furthermore, major and trace elements of both whole-rock and glass samples have been measured, by multiple analytical techniques, revealing distinct compositions between and within boreholes. These observations can be understood in terms of temporal changes in the depth and degree of melting. In summary, petrological, petrophysical and geochemical analysis of this rock core in conjunction with consideration and modelling of wide-angle seismic surveys, gravity and bathymetric data can be used to develop a quantitative understanding of the dynamics of plume-ridge interaction, test hypotheses for the formation of VSRs, and constrain the temporal evolution of the North Atlantic mantle domain.

How to cite: Pearman, C., White, N., Maclennan, J., and Tien, C.-Y. and the IODP Expedition 395 science party: Temporal Record of Plume-Ridge Interaction in the North Atlantic: Interdisciplinary Insights from IODP Expedition 395C, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-557, https://doi.org/10.5194/egusphere-egu24-557, 2024.

17:06–17:16
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EGU24-17413
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ECS
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Highlight
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On-site presentation
Rebekah Pestes, Joao Coelho, Yael Deniz Hernandez, Stéphanie Durand, Nobuaki Fuji, Eric Mittelstaedt, and Véronique Van Elewyck

The origin of Large Low-Velocity Provinces (LLVPs) at the base of mantle remains a mystery, but particle physics may be able to provide another piece of the puzzle. Using a phenomenon known as neutrino oscillation, atmospheric neutrino experiments are sensitive to the electron number density inside the Earth, which is complementary to the information seismology can provide. In order to reveal lateral heterogeneities in density and chemical composition such as those expected for LLSVPs across the Earth’s lower mantle, we have developed a numerical method that allows us to compute the sensitivity of neutrino oscillation data to 3D Earth structure. Based on this approach, we will present some preliminary assessment of the potential resolving power of ongoing and future neutrino experiments.

How to cite: Pestes, R., Coelho, J., Deniz Hernandez, Y., Durand, S., Fuji, N., Mittelstaedt, E., and Van Elewyck, V.: Neutrino oscillations investigation of the base-of-the-mantle structure: simulation tools and preliminary results, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17413, https://doi.org/10.5194/egusphere-egu24-17413, 2024.

17:16–17:26
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EGU24-5439
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solicited
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On-site presentation
Manuele Faccenda

The study of the Earth’s interior has been traditionally based on seismological and geodynamic modelling, the first one providing important information about its present-day structure, composition and state, while the second about its dynamics and compositional evolution. Seismological and geodynamical modelling are very often conducted independently, which creates mechanical and geometrical inconsistencies across the models, hampers the interpretation of seismic observations in terms of geodynamic processes and enhances the non-uniqueness of geodynamic model predictions.

An alternative approach is combining computational seismology and geodynamics with mineral physics, which provides a comprehensive understanding of the Earth's interior processes, seismic behavior, and material properties. In this multidisciplinary methodology, the geodynamic flow calculations are used to compute the rock elastic properties as a function of strain-induced mantle fabrics through micro-mechanical models of crystal aggregate deformation, and of the local P-T conditions with thermodynamically self-consistent models of mantle mineralogy. The obtained seismic mantle structure is then used for seismological synthetics, such that specific hypotheses on mantle dynamics can be tested directly against seismic data. Examples from the South American, North American, and the Central Mediterranean convergent margins will be discussed.

Finally, I will introduce ECOMAN, a recently developed, open-source software package that is intended to overcome the computationally intensive nature of this approach and the lack of a dedicated and comprehensive computational framework for modelling strain-/stress-induced rock fabrics and testing the effects of the resulting mechanical (elastic and viscous) anisotropy on seismic imaging and mantle convection.

How to cite: Faccenda, M.: Constraining Mantle Convection Patterns by Joint Geodynamic and Seismological Modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5439, https://doi.org/10.5194/egusphere-egu24-5439, 2024.

17:26–17:36
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EGU24-10944
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ECS
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On-site presentation
William Sturgeon, Ana M.G. Ferreira, James Panton, and J. Huw Davies

In order to improve our understanding of mantle flow, we require a joint collaboration between all fields of Earth Sciences. Seismic tomography provides key information on the current state of the mantle and therefore can constrain mantle circulation models. We present high-resolution (degree-60) global models of frequency-dependent phase and group velocity measurements from huge a huge dataset of ~47 million Rayleigh and Love waves. These include fundamental mode measurements, which are sensitive to the uppermost mantle and up to 6th overtone, adding sensitivity to the transition zone, covering a period range of 16-375 s. We also present global models of mantle attenuation (degree-20), made from ~10 million Rayleigh wave amplitude measurements, including fundamental and up to 4th overtone measurements (35-275 s). All seismic maps presented also have associated uncertainty maps, which are essential for robust interpretation but also for multidisciplinary interpretations of mantle circulation models.

