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Absolute plate motion (APM) estimates are key to understand the driving forces of plates, particularly the role of the sublithospheric mantle flow, which has recently gained renewed recognition as a dominant driver. Tectonic plates that lack a subducting boundary (e.g., South America and Nubia) are prime examples of dynamics governed by mantle flow. Both the aforementioned plates, however, lack the hotspot space/age coverage required for high-resolution, well-constrained APM estimates. Here we resort to highly-resolved data sets of relative plate motions (RPM) across a number of spreading ridges in order to extract information on APM changes through geological time. Our analyses involve first mitigating the impact of noise in RPM data sets via Bayesian inference. This allows us to identify time periods that feature a relatively high probability of staging RPM changes. By extending these analyses to several neighboring plates, we can assess whether any of them is likely to feature an APM change through geological time. We apply such a method to RPM data sets in the Atlantic realm and identify three time-intervals for changes in the APMs of the Nubia and South America plates. Our analyses are complemented by a quantitative assessment of the forces required to generate such APM changes.

How to cite: Espinoza, V. and Iaffaldano, G.: Mid-Cenozoic absolute plate motion changes in the South Atlantic from relative plate motion analyses., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4206, https://doi.org/10.5194/egusphere-egu22-4206, 2022.

Mantle convection has profound effects on the Earth’s surface, such as inducing vertical motion, which is commonly termed dynamic topography. Sophisticated mantle convection models have been used to study current and past dynamic topography. But many input parameters, like complex rheologies and thermo-chemical flow properties remain poorly known, requiring ad-hoc model parameterization and long range extrapolation. This makes it attractive to explore simple analytic models of upper mantle flow. The existence of a weak asthenosphere allows one to explore upper mantle in the context of Poiseuille/Couette flow. The latter provides an geodynamically plausible link between flow properties and dynamic topography. Here we construct simple upper mantle flow models parameterized in terms of sources/sinks (plumes/slab) of Poiseuille/Couette flow. Our approach provides physical insight into the pattern of upper mantle flow, makes it easy to assess uncertainties of key model parameters, such as poorly resolved asthenospheric thickness and viscosity, and can be extended back in time, given first-order estimates of plume and subduction flux deduced from geological records. Importantly, it demands low computational cost relative to a time dependent geodynamic models. We present results for the Atlantic realm, and link our estimates of upper mantle flow history to Base Hiatus Surfaces (BHS) recently developed by Friedrich etal., (2018), Vibe etal., (2018), Carena etal., (2019),  Hayek metal., (2020) and Hayek metal., (2021). The latter serve as proxy for inferring past dynamic topography variations. We also relate our calculations to seismically inferred anisotropy, as a further proxy for upper mantle flow. Our results indicate that asthenospheric flow pattern can be explained through the concept of source to sink and that this flow type is testable against first order seismic and geologic observables.

References: 

Carena, S., Bunge, H. P., & Friedrich, A. M. (2019). Analysis of geological hiatus surfaces across Africa in the Cenozoic and implications for the timescales of convectively-maintained topography. Canadian Journal of Earth Sciences, 56(12), 1333-1346.

Friedrich, A. M., Bunge, H. P., Rieger, S. M., Colli, L., Ghelichkhan, S., & Nerlich, R. (2018). Stratigraphic framework for the plume mode of mantle convection and the analysis of interregional unconformities on geological maps. Gondwana Research, 53, 159-188.

Hayek, J. N., Vilacís, B., Bunge, H. P., Friedrich, A. M., Carena, S., & Vibe, Y. (2020). Continent-scale Hiatus Maps for the Atlantic Realm and Australia since the Upper Jurassic and links to mantle flow induced dynamic topography. Proceedings of the Royal Society A, 476(2242), 20200390.

Hayek, J. N., Vilacís, B., Bunge, H. P., Friedrich, A. M., Carena, S., & Vibe, Y. (2021). Correction: Continent-scale Hiatus Maps for the Atlantic Realm and Australia since the Upper Jurassic and links to mantle flow-induced dynamic topography. Proceedings of the Royal Society A, 477(2251), 20210437.

Vibe, Y., Friedrich, A. M., Bunge, H. P., & Clark, S. R. (2018). Correlations of oceanic spreading rates and hiatus surface area in the North Atlantic realm. Lithosphere, 10(5), 677-684.

How to cite: Wang, Z., Bunge, H.-P., Stotz, I., Vilacis Baurier, B., Hayek Valencia, J. N., and Friedrich, A.: Asthenospheric flow estimates in the Atlantic realm based on Poiseuille/Couetteflow models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1226, https://doi.org/10.5194/egusphere-egu22-1226, 2022.

