Details regarding the early evolution of the mantle-crust system are still poorly constrained due to the great scarcity of >3.7 Ga rocks in the geological record. The Napier complex (East Antarctica) is an Eoarchean craton that contains some of Earth’s oldest rocks. This complex recorded Meso- and Neoarchean metamorphism that reached extreme conditions corresponding to granulite facies at 2.5 Ga (1050-1120°C and 7-11 kbar). As a consequence, most samples exhibit disturbed radiogenic isotope systematics (e.g., Rb-Sr, Sm-Nd) and zircon crystals found in such samples are very complex rendering isotopic systematics interpretations challenging. The analytical methods employed in previous studies do not allow these complexities to be understood, which motivated the present contribution.

Here we studied two granulitic orthogneisses labelled 78285007 (Mount Sones) and 78285013 (Gage Ridge) that correspond to the oldest available rocks from the Napier Complex. Mount Sones displays typical characteristics of Archean tonalite-trondhjemite-granodiorite (TTG) suites (e.g., high Na₂O/K₂O, high Sr/Y, fractionated REE patterns with low heavy REE concentrations) with a normative composition intermediate between tonalite and trondhjemite whereas Gage Ridge has a composition closer to that of granite despite a strongly fractionated REE pattern and a pronounced positive Eu anomaly. We have conducted zircon texture assessment using cathodoluminescence and back-scattered electron images in annealed and not annealed crystals. We have subsequently combined U-Pb age profiling by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and Lu-Hf isotope systematics measurement by LA-MC-ICP-MS in these zircon crystals. Finally, we analysed ^146,147Sm-^143,142Nd isotopesystematics in corresponding whole-rock samples to better constrain the early history of their source.

Our results reveal that Mount Sones and Gage Ridgeorthogneisses formed at 3794 ± 40 and 3857 ± 39 Ma, respectively, with initial ɛ_Hf of -2.6 ± 1.5 and -3.6 ± 2.5, respectively. Sm-Nd isotope measurements indicate a μ¹⁴²Nd of -8.7 ± 3.9 and a ɛ_Nd of -2.0 ± 0.3 at 3794 Ma for Mount Sones, whereas Gage Ridge exhibits a μ¹⁴²Nd of -12.1 ± 6.2 and a disturbed ¹⁴⁷Sm-¹⁴³Nd systematics. Taken altogether our results indicate that the oldest granitoids of the Napier Complex formed by reworking of 4456-4356 Ma mafic protocrust(s). Our inferred petrogenesis is similar to what has been proposed for other Eoarchean terranes worldwide (e.g., Itsaq Gneiss Complex, the Acasta Gneiss Complex, the Nuvvuagittuq Supracrustal Belt, and the North China craton). We propose that Hadean protocrusts were massively reworked in the Eoarchean to form cratons which, in turn, would account for both the absence of Hadean crust in the geological record and its little influence throughout the Archean despite crustal growth models proposing that ≤ 25% of present-day volume of continental crust was formed by the end of the Hadean.

How to cite: Guitreau, M., Boyet, M., Paquette, J.-L., Gannoun, A., Konc, Z., Benbakkar, M., Suchorski, K., and Hénot, J.-M.: The Hadean origin of the Archean Napier Complex (East Antarctica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16896, https://doi.org/10.5194/egusphere-egu2020-16896, 2020.

Banded Iron Formations (BIF’s) show a typical layering of Fe minerals and quartz that is observed at various scales ranging from micrometers to meters. Millimeter sized micro bands are commonly interpreted as annual layers so larger bands require decades or millennia to form, whereas micrometer sized nano bands have been interpreted to represents sub annual and even diurnal cycles. Because the mineralogical composition of BIF’s is not primary and because single-phase Fe(III) silica gel forms when Fe(III) (oxyhydr-)oxide precipitates in Si rich water, secondary processes are often invoked to explain the banding. However, trace element and isotope data point towards distinct sources for the Fe and Si rich bands, which is difficult to reconcile with a single phase starting material. In addition, the correlation of banding over long distances is inconsistent with most secondary models. Both primary and secondary models struggle to explain the versatile nature of the banding. I will present a conceptual model that could explain BIF layering at all scales and the more widespread formation of granular iron formations (GIF’s) in the Paleoproterozoic.

The concept builds on primary precipitation models postulating that banding forms due to some form of periodicity such as cyclic Fe or nutrient supply to the shelf. Fe(III) is mainly produced by phototrophic iron oxidizing bacteria. These photoferrotrophs are adapted to very low light levels corresponding to about 1% of the light level required by oxygen producing phototrophs allowing them to thrive deep down in the water column. The depth of Fe(III) production is mainly controlled by water turbidity which controls how deep photosynthetically available radiation (PAR) penetrates into the water column. Eutrophic conditions result in turbidity induced by the biomass itself resulting in shallow Fe(III) production depth and the formation of Fe rich bands. During oligotrophic stages, Fe(III) is only produced deep down in the water column, so that silica rich bands can form. In this case, Fe(III)-silica co-precipitation is not an issue because silica precipitates in the Fe(III) free upper water column. Reactive transport modelling shows that besides upwelling and nutrient supply, alternating Fe(III) production depth are mainly associated with changing light conditions. Hence the model predicts annual layering, but also local occurrences of diurnal cycles. Larger periodicities could be associated with: 1) nutrient supply patterns; 2) formation and clearing of atmospheric haze; or 3) additional sources of turbidity in the water column such as silicate particles, MnO₂ particles or metal sulfides. These additional sources of turbidity become more important in the Paleoproterozoic and could be responsible for the more widespread occurrences of GIF’s, indicative of Fe(III) production above storm wave base. The additional factor light, is quite versatile in producing periodicities at variable scales.

How to cite: Herwartz, D.: Is the banding in iron formations controlled by water transparency?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8817, https://doi.org/10.5194/egusphere-egu2020-8817, 2020.

The long-lived radiogenic isotope systems Lu-Hf and Sm-Nd have been widely used by geochemists to study magma sources and crustal residential times of (igneous) rocks in order to understand how early crust formed and to model the production rate and volume of continental crust on global and regional-scales during the last ~4.4 Ga. However, while throughout most of Earth’s history Nd and Hf isotope signatures in terrestrial rocks are well correlated due to their very similar geochemical behavior, some of Earth’s oldest rocks show an apparent inconsistency in their Nd and Hf isotope signatures. While Hf isotopes in early Archean rocks are generally (near) chondritic, Nd isotope signatures can be distinctly super- or sub-chondritic. The super-chondritic Nd isotope values in Eoarchean samples would suggest that these rocks are derived from a mantle reservoir depleted by prior crust extraction. The chondritic Hf isotope values, on the other hand, support a mantle source from which no significant volume of crust had been extracted. While a range of different processes, some of them speculative, might explain this Hf-Nd isotope paradox, recent research [1, 2] has shown that relatively simple, post-magmatic, open-system processes can explain decoupling of the typically correlative Hf-Nd isotope signatures. This talk will focus on the importance of identifying Nd-bearing accessory minerals in (Archean) rocks to understand how the Sm-Nd isotope system is controlled and how in situ isotope and trace element analyses by LA-(MC)-ICP-MS in combination with detailed petrographic observations help to understand when and via which processes the two isotope systems become decoupled. Reconstructing the isotopic evolution of the different isotope systems since formation of the protoliths has important implications for our understanding of early crust formation and questions some of the proposed current models for early crust extraction from the mantle.

[1] Hammerli et al. (2019) Chem. Geol 2; [2] Fisher et al. (2020) EPSL

How to cite: Hammerli, J.: Understanding the role of accessory minerals in the Sm-Nd isotopic evolution of ancient rocks: An in-situ LA-(MC)-ICP-MS approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9103, https://doi.org/10.5194/egusphere-egu2020-9103, 2020.

One of the fundamental tenets of geochemistry is that the Earth’s crust has been extracted from the mantle creating a crustal reservoir enriched—and a mantle depleted—in incompatible elements. The Hf-Nd isotope record has long been used to help understand the timing of this process. Increasingly, however, it has become apparent that these two isotope records do not agree for Earth’s oldest rocks. Hf isotopes of zircon from juvenile, nominally mantle-derived rocks throughout the Eoarchean have broadly chondritic initial isotope compositions and indicate large-scale development of the depleted mantle reservoir started no earlier than ~ 3.8 Ga. In contrast, the long-lived Sm-Nd isotope record shows large variation in Nd isotope compositions. Most notably, Paleo- and Eoarchean terranes with chondritic initial Hf isotope compositions have significantly radiogenic Nd isotope compositions indicative of the development of a widespread depleted mantle reservoir very early in Earth’s history which, by extension, requires extraction of significant volumes of enriched crust. These two isotope systems, therefore, indicate two fundamentally different scenarios for the early Earth and has been called the Hf-Nd paradox. However, an important unresolved question remains: Do these records represent primary isotopic signatures or have they been altered by subsequent thermomagmatic processes? We have been able to provide clarity in the Hf isotope record by analyzing zircon from Eo- and Paleoarchean magmatic rocks by determining its U-Pb crystallization age and linking this to its corresponding Hf isotope composition. We can do this unambiguously—even in complex polymetamorphic gneisses—with the laser ablation split stream (LASS) technique whereby we determine U-Pb age and Hf isotope composition simultaneously in a single zircon volume. The existing Nd isotope data, in contrast, are all from bulk-rock analyses. These analyses are potentially problematic in old, polymetamorphic rocks because of the inability to link the measured isotopic composition to a specific age. In addition, the REE budget in these rocks is hosted by accessory phases that can be easily mobilized during later metamorphic and magmatic events. We can now use the LASS approach in REE rich phases (e.g., monazite, titanite, allanite, apatite) to determine U-Pb age and Nd isotope composition in a single analytical volume. New Nd isotope data from the Acasta Gneiss Complex (Fisher et al., EPSL, 2020) show that REE-rich accessory phases are not in isotopic equilibrium with their bulk rock compositions and clearly demonstrate mobilization after initial magmatic crystallization. This post-magmatic open-system behavior may well explain the disagreement in the Hf-Nd isotope record in high-grade polymetamorphic terranes like Acasta. In less complicated, lower-grade rocks, such as in the Pilbara terrane, these REE-rich phases yield consistent U-Pb and Sm-Nd age and isotope compositions indicating that the Nd isotope system in these rocks has remained closed since formation. Of particular note, in the Pilbara samples, the Hf and Nd isotope systems have consistent, broadly chondritic, initial Hf and Nd isotope compositions. In these less-complicated samples, where the Sm-Nd isotope system has remained closed, the Hf and Nd isotope systems agree and there is no Hf-Nd paradox.

How to cite: Vervoort, J., Fisher, C., and Salerno, R.: Resolving the Hf-Nd paradox of early Earth crust-mantle evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13269, https://doi.org/10.5194/egusphere-egu2020-13269, 2020.

Outgassing of an early magma ocean on Earth plays a dominant role in determining the composition of its secondary atmosphere, and hence bears on the potential for the emergence of life. The stability of gaseous species in such an atmosphere reflects the redox state of the magma ocean. However, the relationship between oxygen fugacity (fO₂) and the oxidation state of the most abundant polyvalent element, Fe, in likely magma ocean compositions is poorly constrained. Here we determine Fe²⁺/Fe³⁺ ratios as a function of fO₂ in peridotite liquids, experimentally synthesised by aerodynamic laser levitation at 1 bar and 2173 K. We show that a magma ocean with Fe³⁺/∑Fe akin to that of contemporary upper mantle peridotite (0.037) would have had fO₂ 0.5 log units higher than the Fe-“FeO” equilibrium. At this relative fO₂, a neutral CO₂-H₂O-dominated atmosphere of ~ 150 bar would have developed on the early Earth, taking into account the solubilities of the major volatiles, H, C, N and O in the magma ocean. Upon cooling, the Earth’s prebiotic atmosphere was likely comprised of CO₂-N₂, in proportions and at pressures akin to that on presently found on Venus.

How to cite: Sossi, P., Burnham, A., Badro, J., Lanzirotti, A., Newville, M., and O'Neill, H.: A Venus-like atmosphere on the early Earth from magma ocean outgassing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11217, https://doi.org/10.5194/egusphere-egu2020-11217, 2020.

Geochronological data in zircon from Archaean tonalite–trondhjemite–tonalite (TTG) gneisses is commonly difficult to interpret. A notable example are TTG gneisses from the Lewisian Gneiss Complex (LGC), northwest Scotland, which have metamorphic zircon ages that define a more-or-less continuous spread through the Neoarchaean, with no clear relationship to zircon textures. These data are generally interpreted to record discrete high-grade events at c. 2.7 Ga and c. 2.5 Ga, with intermediate ages reflecting variable Pb-loss. Although ancient diffusion of Pb is commonly invoked to explain such protracted age spreads, trace element data in zircon may permit identification of otherwise cryptic magmatic and metamorphic episodes. Although zircons from the TTG gneiss analyzed here show a characteristic spread of Neoarchaean ages, they exhibit subtle but key step changes in trace element compositions that are difficult to ascribe to diffusive resetting, but which are consistent with emplacement of regionally-extensive bodies of mafic magma. These data suggest suprasolidus metamorphic temperatures persisted for 200 Myr or more during the Neoarchaean. Such long-lived high-grade metamorphism is supported by data from zircon grains from a nearby monzogranite sheet. These preserve distinctive trace element compositions suggesting derivation from a mafic source, and define a well-constrained U–Pb zircon age of c. 2.6 Ga that is intermediate between the two previously proposed discrete metamorphic episodes. The persistence for hundreds of millions of years of melt-bearing lower crust was probably the norm during the Archaean.

How to cite: Johnson, T., Taylor, R., and Clark, C.: Persistence of melt-bearing Archean lower crust for >200 m.y.— An example from the Lewisian Complex, northwest Scotland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12837, https://doi.org/10.5194/egusphere-egu2020-12837, 2020.

The early evolution of continental crust, particularly its lower layer, during the first 2.0 billion years of Earth history remains enigmatic. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from five localities in the North China craton. The results show that: (1) the oldest (3.82−3.55 Ga) known lower crustal rocks were survived in the southern part of this craton; (2) the Eo-Paleoarchean zircons have predominant sub-chondritic Hf isotope compositions and elevated δ¹⁸O values, suggesting Lu-Hf fractionation and crust-hydrosphere interactions on the Earth can be traced back to Eoarchean or even earlier; (3) a secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials into magmas at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time.

How to cite: Ma, Q., Xu, Y.-G., Huang, X.-L., and Zheng, J.-P.: Eoarchean to Paleoproterozoic crustal evolution in the North China Craton, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4504, https://doi.org/10.5194/egusphere-egu2020-4504, 2020.

More and more convection codes now consider the apparition of melt when the temperature of the mantle exceeds a considered solidus temperature. How melt is treated when it appears varies a lot from one code to another. The convection code StagYY has been using an implementation in which molten eclogite is produced out of melting of mixed mantle. The melt is then teleported above ("erupted") or below ("intruded") the basaltic crust. In a recent study by Jain et al. 2019, we have shown that it is possible to also self-consistently generate continental crust (so-called TTG rocks) if the basaltic crust is entrained in the mantle and remolten. In nature, this only happens if a lot of water is present in the recycled basalt so a numerical treatment of water is necessary.

In this poster, we discuss the details of a new implementation of melting in which each cell of the convection domain is divided in several groups of different composition. Each group has a different solidus and liquidus temperature according to the composition and the water content. The solidus temperature is computed using an interpolation between composition and water concentration end members instead of using an extrapolation from the solidus temperature, as it is usually done. This ensures that TTGs form at a realistic melt fraction and provides a different view on how the continental crust of the early Earth might have formed.

How to cite: Rozel, A., Mojzsis, S., Guitreau, M., Manjón Cabeza Córdoba, A., Ballmer, M., and Tackley, P.: Implementation of partial melting with a water- and composition-dependent solidus temperature adapted to TTG formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8964, https://doi.org/10.5194/egusphere-egu2020-8964, 2020.

Numerical models of mantle convection help our understanding of the complex feedback between the plates and deep interior dynamics through space and time. Did the early Earth have plate tectonics, a stagnant lid, or something in between? The surface dynamics of the early Earth remain poorly understood. Current numerical models of mantle convection are constrained by present-day observations, but the behavior of the hotter, early Earth prior to the onset of plate tectonics is less certain. The early Earth may have possessed a large hot magma ocean trapped near the core-mantle boundary after formation during differentiation, and likely containing different elements from the surrounding mantle. We examine how composition-dependent properties in the deep mantle affect convection dynamics and surface mobility in high Rayleigh number models featuring plastic yielding. Our Newtonian models indicate that increased conductivity or decreased viscosity flattens basal topography while also increasing the potential for surface yielding. We vary the viscosity, thermal conductivity, and internal heating in a compositionally distinct basal magma ocean and explore the compositional topography, insulation effects and surface stresses for non-Newtonian rheology. Models are run using a variety of crustal compositions, such as the inclusion of primordial continental material before the onset of plate tectonics. We monitor the surface for plate-like behavior. Since convective vigour is very strong in the early Earth, specialized tracer methods are employed for increased accuracy. In our models, Stokes flow solutions are obtained using a multigrid method specifically designed to handle large viscosity contrasts and non-Newtonian rheology.

