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Structure, Deformation, and Dynamics of the Lithosphere-Asthenosphere System

The geological processes that we infer from observations of the Earth’s surface, together with the landscape features are direct consequences of the dynamic Earth, and in particular, of the interaction between tectonic plates. Seismological studies are key for unraveling the present structure and fabric of the lithosphere and the asthenosphere. However, interdisciplinary work is required to fully understand the underlying processes and how features such as anisotropies in the crust, lithospheric mantle or the asthenosphere evolved through time and how they are related. Here we want to gather those studies focusing on seismic anisotropy and deformation patterns that can successfully improve our knowledge of the processes, leading to the observed present geometries (of the crust and the upper mantle). The main goal of the session is to establish closer links between seismological observations and process-oriented modelling studies to demonstrate the potential of different methods, and to share ideas of how we can collaboratively study upper mantle structure, and how the present-day fabrics of the lithosphere relates to the contemporary deformation processes and ongoing dynamics within the asthenospheric mantle.
Contributions from studies employing seismic anisotropy observations, tomography and waveform modeling, geodetic data, numerical and analogue modelling are welcome.

Co-organized by SM4
Convener: Ehsan QorbaniECSECS | Co-conveners: Irene Bianchi, Ana MG Ferreira, Jaroslava Plomerova, Ernst Willingshofer
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Wed, 28 Apr, 11:00–11:45

Chairpersons: Ehsan Qorbani, Irene Bianchi, Ernst Willingshofer

Thomas Duvernay et al.

Several of Earth's intra-plate volcanic provinces cannot be explained solely through the classical mantle plume hypothesis. Instead, they are believed to be generated by shallower processes that involve the interplay between uppermost mantle flow and the base of Earth's heterogeneous lithosphere. The mechanisms most commonly invoked are edge-driven convection (EDC) and shear-driven upwelling (SDU), both of which act to focus upwelling flow, and the associated decompression melting, adjacent to steps in lithospheric thickness.

In this study, we first undertake a systematic numerical investigation, in both 2-D and 3-D, to quantify the sensitivity of EDC, SDU and their associated melting to several key controlling parameters, in the absence of mantle plumes. Our simulations demonstrate that the spatial and temporal characteristics of EDC are sensitive to the geometry and material properties of the lithospheric step, in addition to the depth-dependence of upper mantle viscosity. These simulations also indicate that asthenospheric shear can either enhance or reduce upwelling velocities and predicted melt volumes, depending upon the magnitude and orientation of flow relative to the lithospheric step. When combined, such sensitivities explain why step changes in lithospheric thickness, which are common along cratonic edges and passive margins, only produce volcanism at isolated points in space and time. Our predicted trends of melt production suggest that, in the absence of potential interactions with mantle plumes, EDC and SDU are viable mechanisms only for Earth's shorter-lived, low-volume intra-plate volcanic provinces.

To complement the results from our first numerical investigation, we subsequently explore how the upwelling of a mantle plume within our 3-D domain modifies the occurrence of melting, both in terms of spatio-temporal distribution and intensity. Preliminary results indicate that edges close to the location of plume impingement have their melting shut off as a result of the intense flow generated through sub-lithospheric spreading. Additionally, the heterogeneous distribution of continental lithosphere thickness constrains plume material spreading and results in melting patterns that do not directly reflect the path of the plume relative to the lithosphere, as described by classical mantle plume theory.

How to cite: Duvernay, T., Davies, R., Mathews, C., Gibson, A., and Kramer, S.: Linking Lithospheric Structure, Mantle Flow and Intra-Plate Volcanism, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1023, https://doi.org/10.5194/egusphere-egu21-1023, 2021.

David Schlaphorst et al.

The Madeira and Canary archipelagos, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions, but a better understanding of the crust and upper mantle structure beneath these regions is needed to investigate their structure in more detail. With the study of seismic anisotropy, it is possible to assess the rheology and structure of asthenosphere and lithosphere that can reflect a combination of mantle and crustal contributions.

Here, as part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we present the first detailed study of seismic anisotropy beneath both archipelagos, using data collected from over 60 local three-component seismic land stations. Basing our observations on both teleseismic SKS and local S splitting, we are able to distinguish between multiple layers of anisotropy. We observe significant changes in delay time and fast shear-wave orientation patterns on short length-scales on the order of tens of kilometres beneath the western Canary Islands and Madeira Island. In contrast, the eastern Canary Islands and Porto Santo the pattern is much more uniform. The detected delay time increase and more complex orientation patterns beneath the western Canary Islands and Madeira can be attributed to mantle flow disturbed and diverted on small-length scales by a strong vertical component. This is a clear indication of the existence of a plume at each of those archipelagos, nowadays exerting a strong influence on the western and younger islands. We therefore conclude that a plume-like feature beneath Madeira exists in a similar way to the Canary Island hotspot and that regional mantle flow models for the region should be reassessed.