The growth of the lithospheric mantle following tectonic thinning of the plate is well understood and is a primary consequence of the conductive cooling that is a key driver of mantle convection. However, it is less well understood how the lithosphere interacts with and responds to sub-plate temperature anomalies within the the underlying convecting mantle. Here, we investigate the evolution of the lithospheric mantle during and after intraplate magmatism. First, we examine the thickness of lithosphere beneath oceanic intraplate magmatic provinces that are < 10 Ma in age. Modelling of major oxide and rare earth element concentrations, alongside seismic tomography, indicate that these provinces lie upon lithosphere that is significantly thinner than expected given the age of underlying lithosphere. For example, Hawaii overlies lithosphere that is 50–80 km thick, despite the age of the plate suggesting a thickness of > 100 km. Next, we explore the lithospheric thickness beneath ancient intraplate magmatic provinces that have a record extending back to Jurassic times. Geochemical modelling demonstrates that, like recent magmatism, these provinces were also erupted atop thinner than expected lithosphere. Seismic tomography provides a further constraint on the thickness of the lithosphere by constraining the thermal structure of the upper mantle. By exploiting these tomographic images we show that the lithospheric mantle beneath ancient seamounts gets progressively thicker as a function of their eruption age. Importantly however, the lithosphere is consistently thinner by up to 40 km than would be expected if the plate cooled and thickened from a mid ocean ridge without perturbation. Finally, we extend our analysis to ancient continental large igneous provinces (LIPs). LIPs are massive accumulations (>100,000 km³) of magmatic material that are emplaced within a short period of geological time (1-2 Ma). We show that the lithospheric thickness beneath ancient continental LIPs increases as a function of time since eruption, following a similar relationship to oceanic LIPs. Our results suggest that the emplacement of LIPs causes ubiquitous thinning of the lithospheric mantle to thicknesses of 40–80 km, followed by systematic, progressive re-thickening via conductive cooling. Furthermore, they suggest that continental lithospheric mantle re-thickens to depths of > 200 km, supporting the idea that cratons can be destroyed by LIP emplacement and reformed following the end of eruption. Thinning and re-thickening of the lithospheric mantle during and after intraplate magmatism demonstrates that the lithosphere-asthenosphere boundary is routinely perturbed by sub-plate mantle convection. An understanding of LIP formation and its effect on the lithospheric mantle is necessary to reveal causal links with mass extinctions, continental break-up, and regional epeirogenic events.

How to cite: Stephenson, S. and Ball, P.: Destruction and Regrowth of Lithospheric Mantle by Emplacement of Large Igneous Provinces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-524, https://doi.org/10.5194/egusphere-egu22-524, 2022.

Geodynamic inverse models that aim at retrodicting past mantle evolution require accurate estimates of its thermodynamic present-day state. Tomographic models are in principle well suited to provide this information. However, a fundamental problem that impacts the quality of the retrodiction arises from their inherently limited resolving power and the fact that the magnitudes of seismic heterogeneity are difficult to constrain owing to the necessity to regularize the inversions (e.g. by norm damping). To get a better understanding of the magnitudes of heterogeneity in the mantle, one option is to predict seismic velocity variations from the temperature field of forward mantle circulation models (MCMs) in combination with thermodynamic models of mantle mineralogy.  Temperature is not a free parameter in these models, but rather constrained by the underlying conservation equations and relevant input parameters. If the geodynamic models are run at earth-like Rayleigh number, temperature variations are expected to feature realistic magnitudes, which, together with the mineralogical mapping, should lead to realistic magnitudes of seismic heterogeneity. This has been investigated in previous studies by computing secondary predictions for the MCMs, such as seismic body wave traveltimes and geoid undulations. A complicating factor, however, is the trade-off between thermal and compositional variations that both may affect the seismic velocities. A further complexity arises from the fact that the elastic velocities of the mineralogical model need to be corrected for the effects of anelasticity, the parameters of which are poorly known.  Thus, a range of seismic velocity values may still be possible for a given temperature. 

Here, we explore the possibility to use Earth's normal mode spectrum to narrow the range of plausible magnitudes of seismic heterogeneity in the mantle. To this end, we compute free-oscillation spectra with full coupling of modes below 3.5 mHz in our geodynamic models.  In our analysis, we consider different measures to investigate whether the normal mode data may provide complementary information to earlier assessments of MCMs based on body waves. In addition to the direct misfit between spectra of real and synthetic data, the variance of a large number of stacked multiplets can be used to constrain the even degree covariance of lateral heterogeneity under certain assumptions. Using different realizations of seismic MCM structure that differ in terms of the anelastic temperature to velocity mapping, we will analyse the potential of normal mode data to put tighter constraints on the magnitudes of heterogeneity.