We constrain 3D mantle circulation models, known as TERRA models, at the present day. In order to do this, we construct 1D profiles of velocities and density from a suite of TERRA models on a 2x2 degree grid. Forward modelling of each 1D profile using MINEOS provides global predictions of seismic observables at all seismic wave periods, including phase velocity and group velocity. A misfit can then be calculated between the seismic models and predictions from the suite of TERRA models. This provides constraints on which TERRA models are most Earth-like, which will improve our understanding of mantle flow.

How to cite: Sturgeon, W., Ferreira, A. M. G., Panton, J., and Davies, J. H.: Constraining global mantle circulation models with global seismic observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10944, https://doi.org/10.5194/egusphere-egu24-10944, 2024.

17:36–17:46
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EGU24-14786
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ECS
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On-site presentation
Thomas Schouten, Lars Gebraad, Sebastian Noe, Anna Gülcher, Sölvi Thrastarson, Dirk-Philip van Herwaarden, and Andreas Fichtner

Seismic tomography, a critical tool for studying Earth's interior structure and dynamics, has revealed positive seismic wave speed anomalies in the mantle that are commonly interpreted as slabs, the remnants of subducted lithosphere. However, classical travel-time tomography relies on the inversion of travel times of a few easily identifiable body wave phases along ray paths or volumetric sensitivity kernels, which is strongly dependent on the geometry of seismic sources and receivers. Since both of these are primarily clustered on modern convergent plate boundaries, the resulting tomographic resolution is highly variable across the mantle. Full-waveform inversion (FWI) attempts to reduce this dependence by fitting whole seismograms, thereby including many reflected and refracted body wave phases to enhance the volumetric sensitivity of the inversion.

Here, we analyse a new global tomographic model constructed using FWI. The mantle structure imaged in this model reveals significantly more positive seismic wave speed anomalies in the mantle when compared to travel-time tomography, particularly in regions with low seismic activity and limited station coverage. Notably, FWI detects positive wave speed anomalies with slab-like morphologies at ~1000 km depth beneath the Pacific Ocean that fall outside the coverage of classical travel-time tomography. We demonstrate the sensitivity of FWI to wave speed anomalies below the western Pacific using forward wavefield modelling. Importantly, we find that these newly imaged positive wave speed anomalies do not correspond to reconstructed subduction zones in existing global plate reconstructions.

Our work challenges the widespread assumption that positive wave speed anomalies (exclusively) represent subducted slabs, highlighting potential gaps in either global plate reconstructions or the current understanding of the nature of seismic anomalies in the mantle.

How to cite: Schouten, T., Gebraad, L., Noe, S., Gülcher, A., Thrastarson, S., van Herwaarden, D.-P., and Fichtner, A.: Global full-waveform inversion reveals previously undetected positive wave speed anomalies beneath the Pacific Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14786, https://doi.org/10.5194/egusphere-egu24-14786, 2024.

17:46–17:56
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EGU24-14060
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ECS
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Highlight
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On-site presentation
Joshua Guerrero, Frédéric Deschamps, Wen-Pin Hsieh, and Paul Tackley

