Seismic anisotropy provides a unique link between directly observable surface structures and the more elusive dynamic processes in the mantle below. The ability to infer the vertically- and laterally-varying anisotropic structures is of great significance for the geodynamic interpretation of surface-recorded waveform effects.

In the first part of this presentation, we assess the capabilities of different observables for the inversion XKS phases to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for simple models of upper-mantle anisotropy. In addition to waveforms, we consider the effects on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully constrain the anisotropic parameters of a given model, even if complete waveforms are considered. We also discuss advantages and disadvantages of using the different observables.

Recent technological advances have prompted implementations of large-scale seismic experiments producing huge amounts of seismic data. Standard processing procedures, thus, require automatization to facilitate fast and objective data processing. This also applies to the analysis of shear-wave splitting. A recent extension of the SplitRacer software code allows for an automatization of the analysis by choosing a time window based on spectral analyses and by categorization of results based on different splitting methods.

Finally, we will present new results from the application of Neural Networks to the analysis of shear-wave splitting. Our initial approach involves training based on synthetic data and deconvolution of the real waveforms. Current limitations and possibilities for extension will be discussed.

How to cite: Rümpker, G., Kaviani, A., Link, F., Reiss, M., Chakraborty, M., Faber, J., Köhler, J., and Srivastava, N.: Analysis of shear-wave splitting to infer the seismic anisotropy of the lithosphere-asthenosphere system – inversion ambiguities, automatization, and machine-learning approaches, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16052, https://doi.org/10.5194/egusphere-egu23-16052, 2023.

We investigate the flow field and deformation in the mantle wedge and subslab mantle beneath the Kuril-Kamchatka subduction zone using seismological data from a recently deployed seismic network around the Klyuchevskoy Volcanic Group (KVG) complemented by data from previous temporary deployments and permanent stations to reach a total number of 145 seismic stations covering a region defined in the geographic coordinates 150°-167°E and 50°-61°N.

We perform splitting analysis of both local and core-refracted (SKS) shear waves to study mantle seismic anisotropy as a proxy for the pattern of the mantle flow field and deformation. Anisotropy in the mantle wedge is studied by shear splitting analysis (SWS) of waveform data from local mantle events that occurred along the subducting slab (Wadati-Benioff-Zone) and in the mantle wedge. Crustal anisotropy is also studied by SWS analysis of crustal events. The combined data set (SKS and local) allows us to discriminate the source of mantle anisotropy (sub-slab, mantle wedge, or crust). Shear-wave splitting measurements from the local shear waves give small delay times independent of the depth of the events suggesting that the mantle wedge is characterized by a weak anisotropic fabric. The fast directions of mantle wedge anisotropy are predominantly parallel to the strike of the slab indicating either a trench-parallel flow or B-type seismic anisotropy in the mantle wedge. The relatively small delay times from local shear waves suggest that SKS waves are less affected by potential anisotropy in the mantle wedge and that the results of the SKS-splitting analysis are mainly representative of the sub-slab anisotropy. Our SKS-splitting measurements indicate a trench-normal mantle flow beneath the eastern edge of the Kamchatka peninsula that converts to a more complex pattern beneath the KVG region. We argue that this pattern of fast polarization direction suggests the rotational mantle flow beneath the slab that may be related to the change in slab geometry at the junction between the Kuril-Kamchatka and Aleutian arcs. The observation of relatively strong sub-slab anisotropy against weak mantle-wedge anisotropy suggests that slab termination causes some disturbance in mantle flow; however, no significant component of an around-slab flow occurs in the mantle wedge.

How to cite: Kaviani, A., Rümpker, G., Sens‐Schönfelder, C., Abolfazl Komeazi, A. K., and Shapiro, N.: A study of the mantle flow field and lithospheric deformation beneath the Kuril-Kamchatka subduction zone using seismic anisotropy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15669, https://doi.org/10.5194/egusphere-egu23-15669, 2023.

