GD2.5

EDI
Tracing plate and plume modes of mantle convection: approaches to consolidate observations and theory across scales

Mantle convection is a fundamental process responsible for shaping the tectonic evolution of the Earth. Although direct observations of this process are critical, they tend to be limited in space and time; however, significant information can be obtained through a variety of multiscale methods that allow one to estimate fundamental parameters of the Earth's mantle structure (e.g., viscosity, density and temperature). Theoretically, mantle convection can be separated into the plate and plume mode. The former is associated with the cold upper thermal boundary layer (lithosphere), while the plume mode is tied to the hot lower thermal boundary layer. Convective buoyancy associated with these modes are significant, capable of driving plate motions. However, they need to be contrasted with plate boundary forces, where oblique rifting and the influence of magmatic processes highlight some of the difficulties in understanding these forces.

Seismic imaging and gravity data, for instance, provide a snapshot of processes occurring in the present-day mantle. Geochemical analysis of volcanic rocks can be used to estimate temperature and depths of melt generation through time. Numerous observations of continental breakup since the Cretaceous are well documented in the geological archives. They include mapping continental dynamic topography, studying plate kinematic changes, using thermochronological models and petrological observables, and imaging deep structure by seismic tomography to constrain the breakup history. Altogether, these classes of observations, which are commonly studied in isolation, provide powerful constraints for geodynamic forward and inverse models of past mantle convection. Hence, yielding a holistic view of the Earth's mantle and its temporal and structural evolution.

Convener: Ingo StotzECSECS | Co-conveners: Berta VilacísECSECS, Marthe KlöckingECSECS, Jorge Nicolas Hayek ValenciaECSECS, Andrew Schaeffer, Sascha Brune, D. Sarah Stamps
Presentations
| Tue, 24 May, 15:55–18:30 (CEST)
 
Room -2.91

Presentations: Tue, 24 May | Room -2.91

Chairpersons: Ingo Stotz, Marthe Klöcking, Jorge Nicolas Hayek Valencia
15:55–15:58
15:58–16:08
|
EGU22-4069
|
solicited
|
Highlight
|
On-site presentation
Gareth G. Roberts and Victoria Fernandes

This presentation examines how the growing inventory of geologic, geophysical and geomorphic observations constrains amplitudes, wavelengths and histories of mantle convection. Ocean age-depth residuals have become a cornerstone in understanding loci, amplitudes and wavelengths of modern sub-plate support of Earth’s oceanic lithosphere. Improvements in mapping lithospheric structure, especially from seismology, means that quantifying modern sub-plate support of continents is increasingly tractable. The continents offer a great opportunity to constrain temporal and spatial evolution of sup-plate support because of the plethora of available geologic and geomorphic observations. A challenge is to disentangle, often dominant, lithospheric processes that generate uplift or subsidence (e.g. shortening, extension) to extract information about histories of sub-plate processes from geologic and geomorphic observations. This presentation focusses on how data and theory can be combined to quantify [1] modern and recent sub-plate support of the continents, [2] evolution of sub-plate support through time, and [3] test geodynamic models that predict dynamic topography.

 

Highlights from recent work include the use of collocated long wavelength gravity and shear wave velocity anomalies, mafic magmatism and drainage patterns to identify tracts of uplifted continental topography that are maintained by sub-plate support. These observations indicate that chemical and sedimentary fluxes through drainage networks are likely governed by sub-plate support in many places. Observational and theoretical constraints on uplift and subsidence histories of continents and their margins are presented. These include global paleobiological observations of uplifted marine rock, backstripped stratigraphy (e.g. New Jersey margin, Mauritanian basin), inversion of drainage patterns (e.g. North America, Africa) and landscape evolution modelling. This work indicates that re-assessment of the longevity and evolution of Earth’s surface topography, basin formation and histories of glacio-eustasy is required. Reasons for why large-scale vertical lithospheric motions, often associated with dynamic support, are likely to be recorded in modern and ancient continental landscapes are explored using spectral analysis of drainage patterns and physics-based modelling of erosional thresholds. Examples of how seismology, anelastic parameterisations, thermobarometry, geodesy, stratigraphy and geomorphic observations can be reconciled to constrain Cenozoic to Recent histories of upper mantle support are presented. Finally, examples of using geologic observations to test predictions from geodynamic models are given. Opportunities and challenges associated with assessing contributions of sub-plate support to evolution of Earth’s surface, particularly those associated with crust and lithospheric mantle densities, are discussed. Available observations show that Cenozoic mantle convection has been a significant, in places dominant, driver of continental evolution. It has generated and maintains epicontinental seaways, sedimentary basins, continental plateaux, and has determined routing of water and sediment across continents and biodiversity. It is an important driver of Earth surface evolution and is extremely likely to have left its trace throughout the geologic and geomorphic record.