New insights from models of thermo-chemical mantle convection featuring heterogeneous thermal conductivity indicate that heat-producing element (HPE) enrichment in large low shear velocity provinces (LLSVPs) significantly impacts the long-term stability of these regions. Because the internal heating rate was more significant in the past, thermal conductivity's influence on thermal buoyancy (and bulk erosion) must have also been more substantial. As a consequence, the initial volume of the LLSVPs may have been significantly larger than their present-day volume. In numerical models, the evolution and stability of LLSVPs are often initiated by considering a dense and uniformly distributed layer on top of the core-mantle boundary. From energy balance calculations, a thin layer of LLSVP material (small mantle volume fraction) supports more HPE enrichment than a thicker layer (larger mantle volume fraction) to maintain the mantle's heat budget. For example, an initial layer thickness of 160km (~3% mantle volume) implies present-day HPE enrichment factors up to ~70 times the ambient mantle heating rate. This should be compared with more conservative factors of 10 to 20 for similar dense layer thicknesses employed in previous studies of thermochemical pile stability. Thus, HPE enrichment may have been significantly underestimated in earlier models of LLSVP evolution. Conversely, and assuming that LLSVPs formed from a much larger reservoir, HPE enrichment may be overestimated based on the present-day LLSVP volume. Our study considers LLSVPs with a primordial geochemical reservoir composition (consistent with an undegassed 4He/3He signature and HPE enrichment). We examine models of thermo-chemical mantle convection models with time-dependent internal heating rates and HPE enrichment (implied by the initial dense layer thicknesses). In this new context, we re-examine, in particular, the impact of a fully heterogeneous lattice thermal conductivity (derived from conductivity measurements of upper and lower mantle minerals). Furthermore, in light of recent developments with radiative conductivity, we also examine the added effect of a strongly temperature-dependent radiative conductivity component on the stability of LLSVPs. Using LLSVPs' present-day volume and core-mantle boundary coverage as a constraint, we recover potential initial conditions, heating scenarios, and thermal conductivity for an Earth-like model.

How to cite: Guerrero, J., Deschamps, F., Hsieh, W.-P., and Tackley, P.: Constraining LLSVPs initial conditions and heating scenarios from simulations of mantle convection with heterogeneous thermal conductivity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14060, https://doi.org/10.5194/egusphere-egu24-14060, 2024.

17:56–18:00

Posters on site: Fri, 19 Apr, 10:45–12:30 | Hall X2

Display time: Fri, 19 Apr 08:30–Fri, 19 Apr 12:30
Chairpersons: James Panton, Menno Fraters
X2.71
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EGU24-1275
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ECS
Qian Chen, He Liu, Andrea Giuliani, Tim Johnson, Luc Doucet, Lipeng Zhang, and Weidong Sun

Plate tectonics drives the compositional diversity of Earth’s convecting mantle through subduction of lithosphere. In this context, the role of evolving global geodynamics and plate (re)organisation on the spatial and temporal distribution of compositional heterogeneities in the convecting mantle is poorly understood. We test the hypothesis that an increase in the cumulative length of subduction zones associated with supercontinent assembly triggered geochemical enrichment of the convective mantle globally, in particular since the emergence of protracted, cold, deep subduction in the late Neoproterozoic. We compiled the trace element and Nd isotopic compositions of intracontinental basalts formed over the last billion years (1000 Myr).  After careful filtering to eliminate samples with evidence for crustal contamination, the data show that intracontinental basalts formed before 300 Ma exhibit supra-chondritic initial 144Nd/143Nd values. Those with sub-chondritic initial 144Nd/143Nd values become common only after 300 Ma, broadly coeval with the global appearance of kimberlites with geochemically enriched isotopic signatures. We attribute these step-changes in the sources of intraplate magmatism to a rapid increase in the supply of deeply subducted lithosphere due to increased peri-continental subduction during the assembly of Pangea.

How to cite: Chen, Q., Liu, H., Giuliani, A., Johnson, T., Doucet, L., Zhang, L., and Sun, W.: Enhanced subduction flux during the assembly of Pangea recorded by global intracontinental magmatism, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1275, https://doi.org/10.5194/egusphere-egu24-1275, 2024.

X2.72
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EGU24-5452
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ECS
Ronghua Cui, Bernhard Steinberger, and Jian Fang

Mantle convection causes the most important contribution to the geoid and dynamic topography. With high resolution tomography models and numerical simulation methods solving the governing equations of mantle convection, the model geoid can fit well compared to observation. However, if wave speed variations are converted to density variations assuming both are due to temperature variation in the entire mantle, there is still a large discrepancy between the present dynamic topography predicted by mantle flow and that induced from observations: Especially large negative topography is predicted in cratons, contrary to observations. In order to improve the fit of model dynamic topography compared to observations, chemical density anomaly in earth’s lithosphere need to be included. In this study, we will combine these with lateral viscosity structure and study the effect on model dynamic topography and geoid, and investigate which density models would yield a good fit. In the sublithospheric mantle, under the assumption that the density anomalies are thermally induced from temperature variation in the mantle, we use temperature-dependent viscosity. We also include thermo-chemical density anomalies in the Large low-shear-velocity provinces (LLSVPs) in the lowermost mantle to compute their effect on the model geoid and dynamic topography. Our overall objective is a better constraint on the Earth’s interior structure, by achieving good fits of both dynamic topography and geoid to their observations, to provide as a good reference for the Earth’s study.