How to cite: O'Farrell, K. A., Trim, S., and Butler, S.: Mobile or not mobile: exploring the linkage between deep mantle composition and early Earth surface mobility, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10638, https://doi.org/10.5194/egusphere-egu2020-10638, 2020.

Singhbhum Craton, eastern India, exposes some of the oldest known composite Paleoarchean granitoids. These granitoids range from sodic TTGs to evolved, potassic granites.  The whole process of their formation, starting from nucleation of a juvenile continent to its evolution and final stabilization is documented. The central part of the craton started nucleating with the formation of 3.45–3.40Ga juvenile (zircon εHf_t=+0.6 to +7.1) TTGs. These TTGs characterized by slightly depleted HREE and Y, negligible Eu-anomaly (Eu/Eu*=0.90 to 1.00) and moderate Sr/Y (25–64), consistent with derivation from a low-K mafic crust at a pressure near the lower end of the garnet stability field, causing subordinate garnet retention in the residue and negligible role of plagioclase. During 3.32Ga, deeper melting of a juvenile mafic crust (zircon εHf_t=+1.3 to +5.7) caused emplacement of a second generation of TTG. Deeper melting is suggested by depleted HREE and Y, and high Sr/Y (52–155), implying significant amount of residual garnet retention. Subsequently at 3.28 and 3.25Ga, melting of moderately old to juvenile (zircon εHf_t=-1.9 to +4.5), mostly TTG sources at variable depths generated potassic, LILE-enriched, high-silica granites. Intrusion of these potassic granites resulted in a stable and buoyant crust that marked the final Cratonization of the Singhbhum Craton. The sequence of events is interpreted in terms of repeated intracrustal melting and granitoid generation in a gradually thickening oceanic plateau with a progressive change in granitoid source from mafic to felsic in composition. Combination of rock assemblage, regional geology, and structural pattern also supports intraplate nature of the magmatism in Singhbhum Craton, which might have been a significant mechanism of crustal growth worldwide during Paleoarchean.

How to cite: Mitra, A., Dey, S., Zong, K., Liu, Y., and Mitra, A.: Paleoarchean crustal evolution of the Singhbhum Craton, eastern India: Insights from granitoid petrology and zircon U-Pb and Lu-Hf systematics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1142, https://doi.org/10.5194/egusphere-egu2020-1142, 2020.

Available zircon ages indicate that the plutonic protoliths of Neoarchean TTG (tonalitic-trondhjemitic-granodioritic) gneisses in the Eastern Block were emplaced at two phases, with the earlier one at 2.75-2.65 Ga and the younger one at 2.55-2.50 Ga. Although the 2.75-2.65 Ga rock associations are only exposed in the Luxi and Qixia areas, the ~2.7 Ga igneous event must have occurred across the whole Eastern Block and was a major crustal accretionary or mantle-extraction event that formed a thick mafic crust beneath the whole Eastern Block based on the following lines of evidence:

(1) The 2.75-2.65 Ga TTG rocks in the Luxi granite-greenstone terrane have positive εHf(t) values (+2.7 to +10.0), with most zircon Hf model ages close to the rock-forming ages, which provides robust evidence that the ~2.7 Ga event that formed the 2.75-2.65 rock associations was a crustal accretion (mantle extraction) event, not a crust-reworking event.

(2) The 2.55-2.50 Ga TTG rocks in the Eastern Block possess mildly positive to slightly negative εHf(t) values, with most zircon Hf model ages pointing to 2.8-2.6 Ga, similar to rock-forming ages of the 2.75-2.65 Ga TTG gneisses in the Luxi granite-greenstone terrane, suggesting that the 2.55-2.50 Ga rocks in the Eastern Block were mainly derived from the partial melting of an early Neoarchean (2.75-2.65 Ga) juvenile crust that formed at ~2.7 Ga. As the 2.55-2.50 Ga TTG gneisses are ubiquitous over the whole Eastern Block, the 2.7 Ga event must have occurred over the whole Eastern Block, forming an early Neoarchean juvenile crust that experienced partial melting or reworking to form the 2.55-2.50 Ga TTG rocks.

(3) TTG rocks are generally considered to have been derived from the partial melting of a thickened mafic crust (eclogite or rutitle/garnet-bearing amphibolite). This means that an early Neoarchean (2.75-2.65 Ga) juvenile crust formed by the ~2.7 Ga event should be a mafic-dominant crust, which is either a lower continental crust or an oceanic crust. In this case, the ~2.7 Ga event in the Eastern Block may have represented a Large Igneous Province event that formed the main body of the Eastern Block. This study was financially supported by the sub-project of a NSFC Major Project, entitled “Continental Crust Growth-Stabilization and Initiation of the Early Plate Tectonics” (Project Code: 41890831) and HKU Seed Fund for Basic Research (201811159089).

How to cite: Zhao, G.: Ages and Hf isotopes of igneous zircons from Neoarchean TTG gneisses in the Eastern Block, North China Craton: Tectonic implications , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1544, https://doi.org/10.5194/egusphere-egu2020-1544, 2020.

About 2.5 Ga ago, two distinct mantle sources for basalts developed: one with a lower mantle potential temperature (MPT) being today relatively depleted and feeding the mid-ocean ridges, and one with a higher MPT being relatively enriched and pluming today's ocean-island-basalt (OIB) volcanism (Condie et al., 2016). Previous to that, basalts record rather uniform MPTs corresponding to today's higher-temperature OIB reservoir. The cooler mantle domain started forming, when the slowly cooling thermally uniform mantle reached a MPT of 1550-1500 °C (Condie, 2018). We attribute this “Great Thermal Divergence” (Condie et al., 2016) to a transition from non-layered to layered mantle convection. For primitive mantle compositions, a 1530-adiabat propagates precisely to the high-temperature slope break of the 660 phase transition at about 1800 °C/23 GPa. Mantle with MPT higher than that does not experience the suppression of convective passage through the lower-upper mantle boundary, which results from the negative slope of the ringwoodite-to-perovskite-plus-periclase transition. We propose that mantle convection prior to 2.5 Ga was capable of stirring the whole mantle. A 660 phase transition with a negative slope formed only 2.5 Ga ago and thus established a thermomechanical boundary layer that allowed the formation of two thermally distinct mantle reservoirs.

Condie, K. et al. (2016): A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites. Geoscience Frontiers, 7, 543-553.

How to cite: Nagel, T., Petersen, K. D., and Vesterholt, A.: The Great Thermal Divergence and the slope break of the 660 phase transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13615, https://doi.org/10.5194/egusphere-egu2020-13615, 2020.

We present new time series partial melting experiments performed on a natural enstatite chondrite (EL6), aimed at investigating the textural and geochemical changes induced by silicate-metal equilibration during early planetary differentiation. The starting material of our experiments consisted of small fragments (ca. 50 mg) obtained from the interior of the enstatite chondrite MCY 14005 (MacKay Glacier, Antarctica), collected during the XXX° Italian Expedition in Antarctica (PNRA). Experiments were performed in graphite capsules at a pressure of 1 GPa, at temperature ranging from 1100 to 1300 °C, with run durations from 1 to 24 h. The initial phase assemblage of the enstatite chondrite, mostly composed by granular enstatite and Fe-Ni metal (up to 400 µm in size) with minor amounts of sulphides and plagioclase, undergoes significant changes with increasing temperature and run duration. At 1100 °C, no silicate melt is produced and subsolidus reactions occur at the contact between the metal and silicate phases. At 1200 °C, small amounts of silicate melt are produced at the grain boundaries and enstatite grains in contact with the melt grow Fe-enriched rims. The metal portions are characterized by two immiscible liquid phases that exhibit rounded shapes when in contact with the silicate melt, whereas smaller (micrometric) liquid metal spheres occur isolated within the silicate melt throughout the experimental charges. These features are already observed in the 1 h experiment but become increasingly evident with increasing run duration, and at higher temperatures. In the experiments performed at 1300 °C, the amount of silicate melt increases and new silicate minerals form (olivine and low-Ca-pyroxene).

Enstatite chondrites are characterized by an oxygen isotope composition similar to that of the bulk Earth and Moon, and are considered to have initially formed in the terrestrial planetary zone of the solar nebula. For this reason, they represent a suitable material to investigate the early planetary differentiation processes that occurred in the proto-Earth system. Preliminary results from our experiments indicate that, at the investigated oxygen fugacity (1-2 log units below the IW buffer), the Fe-Si exchange between the metal and silicate phases allows the formation of silicate melt and silicate phases such as olivine and low-Ca-pyroxene. At the same time, the change in shape of the metal grains (increasingly circular/spherical with increasing temperature) and the overall reduction of their number density with increasing experimental time point to rapid aggregation of the metal phase and, possibly, to fast silicate-metal differentiation in small planetesimals.

How to cite: Masotta, M., Folco, L., Ziberna, L., and Myhill, R.: Partial melting of an enstatite chondrite at 1 GPa: Implications for early planetary differentiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9935, https://doi.org/10.5194/egusphere-egu2020-9935, 2020.

The biogenicity of proposed stromatolites from deformed greenschist/amphibolite facies Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland, is debated  [1,2; cf. 3]. To assess their promise as primary sedimentary structures – as opposed to artefacts of strain localization in layered ductile rocks – we report new field mapping at the discovery site of Nutman et al. (2016) to guide micro- and macro-structural investigations and geochemical sampling. Discontinuous field relations preclude confident assignment of these outcrops as being structurally overturned as originally argued. The structures are not deformed conical stromatolites, but instead linear inverted ridges aligned with azimuths of local and regional fold axes, and parallel to linear structures. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron back-scattered diffraction (EBSD) patterns on fresh surfaces cut perpendicular and parallel to the ridges show that the structures lack any internal laminae. Seeming internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceal structureless quartz ± dolomite granoblastic cores. These asymmetric boudins sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. Boudinage fabrics reflect viscosity contrasts of the different ductile layers during deformation, and are thus not of primary origin. Collectively, our results show that such structures were probably never stromatolites, but are instead the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structure.

[1] Nutman, A.P. et al. 2016, Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures: Nature, v. 537, p. 535–538; [2] Nutman, A.P. et al., 2019, Cross-examining Earth’s oldest stromatolites: Seeing through the effects of heterogeneous deformation, metamorphism and metasomatism affecting Isua (Greenland) ∼3700 Ma sedimentary rocks: Precambrian Research, v. 331, p. 105347; [3] Allwood, A.C. et al. 2018, Reassessing evidence of life in 3,700-million-year-old rocks of Greenland: Nature, doi: 10.1038/s41586-018-0610-4.

How to cite: Zawaski, M., Kelly, N., Orlandini, O. F., Nichols, C., Allwood, A., and Mojzsis, S.: Chemical and structural analysis of proposed ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) - a reappraisal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6044, https://doi.org/10.5194/egusphere-egu2020-6044, 2020.

The c. 3.5 Ga Dresser Formation of the East Pilbara Craton (Western Australia) contains large amounts of blackish barite. These rocks produce an intense sulfidic odor when crushed, resulting from abundant primary fluid inclusions. In part, the black barites are interbedded with sulfidic stromatolites. Using Raman spectroscopy, microthermometry, and two different online GC–MS approaches, we characterized in detail the chemical composition of the barite-hosted fluid inclusions. Our GC–MS techniques were based on (i) thermodecrepitation at 150-250°C and (ii) solid phase microextraction (SPME)–GC–MS at reduced temperature (50°C), thereby minimizing external contamination and artefact formation. Major fluid inclusion classes yielded mainly H₂O, CO₂, and H₂S in varying abundance, along with minor amounts of COS and  CS₂, N₂, and CH₄ (< 1%). Notably, we also detected a wide range of volatile organic compounds, including short–chain ketones and aldehydes, thiophenes, and various organic (poly)sulfides. Some of these compounds (CH₃SH, acetic acid) have previously been invoked as initials agents for carbon fixation under primordial conditions, but up to now their presence had not been observed in Precambrian materials. Based on our findings, we hypothesize that hydrothermal seepage of organic and inorganic compounds during Dresser times provided both, catabolic and anabolic substrates for early microbial metabolisms.

How to cite: Thiel, V., Duda, J.-P., van den Kerkhof, A. M., Reitner, J., and Mißbach, H.: Volatile organic compounds in barite-hosted fluid inclusions from the 3.5 Ga old Dresser Formation, Western Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9161, https://doi.org/10.5194/egusphere-egu2020-9161, 2020.

Use of trace and rare earth element concentration of terrigenous sedimentary rocks to deduce the composition of their source rocks in the hinterland is a very common and efficient practice. The results of geochemical analysis of the metaquartzarenites located at the basal part of Bababudan and Sigegudda belt, late Archean greenstone sequences of western Dharwar craton show that the sediments were most possibly supplied from Paleo to Mesoarchean granitoids of western Dharwar Craton. Rare earth element patterns of these basal quartzites display fractionated REE pattern in variable degree (La_N/Yb_N =1.47-10.63) with moderate to highly fractionated LREE (La_N/Sm_N=2.67-8.93) and nearly flat to slighly elevated HREE (Gd_N/ Yb_N=0.62-1.29) and a significant Eu negative anomaly (avg. Eu/Eu*=0.67). In general, presence of negative Eu anomaly in clastic rocks reflect the widespread occurrence of granitic rocks in the source area, which possess negative Eu anomaly. On the other hand, mechanical enrichment of zircon (having negative Eu anomaly, high HREE concentration and low La_N/Yb_N), if present, will hamper the whole REE pattern of the sediments and necessarily, do not actually mimic the source composition. Here, in our study, the Th/Sc vs Zr/Sc diagram show mineral Zircon has been concentrated by mechanical concentration in the sedimentary rocks. Few quartzite samples which have high Zr content typically exhibit low La_N/Yb_N values, reflecting pivotal role of mineral zircon in controlling the REE pattern of the sediments. Hence, in this case, we should be cautious in interpreting of the Eu negative anomaly of the basal quartzites for meticulously identifying their source rock composition. More geochemical and other analytical approaches are required in this regard.

How to cite: Mitra, A. and Dey, S.: Eu anomaly- reliability of the proxy in inferring source composition of clastic Sedimentary rocks: A case study from western Dharwar craton, Karnataka, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21526, https://doi.org/10.5194/egusphere-egu2020-21526, 2020.

Physics-based geodynamic modeling of mantle convection provide a unifying framework for solid-Earth sciences, explicitly linking together disparate fields such as tectonophysics, tomographic imaging, basin analysis, mantle mineralogy, geomorphology, global geodesy and the long-term chemical and thermal evolution of the mantle. Studying the evolution of mantle convection in time is particularly powerful as it reduces trade-offs, increase the possible linkages and the opportunities to cross-test hypotheses. But since mantle convection evolves over geologic timescales, its future evolution is precluded from us and we must focus on its past history.

Here I will show how geodynamic modeling of past mantle flow can be combined with tomographic imaging and geologic observations, highlighting the strengths of this approach and some of its potential pitfalls. I will use a series of case studies, starting from simple analytical solutions for channelized flow in the South Atlantic and Caribbean regions. I will move on to an application of sequential assimilation to the South China Sea, ending with computationally demanding large-scale numerical optimizations of past mantle flow.

How to cite: Colli, L.: Combining tomographic images and geodynamic modeling of past mantle flow: from simple analytical solutions to numerical inverse methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6134, https://doi.org/10.5194/egusphere-egu2020-6134, 2020.

The formation of large igneous provinces is a focus of geoscientists and is a major scientific issue in mantle dynamics. A broad consensus holds that the Emeishan large igneous province (ELIP) was generated by an upwelling mantle plume. However, recent geological and seismic studies have challenged this notion. In this study, I redraw and reanalyze previous tomographic images and use images of three velocity perturbation profiles crossing the ELIP. I collected abundant high-quality teleseismic data and performed common conversion point (CCP) stacking of receiver functions in the mantle transition zone (MTZ) of the ELIP. The tomographic images show a high-velocity anomaly of a northeastward-subducted slab-like body beneath the ELIP, which might be a relic of the Paleo-Tethys oceanic lithosphere. Images from CCP stacking of receiver functions indicate that the subducted slab of the Paleo-Tethys oceanic lithosphere retained an imprint on the X-discontinuity and the 410 and 660 km discontinuities. Based on my assessment, the subducted slab might have induced return mantle flow or large-scale mantle upwelling, which possibly played an important role in the formation of the ELIP.

How to cite: He, C.: A subducted slab of the Paleo-Tethys oceanic lithosphere associated with the formation of the Emeishan large igneous province, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1467, https://doi.org/10.5194/egusphere-egu2020-1467, 2020.