This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Schlaphorst, D., Silveira, G., Mata, J., Krüger, F., Dahm, T., and Ferreira, A.: Similarities between the Madeira and Canary Hotspots Revealed by Seismic Anisotropy from Teleseismic and Local Shear-Wave Splitting with the SIGHT Project, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10797, https://doi.org/10.5194/egusphere-egu21-10797, 2021.

Susana Custódio et al.

The Canary and Madeira provinces, located in the central-east Atlantic Ocean, are characterized by irregularly distributed hotspot tracks displaying large age differences and variable distances between volcanoes. For this reason, the geodynamic mechanism(s) that control the spatio-temporal patterns of volcanism are still unclear. Here, we use results from seismic tomography, shear-wave splitting, and gravity to show that the Central-East Atlantic Anomaly (CEAA), rising from the African large low-shear-velocity province and stalled in the topmost lower mantle, is the source of distinct upper-mantle diapirs feeding those provinces. The diapirs detach intermittently from the CEAA and seem to be at different evolutionary stages. Geochemistry data confirm the lower-mantle origin of the diapirs, and plate reconstructions constrain their temporal evolution. Our observations suggest that the accumulation of deep plume material in the topmost lower mantle can play a significant role in governing the spatio-temporal distribution of hotspot volcanism.

This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL.

How to cite: Custódio, S., Civiero, C., Mata, J., Silveira, G., Neres, M., and Schlaphorst, D.: The Central-East Atlantic Anomaly: its role in the genesis of the Canary and Madeira volcanic provinces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14677, https://doi.org/10.5194/egusphere-egu21-14677, 2021.

Eric Löberich et al.

Shear-wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, and to constrain the dominant mechanism of upper mantle deformation. As the interpretation is often ambiguous, it is useful to consider additional information, e.g. based on the non-vertical incidence of core-phases. Depending on the lattice-preferred orientation of anisotropic minerals, this condition leads to a variation of splitting parameters with azimuth and enables a differentiation between various types of olivine deformation. As the fabric of olivine-rich rocks in the upper mantle relates to certain ambient conditions, it is of key importance to further define it. In this study, we predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine, and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain. This can help to constrain the amount of water in the upper mantle beneath an area, known for a consistent, mainly E-W fast orientation, and increased splitting delay in the back-arc of the Cascadia Subduction Zone. Comparing expected and observed variations renders a C-type olivine mechanism unlikely; a differentiation between A- and E-type olivine remains more difficult though. However, the agreement of the amplitude of azimuthal variation of the fast orientation, and the potential to explain larger splitting values, suggest the occurrence of E-type olivine and the presence of a hydrated upper mantle. Along with a discrepancy to predict delay times from azimuthal surface wave anisotropy, deeper sources could further affect shear-wave splitting observations.

How to cite: Löberich, E., Long, M. D., Wagner, L. S., Qorbani, E., and Bokelmann, G.: Constraints on olivine deformation mechanisms from SKS shear-wave splitting beneath the High Lava Plains, Northwestern Basin and Range and Western Yellowstone Snake River Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4590, https://doi.org/10.5194/egusphere-egu21-4590, 2021.

Caroline Eakin

Australia is an old stable continent with a rich geological history. Limitations in sub-surface imaging below the Moho, however, mean that is unclear to what extent, and to what depth, this rich geological history is expressed in the mantle. Scattering of surface waves at ~150km depth by lateral gradients or boundaries in seismic anisotropy, termed Quasi-Love waves, offer potential new insights. The first such analysis for Australia and Zealandia is performed with over 300 new scatterers detected that display striking geographical patterns. Around two-thirds of the scatterers are coincident with either the continental margins, or major crustal boundaries within Australia, suggesting deep mantle roots to such features. Within the continental interior such lateral anisotropic gradients imply pervasive fossilized lithospheric anisotropy, on a scale that mirrors the crustal geology at the surface, and a strong lithosphere that preserves this signal over billions of years. Along the continental margins, lateral anisotropic gradients may indicate either the edge of the thick continental lithosphere, or small-scale dynamic processes in the asthenosphere, such as edge-drive convection, tied to the transition from oceanic to continental crust/lithosphere.

How to cite: Eakin, C.: Tectonic History of Australia Preserved by Mantle Anisotropic Boundaries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6989, https://doi.org/10.5194/egusphere-egu21-6989, 2021.