How to cite: Cui, R., Steinberger, B., and Fang, J.: Modeling geoid and dynamic topography from tomography-based thermo-chemical mantle convection with temperature- and depth-dependent viscosity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5452, https://doi.org/10.5194/egusphere-egu24-5452, 2024.

X2.73
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EGU24-16119
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ECS
Lijun Deng and Ting Yang

Subducted slabs provide the primary driving force for mantle convection, and the slab strength directly controls the transfer of the forces between the slab and the lithospheric plates at the surface. The analysis of subducted slab viscosity structure has been one of the main concerns in geodynamics over the past few decades. Previous studies, using the topography, gravity or geoid as crucial observations, have provided some constraints on the viscosity of subduction plates (Bessat et al., 2020; Hager, 1984; Moresi & Gurnis, 1996). However, slab viscosity constrained by surface topography and gravity data is significantly lower than that suggested by mineral physics laboratory experiments. It is unclear whether the free-slip top boundary condition used in many previous studies affects the inverted slab viscosity with gravity or geoid data.

In this study, we develop 2-D free-surface subduction models that can generate realistic topography by a modified "sticky-air" method using Underworld2 software (Moresi et al., 2019), and we compare the computed topography and gravity in our free-surface subduction models with observations to constrain the subducting slab viscosity. We investigate the influence of slab viscosity at the bending region and below the bending region on the topography and the gravity, respectively. Our model results support relatively weak slabs (20-120 times more viscous than the upper mantle) at the bending region, consistent with previous studies with a free-slip top boundary. The viscosity of the slab below the bending region barely affects the surface topography and gravity field, and both strong and weak slabs fit the observed topography and gravity field, suggesting that extra independent observations are needed to constrain the deep slab viscosity. Besides, in this study, we also find the comprehensive relations between subduction interface viscosity, surface topography and gravity anomaly, and trench motion. Models with trench advance have significantly low topography and gravity above the volcanic arc, contradicting subduction zone observations. Together with present trench motion observations and previous studies, we support the idea that the trench retreats under normal single-slab subduction conditions.

 

Bessat, A., Duretz, T., Hetényi, G., Pilet, S., & Schmalholz, S. M. (2020). Stress and deformation mechanisms at a subduction zone: insights from 2-D thermomechanical numerical modelling. Geophysical Journal International, 221(3), 1605–1625. https://doi.org/10.1093/gji/ggaa092

Hager, B. H. (1984). Subducted slabs and the geoid: Constraints on mantle rheology and flow. Journal of Geophysical Research: Solid Earth, 89(B7), 6003–6015. https://doi.org/10.1029/JB089iB07p06003

Moresi, L., & Gurnis, M. (1996). Constraints on the lateral strength of slabs from three-dimensional dynamic flow models. Earth and Planetary Science Letters, 138(1–4), 15–28. https://doi.org/10.1016/0012-821X(95)00221-W

Moresi, L., Giordani, J., Mansour, J., Kaluza, O., Beucher, R., Farrington, R., et al. (2019, February 18). underworldcode/underworld2: v2.7.1b (Version v2.7.1b). Zenodo. https://doi.org/10.5281/ZENODO.2572036

How to cite: Deng, L. and Yang, T.: Constraining subducting slab viscosity with topography and gravity fields in free-surface mantle convection models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16119, https://doi.org/10.5194/egusphere-egu24-16119, 2024.

X2.74
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EGU24-6325
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ECS
Franck Latallerie, Paula Koelemeijer, Andrew Walker, James Panton, and Huw Davies

Surface-waves carry important information about upper mantle structure, especially in poorly sampled areas such as oceanic regions. Surface-wave tomography models can be used to assess geodynamic simulations by comparing observed and predicted structures. However, surface-wave data are noisy and sparse resulting in tomography models being noisy and blurred pictures of the Earth's structure. As a result, tomography models can hardly be compared directly to geodynamic predictions which aim to predict the true structure of the Earth. Although challenging, assessing geodynamic simulations with surface-wave tomography requires accounting for full 3D resolution and robust uncertainties.