How to cite: Roberts, G. G. and Fernandes, V.: Global geologic and geomorphic observations of mantle convection, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4069, https://doi.org/10.5194/egusphere-egu22-4069, 2022.

16:08–16:14
|
EGU22-345
|
ECS
|
Highlight
Megan Holdt, Nicky White, and Simon Stephenson

Earth’s topography is supported by both crustal and sub-crustal density variations. Dynamic topography results from the vertical displacement of the Earth’s surface due to processes operating within the mantle. Thus, isolating and quantifying observable dynamic topography can yield valuable information about mantle dynamics. An observationally-based approach can be used to investigate dynamic topography by calculating residual depth anomalies in the oceanic realm and residual topographic anomalies on the continents. To constrain the residual topographic contribution that arises from sub-crustal processes it is necessary to correct for crustal and sedimentary loading. We identify and correct for both forms of loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide-angle/refraction surveys and receiver functions. We present a revised global compilation of oceanic residual depth measurements (n = 10,846) and continental residual topographic measurements (n = 3,897). This compilation represents a significant improvement in terms of the quantity and spatial distribution of measurements. In the oceanic realm, the correction methodology has been revised in two ways, which has improved resolution and accuracy. First, the crustal correction now accounts for variations in bulk density as a function of crustal thickness. Secondly, the quartz and clay content of the sedimentary column has been adjusted, which improves the quality of the sedimentary correction. The revised global compilation is used to generate a spherical harmonic representation of observable dynamic topography out to degree 40 (i.e. ~ 1000 km). The resultant power spectrum demonstrates that dynamic topography varies linearly with inverse wavenumber. Our global results are consistent with independent geologic markers of uplift and subsidence.

How to cite: Holdt, M., White, N., and Stephenson, S.: Constraining global dynamic topography using a revised observational database, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-345, https://doi.org/10.5194/egusphere-egu22-345, 2022.

16:14–16:20
|
EGU22-9794
|
ECS
|
Highlight
|
On-site presentation
Berta Vilacís, Jorge N. Hayek, Hans-Peter Bunge, Anke M. Friedrich, and Sara Carena

Mantle convection is a fundamental driving force of plate tectonics. It is commonly perceived that mantle convection is difficult to constrain directly. However, the convection process affects the Earth surface imprints the geological record. In particular, the positive surface deflections driven by mantle convection create erosional/non-depositional environments, which induce gaps in the stratigraphic record (i.e., an absence or thinning of a sedimentary layer). Modern digital geological maps allow us to map the largest of such un/-conformable surfaces at continental scale systematically.
We report our continent-scale hiatus mapping in geological series across America, Europe, Africa, and Australia, from the Upper Jurassic onward. We find significant differences in the spatial extent of hiatus patterns across and between continents, which is on the order of 2000 – 3000 km in diameter. These surfaces change at geological series, ten to a few tens of millions of years (Myrs). This duration is significantly shorter than the timescale of mantle convection of about 100 – 200 Myrs, implying that different timescales for convection and topography in convective support must be an integral component of time-dependent geodynamic Earth models. Our results call for intensified collaboration between geodynamicists and geologists to test geodynamic Earth models by assembling relevant geological observations at continental scales.