The thermal and compositional structure of the lithospheric keels underlying the Precambrian cratonic cores of the continents may shed light on their evolution and long-term stability. A number of seismic studies have found significant 3D seismic heterogeneity in cratonic lithosphere, which is enigmatic because temperature variations in old shields are expected to be small and seismic sensitivity to major-element compositional variations is limited. Previous studies show that metasomatic alteration may lead to significant variations in shield velocities with depth. Here we perform a grid search for thermo-chemical structures including metasomatic compositions, to model Rayleigh-wave phase velocities between 20 and 160 s for the northeastern part of North America comprising the Superior craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We find smooth variations in thermal structure that include variations in thermal thickness within the Superior and decreasing thickness towards the edges of the shield. Four types of distinct compositional structures are required to match the long-period phase velocities. The different types appear to correlate with: (i) the unaltered oldest cores of the Superior, (ii) Archean and Proterozoic lithosphere modified by rifting and plume activity, and two distinct types of subduction signatures: (iii) an Archean/Paleo-Proterozoic signature that includes a high-velocity eclogite layer in the mid-lithosphere and (iv) a post Paleo-Proterozoic signature characterised by strongly altered shallow mantle lithosphere. Thus, processes that have affected the formation and modification of cratonic lithosphere and were previously recognised in xenoliths appear to have also left large-scale imprints in seismic structure.

How to cite: Goes, S., Eeken, T., Altoe, I., Petrescu, L., Foster, A., Pedersen, H., Arndt, N., Darbyshire, F., and Bouilhol, P.: Thermochemical structure of cratons from Rayleigh wave phase velocities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5380, https://doi.org/10.5194/egusphere-egu2020-5380, 2020.

A better understanding of the Earth’s compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660~800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ~30% to 50% basalt fraction, and from ~40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.

How to cite: Yan, J., D. Ballmer, M., and J. Tackley, P.: The evolution and distribution of recycled oceanic crust in the Earth’s mantle: Insight from geodynamic models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20964, https://doi.org/10.5194/egusphere-egu2020-20964, 2020.

Early periods of Earth's history are of great interest for the evolution of plate tectonics. For instance, neither the formation of lithospheric plates nor the nature of Archean plate tectonics is well known. As a remnant of the magma ocean period, a compositionally dense layer at the core-mantle boundary is assumed to interact with the convective flow of the Earth's mantle forming todays LLSVPs. Since plate motions are strongly coupled to the convection of mantle material, stabilizing effects of compositionally dense material have a profound impact on mantle convection and plate tectonics and will be of major importance for its evolution.
To investigate the influence of a dense basal layer, we use a numerical approach employing thermo-chemical mantle convection models with self-consistent plate generation. Considering different possible scenarios of the post magma ocean period we analyze the influence of different parameters, i.e. the density contrast between the dense basal material and the ambient mantle and the volume of the enriched layer.
Generally we observe that a stagnant lid forms which is initially mobilized episodically before turning to a permanently mobile surface. However, the temporal evolution of the episodic stage is considerably altered due to the presence of dense basal material. The time when an episode occurs, is determined by the mechanism which induces this mobilization. The mechanism itself is controlled by the density and volume of the enriched layer. Therefore, we distinguish between four different initiation mechanisms, which occur for different configurations of the density and volume of enriched material.

How to cite: Hellenkamp, P., Stein, C., and Hansen, U.: LLSVPs of primordial origin: Implications for the evolution of plate tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21471, https://doi.org/10.5194/egusphere-egu2020-21471, 2020.

The dynamic topography of the core-mantle boundary (CMB) provides important constraints on dynamic processes in the mantle and core. However, inferences on CMB topography are complicated by the uneven coverage of data with sensitivity to different length scales and strong heterogeneity in the lower mantle. Particularly, a trade-off exists with density variations, which ultimately drive mantle flow and are vital for determining the origin of mantle structures. Here, I review existing models of CMB topography and lower mantle density, focusing on seismological constraints (Koelemeijer, 2020). I develop average models and vote maps with the aim to find model consistencies and discuss what these may teach us about lower mantle structure and dynamics.

While most density models image two areas of dense anomalies beneath Africa and the Pacific, their exact location and relationship to seismic velocity structure differs between studies. CMB topography strongly influences the retrieved density structure, which partially helps to resolve differences between recent studies based on Stoneley modes and tidal measurements. CMB topography models vary both in pattern and amplitude and a discrepancy exists between models based on body-wave and normal-mode data. As existing models typically feature elevated topography below the Large-Low-Velocity Provinces (LLVPs), very dense compositional anomalies may be ruled out as possibility.

To achieve a similar consistency as observed in lower mantle models of S-wave and P-wave velocity, future studies should combine multiple data sets to break existing trade-offs between CMB topography and density. Important considerations in these studies should be the choice of theoretical approximation and parameterisation. Efforts to develop models of CMB topography consistent with both body-wave and normal-mode data should be intensified, which will aid in narrowing down possible explanations for the LLVPs and provide additional insights into mantle dynamics.

Koelemeijer, P. (2020), “Towards consistent seismological models of the core-mantle boundary landscape”. Book chapter in revision for AGU monograph "Mantle upwellings and their surface expressions", edited by Marquardt, Cottaar, Ballmer and Konter

How to cite: Koelemeijer, P.: Towards consistent seismological models of the core-mantle boundary landscape, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3518, https://doi.org/10.5194/egusphere-egu2020-3518, 2020.

Examination of the least-contaminated rocks of the Jurassic Karoo flood basalt province indicates considerable compositional variability in the mantle source. New and previously published Sr, Nd, and Pb isotopic data are suggestive of two main categories of mantle reservoirs: one coincides with the field of depleted mantle (DM) -affinity oceanic crust and the other has low initial eNd (+3.3 to 0.3) and high ⁸⁷Sr/⁸⁶Sr (0.7039 to 0.7057) and Δ8/4 (92 to 138) typical of enriched mantle 1 (EM1) -affinity oceanic crust. Previous studies have proposed the DM type reservoir included domains affected by subduction-related fluids and recycled oceanic components (e.g. Heinonen et al., 2014). The EM1 type reservoir probably also contained subducted crustal components, but the geochemical data are suggestive of an additional primitive mantle (PM) type component (Turunen et al., 2019).

Importantly, the two reservoirs can be geochemically linked to a recently identified bilateral compositional asymmetry in the volumious Karoo flood basalts (Luttinen, 2018): The DM type  reservoir is the most likely source of Nb-depleted flood basalts in the southeastern Karoo subprovince (Lebombo rifted margin and Antarctica), whereas the EM1-PM type reservoir has been identified as the principal source of the Nb-undepleted flood basalts in the northwestern subprovince (Karoo-Kalahari-Zambezi basins). The boundary between the flood basalt subprovinces and the occurrences of the DM-affinity and EM1-PM-affinity rocks overlie the Jurassic location of the margin of the Jurassic sub-African LLSVP. Magmas derived from the EM1-PM type reservoir were largely emplaced above the deep mantle anomaly, whereas those derived from the DM type reservoir were emplaced outside the footprint of the LLSVP.

Based on isotopic similarity, the EM1-PM type reservoir of the Karoo province may record the same overall LLSVP material as the Gough component in the zoned Tristan da Cunha plume (e.g. Hoernle et al., 2015). Furthermore, it is possible that the DM type reservoir of the Karoo province, which has been interpreted to represent depleted upper mantle heated by mantle plume, could also represent a plume component and that the bilateral Karoo flood basalt province as a whole could thus register melting of a large zoned plume source associated with the margin of the sub-African LLSVP.

References

Heinonen, J.S., Carlson, R.W., Riley, T.R., Luttinen, A.V., Horan, M.F. (2014). Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province. Earth and Planetary Science Letters 394, 229–241. http://dx.doi.org/10.1016/j.epsl.2014.03.012

Hoernl, K., Ronde, J., Hauff, F., Garbe-Schönberg, D., Homrighausen, S., Werner, W., Morgan, J.P. (2015).  How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot. Nature Communications 6:7799. doi: 10.1038/ncomms8799

Luttinen, A.V. (2018). Bilateral geochemical asymmetry in the Karoo large igneous province. Scientific Reports 8:5223. doi:10.1038/s41598-018-23661-3

Turunen, S.T., Luttinen, A.V., Heinonen, J.S., Jamal, D.L. (2019). Luenha picrites, Central Mozambique – Messengers from a mantle plume source of Karoo continental flood basalts? Lithos 346–347. https://doi.org/10.1016/j.lithos.2019.105152

How to cite: Luttinen, A., Heinonen, J., Turunen, S., Carlson, R., and Horan, M.: Were Karoo flood basalts derived from a LLSVP-related plume source?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18210, https://doi.org/10.5194/egusphere-egu2020-18210, 2020.

Significant effort has been made to characterize the diversity of geochemical components sampled by oceanic hotspot volcanoes and mid-ocean ridges, and progress has been made to identify the origin of these geochemical components. However, the locations of the key mantle domains sampled at hotspots—EM1 (enriched mantle I), EM2 (enriched mantle II), HIMU (high ‘μ’, or high ²³⁸U/²⁰⁴Pb), and an ancient, high ³He/⁴He component—remain poorly constrained. In turn, the lack of spatial constraints on the locations and geometries of these reservoirs limits understanding of how geodynamic processes (e.g., subduction, mantle convection) operate to modify the Earth’s interior.

We provide an updated compilation the most extreme EM (lowest ¹⁴³Nd/¹⁴⁴Nd) and HIMU (highest ²⁰⁶Pb/²⁰⁴Pb) compositions in lavas from 46 oceanic hotspots with available data, and the highest ³He/⁴He compositions from 44 hotspots globally. We examine the geographic distributions of hotspots hosting extreme geochemical components at the Earth’s surface. We also explore how tilted plume conduits relate the geochemical distributions in hotspots at the Earth’s surface with the two inferred Large Low Shear Wave Velocity Provinces (LLSVP) in the deep mantle.

We find that the most extreme EM signatures are identified in southern hemisphere hotspots, and northern hemisphere hotspots exhibit more geochemically-depleted compositions. Critically, hotspots with the most extreme HIMU compositions show a very different distribution, and are found in, or near, the tropical latitudes. The EM and HIMU domains thus exhibit a clear spatial separation in the deep Earth.

In order to evaluate whether EM and HIMU domains are spatially associated with the LLSVPs, we compare the magnitude of EM and HIMU signatures with minimum hotspot distances from the LLSVP margins. While EM-hosting hotspots show a clear geographic affinity for the LLSVPs, new data make it apparent that HIMU-hosting hotspots show no geographic association with the LLSVPs, further supporting to the spatial decoupling of EM and HIMU mantle domains.

Hotspots hosting ancient high ³He/⁴He domains exhibit a spatial relationship with the LLSVPs (doi: 10.1029/2019GC008437), suggesting that the EM and high ³He/⁴He domains may coexist in the LLSVPs. While the degree of the EM signature exhibits no relationship with hotspot buoyancy flux, maximum high ³He/⁴He values correlate with hotspot buoyancy fluxes, consistent with the hypothesis that high ³He/⁴He mantle reservoirs are hosted in dense regions in the LLSVPs sampled by only the hottest and most buoyant plumes.

These results raise several key questions. First, if subduction of oceanic crust and sediment generate HIMU and EM reservoirs, then why are they spatially separated?  Why are EM2 domains concentrated in the southern hemisphere, and why are they limited to being inside or near the LLSVPs? Why are EM and high ³He/⁴He domains both geographically associated with the LLSVPs, and are they spatially separated within the LLSVPs so that the low ³He/⁴He of the former does not overprint the high ³He/⁴He of the latter? If elevated ³He/⁴He originates in the core, consistent with negative ¹⁸²W anomalies in high ³He/⁴He lavas, why are high ³He/⁴He plumes associated with the LLSVPs?   

How to cite: Jackson, M., Becker, T., and Steinberger, B.: Geochemical and seismological constraints on the locations and geometries of deep mantle reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10547, https://doi.org/10.5194/egusphere-egu2020-10547, 2020.

Earth’s oldest preserved crustal archive, the Jack Hills zircon of Western Australia, has been controversial to interpret in terms of the onset of plate tectonics. Here we conduct time series analysis on hafnium isotopes of the Jack Hills zircon and reveal an array of statistically significant cycles that are reminiscent of plate tectonics, i.e., subduction. At face value, such cycles may suggest early Earth conditions similar to today—the uniformitarian “day one” hypothesis. On the other hand, in the context of expected secular changes due to planetary evolution and geological observations, the cycles could instead imply that modern plate tectonic subduction inherited convective harmonics already facilitated by an early phase of stagnant-lid delamination—the “lid-to-plates” hypothesis. Either way, any model for the initiation of plate tectonics must begin in Hadean time.

How to cite: Mitchell, R. N., Spencer, C. J., Kirscher, U., and Wilde, S. A.: Modern plate tectonic cycles are inherited from Hadean mantle convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21211, https://doi.org/10.5194/egusphere-egu2020-21211, 2020.

Basalts are generated by adiabatic decompression melting of the upper mantle, and thus provide spatial and temporal records of the thermal, compositional, and dynamical conditions of their source regions. Uniquely constraining these factors through the lens of melting is challenging given the complexity of the melting process. To limit the a priori assumptions typically required for forward modeling of mantle melting, and to assess the robustness of the modeling results, we combine a Markov chain Monte Carlo sampling method with the forward melting model REEBOX PRO [1] simulating adiabatic decompression melting of lithologically heterogeneous mantle. Using this method, we invert for mantle potential temperature (Tp), lithologic trace element and isotopic composition and abundance, and melt productivity together with a robust evaluation of the uncertainty in these system properties. We have applied this new methodology to constrain melting beneath the Reykjanes Peninsula (RP) of Iceland [2] and here extend the approach to Iceland’s Northern Volcanic Zone (NVZ). We consider melting of a heterogeneous mantle source involving depleted peridotite and pyroxenite lithologies, e.g., KG1, MIX1G and G2 pyroxenites. Best-fit model sources for Iceland basalts contain more than 90% depleted peridotite and less than 10% pyroxenite with Tp ~125-200 °C above ambient mantle. The trace element and Pb and Nd isotope composition of the depleted source beneath the Reykjanes Peninsula is similar to DMM [3], whereas depleted mantle for the NVZ is isotopically distinct and more trace element enriched. Conversely, inverted pyroxenite trace element compositions are similar for RP and NVZ and are more enriched than previously inferred, despite marked differences in their Pb and Nd isotope composition. We use these new constraints on the Iceland source to investigate their relative importance in basalt genesis along the adjoining Reykjanes and Kolbeinsey Ridges. We find that the proportion of pyroxenite diminishes southward along Reykjanes Ridge and is seemingly absent to the north along the Kolbeinsey Ridge. Moreover, abundances of inverted RP and NVZ depleted mantle also diminish away from Iceland and give way to a common depleted source for the North Atlantic. These findings further illuminate the along-strike variability in source composition along the North Atlantic ridge system influenced by the Iceland melting anomaly, while reconciling geochemical, geophysical and petrologic constraints required to rigorously test plume vs. non-plume models.

[1] Brown & Lesher (2016); G^3, v. 17, p. 3929-2968

[2] Brown et al. (2020); EPSL, v. 532, 116007

[3] Workman and Hart (2005); EPSL, v.231, p. 53-72

How to cite: Brown, E. and Lesher, C.: Constraining Mantle Source Conditions at Iceland and Adjoining Ridges Using Markov Chain Monte Carlo Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22469, https://doi.org/10.5194/egusphere-egu2020-22469, 2020.

In this study we explored the contrasted plate tectonic reconstructions proposed for the proto-South China Sea and SE Asia. We implemented four different end-member plate models into global geodynamic models to test their predicted mantle structure against tomography. All models reproduced the Sunda slabs beneath Peninsular Malaysia, Sumatra and Java and the proto-South China Sea (PSCS) slabs beneath present Palawan, northern Borneo, and offshore Palawan; some models also predicted slabs under the southern South China Sea. PSCS slabs generated from double-sided PSCS subduction and earlier Borneo rotation generated a slightly better fit to tomography but pure southward PSCS subduction was also viable. A smaller Philippine Sea plate (PSP) with a short ~1000 km restored northern slab (i.e. Ryukyu slab) was clearly superior to a very long >3000 km slab. Mantle flows generated from our geodynamic models suggest strong upwellings under Indochina during the late Eocene to Oligocene. Our models generated strong downwellings under the South China Sea in the late Cenozoic that did not support a deep-origin ‘Hainan plume’. 

The following plate models variants were assimilated in the geodynamic models: (1) southward vs. double-sided PSCS subduction; (2) early Borneo counterclockwise rotations during the Oligocene to Early Miocene vs. later rotations (mid- to Late Eocene and Early Miocene); (3) a smaller Philippine Sea plate restored with a shorter ~1000 km northern slab vs. a longer >3000 km slab. This study assimilates four different plate models into the numerical model TERRA (Bunge et al., 1998). We digitally re-built in GPlates (Boyden et al., 2011) the implemented the plate models as a set of continuously closing plates in order to generate a global self-consistent velocity field to be assimilated into the convection models. The temperature fields were converted to seismic velocities assuming a Pyrolite composition and equilibrium mineralogy. We quantify the correlation between our geodynamic models and seismic tomography within SE Asia. For the tomography models S40RTS and LLNL-G3Dv-JPS we explicitly accounted for their finite resolution (Ritsema et al., 2011; Simmons et al. 2019).

