GD7.3

The Hindu-Kush and Pamir are located north of the western syntaxis of the Himalaya, representing one of the most active continental collision zones involving a complicated lithosphere deformation history. Based on the increased seismic data coverage in this region we employ the Multi-Scale Full Waveform Inversion Scheme (MSFWI) to investigate the seismic structure of the crust and uppermost mantle using earthquake waveforms (12-100s) and cross-correlation Green’s Function derived from ambient noise (10-80s). Through the MSFWI joint inversion, we provide high-resolution images for isotropic Vp and radial anisotropic Vs (Vsv and Vsh).

We image the subducting Hindu-Kush slab beneath the interaction zone of the Hindu-Kush and Tajik-Basin at depth and a thin and relatively low-velocity layer is detected on top of the subducting lithosphere, hosting the intense intermediate depth seismicity, indicating the subducting lower crust of the Hindu-Kush slab. The transition from relatively low-to-high velocity indicates the termination of eclogitization of the subducting crust accompanied by a gradual increase of negative buoyancy causing a slab break-off at a depth of around 150 km. This process is ongoing and accompanied by a deep seismicity cluster. Atop of the Hindu-Kush subducting system, low-velocities are imaged within the lower continental crust, dipping to the southeast. This gently dipping low-velocity layer connects the collision zone of the Hindu-Kush and Indian plate, hinting at a complicated lower crust subduction process, which is also accompanied by a very deep Moho up to 80 km.

Beneath the Central Pamir, a narrow low-velocity zone in the lower crust and uppermost mantle (down to 100 km) follows the curvature of the intermediate-depth seismicity and suture (and thrust faults), marking the active collision position of the Indian-Asian plates, which resulted in an exhumation and significant crustal thickening. The thin and southward dipping low-velocity zone in the uppermost mantle is also consistent with the intermediate seismicity, illuminating the subducting lower crust of the Asian plate while meeting the rigid Indian indentation. 

Meanwhile, a strong sharp transition from high-to-low velocity coinciding the Talas-Ferghana fault at mantle lithospheric depth delineates the transition from the Ferghana basin into the Central Tien Shan, indicating the large scale lithosphere delamination beneath the whole Central Tien Shan with some lithospheric remnants existing beneath the central part of Central Tien Shan. This remnant high-velocity lithosphere possibly indicates that the deformation for the Central Tien Shan mainly concentrated on the south and north end due to the compression from the Tarim basin and Kazakh Shield, respectively.

How to cite: Gao, Y., Tilmann, F., Yuan, X., Schurr, B., Rietbrock, A., Fichtner, A., Li, W., Schneider, F., Kufner, S.-K., Thrastarson, S., and van Herwaarden, D.-P.: Seismic structure in the crust and upper mantle beneath the Hindu Kush and Pamir from Full Waveform Inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11377, https://doi.org/10.5194/egusphere-egu22-11377, 2022.

During the Cenozoic, the Circum-Mediterranean and its periphery have experienced extensive and widespread anorogenic igneous magmatism that reflects the response of the upper mantle to the geodynamic evolution of this area. The exact origin of the volcanic activities and its relation to the underlying thin lithosphere especially in the continental areas have been long-lasting debated. We investigate the structure of the Mediterranean lithosphere and the sub-lithospheric mantle by surface waves that are mainly sensitive to the 3-D S-wave velocity structure at those depths. A high-resolution tomographic study based on automated broad-band measurements of inter-station Rayleigh wave phase velocities down to about 300 km depth is presented. We identify shallow asthenospheric volumes, characterized by low S-wave velocities between about 70 km and 250 km depth, and distinguish between five major shallow asthenospheric volumes in the Circum-Mediterranean: the Middle East, the Anatolian-Aegean, the Pannonian, the Central European, and the Western Mediterranean Asthenosphere volumes. Remarkably, they form an almost continuous circular belt of asthenospheric areas interrupted only by the thick Permo-Carboniferous oceanic lithosphere in the eastern Mediterranean.

Integrated thermochemical modelling using surface wave phase velocities, topography, and heat flow as constraints indicates a remarkable variability of the lithospheric thickness across the area. Thick lithosphere is found in the Paris Basin, the East European Craton, and the eastern Mediterranean whereas thin lithosphere is found in areas of pronounced negative shear-wave anomalies at depth between 70 km and 200 km. Cenozoic intraplate volcanic fields are located in areas with thin lithosphere underlain by shallow asthenosphere. Thus, anorogenic intraplate volcanism in the Circum-Mediterranean appears to be associated with thin and hot lithospheric regions and low S-wave sublithospheric velocities. The distribution and properties of the shallow asthenosphere volumes in the region are discussed and related to the spatial-temporal occurrence of intraplate as well as subduction related volcanism in the western Mediterranean, central Europe, the Pannonian Basin, the Anatolian region and the Middle East.