In this study, we present a workflow to quantitatively assess geodynamic model predictions using surface-wave tomography. Specifically, we measure dispersion data for paths crossing the Pacific ocean and estimate data uncertainties including measurement and theoretical errors. We use a finite-frequency forward theory to linearly relate data to the three-dimensional Vsv structure in the upper mantle. Subsequently, we apply the SOLA (Backus-Gilbert-style) method in 3D to control and produce the full three-dimensional resolution and robust model uncertainties together with the Vsv tomography model. Equipped with this, we assess predictions for the Pacific upper mantle from a set of geodynamic simulations based on different input parameters.

Preliminary results highlight physical parameters of mantle convection influencing significantly the misfit between observed and predicted structure in the Pacific upper mantle; and, for quantitative parameters, inform us on values that provide the best fits.

How to cite: Latallerie, F., Koelemeijer, P., Walker, A., Panton, J., and Davies, H.: Assessment of geodynamic predictions with surface-wave tomography in the Pacific upper mantle accounting for full 3D resolution and robust uncertainties., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6325, https://doi.org/10.5194/egusphere-egu24-6325, 2024.

X2.75
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EGU24-11588
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ECS
Gwynfor Morgan, J. Huw Davies, Bob Myhill, James Wookey, and James Panton

Throughout Earth’s mantle, several significant phase transitions occur, with the Ol→Wd and Rw→Brm+Pc reactions (exothermic and endothermic respectively) producing large discontinuities in Earth’s seismic velocity structure at 410 and 660km depth respectively (‘410’ & ‘660’). The equilibrium depth of these reactions is sensitive to temperature, and the resulting topography has been observed with various seismic phases. Numerical modelling from the 1980s onwards has suggested that the topography on endothermic phase transitions can stagnate downwellings and even layer mantle convection for extreme Clapeyron slopes or density changes. The thermodynamic properties of the post-spinel reaction make it unlikely that slabs would stagnate due to effects associated with phase transitions. At cooler temperatures the post-spinel reaction splits into two reactions (Rw + Ak → Ak + Pc → Brm + Pc) which seems to explain well aspects of the observed topography of the ‘660’ discontinuity. It has been suggested that this second reaction (which has a more extreme Clapeyron slope than the post-spinel reaction) could stagnate downwellings. Recently, Ishii et al (2023) suggested that the post-garnet reaction (Gt → Brm + Cor [+ St]) is in fact univariant, producing a sharp reaction that is endothermic for cooler temperatures and exothermic at higher temperatures – and that this may contribute to slab stagnation. Here, we test these slab stagnation mechanisms using realistic mineral physics and whole-mantle convection models (MCMs).

The lack of anti-correlation between the topography of the ‘410’ and ‘660’ discontinuities does not match simple theory if they are controlled solely by temperature variations across the post-olivine and post-spinel reactions respectively. Previous work has shown that the calculated topography on the discontinuities can be markedly different for various single-composition mantles generated from MCMs (Papanagnou et al, 2022). Here we will explore the impact of laterally varying chemistry generated in thermochemical MCMs on global discontinuity topography.

How to cite: Morgan, G., Davies, J. H., Myhill, B., Wookey, J., and Panton, J.: Modelling the global geodynamic and seismological consequences of different phase boundary morphologies. , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11588, https://doi.org/10.5194/egusphere-egu24-11588, 2024.

X2.76
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EGU24-16771
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ECS
James Panton, Huw Davies, and Paul Beguelin

Volumetrically, the most important magmatic source on Earth is beneath mid-ocean ridges, from which mid-ocean ridge basalts (MORBs) are sourced. Second to this is plume related magmatism, the source for ocean-island basalts (OIBs). Decades of geochemical analysis have discerned that MORBs exhibit low isotopic variation, which is interpreted to mean that their source is globally homogeneous at an ocean basin (and possibly global) scale. OIBs, however, exhibit strong isotopic variation not just spatially, but even temporally, indicating a compositionally heterogenous source region. Global scale numerical geodynamic models, driven by reconstructed plate motions, generate both plume and ridge structures at which melting occurs, similar to Earth, however it is not known how well dynamic models can re-create the first-order observation that ridge lavas are typically homogenous compared to plume lavas.