How to cite: Lin, Y.-A., Colli, L., and Wu, J.: Where are the proto-South China Sea slabs? SE Asia plate tectonic and mantle flow insights from TERRA global mantle convection models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12407, https://doi.org/10.5194/egusphere-egu2020-12407, 2020.

The Caribbean region has been proposed as a candidate for outflow of asthenospheric mantle, from a shrinking Pacific region to an expanding Atlantic region. If this flow exists it should be associated to a dynamic topography gradient across the region. Estimating dynamic topography requires constraining the thicknesses and densities of sediment, crust and lithosphere to remove their isostatic response from the total topography. Dynamic topography has been studied globally in areas of ‘normal’ oceanic lithosphere but the Caribbean region, characterized by overthickened oceanic lithosphere, has not been fully analyzed due to the challenges of estimating crustal thicknesses.

Thanks to the wealth of seismic reflection, as well as borehole data, the basement relief and bulk sediment density in the Caribbean are well constrained. We performed a structural inversion of free air gravity anomalies, constrained by seismic refraction data, to established an improved Moho surface which provides more detail than existing global models such as Crust 1.0. With the improved basement and Moho relief, we computed residual basement depth. We obtained a ~300 m dynamic topography high on the Pacific-side of the Caribbean, gradually decaying to 0 m to the east near the Aves ridge.

This result supports the hypothesis of Pacific outflow through the Caribbean. Assuming a ~200 km thick asthenosphere and a flow velocity a few to a few tens of cm/yr, as suggested by tomographic imaging and regional magmatism, our results suggest the viscosity is ~5*10¹⁸ Pa s.

How to cite: Chen, Y.-W., Colli, L., Bird, D. E., Wu, J., and Zhu, H.: Asthenosphere viscosity in the Caribbean region constrained by gravity anomalies, seismic structure and regional magmatism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12682, https://doi.org/10.5194/egusphere-egu2020-12682, 2020.

Two dimensional numerical models of mantle convection in a cylindrical shell provide a possible geodynamic explanation for cold patches in the mantle below India and Mongolia as detected by seismic tomography. We investigate the influence of very high viscosities at mid-mantle and lower-mantle depths, as proposed by Mitrovica and Forte (2004) and Steinberger and Calderwood (2006), on mantle convective flow.  Models are considered with and without mineral phase transitions.  Our viscosity profiles are depth dependent with deep mantle viscosities increasing to values of 300 times the viscosity of the upper mantle, and then decreasing dramatically on approaching the core-mantle boundary.   The decrease of viscosity near the CMB mobilizes the overall mantle-wide flow despite very high mid-mantle viscosities.  However, cold detached slabs sinking below continental collisions become captured by the high viscosity interior and circulate slowly for times exceeding 200 Myr.  The separation of time scales for mantle-wide flow vs slab circulation, is a consequence of the high viscosity of the mid-mantle.

How to cite: Jarvis, G.: Slab Remnant Recycling and Mantle-Wide Convection: A Separation of Time Scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2032, https://doi.org/10.5194/egusphere-egu2020-2032, 2020.

Polarities of seismic reflection of P and S-waves at the discontinuity at the top of  D" are usually assumed to indicate the sign of the velocity contrast across the D" reflector. For reflections in paleo-subduction regions the S-wave reflections off D" (SdS) are the same as ScS and S, indicating a positive velocity contrast at the reflector. In recent years, an opposite polarity of PdP waves (P-reflection at the D" discontinuity) has been observed in some regions, partly dependent on travel direction, partly dependent on distance. This would indicate a velocity reduction in P-waves where a velocity increase is detected in S-waves. This phenomenon can be explained with the presence of post-perovskite below the top of D", but azimuthal dependence of PdP polarities can be better explained with anisotropy. Here we re-analyse PdP and SdS wave polarities and, when modelling the polarities and amplitudes using Zoeppritz equations, we find that a ratio of dVs/dVp= R of larger than 3 reverses polarities of P-waves in the absence of anisotropy, i.e. we find a polarity of PdP that would point to a velocity decrease while modelling a velocity increase. The S-polarity stays the same as S and ScS and does not change even with large R. Values of R up to 4.1 have been reported recently, so these cases do exist in the lower mantle. Using a set of 1 million models with varying minerals and processes across the boundary, we carry out a statistical analysis (Linear Discriminant Analysis, LDA) and find that there is a marked difference in mantle mineralogy to explain R values larger and smaller than 3, respectively. The regime of cases with R-value larger than 3 is mostly due to an increase in MgO and post-perovskite across the discontinuity. In regions where high R is observed, alternate explanations of lowermost mantle composition versus anisotropy can then be tested by measuring polarities in different azimuths.

How to cite: Thomas, C., Cobden, L., and Jonkers, A.: A new look at polarity information in D" reflections, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18402, https://doi.org/10.5194/egusphere-egu2020-18402, 2020.

The Saffman-Taylor instability, a fluid dynamical phenomenon, occurs when a less viscous
fluid is injected into a more viscous fluid, leading to the development of radial miscible viscous
fingers. Approximately five horizontal fingers radiate away from the Icelandic plume at a
depth range of 100 km–200 km. These fingers are manifest as shear wave velocity anomalies in
full-waveform tomographic models. The best resolved fingers lie beneath the British Isles and
beneath western Norway, extending ∼1,000 km away from the Icelandic plume conduit. The
number and wavelength of miscible viscous fingers are controlled by Péclet number (i.e. the
ratio of advective and diffusive transport rates), mobility ratio (i.e. the ratio of fluid viscosities)
and thickness of the horizontal layer into which the fluid is injected. Observational estimates
for the Icelandic plume suggest the Péclet number is O(10⁴), the mobility ratio is at least 20–
50 and the asthenospheric channel thickness is 100 ± 20 km. Appropriately scaled laboratory
experiments play a key role in developing a quantitative understanding of the spatial and
temporal evolution of the Icelandic plume planform. During laboratory experiments, Péclet
number is varied primarily by changing the flow rate as well as the altering the thickness of
the horizontal layer. Viscosity contrasts are generated by using glycerol and water mixtures
which are miscible, like plume material with ambient mantle. However there is no temperature
contrast in the experiments, which is probably significant in the mantle. Comparison between
scaled analogue experiments and observed values suggests the fluid dynamics may be more
complex than the Saffman-Taylor instability alone. Additional processes including influence
of temperature, interaction with the base of the lithospheric plate or small-scale convection,
along with the Saffman-Taylor instabillity, may be the origin of the fingers imaged by seismic
tomography.

How to cite: Galbraith-Olive, P. H., White, N., and Woods, A.: Radial Miscible Viscous Fingering of the Icelandic Plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-524, https://doi.org/10.5194/egusphere-egu2020-524, 2020.

Geodynamic reconstructions are largely based on information contained in mafic igneous rocks, including dykes and sills. The age and isotope-geochemical characteristics of such rocks are inevitable for understanding of geodynamic history of the Proterozoic cratons. The regions in Siberian Craton, where Precambrian mafic dyke swarms are known are following: Anabar Shield and Olenek Uplifts, Aldan-Stanovoi Shield, SE area of Siberian Craton, and smaller Uplifts on the SW margin of Siberian Craton.

The Udzha paleo-rift is located in the northern part of Siberian Craton between Anabar and Olenek Uplifts is also associated with mafic dyke swarm. These dykes cross-cut the pre-Neoproterozoic sedimentary successions. The age of the largest dyke in Udzha paleo-rift (Great Udzha Dyke) presented by medium-grained dolerite was determined to be 1386 ± 30 Ma (Malyshev et al., 2018).

We present new data of Sr, Nd and Pb isotopic composition on the Udzha paleo-rift dykes, determined by TIMS. The initial isotopic composition of Pb in the dykes was obtained using the leaching method by Savatenkov et al., 2019. The Sr isotopic composition of the dykes demonstrates substantial variation (εSr varies from 8.4 to 110.4). We do not consider this fact as a result of crust contamination, because Nd isotopic composition does not vary significantly (εNd varies from -1.4 to 0.7). Obtained results indicate that initial for the Udzha paleo-rift dykes melts were generated from two mantle reservoirs of DM and EMII-type. The initial Pb isotopic composition of the dykes reveals EMII source participation in the melts generation too (²⁰⁶Pb/²⁰⁴Pb varies from 16.133 to 16.266, ²⁰⁷Pb/²⁰⁴Pb varies from 15.343 to 15.458). The presence of enriched component is likely associated with lithospheric mantle, metasomatized by fluids, derived from subducted terrigenous material.

The studies were supported by the Russian Science Foundation project No. 19-77-10048.

References

Malyshev, S. V., Pasenko A. M., Ivanov A. V., Gladkochub D. P., Savatenkov V. M., Meffre S., Abersteiner A., Kamenetsky V. S. & Shcherbakov V. D. (2018): Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data. – Minerals, 8(12), 555

Savatenkov V. M., Malyshev, S. V., Ivanov A. V., Meffre S., Abersteiner A., Kamenetsky V. S., Pasenko A. M. (2019): An advanced stepwise leaching technique for derivation of initial lead isotope ratios in ancient mafic rocks: A case study of Mesoproterozoic intrusions from the Udzha paleo-rift, Siberian Craton. – Chemical Geology, 528, 119253

How to cite: Shpakovich, L., Malyshev, S., and Savatenkov, V.: Pb, Nd, Sr isotopic composition of the Mesoproterozoic mafic intrusions (Udzha paleorift, Northern Siberia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11081, https://doi.org/10.5194/egusphere-egu2020-11081, 2020.

Elucidating the role of deep mantle plumes in mantle convection is challenging because their influence on seismic waveforms – which could be used to map their location – is subtle. Previous seismic studies have mainly focused on waveform modelling and inversion (i.e. tomography). In this study we instead consider the potential visibility of mantle plumes using array methods. We investigate, in particular, how plumes deviate seismic energy from the great-circle path. This requires a multidisciplinary approach: first, we perform geodynamic modelling to generate thermochemical plumes, and convert them to “seismic” plumes via thermodynamic modelling of mineral physics data. Next, spectral element methods are used to model the interaction of seismic waves with the plumes and generate synthetic seismograms. These seismograms are divided into arrays and we generate slowness-backazimuth plots for each array. With recent advances in computational methods and resources, we investigate wave behaviour at previously unattainable frequencies.  We find that plumes do indeed cause seismic waves to change direction, although the exact behaviour may be frequency-dependent, and at low frequencies we observe waves apparently bending around the plume conduit.  We consider how and where these results may be applied to real seismic arrays, to provide new constraints on the location and structure of mantle plumes.

How to cite: Cobden, L., Afanasiev, M., Deschamps, F., Stockmann, F., Thomas, C., Rost, S., and Fichtner, A.: Probing mantle plumes using seismic arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7491, https://doi.org/10.5194/egusphere-egu2020-7491, 2020.

A clear understanding of the transition from a liquid magma ocean (MO) to a convective solid mantle is still lacking. Part of the problem is that there is still no clear view of all the physical phenomena at play during this crucial stage. As the MO cools down, the formation of a solid and therefore very viscous lithosphere at its surface has often been considered to trigger a new pattern of motion where convection occurs below the lithosphere which remains stagnant. However, when the liquid thermal boundary layer at the top of the MO cools down, it first becomes a mushy lithosphere through which melt and exsolved gas bubbles can still percolate to the surface. Using laboratory experiments of thermal convection in colloidal suspensions, we study the formation of this mushy lithosphere and its different regimes of deformation and coupling to mantle convection. We observe that deformation of the lithosphere can include « heat pipe » formation at high heat, melt and volatile flux. On the other hand, rapid thermal contraction of the lithosphere can cause buckling, leading to subduction. Transition from MO to solid-state convection could involve both processes in succession , or in competition, depending on the temperature and volatiles conditions. 

How to cite: Davaille, A. and Massol, H.: From Magma Ocean turbulent convection to lithosphere formation and mantle convection: insights from laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10746, https://doi.org/10.5194/egusphere-egu2020-10746, 2020.

The Middle-Late Permian Emeishan large igneous province (ELIP), located in the western margin of Yangtze craton, SW China, is regarded as the result of the impingement of a mantle plume onto the lithosphere. However, little is known about the petrogenesis of Late Permian basalts in Sichuan Basin, which were previously considered to be located outside the ELIP. Here we report new petrographic, major elements, trace elements and isotopic data (Sr-Nd-Pb) for Late Permian basalts in the boreholes from the northwestern Sichuan Basin. These basaltic rocks are characterized by low SiO₂ contents (47.17-49.40 wt.%), high TiO₂ contents (3.38-4.11 wt.%) and Ti/Y ratio (539-639), moderate total alkalis contents (Na₂O+K₂O, 3.36-6.01 wt.%) and Mg^# values (40.93-46.04), which geochemically resemble the Emeishan high-Ti basalts. These rocks are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREE), and have (La/Yb)_N ranging from 9.95 to 11.78, showing that typical oceanic island basalt (OIB)-like normalized patterns. The fractionation of MREE to HREE suggests that the basalts were generated by low degree of partial melting within the garnet stability field. Low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70572-0.70676; t=260 Ma), Pb isotopic ratios [²⁰⁶Pb/²⁰⁴Pb_(t) (18.062-18.637), ²⁰⁷Pb/²⁰⁴Pb_(t) (15.574-15.641), ²⁰⁸Pb/²⁰⁴Pb_(t) (38.33-38.98)], and slightly high εNd(t) values (-0.03 to +1.34) indicate that the magma formed from a deep mantle source that may possibly be a mantle plume and have negligibly been affected by crustal contamination. This inference is further supported by high Nb/U ratios (20.56-25.70), low Th/Nb (0.17-0.19) and Th/Ta ratios (2.77-3.14), and no visible Nb and Ta anomalie. In addition, thermal history reconstruction using paleogeothermal indicators in the study area shows that the Lower Paleozoic to Middle Permian formations experienced an intensive thermal event and abnormal high heat flow value reached 118.0 mW/m² at the Late Permian, which may be due to the mantle plume magma upwelling. The geochemical and geothermal characteristics all demonstrate that these basalts were probably generated in response to the Emeishan mantle plume. Thus, we conclude that the ELIP may have larger areal extent and has been played an important role on the thermal evolution of source rocks in the Sichuan basin.

How to cite: Liu, X. and Qiu, N.: Late Permian basalts in the northwestern Sichuan Basin, SW China: Implications for the geodynamics and thermal effect of the Emeishan mantle plume, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6821, https://doi.org/10.5194/egusphere-egu2020-6821, 2020.

How to cite: Dongmo wamba, M., Romanowicz, B., Montagner, J.-P., and Barruol, G.: A narrow plume conduit anchored in the lower mantle beneath la Réunion hotspot, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9135, https://doi.org/10.5194/egusphere-egu2020-9135, 2020.

How to cite: Mandea, M., Dehant, V., and Cazenave, A.: GRACEFUL: Probing the deep Earth interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13515, https://doi.org/10.5194/egusphere-egu2020-13515, 2020.

Radio signals from distant quasars allow us to determine Earth's rotation variations with exquisite accuracy. These observations can be used to estimate the amplitudes, frequencies and damping constants associated with Earth's rotational modes, particularly the Free Core Nutation (FCN) and the Free Inner Core Nutation (FICN). These estimates suggest, however, fluid core viscosities many orders of magnitude higher than expected, or rms magnetic fields at the core-mantle boundary (CMB) incompatible with downward continuation of the observed surface field. Aiming at resolve this difficulty, we have developed a proof-of-concept model where we incorporate an approximate fluid-dynamical treatment of the core flow associated with the FCN and the FICN. We show that, at least for the FCN, no abnormally high viscosities or magnetic fields are required. The model might provide in fact a robust, independent estimate of the rms magnetic field strength in the fluid core. Additionally, the model illustrates the importance of considering inter-mode resonances involving inertial modes (i.e. Coriolis-restored) and the rotational normal modes.

How to cite: Triana, S., Trinh, A., Rekier, J., and Dehant, V.: Ohmic dissipation induced by Earth's nutation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11740, https://doi.org/10.5194/egusphere-egu2020-11740, 2020.

Nonlinear analysis in a rotating Rayleigh-Benard system of electrically conducting fluid is studied numerically in the presence of externally applied horizontal magnetic field with rigid-rigid boundary conditions [1, 2]. This DNS approach is carried near the onset of convection to study the flow behaviour in the limiting case of Prandtl number [2]. The flow topology is verified with respect to the Euler number. The fluid flow is visualized in terms of streamlines, limiting streamlines, isotherms and heatlines. The dependence of the Nusselt number on the Rayleigh number, Ekman number, Elsasser number is examined.

References:

[1] S. Chandrasekhar, Hydrodynamic and Hydromagnetic Stability, 1961, Oxford University Press, London.

[2] P.H. Roberts and C.A. Jones, The onset of magnetoconvection at large Prandtl number in a rotating layer I. Finite Magnetic Diffusion, Geophysical and Astrophysical Fluid Dynamics, 92, 289-325 (2000).