How to cite: Vilacís, B., Hayek, J. N., Bunge, H.-P., Friedrich, A. M., and Carena, S.: Tracing upper mantle flow patterns through continent-scale hiatus surfaces in the Indo-Atlantic Realms since the Upper Jurassic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9794, https://doi.org/10.5194/egusphere-egu22-9794, 2022.

16:20–16:26
|
EGU22-4206
|
ECS
|
On-site presentation
Valentina Espinoza and Giampiero Iaffaldano

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.

16:26–16:32
|
EGU22-1226
|
ECS
|
Virtual presentation
Zhirui Wang, Hans-Peter Bunge, Ingo Stotz, Berta Vilacis Baurier, Jorge Nicolas Hayek Valencia, and Anke Friedrich

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.

16:32–16:38
|
EGU22-524
Simon Stephenson and Patrick Ball

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.

16:38–16:40
Coffee break
Chairpersons: Ingo Stotz, Marthe Klöcking, Berta Vilacís
17:00–17:06
|
EGU22-11623
|
ECS
|
On-site presentation
Gianmaria Tortelli, Anna Gioncada, Carolina Pagli, Eleonora Braschi, Ermias Gebru, and Derek Keir

In this work we investigate the genesis of widespread continental basaltic volcanism and the transition to localised magmatic segments at the Afar Rift-Rift-Rift triple junction. Basing on major and trace elements we investigated the thick (up to 1500m) and widespread (~55.000 km2) Lower (4.5-2.6 Ma) and Upper Stratoids (2.6-1.1 Ma) Series and the subsequent, less voluminous and focalised, Gulf Series (1.1-0.6 Ma). Our results, together with published geophysical and stratigraphical evidence, allow us to interpret the evolution of the Red Sea rift and the associated break-up process in Southern and Central Afar. The three Series are characterised by E-MORB magmatism and residual amphibole (K, Rb trough and Ba, Nb-Ta peak), with subordinately pyroxenite (Rb peak, Ba trough and MREE fractionation), in their mantle source, suggesting partial melting of the diffusely metasomatized sub-continental mantle. Marked differences in garnet-compatible trace elements reveal a deeper melting column for the Upper Stratoids (TbN/YbN > 1.7) with respect to the Lower Stratoids and the Gulf Series (TbN/YbN < 1.7), indicating distinct mantle sources for the three Series. Lower values of the incompatible element ratios Th/Nb, Th/Zr and LaN/SmN of the Gulf Series with respect to the Upper Stratoids indicate a higher degree of partial melting for the Gulf Series mantle source. The spatial variation in the volume and sources of Afar magmatism between 4.5-0.6 Ma correlates well with spatial changes in the locus of strain with two distinct episodes of rifting: (1) The late Miocene rifting episode (7-2.6 Ma), associated with thinned lithosphere and the Hadar Basin formation (3.8-2.9 Ma), erupted the Lower Stratoids in South Afar; (2) The Pleistocene rift (2.6-0.01 Ma), relocated in Central Afar, erupted the Upper Stratoids first (~2.6-1.1 Ma) and, subsequently, along with the stretching of the lithosphere and focalization of the rift, the Gulf Series (~1.1 Ma). Accordingly, our data supports the interpretation that the Afar strain localisation and associated magmatism migrated north-eastward from South to Central Afar through time, potentially in response to triple junction tectonics.

How to cite: Tortelli, G., Gioncada, A., Pagli, C., Braschi, E., Gebru, E., and Keir, D.: Stratoids flood basalt volcanism at the Afar rift: new insights from trace elements geochemistry , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11623, https://doi.org/10.5194/egusphere-egu22-11623, 2022.