Ziqi Zhang and Tolulope Olugboji

While the receiver function technique has been successfully applied to high-resolution imaging of sharp discontinuities within and across the lithosphere, it has been shown, however, that it suffers from severe limitations when applied to seafloor seismic recordings. This is because the water and sediment layer could strongly influence the receiver function traces, making detection and interpretation of crust and mantle layering difficult. This effect is often referred to as the singing phenomena in marine environments. Here, we show how one can silence this singing effect. We demonstrate, using analytical and synthetic waveform modeling, that this singing effect can be reversed using dereverberation filters tuned to match the elastic property of each layer. We apply the filter approach to high-quality earthquake records collected from the NoMelt seismic array deployed on normal, mature (~70 Ma) Pacific seafloor. An appropriate filter designed using the elastic properties of the underlying sediments, and obtained from prior studies, greatly improves the detection of Ps conversions generated from the moho (~8.6 km) and from a sharp discontinuity (<~ 5 km) across the lithosphere-asthenosphere transition (~72 km). Sensitivity tests show that the filter is robust to small errors in the sediment properties. Our analysis suggests that appropriately filtering out the sediment reverberations from ocean seismic data could make inferences on subsurface structure more robust. We expect that this study will enable high-resolution receiver function imaging of the base of the oceanic plate across a growing fleet of ocean bottom seismic arrays being deployed in the global oceans.

How to cite: Zhang, Z. and Olugboji, T.: The Signature and Elimination of Sediment Reverberations on Submarine Receiver Functions: Imaging the Lithosphere of a Normal Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3157, https://doi.org/10.5194/egusphere-egu21-3157, 2021.

Andrey Goev

The Kola region of the Russian Arctic is located in the northeast of the Baltic Shield and is widely known for its unique geology in regards to the presence of massive Paleozoic intrusions. Multidisciplinary researches have been carried out to provide a comprehensive reconstruction of Khibiny and Lovozero plutons’ formation and their structure models The main source of geochronological data comes from isotope analysis of the arrays’ rocks. The amount of research focuses on the deep structure beneath the Khibiny pluton is scarce. To investigate velocity structure of the investigated region we used receiver function technique. Essence of the method is to analyze P-S (PRF) and S-P (SRF) converted waves form seismic boundaries along with their multiples. For the given research we used seismograms of the teleseismic events recorded by the Apatity (APA) and Lovozero (LVZ) broadband seismic stations since 2000. We selected 220 and 232 individual PRF;147 and 122 individual SRF for LVZ and APA station respectively. As both LVZ and APA are located relatively close to each other, we combined all 452 PRF to get a robust estimation of delay times of P410s and P660s phases. Our estimations of P410s and P660s phases are 43.6 and 67.6 sec respectively. Delay time between these phases is 24 sec that is close to “standard” according to the IASP91 model. The individual times of each phase are slightly less than predicted by IASP91 (by 0.4 sec) and could indicate an increase of velocities in the upper mantle, but it is not unusual for cratonic regions. Joint inversion of PRF and SRF was used to restore velocity sections for the depth up to 300 km. All models have shown a gradient increase in velocities in the earth's crust and sharp crust-mantle boundary at depth of 40 ± 1 km with a velocity jump from 3.9 to 4.4 km/s. The most prominent feature of the upper mantle structure is the presence of the low-velocity zone at a depth from 90 to 140 km. One of the possible explanation of this discontinuity could be the presence of deep fluids and the high porosity of this zone. This study was partially supported by the RFBR grant 18-05-70082 and the SRW theme No. АААА-А19-119022090015-6.

How to cite: Goev, A.: Lithosphere velocity structure of the Khibiny and Lovozero plutons (Eastern part of the Baltic shield) from receiver functions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9201, https://doi.org/10.5194/egusphere-egu21-9201, 2021.

Luan C. Nguyen et al.

The Gulf of Mexico formed as a result of continental breakup between the North and SouthAmerican plates and a short period of seafloor spreading in the Late Jurassic-Early Cretaceous. This small ocean basin offers an opportunity to further our understanding of continental rifting processes and the geologic evolution of continental margins during and after rifting. However, previous knowledge of lithospheric structure has been limited to crustal investigations. We constructed a 3D shear-wave velocity model for the Gulf of Mexico region using cross-correlations of the ambient noise field and measurement of vertical component Rayleigh wave phase velocities in the period band 15 to 95 s. We employed continuous data recorded by more than 500 stations in seismic networks in the US, Mexico and Cuba. Our model shows distinct variation in lithospheric structures that reliably identify and constrain the properties of extended continental and oceanic domains. We estimate the depth of the lithosphere-asthenosphere boundary to be in the range of 85-100 km with the thinnest lithosphere under the oceanic region. A low velocity zone is observed below the lithosphere centered at ~150 km depth with a minimum shear-wave velocity of ~4.45 km/s. Lithospheric mantle underlying the offshore Texas Gulf Coast between oceanic lithosphere and unextended continental lithosphere is characterized by reduced shear-wave velocity. This might indicate that extension resulted in permanent deformation of the continental lithosphere. The differential thinning between the crystalline crust and mantle lithosphere suggests that the extended continental lithosphere has cooled and thickened by approximately 30 km since breakup.