How to cite: Rani, H. P., Rameshwar, Y., Brestensky, J., and Filippi, E.: Nonlinear Convection of Electrically Conducting Fluid in a Rotating Magnetic System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21322, https://doi.org/10.5194/egusphere-egu2020-21322, 2020.

The magnetohydrodynamics of Earth has been explored at the University of Maryland through experiments and numerical models. Experimentally, the interaction between Earth's magnetic fields and its outer core is replicated using a three-meter spherical Couette device filled with liquid sodium that is driven by two independently rotating concentric shells and a dipole magnetic field applied from external electromagnets. Currently, this experiment is being prepared for design modifications that aim to increase the helical flows in the poloidal direction in order to match the turbulence of convection-driven flows of Earth. The experiment currently has 33 hall probes measuring the magnetic field, 4 pressure probes, and torque measurements on each sphere. We supplement the experiment with a numerical model, XSHELLS, that uses pseudospectral and finite difference methods to give a full picture of the velocity and magnetic field in the liquid and stainless steel shells. However, its impracticable to resolve all the turbulence. Our ultimate goal is to implement data assimilation by synchronizing the experimental observations with the numerical model, in order to uncover the unmeasured velocity field in the experiment and the full magnetic field as well as to predict the magnetic fields of the experiment. Through numerical simulations (XSHELLS) and data analysis we probe the behavior of the experiment in order to (i) suggest the best locations for new measurements and (ii) find what parameters are most feasible for data assimilation. These computational studies provide insight on the dynamics of this experiment and the measurements required to predict Earth's magnetic field. We gratefully acknowledge the support of NSF Grant No. EAR1417148 & DGE1322106.

How to cite: Burnett, S., Schaeffer, N., Ide, K., and Lathrop, D.: Exploring Alternative Instrumentation in the Three-Meter Spherical Couette Experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10723, https://doi.org/10.5194/egusphere-egu2020-10723, 2020.

We have studied theoretically the weakly nonlinear analysis in a rotating Rayleigh-Benard system of electrically conducting fluid in the presence of applied horizontal magnetic field with free-free boundary conditions [1]. This theoretical approach is carried near the onset of convection to study the flow behavior at the occurrence of cross rolls, which are perpendicular to the applied magnetic field. The nonlinear problem is solved by using the Fourier analysis of perturbations up to the O(ε⁸) to study the cross rolls visualization [2,3]. The dependence of the Nusselt number on the Rayleigh number, Ekman number, Elsasser number is extensively examined. The fluid flow is visualized in terms of kinetic energy, potential energy, streamlines, isotherms, and heatlines.

References :

[1] P. H. Roberts and C. A. Jones , The Onset of Magnetoconvection at Large Prandtl Number in a Rotating Layer I. Finite Magnetic Diffusion, Geophysical and Astrophysical Fluid Dynamics, Vol. 92, pp. 289-325 (2000).

[2] H.L. Kuo, Solution of the non-linear equations of the cellular convection and heat transport,  Journal of Fluid Mechanics,  Vol.10, pp.611-630 (1961).

[3] Y. Rameshwar, M. A. Rawoof Sayeed, H. P. Rani, D. Laroze, Finite amplitude cellular convection under the influence of a vertical magnetic field, International Journal of Heat and Mass Transfer, Vol. 114, pp.  559-577 (2017).

How to cite: Rameshwar, Y., Srinivas, G., Rani, H. P., Brestensky, J., and Filippi, E.: Convection of Electrically Conducting Fluid in a Rotating Magnetic System: Cross rolls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21632, https://doi.org/10.5194/egusphere-egu2020-21632, 2020.

The Earth is submitted to the gravitational effect of different objects,  resulting  in  small  variations  of  the  orientation  of  its  axis  rotation.   The  precession corresponds to the rotation of the body spin axis around the normal to the elliptic plane. The primary flow forced by precession in a sphere is mainly a tilted solid body rotation, a flow of uniform vorticity. In this study we focused on the pseudo-resonance between the precessional forcing  and  the  spin-over  mode,  detected  as  a  peak  of  amplitude  of  the  norm  of the  vorticity  of  the  fluid.   We  show  the  influences  of  both  the  geometry and the application of an uniform external magnetic field on the external boundary, onto this pseudo-resonance.  The major purpose is to validate a semi-analytical model to allow its interpolation to planetary bodies.  We compared the semi-analytical model [Noir and C ́ebron, 2013] with numerical simulations performed with XSHELLS [Schaeffer, 2013],  which give us the components of the fluid vorticity in a precessing frame. We compared also the spin-over mode coefficients, used to simulate the viscous  effect  on  the  model,  with  two  methods :  an  empirical  equation  and  the numerical solver Tintin [Triana et al., 2019], taking into account the solid inner-core size (η=RI/R).  The differential rotation between the flow and the container, obtained with the model and the XSHELLS simulations, show us a verygood agreement especially for a small Ekman number (E= 10^−5), thus the spin-over mode coefficients for small E and η≤0.5.  An increase of the inner-core size  implies  a  decrease  of  the  resonance  amplitude  caused  by  the  supplementary Ekman layer added at the Inner Core Boundary (ICB); nevertheless thecolatitude (αf) and the longitude (φf) of the fluid don’t change significantly.The  application  of  a  uniform  magnetic  field  at  the  CMB  implies  a  decrease of the resonance amplitude, but also a modification of the mean rotation axis direction.  Indeed, the coupling between the viscous flow and the magnetic field induces a modification of the αfand φf, which follow the main direction angle of the magnetic field axis.  We observe small discrepancies between the simulations (XSHELLS and Tintin) and the model but the behavior following different parameters (Po,α angle,Ro,η,β angle, Λ) is well understood.  As a result, we applied the models at few parameter ”realistic values” of planetary objects like terrestrial planets but also ice’s satellites.

References

[Noir and C ́ebron, 2013]  Noir,  J.  and  C ́ebron,  D.  (2013).    Precession-driven flows in non-axisymmetric ellipsoids.Journal of Fluid Mechanics, 737:412–439.

[Schaeffer, 2013]  Schaeffer, N. (2013).  Efficient spherical harmonic transforms aimed  at pseudospectral numerical  simulations.Geochemistry, Geophysics,Geosystems, 14(3):751–758.

[Triana et al., 2019]  Triana, S. A., Rekier, J., Trinh, A., and Dehant, V. (2019).The coupling between inertial and rotational eigenmodes in planets with liq-uid cores.Geophysical Journal International.

How to cite: Laguerre, R., Houliez, A., Cébron, D., and Dehant, V.: Steady flows in the core of precessing planets : effects of the geometry and an applied magnetic field. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14058, https://doi.org/10.5194/egusphere-egu2020-14058, 2020.

The release of latent heat and lighter materials during inner core solidification is the driving force of the liquid iron flow in the outer core which generates the Earth's magnetic field. It is well known that the behaviour of the magnetic field varies over long time scales. Two clearly identifiable regimes are recognized, (i) superchrons and (ii) periods of hyperactivity (Biggin et al. 2012). Superchrons are characterized by an exceptionally low reversal rate of the magnetic pole and are associated with a low core mantle boundary (CMB) heat flux. Hyperactive periods are defined by a high reversal rate and have a high CMB heat flux.

Here we investigate whether the occurrence of these two regimes is related to radial variations in inner core seismic structure. Using seismic body-wave observations of compressional PKIKP-waves (Irving & Deuss 2011, Waszek & Deuss 2011, Lythgoe et al. 2013)., we construct a model of inner core anisotropy by comparing the difference between travel times for polar and equatorial rays. Anisotropy is the directional dependence of wave velocity and is determined by the structure of iron crystals in the inner core, hence changes in seismic anisotropy are due to changes in inner core crystal texture. We invert for radial changes in anisotropy while allowing for lateral variations and find that a model of the inner core containing five layers best fits our data. The model contains an isotropic uppermost inner core and four deeper layers with varying degrees of anisotropy.

Texture differences of the inner core iron crystals have been linked to changes in the solidification process of the inner core (Bergman et al. 2005), i.e. the motor of outer core flow. Therefore, the observed anisotropy variation, caused by variations of inner core solidification, might be related to changes in the behaviour of the magnetic field. Using an inner core growth model (Buffett et al. 1996) we convert depth to time for a range of inner core nucleation ages between 3.0 and 0.5 Ga (Olsen 2016). We find a remarkable correlation between the solidification time of the seismically observed layers and the occurrence of the magnetic regimes for two inner core ages; one with a nucleation at 1.4 Ga and one at 0.6 Ga, corresponding to an average CMB heat flux of 7.6 TW and 16.7 TW respectively.

Although we currently cannot differentiate between these two inner core ages considering our results alone, they do show that a relation between inner core structure and the behaviour of the magnetic field is possible, and suggest that seismic observations of inner core structure might provide new and independent insights into the magnetic field and its history.

How to cite: de Jong, J., de Groot, L., and Deuss, A.: Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20504, https://doi.org/10.5194/egusphere-egu2020-20504, 2020.

Determining the high pressure and temperature behavior of iron (Fe) provides valuable insight into the evolution and dynamics of the Earth’s core. Shock compression using lasers can achieve extreme pressure and temperature conditions simultaneously. The duration of the extreme conditions state is on the order of nanoseconds. This is a challenge for in situ measurements of the shocked material’s properties. In this work, we shock-compressed polycrystalline iron at the Matter in Extreme Conditions End Station at the Linac Coherent Light Source, SLAC National Accelerator Laboratory and performed in situ X-ray diffraction (XRD) measurements with sub-picosecond time resolution. The final aim of these experiments is the study of stress of texure in Fe under extreme conditions of pressure and temperature. The presentation will highlight the strategies for such experiment and data processing and present our premilinary results.

How to cite: Merkel, S., Hok, S., Bolme, C., Gleason, A., and Mao, W.: Understanding strength and texture in Fe at planetary core pressures and temperatures: insights from laser compression experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22379, https://doi.org/10.5194/egusphere-egu2020-22379, 2020.

We used more than one decade of core-refracted teleseismic shear (SKS) waveforms recorded at more than 160 broadband seismic stations across the Iranian plateau and Zagros to investigate seismic anisotropy beneath the region. Splitting analysis of SKS waveforms provides two main parameters, i.e., fast polarization direction and split delay time, which serve as proxies for the trend and strength of seismic anisotropy beneath the stations. Our observation revealed a complex pattern of splitting parameters with variations in the trend and strength of anisotropy across the tectonic boundaries. We also verified the presence of multiple layers of anisotropy in conjunction with the lithosphere deformation and mantle flow field. Our observation and modeling imply that a combined system of lithosphere deformation and asthenospheric flow is likely responsible for the observed pattern of anisotropy across the Iranian Plateau and Zagros. The rotational pattern of the fast polarization directions observed locally in Central Zagros may indicate the diversion of mantle flow around a continental keel beneath the Zagros. The correlation between the variation in lithosphere thickness and the trend of anisotropy in the study area implies that the topography of the base of lithosphere is also a determining factor for the pattern of mantle flow inferred from the observations.

How to cite: Kaviani, A., Mahmoodabadi, M., Rümpker, G., Yamini-Fard, F., Tatar, M., Moradi, A., and Nilfouroushan, F.: SKS splitting observations across the Iranian plateau and Zagros: the role of lithosphere deformation and mantle flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8071, https://doi.org/10.5194/egusphere-egu2020-8071, 2020.

Constraining the seismic structure of the West Antarctic mantle is important for understanding its viscosity structure, and thus for accurately predicting the evolution of the West Antarctic Ice Sheet.  Seismic anisotropy, which is the dependence of seismic velocities on the propagation and polarization direction of seismic waves, is a valuable tool for understanding mantle deformation and flow.  We provide petrological and microstructural data from a suite of 44 spinel peridotite xenoliths entrained in Cenozoic (1.4 Ma) basalts of 7 volcanic centers located in Marie Byrd Land, West Antarctica.  Equilibration temperatures obtained from three different calibrations of the two-pyroxene geothermometer and the olivine-spinel Fe-Mg exchange geothermometer range from 780°C to 1200°C, calculated at a pressure of 1500 MPa.  This range of temperatures corresponds to extraction depths between 39 and 72 km, constraining the source of the xenoliths within the lithospheric mantle above the low velocity zone modelled by seismic studies.

The Marie Byrd Land xenoliths are fertile with average clinopyroxene mode that ranges between 15 and 24%.  Based on their modal composition, xenoliths are predominantly classified as lherzolites (n=30), with lesser occurrences of harzburgite (n=4), wehrlite (n=3), dunite (n=3), olivine websterite (n=1), websterite (n=1), and clinopyroxenite (n=2).  Petrological data suggest that the xenoliths have been affected by various degrees of partial melting as well as by reaction with silicate melts or fluids.  For example, clinopyroxenes in the more fertile lherzolites and wehrlites show a constant TiO₂ concentration at 0.65 wt% and 0.8 wt% over a range of olivine Mg# values, while TiO₂ decreases rapidly with increasing Mg#, down to 0.01 wt% in the more refractory harzburgites and dunites.  The observed trend is interpreted to indicate a refertilization process.  Microstructures also indicate multiple episodes of reactive melt percolation under either static conditions or during the late stages of deformation.  Pyroxenes may enclose rounded olivine grains in crystallographic continuity with neighbouring grains, cross-cut the subgrain boundaries of olivine grains, or show an interstitial habit, either forming cuspate-shaped grains in olivine triple junctions or films along olivine-olivine grain boundaries.  Olivine shows a range of crystallographic preferred orientation (CPO) patterns, including the A-type, axial-[010], axial-[100], and B-type.  Pyroxenes have weaker but not random CPOs with [001] axes having similar orientation to olivine [100] axes in the majority of the xenoliths.  Calculated P and S waves anisotropy is variable (2–12%) and increases with olivine fraction but decreases with both increasing ortho- or clinopyroxene content.  P-wave anisotropy is correlated with the strength of olivine CPO expressed with the M-index and increases with increasing strength of the orthopyroxene CPO, but seems to be less correlated with the strength of the clinopyroxene CPO.

How to cite: Kruckenberg, S. and Chatzaras, V.: Seismic anisotropy of the lithospheric mantle beneath Marie Byrd Land, West Antarctica: Constraints from peridotite xenoliths, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11597, https://doi.org/10.5194/egusphere-egu2020-11597, 2020.

The Australian plate is composed of tectonic features showing progression of the age from dominantly Phanerozoic in the east, Proterozoic in the centre, and Archean in the west. These tectonic structures have been investigated in the last three decades using a variety of geophysical methods, but it is still a matter of debates of how temperature and strength are distributed within the lithosphere. We construct a thermal crustal model assuming steady state variations and using surface heat flow data, provided by regional and global database, and heat generation values, calculated from existing empirical relations with seismic velocity variations, which are provided by AusREM seismic tomography model. The lowest crustal temperatures are observed in the eastern part of the WAC and the Officer basin, while Central and South Australia are regions with anomalously elevated heat flow values and temperatures caused by high heat production in the crustal rocks. On the other hand, the mantle temperatures, estimated in a previous study, applying a joint interpretation of the seismic tomography and gravity data, show that the Precambrian West and North Australian Craton (WAC and NAC) are characterized by thick and relatively cold lithosphere that has depleted composition (Mg# > 90). The depletion is stronger in the older WAC than the younger NAC. Substantially hotter and less dense lithosphere is seen fringing the eastern and southeastern margin of the continent. Both crustal and mantle thermal models are used as input for the lithospheric strength calculation. Another input parameter is the crustal rheology, which has been determined based on the seismic velocity distribution, assuming that low (high) velocities reflect more sialic (mafic) compositions and thus weaker (stiffer) rheologies. Furthermore, we use strain rate values obtained from a global mantle flow model constrained by seismic and gravity data. The combination of the values of the different parameters produce a large variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. The sharp lateral strength variations are coincident with intraplate earthquakes location. The strength variations in the crust and upper mantle is also not uniformly distributed: In the Archean WAC most of the strength is concentrated in the mantle, while the Proterozoic Officer basin shows the largest values of the crustal strength. On the other hand, the younger eastern terranes are uniformly weak, due to the high temperatures.

How to cite: Tesauro, M., Kaban, M., Petrunin, A., and Aitken, A.: Temperature, strain rates, and rheology: the key parameters controling strength variations in the Australian lithosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5663, https://doi.org/10.5194/egusphere-egu2020-5663, 2020.

The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation φ and delay time Δt, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations φ show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters φ and Δt, and in the backazimuthal variation of δφ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.

How to cite: Löberich, E. and Bokelmann, G.: Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7450, https://doi.org/10.5194/egusphere-egu2020-7450, 2020.

The passive seismic experiment AniMaLS was organized in 2017 in the Sudetes in Poland. One of the objectives was to study the anisotropy of the sub-crustal lithosphere and asthenosphere beneath the NE termination of the Bohemian Massif. Temporary seismic network of 23 broadband stations was operating in the area of Sudetes mountains and Fore-Sudetic Block, covering a ~200 x 100 km large area, with ~30 km spacing between stations. Obtained recordings were supplemented with data from permanent stations of Czech and Polish seismological networks located in the study area.