17:06–17:12
|
EGU22-3343
Ameha Muluneh, Sascha Brune, Tesfaye Kidane, Carolina Pagli, Derek Keir, and Giacomo Corti

The Afar rift, at the northern part of the East African Rift System (EARS), is a classic natural laboratory to study the formation of sea-floor spreading centers. Several geo-physical monitoring studies have been conducted mainly following the 2005 Dabbahu-Manda Harraro (DMH) and the 1978 Asal segments volcano-seismic crises. The two segments are located at the tips of the Red Sea and the Gulf of Aden rifts, respectively, hence how the two segments propagate towards each other is crucial to our understanding on deformation during rift linkage. To this end, we use GPS data from central Afar to model the strain and rotation rates in the region. Our results show that both the DMH and Asal segments are characterized by high shear strain and rotation rates, in agreement with independent geophysical and geological observations. No significant strain concentration occurs between the two rift propagators. By combining our results with previous geophysical observations, we suggest that linkage between the DMH and Asal segments occurs via ∼E-W oriented strike-slip fault at the tip of DMH and a broad region of NW-SE oriented normal fault bounded en echelon grabens, which are almost parallel to the Asal segment. Our preliminary results show that the style of deformation in the central Afar region is more complex and distributed than at ocean ridges where rift segments connect with localized transform faults. However, our results may inform on how transform faults initiate. 

How to cite: Muluneh, A., Brune, S., Kidane, T., Pagli, C., Keir, D., and Corti, G.: Complex strain accommodation mechanisms during rift linkage: an example from the central Afar, East Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3343, https://doi.org/10.5194/egusphere-egu22-3343, 2022.

17:12–17:18
|
EGU22-963
|
Virtual presentation
D. Sarah Stamps, Emmanuel Njinju, Asenath Kwagalakwe, John Naliboff, and Tahiry Rajaonarison

Continental rifting processes are influenced by viscous coupling of the deforming lithosphere to asthenospheric flow, as well as magma that migrates upward from the upper asthenosphere. Over the past few decades, significant advances have been made in finite element numerical methods that enable modeling of lithospheric deformation, viscous coupling to asthenospheric flow, and melt generation in the upper asthenosphere. In this work, we present new developments based in the NSF Computational Infrastructure for Geodynamics finite element code ASPECT (Advanced Solver for Problems in Earth’s Convection) that allow users to investigate lithospheric deformation, asthenospheric flow, and melt generation in the upper asthenosphere. Users have the options to constrain their initial temperature and density conditions with laterally varying lithospheric thickness, layers of crustal thickness, and shear wave seismic velocity models in the sublithospheric mantle. We present case studies from regions along the East African Rift System that demonstrate these capabilities. 

How to cite: Stamps, D. S., Njinju, E., Kwagalakwe, A., Naliboff, J., and Rajaonarison, T.: Continental Rifting Advances Using 3D Computational Modeling of Lithospheric Deformation, Asthenospheric Flow, and Deep Melt Generation with ASPECT , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-963, https://doi.org/10.5194/egusphere-egu22-963, 2022.

17:18–17:24
|
EGU22-12414
|
Virtual presentation
Bernhard Schuberth, Dominik Strutz, and Anna Schneider

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.

17:24–17:30
|
EGU22-815
|
ECS
|
On-site presentation
Sujania Talavera-Soza, Laura Cobden, Ulrich H. Faul, and Arwen Deuss

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.

17:30–17:36
|
EGU22-8883
|
ECS
|
On-site presentation
Lisanne Jagt, Sujania Talavera-Soza, and Arwen Deuss
Free oscillations, or normal modes, of the Earth provide important constraints on large-scale structures in the mantle, both elastic and anelastic. In addition to shear-wave (Vs), compressional-wave velocity (Vp) and density (ρ), normal modes are sensitive to perturbations in attenuation, or loss of energy. Attenuation is key in imaging partial melt, water, grain size differences and temperature. Surface waves have imaged the upper mantle attenuation, but lower mantle attenuation is still unknown. Normal mode observations of attenuation in the lower mantle provide constraints on the origin and nature of the two lower mantle Large Low Shear wave Velocity Provinces (LLSVPs). Scattering and focussing, which are hard to separate from intrinsic attenuation in the case of body waves, are included by cross-coupling between normal modes. 
 