Using the 3D spherical mantle convection code, TERRA, constrained by plate motion reconstructions spanning 1Gyr of Earth’s history, ridge and plume structures are simultaneously generated. The location of ridges is known from the input plate reconstruction model, while plumes are identified using the simulated temperature and radial velocity fields and a combination of K-means and density-based clustering. Using our approach, we can not only compare the properties of ridges and plumes, but can also compare the properties of ridges across different ocean basins and of individual simulated mantle plumes. Analysis of the bulk composition and melting age of tracer particles associated with ridges and plumes allows us to better interpret the history of material found in these regions. We analyse the U ratio to see if recent (~600 Ma) changes in the subducted U flux are evident in differences in the U composition of plume and ridge material.

How to cite: Panton, J., Davies, H., and Beguelin, P.: Trace element and bulk chemistry of plumes and ridges in geodynamic simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16771, https://doi.org/10.5194/egusphere-egu24-16771, 2024.

X2.77
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EGU24-12417
Cedric Thieulot, Erik van der Wiel, and Douwe van Hinsbergen

Geodynamic models of mantle convection provide a powerful tool to obtain insights into the structure and composition of the Earth’s mantle that resulted from a long history of differentiating and mixing. Comparing such models with geophysical and geochemical observations is challenging as these datasets often sample entirely different temporal and spatial scales. Here, we explore the use of configurational entropy, based on tracer and compositional distribution on a global and local scale. We show means to calculate configurational entropy in a 2D annulus and find that these calculations may be used to quantitatively compare long-term geodynamic models with each other. The entropy may be used to analyze, with a single measure, the mixed state of the mantle as a whole and may also be useful to validate numerical models against local anomalies in the mantle that may be inferred from seismological or geochemical observations.

How to cite: Thieulot, C., van der Wiel, E., and van Hinsbergen, D.: Quantifying mantle mixing through configurational Entropy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12417, https://doi.org/10.5194/egusphere-egu24-12417, 2024.

X2.78
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EGU24-8992
Christine Thomas and Björn Holger Heyn

The D" region, located just above the core-mantle boundary (CMB), is a geologically interesting region that has been imaged using both tomographic and reflection techniques. However, reflection studies often rely on array analysis techniques, and the lack of suitable seismic arrays in the oceans has left large areas of D" unmapped. One notable area, that is currently sparsely sampled, is beneath the Indian Ocean, where ancient subducted lithosphere has been imaged near the CMB in global tomography studies. We take advantage of the long-running history of five GEOSCOPE stations located in the western Indian Ocean and Antarctica, to investigate the possibility of using source arrays to detect P-wave reflections from the discontinuity above the D" layer. Despite restricting the selected earthquakes around Indonesia to a 120 km depth range and implementing several source normalization techniques, source-array stacks (i.e., source vespagrams) were difficult to interpret. We infer that this complication arises from differing earthquake depths, violating the plane wave assumption made when constructing these stacks. Therefore, we extend our method to a source-array scatter imaging method, which we call source migration, that does not rely on travel-times calculated for a plane wave. Using this technique in conjunction with source normalization, we found clear evidence for a D" P-wave reflector at four of the six GEOSCOPE stations considered in the study. The depth of the reflector for our imaged region varies between 190 km above the CMB beneath the Great Australian Bight and 220 to 270 km beneath the Indian Ocean west of Australia. Our determined depth in the northern portion of our study area is consistent with previous studies of D" depths using S-waves. We suggest that our D" reflections are the result of the previously imaged subducted lithosphere in the region and find that this lithosphere likely thins to the southeast. Additionally, our work more broadly indicates that the long-running history of single global seismic stations combined with source array techniques may be utilized to compliment and extend previous work imaging D" using conventional receiver-array techniques.

How to cite: Thomas, C. and Heyn, B. H.: Imaging deep subducted lithosphere beneath the Indian Ocean with seismic source array recordings, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8992, https://doi.org/10.5194/egusphere-egu24-8992, 2024.