How to cite: Schuberth, B., Strutz, D., and Schneider, A.: Earth's free-oscillation spectrum as a tool to assess mantle circulation models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12414, https://doi.org/10.5194/egusphere-egu22-12414, 2022.

Seismic tomographic models based only on wave velocities have limited ability to differentiate between a compositional or temperature origin for the Earth's 3D structure variations. Complementing wave velocities with attenuation (conversion of energy to heat) can help make that distinction, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the lower mantle large low shear velocity provinces (LLSVPs) will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming stable 'anchors' and influencing the pattern of mantle convection. So far, only global 3D attenuation models built using seismic body waves and surface waves have been available for the upper mantle. Here, we use whole Earth oscillations or normal modes to measure 3D variations in mantle attenuation, which allow us to include focussing and scattering without the need for approximations. We achieve this by jointly measuring 3D variations in velocity and attenuation using splitting functions, which are depth-averaged models of how a mode 'sees' the Earth. 

Splitting functions are linearly dependent on heterogeneous structure and can be easily incorporated in tomographic models. We measured 14 anelastic splitting functions and used those to build a 3D global model of attenuation for the whole mantle. For comparison purposes, we have also constructed a 3D shear-velocity model using the same number of modes and model parametrization. In the upper mantle, we find high attenuation in the low velocity spreading ridges, which suggests a thermal origin and agrees with previous surface wave studies. In the lower mantle, we find the highest attenuation in the 'ring around the Pacific' high velocity region, which is thought to be the 'graveyard' of subducted slabs, and not in the LLSVPs beneath Africa and the Pacific. We compare our 3D attenuation model to the wave-speeds and attenuation predictions of a laboratory-based viscoelastic model. Our comparison indicates that the higher attenuation seen in the slab regions can be explained by a small grain-size in combination with cold temperatures, while the lower attenuation in the LLSVPs can be explained by a large grain-size in combination with high temperatures. Grain-size is related to viscosity in diffusion creep, which would mean that the LLSVPs have larger viscosity making them long-lived stable features, while the graveyard of slabs would have a lower viscosity making them shorter lived. 

How to cite: Talavera-Soza, S., Cobden, L., Faul, U. H., and Deuss, A.: Global 3D model of mantle attenuation using normal modes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-815, https://doi.org/10.5194/egusphere-egu22-815, 2022.

How to cite: Jagt, L., Talavera-Soza, S., and Deuss, A.: Normal Mode Constraints on Anelastic Structures in the Earth's Lower Mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8883, https://doi.org/10.5194/egusphere-egu22-8883, 2022.

Mantle plumes are commonly envisioned as thin, buoyant conduits rising vertically from the core-mantle boundary (CMB) to the earth's surface, where they produce volcanic hot spots. Most hotspots are located in the sparsely instrumented oceans, creating poor prospects for the seismic resolution of thin conduits in the deep mantle. 

The RHUM-RUM experiment remedied this issue around the hotspot island of La Réunion by instrumenting 2000x2000 km2 of seafloor for 13 months with 57 broadband ocean-bottom seismometers (OBS). We present a 3-D P-wave tomography model computed from the RHUM-RUM waveform data, supplemented by a global data set of P-diffracted measurements and a selection of ISC picks. Multifrequency travel times were measured on the waveforms and inverted in a finite-frequency framework. We achieve high image resolution beneath the Indian Ocean hemisphere, and especially beneath La Réunion, from upper mantle to CMB.

We observe the Large Low-Velocity Province (LLVP) rising 800 km above the CMB, forming a cusp beneath South Africa. A low-velocity branch undulates obliquely from this cusp region towards the uppermost mantle beneath La Réunion. Hence La Réunion's connection to the lower mantle is more complex than previously envisioned, being neither a thin vertical conduit nor projecting down to an edge of the LLVP. The deep-mantle connections of the Afar and Kerguelen hotspots emerge from the same LLVP cusp beneath South Africa and extend towards the surface through tilted low-velocity branches. 

Our results provide the first high-resolution image of a western Indian Ocean plume cluster from the surface to the CMB. This represents a key advance for linking geophysical, geodynamic and geochemical observations.

How to cite: Tsekhmistrenko, M., Sigloch, K., Hosseini, K., and Barruol, G.: A tree of Indo-African mantle plumes imaged by seismic tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6182, https://doi.org/10.5194/egusphere-egu22-6182, 2022.