Here, we will use normal mode spectra to make tomographic models of 3D variations in shear-wave velocity and shear attenuation qμ = 1/Qμ. Normal mode spectra can be inverted in two ways, using either 1) a direct spectrum one-step inversion or 2) a two-step inversion with splitting function measurements as intermediate step. We will image attenuation using the first method. In synthetic tests, we are able to recover Vs and Qμ structure very well. In our real data inversions, we find anti-correlation in the upper mante, i.e. strong attenuation in low-velocity zones, agreeing with the idea of ridges being hotter and containing melt. In the lower mantle, we find weak attenuation in the center of LLSVPs, and stronger attenuation in the ring around them, which is in agreement with results for the two-step splitting function inversion. 

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.

17:36–17:46
|
EGU22-6182
|
ECS
|
solicited
|
Highlight
Maria Tsekhmistrenko, Karin Sigloch, Kasra Hosseini, and Guilhem Barruol

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.

17:46–17:52
|
EGU22-2093
Frederic Deschamps and Anselme Borgeaud

The temperature at the Earth’s core-mantle boundary (CMB), TCMB, is a key property for a better understanding of our planet dynamics and evolution, but remains poorly constrained. Because the CMB is a material boundary between silicate rocks and molten iron, TCMB cannot be deduced directly from a single phase diagram. Estimates from mineral physics extrapolate either the solidus of mantle rocks to large pressure, or the ICB temperature to lower pressure, assuming that the outer core is adiabatic. Here, we propose a new approach to the determination of TCMB based on the analysis of observed lateral variations in shear-wave seismic velocity, VS, and attenuation, measured with the quality factor QS, in the lowermost mantle. Shear velocity is sensitive to the presence of post-perovskite (pPv), a high pressure phase of bridgmanite that is stable at lowermost mantle conditions. Because the stability field of pPv strongly depends on temperature, the presence of this phase and its impact on VS provides a constraint on the horizontally averaged temperature in the lowermost mantle. On another hand, seismic attenuation is a thermally activated process, implying that its amplitude depends on temperature. At a given depth, and using an appropriate modeling of QS, its deviation from a global average can then give access to the average temperature at this depth. Our approach is based on the knowledge of several parameters, of which the temperature of the phase transition to pPv, TpPv, appears to be the most sensitive. We then performed a preliminary application using models of VS and QS obtained for beneath the Central America and the Northern Pacific, and found that for TpPv = 3150 K, the CMB temperature should be in the range 3500-3800 K.

How to cite: Deschamps, F. and Borgeaud, A.: Estimating core-mantle boundary temperature from lateral variations in shear velocity and seismic attenuation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2093, https://doi.org/10.5194/egusphere-egu22-2093, 2022.

17:52–17:58
|
EGU22-11686
|
ECS
|
Virtual presentation
Roman Freissler, Bernhard S.A. Schuberth, and Christophe Zaroli

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.

17:58–18:04
|
EGU22-8964
|
ECS
|
Virtual presentation
Ayodeji Taiwo, Hans-Peter Bunge, and Bernhard Schuberth

A crucial goal in geodynamics is the development of time-dependent earth models so that poorly known mantle convection parameters can be tested against observables gleaned from the geologic record. To this end one must construct model trajectories to link estimates of the current heterogeneity state to future or past flow structures via forward or inverse mantle convection models. Unfortunately, the current heterogeneity state which is derived from seismic imaging methods is subject to substantial uncertainty due to the finite resolution of seismic tomography. These uncertainties are likely to considerably affect the computed flow trajectory, in what is known as the butterfly effect. Here we study mantle convection models to assess the effects of varying initial conditions on the evolution of mantle flow. We perform twin experiments (Lorenz 1965), that is, we compute convection calculations with identical flow parameters but different initial temperature fields. A base temperature field is generated by allowing a mantle convection calculation to evolve until a statistical steady state is reached. This temperature field is then used to initialize our reference case. We proceed to modify this reference temperature field in a number of different forms to reflect tomographic choices of damping and smoothing. In all cases we track the divergence of the perturbed models from the reference model. Furthermore, we test the efficiency of surface velocity assimilation, following from the work of Colli et al (2015), in locking two convecting systems and driving their divergence to a minimum.