How to cite: El-Sharkawy, A., Hansteen, T., Clemente-Gomez, C., Fullea, J., Lebedev, S., and Meier, T.: Spatial Correlation between Intraplate Volcanism and Thin Lithosphere in the Circum-Mediterranean: New Evidences from Surface Wave Tomography and Thermomechanical Modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8282, https://doi.org/10.5194/egusphere-egu22-8282, 2022.

Mantle viscosity controls a variety of geodynamic processes such as glacial isostatic adjustment (GIA), but it is poorly constrained because it cannot be measured directly from geophysical measurements. To improve viscosity estimates, we develop a method that computes viscosity using empirical viscosity flow laws and mantle parameters (temperature and water content) inferred from geophysical observations. We find that combining both seismic and magnetotelluric constraints allows us to place significantly tighter bounds on viscosity estimates compared to either geophysical observation by itself. In particular, electrical conductivity inferred from MT data can determine whether upper mantle minerals are hydrated, which is not seismically detectible but significantly reduces viscosity. Additionally, we show that rock composition should be considered when estimating viscosity from geophysical data because composition directly affects both seismic velocity and electrical conductivity. Therefore, temperature and water content is more uncertain for rocks of unknown composition, which makes viscosity also more uncertain. Furthermore, calculations that assume pure thermal control of seismic velocity may misinterpret compositional heterogeneity for temperature variations, producing erroneous predictions of mantle temperature and viscosity. Stress and grain size also affect the viscosity and its associated uncertainty, particularly via their controls on deformation regime. Overall, mantle viscosity can be estimated best when both seismic and MT data are available and the mantle composition, grain size and stress can be estimated. Collecting additional MT data probably offers the greatest opportunity to improve geodynamic or GIA models that rely on viscosity estimates.

How to cite: Ramirez, F., Selway, K., Conrad, C., and Lithgow-Bertelloni, C.: Constraining Upper Mantle Viscosity Using Temperature and Water Content Inferred from Seismic and Magnetotelluric Data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2420, https://doi.org/10.5194/egusphere-egu22-2420, 2022.

While the lateral limits of tectonic plates are well mapped by seismicity, the bottom boundary of the lithosphere, the uppermost rigid layer of the Earth comprising both crust and shallow mantle, remains elusive. Lithospheric plates are usually viewed as cold, rigid, internally undeformed blocks that translate coherently. The base of the lithosphere, designated as the lithosphere-asthenosphere boundary (LAB), could thus theoretically be characterized from either temperature, viscosity, strain rate and horizontal velocity.

Several LABs as defined from these different fields are investigated here using thermo-mechanical models of plate and upper mantle dynamics, either in a transient subduction or in a steady-state plate-driven set-up. Mantle material is modelled as homogeneous in composition with a viscosity that depends on temperature, pressure and strain rate. In such systems, the thermo-mechanical transition between lithosphere and asthenosphere occurs over a finite depth interval in temperature, strain rate and velocity. We propose that the most useful dynamical LAB is defined as the base of a “constant-velocity” plate (i.e. the material translating at constant horizontal velocity). The bottom part of this plate deforms at strain rates comparable to those in the underlying asthenosphere mantle: the translating block is not fully rigid.

Thermal structure exerts a major control on this dynamical LAB, which deepens with increasing plate age. However, the surface plate velocity and more generally the asthenospheric flow geometry and magnitude also impact the depth of the dynamical LAB, as well as the thickness of the deformed region at the base of the constant-velocity plate. Moreover, the mechanical transitions from lithosphere to asthenosphere adjust when mantle dynamics evolves.

The dynamical and thermo-mechanical LABs occur within a thermal lithosphere-asthenophere gradual transition, in agreement with the results obtained from geophysical proxies. The concept of a constant-velocity plate can be extended to a constant-velocity subducting slab, which also deforms at its borders and drags the surrounding mantle. This dynamical definition of a lithospheric plate is relevant to interpret mantle seismic anisotropy in terms of (past) flow direction, and to quantify mass transport within the Earth’s mantle.