The complex tectonic structure of eastern Anatolia results from the superposition of subduction and collisional structures along a long-lived convergent margin between the Gondwanan (Arabian) and Eurasian plates. The geodynamic processes shaping the tectonic setting and uplifting history of the region still remain enigmatic despite the fact that the number of geophysical, geological, and petrographic-based models/interpretations in recent years has increased notably. Further issues, i.e., how the spatiotemporal patterns of seismic activity are controlled by pre-existing deformational zones in the lithosphere and/or modern convergent stresses, and how magmatism is related to the lithospheric variability along the margin, are unclear. Models of seismological features of the Earth’s interiors provide insights on isotropic heterogeneity that are of great importance for constraining the current physical and chemical conditions, as they likely control the localization of structures. For this purpose, the present study aims to constrain lateral variations of crustal thickness, Moho topography, and average seismic velocities (Vp, Vp/Vs) by leveraging information from both teleseismic scattered (receiver function) and reflected (autocorrelation) waves (H-k-Vp stacking). Incorporating teleseismic autocorrelation waveforms from the P-wave coda, we can better constrain average crustal P-wave velocities (Vp) by highlighting the amplitude term of the Moho-reflected Pmp phase. Our dataset consists of digital waveforms extracted from 512 teleseismic events (within the epicentral distance range from 30° to100° and with Mw>6) observed at 33 permanent broadband seismic stations operated under the KOERI network between 2013 and 2022 and will result in a new map of crustal architecture and its physical properties (crustal thickness, Vp, and Vp/Vs) below eastern Anatolia. Preliminary results indicate a thickening crust from south to north reaching down to depths of ~50 km. High Vp/Vs ratios mark volcanic provinces as well as fault damage areas presumably characterized by highly fractured rocks with high amounts of water content. Lateral variations of P-wave velocities along two continental fault zones (EAFZ and NAFZ) of the region imply that the degree of shear deformation and resultant seismic activity is well-correlated with density/seismic wave speed variations. Moho depth variations across the NAFZ further suggest a much narrow and localized distribution of deformation in the lower crust and upper mantle compared to the EAFZ. Further analysis of these results will lead to a better understanding of the controlling mechanisms behind seismicity and magmatism in the Eastern Anatolian Plateau.

How to cite: Aygün, H., Eken, T., Keleş, D., Kaya-Eken, T., Cammarano, F., Delph, J. R., and Taymaz, T.: Crustal Features of Eastern Anatolia based on a Joint Grid Search Performed over Receiver Functions and P-wave Coda Autocorrelation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8306, https://doi.org/10.5194/egusphere-egu23-8306, 2023.

The western edge of the Baltic Shield is covered by the northeast – southwest oriented, 2500 m high mountain range, the Scandes at the northwestern Atlantic Ocean. This mountain range is located far from any active plate boundary and lack of sedimentary sequences precludes direct knowledge of the timing of uplift.

We present a crust and upper mantle scale velocity model, obtained along thea 600 km long Silver-Road seismic profile, which extends in a WNW to ESE direction in the northeastern Baltic Shield perpendicular to the coast between 8^oE and 20^oE. The profile has a 300 km long offshore section on the continental shelf and the deep ocean as well as a 300 km onshore section across Caledonian to Svecofennian units. The seismic data were acquired with 5 onshore explosive sources and offshore air gun shots from the vessel Hakon Mosby along the whole offshore profile. Data was acquired by 270 onshore stations at nominally 1.5 km distance and 16 ocean bottom seismometers on the shelf, slope and oceanic environment. The results of this study will provide new input to interpretation of the anomalous topography the Scandes and continental shelf in the northeast Baltic Shield.

We present results of ray tracing and gravity modeling along the profile. The vertical crustal structure in the upper, middle and lower crust are almost constant across the Caledonian and Svecofennian parts of the profile. The crust is 45 km thick along the whole onshore profile and abruptly thins to 25 km thickness in the continental shelf. Pn velocity is low ~7.6-7.8 km/s below the high topography areas with Caledonian nappes, whereas it is 8.4 km/s below the Svecofennian parts. Our gravity models, based on the seismic velocity structure, suggest a low density 3.20 g/cm³ for the low Pn zone below the high Caledonian topography in contrast to the very high density 3.48 g/cm³ below the Svecofennian parts with relatively low topography. We interpret these bodies as eclogitizised basaltic crustal material at different metamorphic grades. Isostatic calculation with a 60 km depth compensation depth predicts 2 km high topography which is ~1 km higher than observed. We therefore propose that the low-grade metamorphic unit below the high topography is underlain by a sequence with relatively high mantle density to 120 km depth.

How to cite: Kahraman, M., Thybo, H., Artemieva, I., Shulgin, A., Hedin, P., and Mjelde, R.: Crustal Structure across Central Scandinavia along the Silver-Road refraction profile, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15239, https://doi.org/10.5194/egusphere-egu23-15239, 2023.