Tomographic-geodynamic model comparisons are a key component in studies of the present-day thermodynamic state of the mantle. A fundamental prerequisite for quantitatively meaningful comparisons is “tomographic filtering” of the geodynamic model. This means that geodynamically predicted mantle structures have to be modified to account for the spatially variable resolving power of tomographic images, i.e. to mimic the effects of uneven data coverage and regularization. Different approaches for tomographic filtering are available, but it is so far unclear which one will be the method of choice in the context of computationally demanding retrodictions of past mantle flow.

Here, we investigate the impact of the possible filtering approaches in a fully synthetic framework. For the first time in a mantle circulation model (MCM), we simulate 3D-wavefields and seismograms for an entire tomographic earthquake catalogue with over 4,200 events using SPECFEM3D_GLOBE. We use both classic filtering with the resolution operator R, as well as the recently introduced “generalized inverse projection” (GIP; Freissler et al. 2020) to generate tomographically filtered versions of the MCM.

In the GIP method, the generalized inverse operator of a given tomographic image is applied to synthetic seismic data predicted from the geodynamic model, as well as to potential data errors, to obtain the filtered MCM plus the propagated error. Important to note, the same generalized inverse operator is applied to an observed data set to build the tomographic model. A physically accurate prediction of synthetic data, here realized with the seismograms from numerical wave propagation, thus enables GIP filtering to consistently reproduce the tomographic imaging process. This is an important methodological advantage over classic filtering with R, where an unphysically reparametrized version of the MCM is filtered directly in model space and seismic data errors can not be considered.

In our study, GIP-filtered models are computed with cross-correlation S-wave traveltime residuals from the synthetic seismograms, as well as with banana-doughnut kernel and ray-theoretical traveltime predictions. The differently filtered models are compared against each other using statistical measures. By taking the GIP-filtered model that is based on the 3D-wavefield simulations as a reference, we can quantify the impact of reparametrization in classic filtering versus the lack of exact wave physics when using less accurate methods for traveltime predictions in the GIP filtering. Additionally, all filtered models can be compared to the underlying original structure of the MCM.

Detailed knowledge of tomographic filtering effects with different strategies is required prior to efforts on the associated uncertainty quantification in data-driven geodynamic retrodictions of mantle evolution.

How to cite: Freissler, R., Schuberth, B. S. A., and Zaroli, C.: The relevance of full 3D-wavefield simulations for the tomographic filtering of geodynamic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11686, https://doi.org/10.5194/egusphere-egu22-11686, 2022.

The post-perovskite (pPv) phase is often invoked as an explanation for seismic observations of discontinuities, anisotropy and anti-correlation between velocities in the lower mantle. Accurate interpretations of these features in terms of pPv are important, as the phase transition provides a much-needed temperature probe in the lowermost mantle. Robust observations of this phase transition have the potential to constrain the temperature of and heat flow across the core-mantle boundary and thus provide estimates of the heat budget and thermal evolution of the Earth.

Traditionally, the presence of post-perovskite (pPv) has been inferred from observations of seismic discontinuities in the lowermost mantle. However, these only give a very patchy image of lateral variations in the presence of pPv due to the heterogeneous coverage of seismic data. In addition, interpretations are complicated by the fact that the properties and stability field of pPv remain uncertain from a mineral physics point of view.

Here, we describe different proxies for the presence of post-perovskite, proposed based on global seismic tomography. To investigate their accuracy, we utilize synthetic tomography models derived from geodynamic modelling in combination with mineral physics and we compare the predicted presence to the true occurrence of pPv in the model. By using both high-resolution geodynamic models as well as filtered models that have been corrected for the limited resolution of seismic tomography, we can investigate whether a proxy works in theory (on the high-resolution versions) and also in practice (on the filtered models). We will discuss how we may be able to constrain the stability field of pPv based on comparisons with published tomographic models and make recommendations as to what has to improve in seismic tomography to make different proxies work.

How to cite: Koelemeijer, P., Pagu, A., Schuberth, B., and Davies, R.: Proxies for the presence of post-perovskite in the lowermost mantle based on seismic tomography and geodynamic modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9966, https://doi.org/10.5194/egusphere-egu22-9966, 2022.

The mantle transition zone (TZ) is expected to influence convective flow, but neither its structural characteristics nor dynamic effects have been conclusively constrained. Lateral temperature variations modulate the topography of associated seismic discontinuities at approximately 410 and 660 km depth (‘410’ and ‘660’). These discontinuities are related to mineral phase transitions and thus also sensitive to composition. Consequently, discontinuity topography can potentially provide insight on temperature and even phase composition at depth. It has been recognized that, in addition to phase transitions in olivine polymorphs, the transition of garnet to lower mantle minerals may impact particularly the ‘660’ at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified.