How to cite: Garel, F. and Thoraval, C.: What is a plate ? Dynamical definition of the transition between lithosphere and asthenosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6274, https://doi.org/10.5194/egusphere-egu22-6274, 2022.

The North Tanzanian Divergence (NTD) is a rift initiation zone situated at the southern tip of the Eastern Branch of the East African Rift. This zone is a unique continental open-air laboratory to study the beginning of the continental break-up. The rift surface expression results from the interaction between tectonic and magmatic processes. However, the role of each process on the observed surface activity is still debated, as their respective signal is difficult to differentiate. In order to consider the various factors that may interact in this complex zone, a multi-disciplinary study was carried out, combining seismological and petrophysical approaches.

First, our recent development of a new hybrid tomographic method for both P and S-body waves permitted to image at depth the main suture zones between the inherited structures (Archean craton and Proterozoic orogenic belts) and the mantle plume extension (Clutier et al. 2021). We also inferred zones of fluid (melt or gas) presence from the Vp/Vs ratio maps deduced from these P and S independent inversions. Then, to quantify the proportion of fluid from the tomographic images, we carried out a petrophysical study on mantle xenoliths from the Pello Hills volcano, situated in the rift axis. The clinopyroxene-amphibole-phlogopite vein-bearing xenoliths allowed to compute, at a sample scale, the seismic properties of the mantle with and without crystallised or fluid-filled veins. By varying the composition and increasing the proportion veins in the samples, the P and S-wave maximum velocities can decrease from 9.2 down to 5.3 km/s and from 5.1 down to 3.1 km/s, respectively. Those velocity models point out anisotropy in the mantle below the NTD, and particularly in highly metasomatized zones. Finally, despite the difference in spatial and temporal scales between the petrological and geophysical studies, we managed to combine the tomographic velocity anomalies and the xenolith’s seismic properties to infer a maximum volume of fluid in the lithospheric mantle below Pello Hills volcano. This volume may be intermediate between 20% of clinopyroxene-phlogopite-amphibole crystallised vein and 10% melt/fluid-filled vein.

How to cite: Clutier, A., Gautier, S., Parat, F., and Tiberi, C.: Seismological and petrophysical properties of the lithospheric mantle in a nascent rift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7462, https://doi.org/10.5194/egusphere-egu22-7462, 2022.

The classic definition of plate tectonics suggests that mid-ocean ridges (MORs) are places of passive mantle upwelling driven by plate divergence, and that the oceanic lithosphere forms by conductive cooling away from the ridge. This model predicts the symmetry of the partially-molten region beneath the ridge axis, and the lithosphere thickening with age (i.e., half-space cooling model). New and classic observations show some inconsistency with these predictions. Here we present dynamic, two-phase flow numerical models of MORs that reconcile theory and observations by incorporating buoyancy-driven flow associated with temperature, composition and porosity.

First, geophysical observations at various MOR segments indicate strong asymmetry in melt production, upwelling and seamount distribution across the axis at fast spreading centers such as the MELT region (Melt Seismic Team, 1998), intermediate-spreading centers such as Juan de Fuca Ridge (Bell et al., 2016) and the Mid-Atlantic Ridge (Wang et al., 2020), and slow-spreading centers such as the Mohns Ridge (Johansen et al., 2019). Passive flow models cannot explain this asymmetry, as they require unrealistically large forcing (Toomey et al., 2002).

Second, both seismic and electromagnetic studies have inferred variations in the lithosphere-asthenosphere boundary (LAB) and plate thickness that do not monotonically increase with age (e.g., Rychert et al., 2020). Sublithospheric small-scale convection (SSC) is generally the preferred explanation of these oscillations (e.g., Parsons and McKenzie, 1978, Likerman et al., 2021). However, seismic anomalies cannot be explained using solely solid-state thermal variations. While other mechanisms have been proposed to match the sharp discontinuities in seismic data, small amounts of melt (1-5.5%) could be the most straightforward explanation (Rychert et al., 2021). Sub-plate partial melt could also explain the cause of intraplate volcanism or petit-spot volcanoes observed on the outer rise in some subduction centers (Hirano et al., 2006). 

We show that melting-induced buoyancy effects may provide an explanation for both the asymmetric distribution of melt beneath the axis and LAB variations. Here, we extend our 2D mid-ocean ridge calculations to incorporate chemical (residue depletion) and thermal buoyancy, in order to investigate how the dynamics of melt generation and migration may influence small-scale convection at the LAB.