Posters virtual: Fri, 19 Apr, 14:00–15:45 | vHall X2

Display time: Fri, 19 Apr 08:30–Fri, 19 Apr 18:00
Chairpersons: Franck Latallerie, Sarah Jane Fowler, James Ward
vX2.5
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EGU24-6554
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ECS
Steve Carr and Tolulope Olugboji

On a planet that dissipates heat through whole mantle convection, no sharp changes in elastic properties are expected in the mid-mantle: ~750-1300 km. Yet, a growing number of seismic studies continue to document evidence of discontinuities across these depths. Compared to the upper mantle, the global prevalence and causal origins of such features remain relatively enigmatic. Here, we investigate mid-mantle layering beneath two large seismic arrays (US and Alaska) using high-resolution Ps-converted waves. The challenge is that top-side reflections (reverberations) from the mantle transition zone interfere with and contaminate desired mid-mantle conversions and make their interpretation difficult. In the past, the slowness slant stack (vespagram) approach has been used. We extend the resolution of this stacking scheme using a newly developed sparsity-promoting, non-linear, CRISP-RF technique (Clean Receiver function Imaging with Sparse Radon Filters). Preliminary results suggest that CRISP-RF can isolate high-frequency (0.5Hz) mid-mantle body wave conversions buried within transition zone reverberations. With our filtered Ps-RFs and machine learning, we will present tighter constraints on mid-mantle layering (depth, sharpness, spatial variation)  exploring important implications for its origin.  

How to cite: Carr, S. and Olugboji, T.: Mid-mantle imaging through a reverberant transition zone: A CRISP-RF approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6554, https://doi.org/10.5194/egusphere-egu24-6554, 2024.

vX2.6
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EGU24-2497
Frederic Deschamps

The temperature at Earth’s core-mantle boundary (CMB) is a key parameter to understand the dynamics of our planet’s interior. However, it remains poorly known, with current estimate ranging from about 3000 K to 4500 K and more. Here, I introduce a new approach based on joint measurements of shear-wave velocity, VS, and quality factor, QS, in the lowermost mantle.  Lateral changes in both VS and QS above the CMB provide constraints on lateral temperature anomalies with respect to a reference temperature, Tref, defined as the average temperature in the layer immediately above the CMB. The request that, at a given location, temperature anomalies inferred independently from VS and QS should be equal gives a constraint on Tref. Correcting Tref for radial adiabatic and super-adiabatic increases in temperature gives an estimate of the CMB temperature, TCMB. This approach further relies on the presence of post-perovskite (pPv) phase in the deep mantle and on the fact that VS-anomalies are affected by the geographical distribution of phis phase. As a result, the inferred Tref is linked to the temperature TpPv at which the transition from bridgmanite to pPv occurs close to the CMB. A preliminary application to VS and QS measured beneath Central America and the Northern Pacific suggest that for TpPv = 3500 K, TCMB lies in the range 3470-3880 K with a 95 % likelihood. Additional measurements in various regions, together with a better knowledge of TpPv, are needed to determine a precise value of TCMB with this method.

How to cite: Deschamps, F.: Estimating the temperature at the core-mantle boundary from measurements of shear-wave velocity and seismic attenuation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2497, https://doi.org/10.5194/egusphere-egu24-2497, 2024.

vX2.7
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EGU24-322
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ECS
Jun Su, Christine Houser, and John Hernlund

Many large-scale structures in the mantle have been proposed to explain seismic observations and constrain geodynamic models. While the geophysical community cannot agree on the morphology and nature(s) of large low shear velocity provinces (LLSVPs) due to the difference in approaches, decorrelated P and S velocity anomaly (dVno longer proportional to dVS), inherently associated with changes in composition and/or phase, can help examine geodynamic models and imply the thermal/chemical evolution of the mantle. To further apply the inference to finer structures and to improve the precision for quantitative mineral physical implications, it is necessary to build a new seismic dataset for P and S waves measured in a self-consistent manner.

In this study, we trained a phase-picking model using code modified from EQTransformer (Mousavi et al., 2020). Our training dataset includes 65,298 traces, where teleseismic P and S arrivals are manually picked at the long-period (~20 sec) onset. Based on the machine-learning architecture proven useful for seismicity at local to regional distances, we managed to reproduce the manual picking results by machine and extend the picking catalog for seismic data to the present. We also conduct tomographic inversion for the global mantle to obtain a three-dimensional velocity model for both P and S waves. The new model has a higher resolution, allowing interpretations to understand geodynamics better.

How to cite: Su, J., Houser, C., and Hernlund, J.: Joint long-period P and S velocity inversion for Earth's mantle based on deep learning, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-322, https://doi.org/10.5194/egusphere-egu24-322, 2024.