The Sudetes belong to internal zone of Variscan Orogen and are located in the NE part of the Bohemian Massif, between the Elbe Fault in SW and the Odra Fault in NE. The sudetic lithosphere represents a complex mosaic of several units with distinct histories of tectonic evolution and with consolidation ages ranging from the upper Proterozoic to the Quaternary. The aim of the project is to study seismic structure and anisotropy of the lithosphere-asthenosphere system based on broadband seismograms of local, regional and teleseismic events. The obtained data will be analysed using several interpretation methods. The poster presents the results of analysis by shear wave splitting method.

The analysis was done based on SKS and SKKS phases recorded during a ~2 years observation period. For analysis, three single-station methods were used: cross-correlation, eigenvalue minimization and transverse energy minimization. The dependence of resulting splitting parameters on the backazimuth of the event was also analysed. The results show that time delays between slow and fast S-wave components are typically in the range of ~0.5-1.6 sec, with average 1.2 sec. The splitting is interpreted as a result of lattice-preferred orientation (LPO) of mantle olivine. The azimuths of fast velocity axis are mostly consistent and showed largely WNW-ESE direction. They correlate well with trends of tectonic units observed at the surface and with strike directions of major fault zones. This suggests vertically coherent deformation throughout the lithosphere and frozen-in LPO, reflecting last tectonic episode which shaped Sudetic area. Obtained results were also compared with previous seismic studies of the upper mantle anisotropy in the neighboring areas by various methods.

How to cite: Rewers, J., Środa, P., and Working Group, A.: Upper mantle anisotropy beneath Sudetes from shear wave splitting - passive seismic experiment AniMaLS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3850, https://doi.org/10.5194/egusphere-egu2020-3850, 2020.

The Ontong Java Plateau (OJP), one of the largest oceanic plateaus located in the western Pacific Ocean, was first formed by a massive volcanism at 122 Ma, which had a major effect on the Earth's environments, including global climate change, oceanic anoxic events, and mass extinction of marine life. However, the cause of the volcanism remainscontroversial since the underground structure beneath the OJP has been poorly understood due to limited geophysical and geochemical data. To improve such situation, we conducted about 1.6-year long-term seafloor observation on the OJP and its vicinity. Using seismograms obtained by this observation as well as those from existing seismic stations, we obtained three dimensional radially anisotropic shear wave velocity structure beneath the OJP at depths down to 300 km.

Obtained structure shows the following new features:

(1) Beneath the Caroline Islands, in the north of the OJP, 1 % slow anomalies exist, which may be associated with the Caroline hotspot activity;

(2) In the center of the OJP at depths between 70–130 km, about 2% fast anomalies, whose shear wave speed is about 4.45-4.55 km/s, exists.

(3) The seismic structure clearly shows that the lithosphere–asthenosphere boundary (LAB) beneath the center of the OJP is located about 40 km deeper than that beneath the surrounding normal oceanic seafloor.

Judging from our results and petrological/rheological constraints given by previous studies, we interpret that the LAB is deepened by dehydrated residual material from hot mantle plume underplating a pre-existing lithosphere during a formation of OJP.

How to cite: Isse, T., Suetsugu, D., Ishikawa, A., Shiobara, H., Sugioka, H., Ito, A., Kawano, Y., Yoshizawa, K., Ishihara, Y., Tanaka, S., Obayashi, M., Tonegawa, T., and Yoshimitsu, J.: Seismic evidence for residual mantle underplating the lithosphere beneath the Ontong Java Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6480, https://doi.org/10.5194/egusphere-egu2020-6480, 2020.

The deformation of Qilian Orogenic Belt, which is the uplifting front of the northeastern Tibet Plateau, plays a decisive role in understanding the dynamic process of the area uplift. Many of the tectonic processes models of the Tibetan Plateau growth, which are based on geophysical and geological studies, have been conducted in recent years. However, the deformation mode of northeastern Tibetan Plateau (NETP) remains controversial for lack of sufficient proofs. We used teleseismic waveform data collected from the China Array seismic experiment during 2013-2015 and QL temporary stations during 2016-2017. In this study, we used the 3-D Common Conversion Point (CCP) technique (with the P/S receiver functions) to obtain detailed seismic velocity discontinuities structure of lithosphere beneath the NETP and Alxa block. Our preliminary results can be summarized as follows: 1) The Lithosphere asthenosphere boundary (LAB) lies at a depth pf 110-140 km in Alxa platform, deepens below the North Qilian mountain (160-170 km ) which has been inserted by lithosphere of Central Qilian, between the South Qilian suture zone (SQL) and the north of the Songpan-Ganzi Terranes (160-170 km). 2) The main features in the crust include offset of Moho beneath NQLF, shallower crust thickness below between the NQLF and LSSF and a continuous positive interface over the Moho in the north of the LSSF. 3) According to our observation and previous studies, we suppose that lithosphere had been passive underthrust and localized crust had been shortened and thickened in the NETP.

How to cite: chen, Y. and chen, J.: Deep deformation process of the NE Tibetan Plateau: evidence from receiver function imaging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7581, https://doi.org/10.5194/egusphere-egu2020-7581, 2020.

The focus of this study is the mantle structure beneath the Apennines, and aims to understanding how deep processes are connected to shallow deformations. We present new observations from a rich receiver function data set from stations located along the North and Central Apennine chain, and use it for comparison and to strengthen the observations of previous seismic tomography images. The two methodologies define a low shear wave velocity zone (decrease of Vs in the order of 5%) and an increase of Vp/Vs (about 3%) in the shallow mantle between 50 and 90 km depth beneath the orogenic belt. The low Vs melt zone is not restricted to the mantle beneath the Quaternary volcanic areas, as previously thought, but is detected under the whole central Apennines suggesting future broad effects on a large scale.  Our interpretation of the teleseismic RFs and tomography, reveals consistently a diffuse mantle upwelling beneath the Apennines, and we hypothesize that slab-derived fluids might interact with the sub-lithospheric mantle generating melts that accumulate at the top of the mantle feeding post-collisional extension. This mechanism can be potentially applied to other cases of extension that spread over wide continental regions.

How to cite: Bianchi, I., Chiarabba, C., De Gori, P., and Piana Agostinetti, N.: Diffuse mantle upwelling and melts accumulation beneath the Italian Apennines., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11320, https://doi.org/10.5194/egusphere-egu2020-11320, 2020.

In this study, we show results from ambient noise tomography at the KTB drilling site, Germany. The Continental Deep Drilling Project, or ‘Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland’ (KTB) is at the northwestern edge of the Bohemian Massif and is located on the Variscan belt of Europe. During the KTB project crustal rocks have been drilled down to 9 km depth and several active seismic studies have been performed in the surrounding. The KTB area therefore presents an ideal test area for testing and verifying the potential resolution of passive seismic techniques. The aim of this study is to present a new shear-wave velocity model of the area while comparing the results to the previous velocity models and hints for anisotropy depicted by former passive and active seismological studies. We use a unique data set composed of two years of continuous data recorded at nine 3-component temporary stations installed from July 2012 to July 2014 located on top and vicinity of the drilling site. Moreover, we included a number of permanent stations in the region in order to improve the path coverage and density. Cross correlations of ambient noise are computed between the station pairs using all possible combination of three-component data. Dispersion curves of surface waves are extracted and are then inverted to obtain group velocity maps. We present here a new velocity model of the upper crust of the area, which shows velocity variations at short scales that correlate well with geology in the region.

How to cite: Qorbani, E., Bianchi, I., Kolínský, P., Zigone, D., and Bokelmann, G.: Upper crustal structure at the KTB drilling site from ambient noise tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6842, https://doi.org/10.5194/egusphere-egu2020-6842, 2020.

Detailed knowledge of the seismic structure, fabric, and dynamics that surround the oceanic LAB continue to be refined through offshore seismic studies. Previous high-resolution studies in the Pacific basin far from plate boundaries show asthenospheric fabric that aligns neither with the lithospheric fabric (the paleo-spreading direction) nor with absolute plate motion, but rather in between. Here we present preliminary results from the Blanco Transform and Cascadia Initiative experiments, investigating the structure of the Juan de Fuca and Pacific plates on either side of the Blanco Transform. We measure ambient-noise and teleseismic Rayleigh-wave phase velocities, and solve for the period-dependent azimuthal anisotropy on either side of the transform. We will contextualize and interpret the fabrics based on mantle flow inferred from these previous Pacific basin studies. 

How to cite: Hawley, W. and Gaherty, J.: High-resolution constraints on LAB structure at the Blanco transform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12595, https://doi.org/10.5194/egusphere-egu2020-12595, 2020.

    There exists an important polymetallic ore belt in Nanling of the southeastern China. Previous studies suggest that the mineralization of Nanling is probably related to the bottom intrusion of magmatic rocks in the late Mesozoic. In this study, a natural seismic section was installed by using 81 portable stations with an interval of 5 km from July 2017 to August 2019, which runs across the Nanling belt in the south of Fujian and Jiangxi provinces. As a result, we have picked up 3,818 relative residual data from 215 teleseismic events with magnitude greater than 5.5. And we have applied the teleseismic full-waveform tomography and the teleseismic travel-time tomography to study the crust and the mantle velocity structure beneath the Nanling metallogenic belt, respectively. Our preliminary results show that: (1) a clear low-velocity anomaly exists in the crust beneath the Zhenghe-Dapu fault and its east side, which might be related to the rich ore deposits in Nanling; (2) some high-velocity anomalies in the uppermost mantle beneath the Wuyi metallogenic belt may be relevant to the igneous rock cooling and the lithospheric thickening; (3) there are obvious low-velocity anomalies at the upper mantle beneath the Wuyi and Nanling metallogenic belts, which are speculated to be hot materials from asthenosphere upwelling into the bottom of the lithosphere. Our results provide a new insight for investigating the deep structures and deep dynamic processes of Nanling tectonic belt.

How to cite: xi, J., jiang, G., zhang, G., and he, X.: 2D crust-upper mantle velocity structure along a seismic section in Nanling, South China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6416, https://doi.org/10.5194/egusphere-egu2020-6416, 2020.

We calculated the thermal lithosphere structure of Tibet and adjacent regions based on the new thermal isostasy method. Moho depth is constrained by the published receiver function results. The calculated surface heat flow in the surrounded Tarim, North China, and Yangtze cratons have a good match with the real measurements of surface heat flow. We recognize the northern Tibet anomaly where has a relatively thin lithosphere with a thermal thickness of <80 km and surface heat flow of >80 - 100 mW/m 2 may cause by the removal of lithospheric mantle and upwelling of asthenosphere. In Lhasa Block, the cold and thick lithosphere (>200 km) with a surface heat flow of 40 - 50 mW/m 2. In the east Tibet, the heterogeneous thermal lithosphere does not follow the widely spread large scale strike-slip faults and suggested that the faults do not cut down to the lithosphere. The surrounding cratons have different thermal lithosphere features. The Tarim and Yangtze cratons show typical cold and thick lithosphere with a lithosphere of >200km and surface heat flow of <50 mW/m2. The western North China Craton has an intermated lithosphere with a thickness of 120-200km and surface heat flow of 45-60 mW/m2. Our result suggested that high and flat Tibet has different isostatic compensation in different blocks. The heterogeneous lithosphere thermal structure of the Tibet suggested that the uplife force drive are difference in Tibet.

How to cite: Xia, B., Artemieva, I., and Thybo, H.: The Tibet lithosphere is not all hot, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2181, https://doi.org/10.5194/egusphere-egu2020-2181, 2020.

In this work, we studied the mantle flow around cratonic keels using numerical models to simulate the thermochemical convection in the terrestrial mantle taking into account the relative displacement between the lithosphere and asthenosphere. The numerical simulations were performed using the finite element code developed by Sacek (2017) to solve the Stokes Flow for an incompressible Newtonian fluid. Several synthetic models in 2D and 3D were constructed considering different keel geometries and different regimes of relative displacement between the lithosphere and asthenosphere. In the present numerical experiments, we adopted a rheology in which the viscosity of the mantle is controlled by temperature, pressure and composition, assuming that the cratonic keel is compositionally more viscous than the surrounding asthenosphere, using a factor f to rescale the lithospheric viscosity compared to the asthenospheric one. We tested different f values, reference viscosity for the asthenosphere, and relative velocity between the lithosphere and the base of the upper mantle, quantifying the amount of deformation of the cratonic keel in each scenario. In general, we conclude that for a relatively low compositional factor (f < 20), the lithospheric keel can be significantly deformed in a time interval of few tens of million years when the lithosphere is moving horizontally relative to the base of the upper mantle, does not preserving its initial geometry. The synthetic models can be helpful for a better understanding of the interaction in the lithosphere-asthenosphere interface such as the deformation and flow patterns in the mantle around the keels, the rate of erosion of the root of the continental lithosphere due to the convection in the upper mantle and how it affects the thermal flow to the surface.

Sacek, V. (2017). Post-rift influence of small-scale convection on the landscape evolution at divergent continental margins. Earth and Planetary Science Letters, 459, 48-57.

How to cite: Santos, E. and Sacek, V.: Numerical simulation of asthenospheric flow around cratonic keels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9668, https://doi.org/10.5194/egusphere-egu2020-9668, 2020.

The subduction zone is a convergent plate boundary, and where most seismic activity is concentrated and megathrust may occur. To evaluate the potential hazard in subduction zones always relates to the plate coupling status. From previous studies, the status of plate coupling between plates can be reflected by the vibration of the buoyancy of mantle lithosphere (Hm). As far as the respective plate coupling states are concerned, more than a dozen Hm profiles across different subduction zones have been successfully verified. It is normally to determine the coupling status depending on the Hm vibration without manifest definition. We therefore propose a method to estimate the plate coupling factor (pcf) quantitatively. The pcf is defined as the difference of the Hm caused by the respective subduction and overriding plates between the distances where Hm deviated from the normal lithospheric Hm value across the plate boundary. The collected Hm profiles are calculated by the proposed method, the results show that the pcf value is corresponding well to the plate coupling status in the respective subduction zone. The small pcf is for strong plate coupling, such as the northern Sumatra and the southern central Andes subduction zones, while the large pcf is for weak coupling, such as the Calabria and the northern Manila subduction zones. The calculation of pcf is a feasible solution for determination of plate coupling status, but more Hm profiles across subduction zones will help the estimation more reliable.

How to cite: Lo, C.-L., Doo, W.-B., and Hsu, S.-K.: The rule of thumb for inferring plate coupling status based on the mantle lithospheric buoyancy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18448, https://doi.org/10.5194/egusphere-egu2020-18448, 2020.

East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western Indo-Asian continental collision and the eastern oceanic subduction mainly from Pacific plate. Till now, mantle dynamics beneath this area is not well understood due to its complex mantle structure, especially in the framework of global spherical mantle convection. Hence, a series of numerical models are conducted in this study to reveal the key controlling parameters in shaping the present-day observed mantle structure beneath East Asia under 3-D global mantle flow models. Global mantle flow models with coarse mesh are firstly applied to give a rough constraint on global mantle convection. The detailed description of upper mantle dynamics of East Asia is left with regional refined mesh. A power-law rheology and absolute plate field are applied subsequently to get a better constraint on the related regional mantle rheological structure and surficial boundary conditions. Thus, the refined and reasonable velocity and stress distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations. The derived large shallow mantle flow beneath the Tibetan Plateau causes significant lithospheric shear drag and dynamic topography that result in prominent tectonic evolution of this area. And the Indo–Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo–Asian collision has occurred for 50 Ma, and this collision can still continue to accelerate uplift in the Tibetan plateau. Finally, we also consider the possible implementations of 3-D numerical simulations combined with global lithosphere and deep mantle dynamics so as to discuss the relevant influences.

How to cite: Zheng, Q. and Zhang, H.: Mantle convection beneath East Asia under global mantle convection spherical framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12041, https://doi.org/10.5194/egusphere-egu2020-12041, 2020.

It is well-accepted that stresses and deformation are controlled by active forces, such as tractions applied along lateral boundaries and base of the lithosphere and body forces raised from density heterogeneities within or below the lithosphere. Here we analyze how structure, geometry and strength distribution, of the Earth crust and upper mantle can affect the pattern of stresses and deformation. As an application example, we use the North Atlantic realm which characterized by strong topography and rheological variations and subjected to active forces from, e.g., the Iceland hot spot. We conduct a series of numerical experiments modelling the lithosphere as an elastic shell of altering geometries influenced by various mechanisms. The first set of experiments demonstrates that lithosphere, as a part of the spherical Earth, is structurally stronger than the flat lithosphere if boundary moments applied. An application of more realistic, topography derived, geometry of the lithospheric shell in the second set of experiments demonstrates the importance of strong topography changes, for example along continent-ocean transition, as a concentrator of bending stresses and deformations. In the third set, we show how viscous properties of the sub-lithospheric asthenosphere may control the lateral extent of the membrane stresses in the lithosphere.

How to cite: Medvedev, S. and Minakov, A.: Structural controls on stresses and deformations in a large-scale lithospheric shell, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9861, https://doi.org/10.5194/egusphere-egu2020-9861, 2020.