We address this question by predicting synthetic seismic structure and discontinuity topography of the TZ based on the temperature field of a 3-D mantle circulation model (MCM) for a range of relevant bulk compositions and associated mineralogy models. The models differ in complexity in terms of the number of incorporated oxide-components and include pyrolite, depleted mantle and mechanical mixing (MM) models. We thus create a suite of relevant hypothetical realizations of TZ seismic structure and major discontinuities.

Our theoretical approach allows us to systematically investigate the effects of varying mineralogy, in combination with a dynamically constrained temperature field, on TZ structure. We explicitly relate major phase transitions as given by the mineralogical tables to specific topographic features of the ‘410’ and ‘660’ and quantify the relative impact of the different phases. Analyzing a number of statistical measures for our synthetic discontinuity topographies provides theoretical predictions on possible distribution and magnitude of real-world depth variations. Our study thus provides a framework for dynamically informed interpretations of seismically derived TZ structure in terms of mantle temperature and composition. It moreover gives insights on the potential dynamic behavior of the TZ by constraining the importance of garnet in our theoretical models.

We find that garnet only occurs in regions with excess temperatures above 150 - 300 K, depending on phase composition. This leads to ~ 3 % garnet at the ‘660’ in a pyrolite mantle and ~ 1 % in MM. Absolute base temperatures could however be higher (or lower) than predicted by the MCM’s geotherm. For different plausible background temperature fields the garnet proportion at the ‘660’ could vary between ~ 1 and 39 % in pyrolite, while remaining largely unaffected in MM. Since not all warmer than average but only the hottest mantle regions see the garnet transition, dynamic effects of the ‘660’ might be even more complex than previously assumed.

How to cite: Papanagnou, I., Schuberth, B. S. A., and Thomas, C.: Geodynamic predictions of seismic structure and discontinuity topography of the mantle transition zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11433, https://doi.org/10.5194/egusphere-egu22-11433, 2022.

A quantitative understanding of Earth's deep compositional structure remains elusive. Geophysical and geochemical observations illuminate heterogeneous features on various scales in the lower mantle: however, the origin and interaction of such heterogeneities are not yet fully explained in the context of global mantle dynamics. Conversely, numerical geodynamic models predict a wide range of viable scenarios of mantle convection and heterogeneity preservation. In the "marble cake" end-member mantle model, slabs of Recycled Oceanic Crust (ROC) are subducted and deformed but never fully homogenized in the convecting mantle. In the "plum pudding" model, MgSiO₃-rich primordial material may resist convective entrainment due to its intrinsic strength. Only few geodynamic studies have explored the effects of subducted ROC properties on mantle dynamics while also accounting for the influence of primordial heterogeneity. Furthermore, predictions from numerical models need to be tested against geophysical data. However, current imaging techniques poorly resolve the lower mantle and may be unable to distinguish between both end-member models above.

Here, we use the finite-volume code StagYY to model mantle convection in a 2D spherical-annulus geometry. We investigate the style of heterogeneity preservation as a function of two parameters: the intrinsic density and the intrinsic strength (viscosity) of basalt at lower-mantle conditions.  Additionally, we employ the thermodynamic code Perple_X and the spectral-element code AxiSEM to compute, respectively, seismic velocities and synthetic seismograms from the predictions of our models.

We obtain two main regimes of mantle convection: low-density basalt leads to a well-mixed, "marble cake"-like mantle, while dense basalt aids the preservation of primordial blobs at mid-mantle depths as in a "plum pudding". Intrinsically viscous basalt also promotes the preservation of primordial material. These trends are well explained by smaller convective vigour of the mantle as intrinsically dense (and viscous) piles of basalt shield the core. In order to test these model predictions, we convert model temperatures and compositions to thermoelastic properties for two characteristic models of each regime. These are then used to compute synthetic seismic velocity models, through which we simulate wave propagation using AxiSEM. Finally, we  discriminate between these two end-members by comparing statistical properties of the corresponding ensembles of synthetic seismograms. Our results highlight how the interaction of mantle materials drives the long-term thermochemical evolution of terrestrial planets. Furthermore, they provide a framework for relating the style of heterogeneity preservation in the Earth's lower mantle with specific features of the seismic waveforms.

How to cite: Desiderio, M., Gülcher, A. J. P., and Ballmer, M. D.: The interplay between recycled and primordial heterogeneities: constraints on Earth mantle dynamics via numerical modeling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5307, https://doi.org/10.5194/egusphere-egu22-5307, 2022.