How to cite: Nguyen, L. C., Levander, A., Niu, F., and Li, G.: Crustal and lithospheric architecture of the Gulf of Mexico and its continental margins from ambient noise Rayleigh wave tomography, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13785, https://doi.org/10.5194/egusphere-egu21-13785, 2021.

Ehsan Qorbani et al.

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. 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 2021, online, 19–30 Apr 2021, EGU21-9817, https://doi.org/10.5194/egusphere-egu21-9817, 2021.

Mingju Xu et al.

Lithosphere motion is one of the fundamental processes in Earth tectonics. To understand the processes involving the nature of tectonic evolution and dynamics, it is critical to figure out the lithosphere flexure of tectonic plates. Over long-term (> 105 yr) geological timescales, the lithosphere can be modelled as flexing like a thin, elastic plate, using the partial differential equation for flexure of an orthotropic plate. The partial differential equation is used indirectly to form theoretical admittance and coherence curves, which are then compared against the observed admittance and coherence to invert a non-uniform flexural rigidity (or effective elastic thickness, Te) plate. The non-uniform flexural rigidity lithosphere flexure amplitude can be estimated after that.

In this presentation, we use the classic lithosphere model with applied surface load at ground and internal load at Moho, but assume that the compensation material is denser than the mantle material beneath Moho. The density contrast between compensation material and mantle material beneath Moho is set to be 200 kg/m3 referring to the density contrast of the uppermost and bottom lithosphere mantle. In such a lithosphere model, errors of lithosphere flexure estimation are mainly contributed by the errors of Te and Moho recovering. Synthetic modelling is then performed to analyze the incoming influence deriving from Te and Moho errors.

The synthetic modelling reflects 1) the lithosphere flexure estimation errors are not sensitive to the errors of Te recovering, even an error of about 10 km of Te only result in an error within 1km of lithosphere flexure, 2) the influence of Moho errors to lithosphere flexure errors will be magnified in regions where Te is low, as lithosphere flexure errors over 1km mainly occur in regions where Te is lower than 8km.

How to cite: Xu, M., Wu, Z., Ji, F., Ruan, A., and Li, C.: Lithosphere flexure estimation of an non-uniform flexural rigidity plate:A quantitative modeling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14023, https://doi.org/10.5194/egusphere-egu21-14023, 2021.

Olga Usoltseva and Vladimir Ovtchinnikov

Study of the contact zone between the inner and outer core represents considerable interest for understanding of properties, structures and dynamic of the Earth's core. One of the sources of the data about the processes proceeding in the top part of the inner core is the seismic wave PKIIKP once reflected from an undersize inner core boundary. Amplitudes of these waves are sensitive to the shear velocity in the top part of the inner core and are small. Therefore their identification at a single seismic station is not reliable without application of additional methods of analysis. Significant in this regard is the discussion about the source (in inner core or in mantle) of anomalous arrivals detected at the TAM station in North Africa [1,2] in the time range of PKIIKP phase.

To estimate influence of model parameters (S and P seismic velocity) on the characteristics of PKIIKP wave (amplitude and travel time) we calculated sensitivity kernels for upper mantle and inner core for dominant period 1.2 s, azimuth step 0.2 degrees and radius step 20 km by using DSM Kernel Suite algorithm. It was revealed that PKIIKP amplitude is more sensitivities to mantle heterogeneities than to inner core ones. For reducing the effects of the overlying structures we suppose to use а joint analysis PKIIKP and pPKIIKP waves. With this approach, an incorrect identification of the PKIIKP wave is most likely excluded. We demonstrate the effectiveness of the approach on the example of processing the seismogram of the 11.02.2015 earthquake reсorded at the GZH station in China at a distance of 179.4 degrees.

1. Wang W., Song X. Analyses of anomalous amplitudes of antipodal PKIIKP waves, EaPP. 2019. V. 3. P. 212-217. doi: 10.26464/epp2019023

2. Tsuboi S., Butler R. Inner core differential rotation inferred from antipodal seismic observations, PEPI, 2020. V.301. 106451.

How to cite: Usoltseva, O. and Ovtchinnikov, V.: On the reliability of PKIIKP phase identification at a single station, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6554, https://doi.org/10.5194/egusphere-egu21-6554, 2021.

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