The Congo basin (CB) is an intracratonic basin that occupies a large part of the Congo Craton (1.2 million km²) covering approximately 10% of the continent [1]. It contains up to 9 km of sedimentary rocks from the Mesoproterozoic until Cenozoic age. The formation of the CB started with a rifting phase during Mesoproterozoic with the amalgamation of the Rodinia supercontinent (1.2 Gyr). Afterwards, the main episodes of subsidence occurred during the subsequent Neoproterozoic post-rift phases, which were followed by phases of compression at the end of the Permian and during the Early Jurassic age and other sedimentation episodes during Upper Cretaceous and Cenozoic [2].

 We reconstruct the stratigraphy and tectonic evolution of the basin by analyzing seismic reflection profiles. Furthermore, we estimated the velocity, density, and thickness of the sedimentary layers in order to calculate their gravity effect. Afterwards, we calculate the gravity disturbance and Bouguer anomalies using a combined satellite and terrestrial data gravity model. The gravity disturbance obtained from the EIGEN-6C4 gravity model [3] shows two types of anomalies. One with a long wavelength (~50 mGal) that covers the entire area of the Congo basin and a second one with a short wavelength (~130 mGal), having a NW-SE trend, which corresponds to the main depocenters of sediments detected by the interpretation of seismic reflection profiles. These results have been used as input parameters for 3D numerical simulations to test the main mechanisms of formation and evolution of the CB. For this aim, we used the thermomechanical I3ELVIS code [4] to simulate the initial rift phase. The numerical tests have been conducted considering a sub-circular weak zone in the central part of the cratonic lithosphere [2] and applying a velocity of 2.5 cm/yr in two orthogonal directions (NS and EW), to test the hypothesis of the formation of a multi extensional rift in a cratonic area. We repeated these numerical tests by increasing the size of the weak zone and varying its lithospheric thickness. The results of these first numerical experiments show the formation of a circular basin in the central part of the cratonic lithosphere, in response to extensional stress, inducing the uplift of the asthenosphere.

[1] Kadima, et al. (2011), Structure and geological history of the Congo Basin: an integrated interpretation of gravity, magnetic and reflection seismic data, doi:10.1111/j.1365-2117.2011.00500.x.
[2] De Wit, et al. (2008), Restoring Pan-African-Brasiliano connections: more Gondwana control, less Trans-Atlantic corruption, doi:10.1144/SP294.20
[3] Förste et al. (2014) EIGEN-6C4 The latest combined global gravity field model including GOCE data up to degree and order 2190 of GFZ Potsdam and GRGS Toulouse; doi: 10.5880/ICGEM.2015.1, 2014
[4] Gerya (2009), Introduction to numerical geodynamic modelling, Cambridge University Press

How to cite: Maddaloni, F., Delvaux, D., Tesauro, M., Gerya, T., and Braitenberg, C.: Tectonic evolution of the Congo Basin using geophysical data and 3D numerical simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13630, https://doi.org/10.5194/egusphere-egu2020-13630, 2020.

East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western continental collision from the cratonic Indian plate and the eastern oceanic subduction mainly from Pacific plate. Studies have suggested that the Indo–Asian continental collision may have driven significant lateral mantle flow, but the velocity, range and effect of the mantle flow remain uncertain. Hence, a series of 3-D numerical models are conducted in this study to reveal the impacts of the Indo–Asian collision on mantle dynamics beneath the East Asia, especially on the asthenospheric mantle. Global model domain encompasses the lithosphere, upper mantle and the lower mantle with different viscosity for each layer. A global temperature structure built from seismic tomography and absolute plate field are applied subsequently to get a better constraint of the initial temperature condition and surficial velocity boundary condition. Thus, the reasonable velocity and temperature distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations, and the key controlling parameters in shaping the present-day observed mantle structure are investigated. The results show different scales of convection beneath East Asia.

Our results suggest that Indo–Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo–Asian collision has occurred since 50 Ma, and this collision can still continue to accelerate in the Tibetan Plateau. The simulation results also show the lithospheric delamination and the induced mantle upwelling, which is consistent with the general understanding from previous observations. The Indian lithosphere and its asthenosphere move northward, while the Yunnan lithosphere and its asthenosphere move southward, that may reflect the differences in deep mantle dynamics between the eastern and western Himalayan Syntaxis.

How to cite: Zhang, H., Zheng, Q., and Zhang, Z.: Deep Mantle Dynamics in East Asia: Numerical Simulation of Mantle Convection Based on Seismic Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12137, https://doi.org/10.5194/egusphere-egu2020-12137, 2020.

We present an integrated geophysical-petrological model of the Eifel region. The Eifel is a volcanic active region in West Germany that exhibits Tertiary as well as Quaternary volcanism. One suggestion for the source of this volcanism is a small-scale upper mantle plume.

The 3D model includes the crust and upper mantle and was generated by combined modelling of topography and the gravity field with constraints from seismology and geochemistry. In the best-fit model, the subcontinental lithospheric mantle is associated with a Phanerozoic-type composition, resulting in a depth of 80 km for the lithosphere-asthenosphere boundary (LAB) beneath the Eifel and in comparison 110 - 130 km beneath the Paris basin. A Proterozoic-type composition in contrast results in a LAB depth of 120 km in the Eifel. While the model fits the geophysical observables and features a thin lithosphere, it does not lead to a plume-like structure and does not feature a seismic low-velocity anomaly.

The measured low-velocity anomaly can be reproduced by introducing (1) an even thinner lithosphere or (2) a plume-like body above the thermal LAB with a composition based on data from Eifel xenoliths, which have a mainly basanitic composition. This additional structure results in a thermal anomaly and has an effect on the isostatic elevation of c. 360 m, but it does not result in a significant signal in the gravity anomalies. Further modelling showed how crustal intrusions could additionally mask the gravitational effect from such a small-scale upper mantle plume.

The model does not conclusively explain the source of the Eifel volcanism, but the models and the calculation of synthetic dispersion curves help to assess the possibility to resolve a small-scale upper mantle plume with joint inversion in future analysis.

How to cite: Wansing, A., Ebbing, J., and Bredow, E.: Integrated geophysical-petrological modelling of the Eifel region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2305, https://doi.org/10.5194/egusphere-egu2020-2305, 2020.

The exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile-brittle deformations until the complete emplacement in the oceanic lithosphere. São Pedro and São Paulo Archipelago, is located at 1° N latitude, 1000km from the Brazilian mainlad. Ten isles compose the archipelago with a total exposed area of 17 km². Those isles record the deformational products of ductile, brittle and the rocks/fluid interaction generating specific structures in each domain. The deformational stages are related to the transpressional and transtensional geodynamics of São Paulo Transform Fault (SPTF). The ductile-brittle fabrics were observed in a multiscale context, using drone images, geological mapping, fault analysis, and microstructural studies. Using all these tools to define the tectonic tensions and structures associated with a transition between ductile to the brittle deformational settings. Firstly during the transpressional context, the exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700° - 800°C. Leading to olivine and orthopyroxene recrystallization forming such as well-marked mylonitic foliation and rotated porphyroclast with left-lateral kinematic. The interaction with fluids initially originated from the mantle, generates fragmented crystals of amphibole and oxide-rich levels, marking the transition to semi-brittle deformation. The continuous and rapid uplift led to the superposition of deformation mechanisms, with reactivation of pre-existing structures and predominance of brittle deformation mechanisms. The tectonics associated with an NW-SE shortening in the transpressional tectonics context led to greater availability of hydrothermal fluids. Consequently, the formation of four serpentinization episodes, which are associated with semi-brittle to brittle transition, with temperatures between 300 - 400° C. The presence of serpentine marks the transition between semi-brittle to brittle regimes, whose dextral kinematics is marked by the domino faults, microfaults and gash veins. The kinematics at the brittle moment is compatible with the current movement of the SPTF. Finally, the complete exhumation and establishment of brittle mechanisms led to the carbonatation phase near the surface, with temperatures ranging from 150 - 300°C. The active NW-SE tectonic stress generated an E-W strike-slip faults that filled by carbonates, symbolizing the final exhumation stage.

How to cite: Barão, L. M., Trzaskos, B., Angulo, R. J., and Souza, M. C. D.: Evolution of mantle peridotite rocks - structures generated in a transition from the ductile-brittle regime in the Equatorial Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1644, https://doi.org/10.5194/egusphere-egu2020-1644, 2020.

Strong seismic anisotropy is generally observed in subduction zones. Lattice preferred orientation (LPO) of olivine and elastically anisotropic hydrous minerals has been considered to be an important factor causing anomalous seismic anisotropy. For the first time, we report on measured LPOs of polycrystalline talc. The study comprises subduction-related ultra-high-pressure metamorphic schists from the Makbal Complex in Kyrgyzstan-Kazakhstan and amphibolite-facies metasomatic schists from the Valla Field Block in Unst, Scotland. The here studied talc revealed a strong alignment of [001] axes (sub)normal to the foliation and a girdle distribution of [100] axes and (010) poles (sub)parallel to the foliation. The LPOs of polycrystalline talc produced a significant P–wave anisotropy (AVp = 72%) and a high S–wave anisotropy (AVs = 24%). The results imply that the LPO of talc influence both the strong trench-parallel azimuthal anisotropy and positive/negative radial anisotropy of P–waves, and the trench-parallel seismic anisotropy of S–waves in subduction zones.

How to cite: Lee, J., Jung, H., Klemd, R., Tarling, M., and Konopelko, D.: Lattice preferred orientation of talc and implications for seismic anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4440, https://doi.org/10.5194/egusphere-egu2020-4440, 2020.

When modeling subduction processes, the results are usually constrained by looking at the geological surface expressions, geochemistry and geophysical observations such as tomography and seismic anisotropy. Of these observations, seismic anisotropy is the only type of observation that can potentially be directly linked to the spatial flow pattern in the mantle. Seismic anisotropy in the mantle is due to lattice-preferred orientation (LPO) of olivine minerals. In subduction environments, which can have complex and changing flow patterns, it is not expected that the LPO necessarily aligns with the flow pattern. This is partly due to the fact that it takes time to realign the LPO and partly because the olivine fast axis alignment depends on the water content and the magnitude of stress. To overcome this problem, the LPO must be computed for realistic and end member subduction zones in order to be able to relate seismic anisotropy to mantle flow and thereby slab dynamics.

There are many ways to compute LPO. For this study we have used DREX (Kaminski et al., 2004), because the underlying method is accurate and fast enough for use in geodynamic models. To achieve a good and native integration with ASPECT (Kronbichler et al., 2012; Heister et al., 2017; Bangerth et al,. 2019), we have rewritten DREX in CPP as a plugin for ASPECT. In this presentation we will show how it was implemented and what the limitations and possibilities are. Furthermore, we will show initial results from 3D subduction models to study the link between seismic anisotropy and mantle flow.

How to cite: Billen, M. and Fraters, M.: Computing LPO for Geodynamic Models in ASPECT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12579, https://doi.org/10.5194/egusphere-egu2020-12579, 2020.

We present a new model, EUNA-rho (Shulgin and Artemieva, 2019, JGR), for the density structure of the European and the North Atlantics upper mantle based on 3D tesseroid gravity modeling and a new regional model for the lithosphere thickness in Europe, Greenland, the adjacent off-shore regions (Artemieva, 2019ab, ESR), and Anatolia (Artemieva and Shulgin, 2019, Tectonics). On continent, there is no clear difference in lithosphere mantle (LM) density between the cratonic and Phanerozoic Europe, yet a ca. 300 km wide zone of a high-density LM along the Trans-European Suture Zone may image a paleosubduction. Kimberlite provinces of the Baltica and Greenland cratons have a low density mantle, while the correlation between LM density and the depth of sedimentary basins indicates an important role of eclogitization in basin subsidence, with the presence of 10-20% of eclogite in LM beneath the super-deep platform basins and the East Barents shelf. The Barents Sea has a sharp transition in lithosphere thickness from 120-150 km in the west to 175-230 km in the eastern Barents. Highly heterogeneous lithosphere structure of Anatolia is explained by the interplay of subduction systems of different ages. The block with 150 km thick lithosphere in the North Atlantics east of the Aegir paleo-spreading may represent a continental terrane. In the North Atlantics, south of the Charlie Gibbs fracture zone (CGFZ) bathymetry, heat flow and mantle density follows half-space cooling model with significant deviations at volcanic provinces. Strong low-density LM anomalies (<-3%) beneath the Azores and north of the CGFZ correlate with geochemical anomalies and indicate the presence of continental fragments and heterogeneous melting sources. Thermal anomalies in the upper mantle averaged down to the transition zone are 100-150^o C at the Azores and can be detected seismically, while a <50^o C anomaly around Iceland is at the limit of seismic resolution.

References:

• Artemieva I.M., 2019. The lithosphere structure of the European continent from thermal isostasy. Earth-Science Reviews, 188, 454-468.     
• Artemieva I.M., 2019. Lithosphere thermal thickness and geothermal heat flux in Greenland from a new thermal isostasy method. Earth-Science Reviews, 188, 469-481.
• Shulgin A. and Artemieva I.M., 2019. Thermochemical heterogeneity and density of continental and oceanic upper mantle in the European‐North Atlantic region. Journal of Geophysical Research: Solid Earth, 124, 1-33, doi: 10.1029/2018JB017025 (open access)       
• Artemieva I.M. and Shulgin A., 2019. Geodynamics of Anatolia: Lithosphere thermal structure and thickness. Tectonics, 38, 1-23, doi: 10.1029/2019TC005594

How to cite: Artemieva, I. and Shulgin, A.: Lithosphere thermo-chemical heterogeneity in the European-North Atlantic region, Greenland and Anatolia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5000, https://doi.org/10.5194/egusphere-egu2020-5000, 2020.

The causes of the high topography in Scandinavia along the North Atlantic passive continental margins are enigmatic, and two end-member models have been proposed. One opinion is that the high topography has been maintained since the Caledonian orogeny, because isostatic rebound has compensated for most of the erosion over >400 My. The other opinion is that the topography is Cenozoic and that it is related to plate tectonic or deep thermal / geodynamic processes. Onshore uplift is related to simultaneous offshore subsidence, and the rapid topographic changes may be the combined result of a series of complementary processes.

Here, we provide new evidence for the upper mantle structure by calculating a tomographic model for Fennoscandia (Scandinavia and Finland) by teleseismic inversion of finite-frequency P- and S- wave travel-time residuals. We use seismic signals from earthquakes at epicentral distances between 30° and 104° and with magnitudes larger than 5.5, gathered on 200 broad-band seismic stations installed by the ScanArray project in Norway, Sweden and Finland, which operated during 2012-2017, together with data from earlier projects and stationary stations..

We measure relative travel-time residuals of direct body waves in high- and low-frequency bands, and carry out an appropriate frequency-dependent crustal correction. The average residuals vary over the region, and show clear trends depending on location and and back-azimuthal directions. This demonstrates the presence of significant heterogeneity of seismic velocities in the upper mantle across the region. Based on the travel-time residuals, we carry out finite-frequency body-wave tomographic inversion to determine the P and S wave seismic velocity structure of the upper-mantle. By use of “relative kernels” we reduce problems related to station coverage with asynchronous datasets, which allows the use of data from different deployments for the inversion. The resulting seismic model is compared to the existing and past topography in order to contribute to the understanding of mechanisms responsible for the topographic changes in the Fennoscandian region, which we relate to the general tectonic and geological evolution of the North Atlantic region. The models provide basis for deriving high-resolution models of temperature and compositional anomalies that may contribute to the understanding of the observed, enigmatic topography.

How to cite: Bulut, N., Maupin, V., and Thybo, H.: Seismic P- and S-wave velocity Tomography in Scandinavia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22265, https://doi.org/10.5194/egusphere-egu2020-22265, 2020.

Upper mantle structures under cratons have recently been investigated by many researchers using receiver functions and surface waves to clarify the nature of the Lithosphere-Asthenosphere Boundary (LAB) and Mid-Lithosphere Discontinuity (MLD). Majority of seismological studies of joint inversions using receiver functions and surface waves have employed dispersion curves of fundamental-mode only, but higher-mode information is essential for resolving the whole depth range of thick continental lithosphere (over 200 km) and its underlying asthenosphere.

In this study, we reconstructed radially anisotropic S wave models including multiple discontinuities in the upper mantle under seismic stations in Australia, using multi-mode surface waves and receiver functions in the framework of the Bayesian inference. We employed a fully nonlinear method of joint inversions incorporating P-to-S receiver functions and multi-mode Rayleigh and Love waves, based on the trans-dimensional hierarchical Bayesian formulation. The method allows us to estimate a probabilistic Earth model taking account of the complexity and uncertainty of Earth structure, by treating the model parameters and data errors as unknowns. The Parallel Tempering algorithm is incorporated for the effective parameter search based on the reversible-jump Markov Chain Monte Carlo method.