 

We also introduce a framework for the comparison of model output with geological observables. To this end, we perform a comparison between the dynamic topography maps of our reference and perturbed models. We calculate simple traditional metrics such as RMSE, correlation, difference fields and Taylor diagrams. Such traditional grid-point based error measures, however, suffer from the “double-penalty” problem and as such we introduce scale-decomposition methods that allow a computation of correlation, RMSE and ratios of variances for every spatial scale (see Surcel et al (2015), Casati et al (2005) for examples). Furthermore, we introduce object-based verification measures that identify and match uplift and subsidence objects in the dynamic topography maps for both reference and perturbed models similar to what a human observer would identify. Borrowing from the wealth of work in meteorology, we calculate SAL scores (Wernli et al 2008) and a Critical Success Index (Schaefer 1990). Finally, for successfully matched objects, a Procrustes shape analyis (Michaes et al 2007) is performed to compare the similarities in area, shape, orientation and intensity, after which a final score is calculated based on these properties. We believe that the measures introduced here represent the next step in geodynamics as mantle convection models become increasingly complex and more focus is placed on matching model observations with the geological record.

How to cite: Taiwo, A., Bunge, H.-P., and Schuberth, B.: Mantle Flow Trajectories in the Presence of Poorly Constrained Initial Conditions: Analysis of an Ensemble of Models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8964, https://doi.org/10.5194/egusphere-egu22-8964, 2022.

18:04–18:10
|
EGU22-9966
Paula Koelemeijer, Ana Pagu, Bernhard Schuberth, and Rhodri Davies

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.

18:10–18:16
|
EGU22-11433
|
ECS
|
Virtual presentation
Isabel Papanagnou, Bernhard S.A. Schuberth, and Christine Thomas

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.

18:16–18:22
|
EGU22-5307
|
ECS
|
On-site presentation
Matteo Desiderio, Anna J. P. Gülcher, and Maxim D. Ballmer

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.

18:22–18:28
|
EGU22-7608
|
ECS
|
On-site presentation
Anna Gülcher, Gregor Golabek, Marcel Thielmann, Maxim Ballmer, and Paul Tackley

The rheological properties of­­­ Earth’s lower mantle materials are key for mantle dynamics and  planetary evolution. The main rock-forming minerals in the lower mantle are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite minerals are intrinsically much stronger than ferropericlase minerals, resulting in significant variations in lower-mantle rheological behavior depending on the quantity and degree of interconnectivity of the weak phase. The resulting effective bulk rock viscosity decreases with accumulating strain when the weaker Fp minerals become elongated and eventually interconnected. This implies that strain localization may occur in Earth’s lower mantle, which would in turn influence the pattern of mantle flow and could potentially aid the preservation of compositionally distinct, “hidden” reservoirs. So far, there have been no studies on global-scale mantle convection in the presence of such strain-weakening (SW) rheology.

Here, we present 2D numerical models of thermo-chemical convection in spherical annulus geometry including a new strain-weakening (SW) rheology formulation for lower-mantle materials. This macro-scale SW rheology is based on micro-scale rheological behavior found in prior studies, and combining rheological weakening and healing terms. We determine the effects of SW rheology on the planform of mantle flow, the mixing of chemical reservoirs, and the dynamics of mantle plumes.

We find that, in particular, plume conduits are weakened and act as lubrication channels which allow for the rapid ascent of mantle material. Their thermal anomalies and geometries are significantly different than those of mantle plumes which are not rheologically weakened. Moreover, larger thermochemical piles at the base of the mantle are stabilized by SW rheology, with implications for preservation of chemically-distinct materials over long timescales. Finally, we put our results into context with observations and existing hypotheses on the style of Earth's mantle convection and mixing. Most importantly, we suggest that the new kind of plume dynamics may explain the discrepancy between expected and observed thermal anomalies of deep-seated mantle plumes on Earth.

How to cite: Gülcher, A., Golabek, G., Thielmann, M., Ballmer, M., and Tackley, P.: Narrow, fast, and "cold" mantle plumes on Earth explained by strain-weakening rheology in the lower mantle , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7608, https://doi.org/10.5194/egusphere-egu22-7608, 2022.

18:28–18:30