We run two types of models: closer to the ridge axis, where melt is generated over an extended region, and further away from the axis, where active flow may induce small amounts of partial-melting. Results show that MOR models with both chemical and porous buoyancy are sensitive to background forcing and can readily induce asymmetry and small-scale, time-dependent convection beneath the axis. Melting and crystallization of enriched material leads to a dynamic LAB closer to the ridge axis. Models of older oceanic LAB are more susceptible to the influence of thermal instabilities, which can erode the lithosphere and limit the base of the ocean lithosphere from cooling. 

References

Bell et al., 2016, DOI:10.1002/2016JB012990

Hirano et al., 2006, DOI:10.1126/science.1128235

Johansen et al., 2019, DOI:10.1038/s41586-019-1010-0

Likerman et al., 2021, DOI:10.1093/gji/ggab286

Melt Seismic Team, 1998, DOI:10.1126/science.280.5367.1215

Parsons and McKenzie, 1978, DOI:10.1029/JB083iB09p04485

Rychert et al., 2020, DOI:10.1029/2018JB016463

Rychert et al., 2021, DOI:10.1016/j.epsl.2021.116949

Toomey et al., 2002, DOI:10.1016/S0012-821X(02)00655-6

Wang et al., 2020, DOI:10.1029/2020GC009177

How to cite: Pusok, A. E., Dale, K., Katz, R. F., May, D. A., and Li, Y.: The role of buoyancy-driven flow at the lithosphere-asthenosphere boundary: from mid-ocean ridge to old sub-lithosphere models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4614, https://doi.org/10.5194/egusphere-egu22-4614, 2022.

Earth science has been heavily data-driven due to the abundance in data. Yet, when there are relatively a small number of hypotheses to verify, the inverse problem becomes a classification problem. It is then worth directly examining observed seismic data against predicted data. Concretely, we chain forward modelling from geodynamics  to seismology. We call this process ‘waveform Seismic Low Filtering of Earth’s models’ (SeLFiE). We take seismic signals of the snapshots of forwardly generated Earth models with that of the actual Earth, as if we took a photo of ourselves. Although there have several studies on how the seismological tomographic technique can perceive the geodynamical models, there are few studies on the seismic waveform sensitivity to geodynamical or petrological parameters. A pilot test of our SeLFiE methodology was encouraging, since we used only one seismic station to constrain the melt transportation manner beneath the Réunion island (Franken et al. 2020). Here in this contribution we present our strategy and developed tools towards the waveform filtering that have been developed during and after the CLEEDI week in August, 2020.

How to cite: Fuji, N., Dhabaria, N., Roncoroni, G., Myhill, R., Durand, S., Borgeaud, A., Tackley, P., Nakagawa, T., and Deschamps, F.: Towards waveform seismic filtering of mantle convection models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3554, https://doi.org/10.5194/egusphere-egu22-3554, 2022.

Seismic tomography is a powerful tool to study the deep Earth, given the lack of direct observations. Seismic structures can be interpreted together with constraints from other disciplines, such as geodynamics and mineral physics, to provides valuable information about the structure, dynamics and evolution of the mantle. Nevertheless, a robust physical interpretation of seismic images remains challenging as tomographic models typically lack uncertainty information and may have biased amplitudes due to uneven data coverage and regularisation.

We aim to build tomographic models of the mantle with associated uncertainties and unbiased amplitudes. For this, we use the SOLA method (Zaroli, 2016) applied to normal mode data, the Earth’s free oscillations. SOLA is based on a Backus-Gilbert approach, which explicitly constrains the amplitudes to be unbiased and inherently computes the model uncertainty and resolution. This approach enables us to perform meaningful physical interpretations of the imaged structures. By applying this method to normal modes, we obtain valuable insights on the long wavelength structure of the mantle. The use of normal modes also has several advantages: these data are sensitive to multiple parameters, including both Vs and Vp anisotropy as well as density, and they provide global data coverage.

Here, we report on our progress towards a new 3-D mantle model based on the inversion of normal mode splitting function data. We discuss initial results from synthetic tests and isotropic inversions in terms of model estimates, uncertainties and resolution.

How to cite: Restelli, F., Koelemeijer, P., and Zaroli, C.: Normal mode models of the mantle using Backus-Gilbert tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8734, https://doi.org/10.5194/egusphere-egu22-8734, 2022.