Multi-mode phase speed maps of surface waves developed by Yoshizawa (2014) are used to extract localized multi-mode dispersion curves. The use of higher-mode surface waves enables us to enhance the sensitivity to the depth below the continental asthenosphere, while the receiver functions allows us to better constrain the depths of discontinuities and velocity jumps. Synthetic experiments indicate the importance of higher-mode information for the better recovery of radial anisotropy in the whole depth range of the upper mantle.

The method has been applied to Global Seismographic Network stations in Australia. While the S-wave models in eastern Australia show shallow LAB above 100 km depth, those in central and western Australia exhibit both MLD and LAB. Also, seismic velocity jumps equivalent to the Lehmann Discontinuity (LD) are found in all seismic stations in Australia. The LDs under the Australian continents are found at the depth of around 200 - 300 km, depending on locations. Radial anisotropy in the depth range between LAB and LD tends to show faster SH anomalies, which may indicate the effects of horizontal shear underneath the fast-moving Australian plate.

How to cite: Yoshizawa, K. and Taira, T.: Upper mantle discontinuities beneath Australia from trans-dimensional hierarchical Bayesian inversions using receiver functions and multi-mode surface waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19418, https://doi.org/10.5194/egusphere-egu2020-19418, 2020.

A variety of anthropogenic activities are now known to have triggered earthquakes. These include mining, filling of artificial water reservoirs, production of petroleum and geothermal energy, high- pressure fluid injections into shallow crust and many more. Among these, artificial water reservoir triggered seismicity (RTS) is most prominent, with the largest triggered earthquake of M 6.3 having occurred at Koyna, India in 1967. Whether, the devastating Mw 7.8 Sichuan, China earthquake of 8 May 2008 that claimed some 80,000 human lives was triggered by filling of the nearby Zipingpu reservoir, continues to be debated.

There are over 100 sites globally where RTS events of M ≥ 4 have occurred. Here we present an over view of RTS, common characteristics of the RTS earthquake sequences that help to discriminate them from normal earthquake sequences and also help selection of safer sites for locating dams to create artificial water reservoirs.

Koyna, near the west coast of India continues to be most prominent site where triggered- earthquakes have been occurring since the impoundment of the reservoir in 1962 and have continued till now with 22 M ≥ 5, ~ 200 M ≥ 4 and several thousands smaller earthquakes. It was argued that Koyna is a very suitable site for near field investigations of triggered earthquakes. Discussions were held in dedicated ICDP workshops and finally a go ahead was given. As a precursor to setting up a near field laboratory at ~ 7 km depth, a 3 km deep Pilot Borehole has been completed in June 2017 and investigations are being carried out for necessary input for setting up the deep borehole laboratory. Salient features of this project are also presented.

How to cite: Gupta, H.: Artificial Water Reservoir Triggered Seismicity (RTS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4745, https://doi.org/10.5194/egusphere-egu2020-4745, 2020.

Sub-Moho reflectors have been identified in seismic refraction and wide-angle reflection recordings in western Iberia since the late ‘80s. These control source seismic wide-angle shot records have energy large enough to illuminate the uppermost mantle showing strong sub-Moho arrivals at distant offsets (>180 km) with amplitudes significantly higher than the Pn and a relatively long coda. The kinematics and wavelet characteristics of these features are probably produced by an increase in P-wave velocity, and forward modeling indicates that these arrivals reflect off an interface in the 60-80 km depth range beneath the Iberian Massif. The waveform and time length of this arrival suggests that it can result from the interaction of the seismic energy with a ~10 km thick heterogeneous layer. To test this hypothesis, we used a 2D second-order finite-difference acoustic and elastic full wave-field scheme with a layer consisting of randomly distributed bodies smaller than ¼ of the wavelength of the seismic waves in thickness and ΔVp=±0.2 km/s at the considered depth range. Resulting synthetic shot gathers reproduce well the observed amplitudes and codas as a result of the constructive interference caused by the tuning effect produced by this gradient heterogeneous zone. The contrast in physical properties and depth level of this feature are consistent with the top of the phase transition from spinel to garnet lherzolite, the so-called Hales discontinuity.

Some of the available gathers show a second and deeper reflection. Detailed analysis of the reflected wave-forms suggests that the reflected wavelet has reversed polarity, a feature suggesting. a velocity decrease with depth. Finite difference acoustic and elastic full wave-field modeling places this discontinuity around 90 km depth beneath the Ossa-Morena Zone (south Iberian Massif). A lateral change is observed beneath the Centro-Iberian Zone (central Iberian Massif) where it is imaged at 103-110 km depth on the southeast and shallows up to 80 km depth on the northeast. The indicated depth would be consistent with the depth location of the LAB, which is relatively well constrained for the target area by other geophysical observations.

Funding resources: EU EIT-RawMaterials Ref: 17024_20170331_92304; MINECO: CGL2016-81964-REDE CGL2014-56548-P: JCYL: SA065P17). 

How to cite: Palomeras, I., Ayarza, P., Díaz, J., Andrés, J., and Carbonell, R.: Seismic Modeling of the Subcrustal Reflectivity Beneath the Iberian Massif (Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18492, https://doi.org/10.5194/egusphere-egu2020-18492, 2020.

The receiver function analysis (RF) is a commonly used and well-established method to investigate subsurface crustal and upper mantle structures, removing the source, ray-path and instrument signatures. RF gives the unique signature of sharp seismic discontinuities and information about P-wave (P) and shear-wave (Ps) velocity below the seismic station. In particular using the direct P-wave as a known reference arrival time, and the relative arrival time of P-to-S (Ps) conversions as well as PpPs, PsPs and PsSs reflections allow constraining the principal crustal structures and allows us to study the effects of dipping interfaces and crustal layering.

The aim of this work is to use the RF non-conventional analysis to study the crustal structures of Tenerife. Previous studies on receiver functions analysis an active oceanic volcanic island, showed that the Moho topography have a high dipping under the volcanic edifice and a depth ranging between 11 and 18 km depth. Furthermore, it has been observed that some phases related with a layer of volcanic rocks having a thickness of about 5.5 km and a P-wave velocity (Vp) of approximately 6 Km/s, lies above an old oceanic crust having a thickness of about 7 km and a Vp of about 6.8 km/s.

For this study we applied both time and frequency domain deconvolution to obtain receiver functions. The determination of the average crustal thickness and has been achieved by using the commonly uses H-k method. To constrain the internal crustal layering, we used a non-linear inversion algorithm based on full waveform modeling of the receiver function. Finally, we realized a modelling of the reflected and converted phases in the crust using seismic ray tracing. Our modelling takes into account the surface topography as well as an arbitrary geometry of the Moho.

In conclusion our results showed the presence of a thick layer (up to 5.5 km) of volcanic rocks in the central part of the island overlying an oceanic crust whose total thickness varied from 18 km in the central part to about 11 km in the peripheral areas. This work represents the first step toward further studies devoted at a finer imaging of the crustal structures of Tenerife using receiver function analysis.

How to cite: Ortega, V. and D'Auria, L.: Receiver function analysis for determining the crustal structure of Tenerife (Canary Islands, Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-309, https://doi.org/10.5194/egusphere-egu2020-309, 2020.

The southern Baltic Sea area is located in the transition zone between the East European Craton (EEC; Baltica) and the West European Platform (Avalonia). The most prominent tectonic feature in the area is the NW–SE trending Tornquist Zone (TZ), crossing the southern Baltic Sea area between Scania in Sweden and Pomerania in Poland. A peculiar feature of the TZ and its southern prolongation (Teisseyre-Tornquist Zone, TTZ) is possibly a crustal keel that was recently postulated for northern Poland based on potential field modelling. A crustal keel was also imaged in the Baltic Sea by BABEL profile A, which crossed the TZ northwest of Bornholm, and by two TTZ’92 profiles crossing the TTZ south of Bornholm. However, the DEKORP-PQ profile shows a flat Moho across the TTZ.

In order to reconcile those contrasting interpretations of the crustal structure around the TTZ offshore Poland, a 230-km long refraction/wide-angle reflection profile was acquired across the TTZ in the course of RV/MARIA S. MERIAN expedition MSM52 (BalTec) in March 2016. This profile is nearly parallel to the western Polish coast, in half a distance to Bornholm. The data acquisition was conducted with 15 ocean bottom seismometers (OBS) and 3 land stations. The source array consisted of 8 G-guns with the total volume of 32 litres. In total 2227 shot points were recorded. Hydrophone data are of high quality and despite the relatively small source volume, sharp first arrivals of Pg and Pn are observed at over 120 km offsets. Some seismic record sections show clear PmP phases beginning at offsets of 70 km, continuing till the end of the profile.

Two variants of seismic modelling were performed, which results proved to be similar in terms of P-wave velocities and observed features. Tomographic joint inversion of both first arrivals and Moho reflections was used to extend velocity model depth range. Second was trial-and-error forward modelling technique using all identified seismic phases, paying attention to minimize misfit between calculated and observed P-wave travel times for each individual layer.

In the area of the TTZ, a complex upper crustal structure deepening towards the southwest is observed. One of the most interesting features is an increase in Vp (>6.5 km/s) at a depth of 16-25 km, offset by ~40 km from the TTZ on the EEC side. Similar feature was observed along the TTZ in SE Poland. Due to the lack of information from refraction, the presented ray-tracing model is the result of testing various possible velocity values for the lower crust in different parts of the model. A layer with Vp>7 km/s with a thickness of ~6 km along the entire model seems to be the best solution The Moho boundary was inferred at 33-38 km depth, deepening towards the EEC, with ~3 km uplift (but not keel) corresponding to the location of the elevated middle-crust velocities. Final velocity models were further verified by forward potential field modelling, testing various Vp – density relations.

This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.

How to cite: Wójcik, D., Janik, T., Malinowski, M., Ponikowska, M., Mazur, S., Skrzynik, T., and Hübscher, C.: New refraction/wide-angle reflection profile across the Teisseyre-Tornquist Zone offshore Poland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7394, https://doi.org/10.5194/egusphere-egu2020-7394, 2020.

The distribution of temperature inside active continental margins plays a fundamental role on regulating first order geodynamic processes as the isostatic balance, rheologic behavior of crust and mantle, magmagenesis, volcanism and seismogenesis. In spite of these major implications, well-constrained 3D thermal models are known for few regions of the world (Europe, Western USA, China) where large geophysical databases have been integrated into compositional and structural models of crust and lithospheric mantle from which a thermal model is derived. Here we present a three-dimensional representation of the distribution of temperature underneath the Andean active margin of South America (10°-45°S) that is based on a geophysically-constrained model for the geometry of the subducted slab, continental lithosphere-asthenosphere boundary (LAB), Moho discontinuity and an intracrustal discontinuity (ICD). This input model was constructed by forward modelling the satellite gravity anomaly under the constraint of most of the seismic information published for this region. We use analytical expressions of 1D conductive continental geotherms with adequate boundary conditions that consider the compositional stratification of crust and mantle included in the input model, and the advective thermal effect of slab subduction. The 1D geotherms are assembled into a 3D volume defining the thermal structure of the study region. We test the influence of several thermal parameters and structural configurations of the Andean lithosphere by comparing the resulting surface heat flow distribution of these different models against a database containing heat flow measurements that we compile from the literature. Our results show that the thermal structure and derived surface heat flow is dominantly controlled by the geometry of the thermal boundary layer at the base of the lithosphere, i.e. the slab upper surface below the forearc and LAB inland. Variations on the modeled configuration of the continental lithosphere (i.e. the way on which the geometry of the continental Moho and ICD are considered into the definition of a space-variable thermal conductivity or the length scale for radiogenic heat production) have an effect on surface heat flow that is lower than the average uncertainty of the measurements and therefore can be considered as second-order. The simplicity of our analytical approach allows us to compute hundreds of different models in order to test the sensitivity of results to changes on thermal parameters (conductivity, heat production, mantle potential temperature, etc), which provides a tool for discussing their possible range of values in the context of a subduction margin. We will also show how variations of these models impact on the Moho temperature and therefore in the expected mechanical behavior of crust and mantle in this geotectonic context

How to cite: Tassara, A., Julve, J., Echeverría, I., and Stotz, I.: Temperature structure of the Andean subduction zone as derived from the 3D geometry of crustal and upper mantle discontinuities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20322, https://doi.org/10.5194/egusphere-egu2020-20322, 2020.

Margins of craton lithosphere are prone to ongoing modification process. Marginal tectonism such as slab subduction, continental collision, and mantle dynamics significantly influence properties of lithosphere in various scales. Thus, constraints on the detailed properties of craton margin are essential to understand the evolution of continental lithosphere. The eastern margin of the Eurasian plate is a natural laboratory that allows us to study the strong effects from multiple episodes of continental collision and subduction of different oceanic plates since their formation. Extensive reworking and destruction of the cratonic lithosphere mainly occurred in eastern China during the Mesozoic to Cenozoic, which leaves distinct geochemical and geophysical signatures. Specifically, the Korean Peninsula (KP) is known to consist of Archean–Proterozoic massifs (e.g., Gyeonggi, Yeongnam Massif) located in the forefront in northeast Asia, where current dynamics in the upper mantle and effects due to nearby subducting slabs are the most significant.

Here we present, for the first time in detail, 3-D velocity structure of KP by teleseismic body wave traveltime tomography. Detailed P-wave and S-wave images of the crust and upper mantle were constructed by approximately 5 years of data from dense arrays of seismometers. We newly found a thick high-velocity body beneath the southwestern KP with a thickness of ~150 km, which is thought as a fragment of lithospheric root beneath the Proterozoic Yeongnam Massif. Also, we found low velocities beneath the Gyeonggi Massif, eastern KP margin, and Gyeongsang continental arc-back-arc system, showing significant velocity contrasts (dlnVp of ~4.0% and dlnVs of ~6.0%) to the high-velocity structure. These features indicate significantly modified regions. In addition, there was a clear correlation of the upper mantle low-velocity anomalies and areas characterized by Cenozoic basaltic eruptions, high heat flow, and high tomography, suggesting that there are close associations between mantle dynamics and recent tectonic reactivation.

The presence of a remnant cratonic root beneath the KP and contrasting lithospheric structures across the different Precambrian massifs suggests highly heterogeneous modification along the Sino-Korean craton margin, which includes the KP and North China Craton. A striking localization of lithosphere modification among the different Precambrian massifs within the KP suggests that the structural heterogeneity of the craton margin is likely sharp in scale and thickness within a confined area. We suggest that intense interaction of upper mantle dynamics and inherent structural heterogeneities of a craton margin played an important role in shaping the current marginal lithosphere structure in northeast Asia.

How to cite: Song, J.-H., Kim, S., and Rhie, J.: Heterogeneous modification and reactivation of craton margin in northeast Asia: insight from teleseismic traveltime tomography of the Korean Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12562, https://doi.org/10.5194/egusphere-egu2020-12562, 2020.

Classical field studies are vital for mapping and understanding volcano-tectonic processes, particularly for those that produce superficial deformation consequently to magmatic and tectonic activity. Unfortunately, very often, key outcrops are inaccessible due to harsh logistic conditions or their location in remote or dangerous areas. In the framework of the ILP Task Force II, we developed and tested modern and innovative methods aimed at overcoming these limitations in field research and data collection, that we combined with classical field mapping. Such methods have been used to provide a more complete picture of the deformation processes that have been taking place in the Theistareykir Fissure Swarm within the Northern Volcanic Zone of Iceland. This rift is characterized by the presence of huge normal faults, several extension fractures and volcanic centres. The modern methods we used derive from the use of UAVs (drones) combined with Structure from Motion (SfM) photogrammetry techniques. The first innovative method consists of analysing UAV-based SfM-derived high resolution orthomosaics and digital surface models where we collected hundreds of quantitative measurements of the amount of opening and opening direction of Holocene extension fractures and measurements of fault scarp height. The second and more innovative method we used is the Immersive Virtual Reality that can be applied to 3D digital outcrop models (DOMs), reconstructed with UAV-based SfM photogrammetry techniques; several sites within the Theistareykir Fissure Swarm have been reconstructed in the framework of the Italian Argo3D project. The reconstructed 3D DOMs were explored using different modalities: on foot, as is often the case during field activity, moving like a drone, above and around the target, as well as flying like an airplane. Thanks to these modes of exploration we were capable of better understanding the geometry of extension fractures, volcanic centres and normal faults. We also measured, in the virtual environment, the opening direction and the amount of dilation along the extensional fractures, the direction of magma-feeding fractures underlying cones and volcanic vents, as well as the amount of vertical offset along normal faults. The quantification and mapping of these features was accomplished through some tools tailored for virtual field activity in the framework of Italian Argo3D project and the Erasmus+ Key Action 2 2017-1-UK01-KA203-036719. Thanks to the merging of classical and modern approaches we are able of providing a complete picture related to the post-LGM deformation field affecting this part of the Icelandic rift, particularly focusing on the spreading direction and the stretch ratio across the whole Theistareykir Fissure Swarm.

How to cite: Bonali, F. L., Tibaldi, A., Pasquaré Mariotto, F., Russo, E., and Corti, N.: Advances in field data collection in volcano-tectonic sensitive areas: examples and results from the Northern Volcanic Zone of Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4517, https://doi.org/10.5194/egusphere-egu2020-4517, 2020.