It is well known that oscillating stress sources play a relevant role in the stability of mechanical systems. The Earth is routinely subject to stress loading due to tides, hydrological cycles, atmospheric pressure variations and anthropical activities. However, the shallow part of our planet is far from being a simple system, so each component showcases a different response to perturbations depending on its physical properties. Macroscopically, the outer layers of the Earth form a two-tier system with respect to periodic stress changes: the brittle crust reacts forthwith to additional loads; conversely, the viscous lithosphere behaves as a low-pass filter. Such a dichotomy produces a wide range of different geodynamic, tectonic, and seismological processes. Seismicity becomes more and more sensitive to stress perturbations as strain accumulates so that earthquakes tend to occur, on average, during phases close to stress peak. We analyse the effect of solid and liquid tides in modulating seismicity during the seismic cycle in several regions of tectonic interest. Our study shows that the correlation between the amplitude of tidal CFS and seismic energy rate usually increases before large shocks, while it undergoes drops during foreshock activity and after the mainshock. A preseismic phase, featured by increasing correlation, is detected before large and intermediate (Mw > 4.5) shallow earthquakes in about 2/3 of cases. The duration of the anomaly T appears to be related to the seismic moment M of the future mainshock via the relationship T ∝ M^0.3 if the magnitude of the largest event is below 6.5. This power exponent, 1/3, is typical of seismic nucleation scaling of single seismic events; therefore, the increase of correlation between seismic rates and tidal stress on fault may be understood in the light of diffuse nucleation phases throughout the crust due to incoming large-scale destabilization. We also consider tremors and low-frequency earthquakes in the Cascadia region along the West coasts of British Columbia, Washington, Oregon and Northern California and the Nankai thrust in Japan. Their sensitivity to stress perturbations increases as the surrounding fault interface is seismically locked, showing an analogous response to fast seismic events. On the other hand, viscous layers of the lithosphere are almost unresponsive to high-frequency stress perturbations (e.g., at least up to annual periods); however, they can flow plastically under the action of long-lasting loading: it is the case of low-frequency Earth tides (e.g., lunar nodal 18.61-years-long cycle) which can be detected as millimetric modulations in relative plate velocities using single-station- and baseline- modes GNSS time series. On the light of thin ultralow viscosity zones spreading at the lithosphere-asthenosphere boundary and inside the asthenosphere, and of thermally active small-cell stratified convection in the super-adiabatic zones of the upper mantle, it is reasonable that such modulations may have geodynamic implications. This conclusion is also supported by several observations proving a worldwide asymmetry in global geodynamics such as the westerly oriented motions of plates which follow a mainstream with a 0.2-1.2°/Myr drift relative to the sub-asthenospheric mantle in the hotspot reference frame.

How to cite: Zaccagnino, D. and Doglioni, C.: Oscillating tidal stress loading on the lithosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-218, https://doi.org/10.5194/egusphere-egu23-218, 2023.

Cratons are the ancient cores of continents, stable over billions of years. The thermochemical properties of their lithosphere are debated, with a number of open questions regarding their composition, the presence of volatiles and the degree of metasomatism.  Cratonic mantle lithosphere is thought to be dominated by depleted mantle peridotites, primarily harzburgites, which can provide chemical buoyancy and, therefore, long-term stability. Some recently proposed models, however, featured substantially metasomatised shallow mantle lithosphere, modified by the addition of volatiles (Eeken et al. 2018) or significant proportions of eclogite and diamond within the lithosphere (Garber et al. 2018). The broad range of the compositions proposed highlights the persisting uncertainty over what cratons are made of.

Arguments for cratonic lithosphere complexity often follow from difficulties in fitting seismic velocity profiles (taken from tomographic models beneath cratons) using peridotitic compositions. Some Rayleigh-wave inversions have also found difficulty fitting phase velocity dispersion curves without significant metasomatism, including models with up to 5wt% CO₂.

Recently developed methods of petrological inversion can relate geophysical and geological observations directly to the thermochemical structure of the lithosphere and asthenosphere. Here, we invert Rayleigh and Love surface wave phase velocities, elevation and heat flow data for temperature and composition at depth (Fullea et al. 2021) beneath a selection of cratons around the world and a global craton average. We aimed to assemble the most accurate surface-wave dispersion data, with broad period ranges and small errors. The models fit the data within 0.1-0.2% of the phase-velocity values. This accuracy is important in order to extract the information on the radial structure of the lithosphere from the dispersion data.

Our models use a harzburgitic (depleted peridotite) composition with major oxide weight percentages taken from prior global modelling (Fullea et al. 2021) and produce very close fits for the Rayleigh and Love dispersion curves averaged over cratons globally, as well as the Rayleigh and Love dispersion data measured in several cratons around the world. The cratonic lithospheric thicknesses range from 180 km (Guyana) to almost 300 km (Congo). We demonstrate that these new models can also be produced by careful regularisation of purely seismic inversions of the same data.

Our results do not rule out extensive metasomatism in the cratonic uppermost mantle but suggest that it is likely to be a rare anomaly in particular locations, rather than a common occurrence. Ubiquitous presence of substantial quantities of eclogite and diamond in cratonic lithosphere is not required by the data.

References:

Eeken, T., et al., 2018. Seismic evidence for depth-dependent metasomatism in cratons. Earth Planet. Sci. Lett. 491, 148-159.

Fullea, J., Lebedev, S., Martinec, Z. et al., 2021. WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophys. J. Int. 226, 146-191.

Garber, J.M., et al., 2018. Multidisciplinary constraints on the abundance of diamond and eclogite in the cratonic lithosphere. Geochem., Geophys., Geosyst. 19, 2062-2086. 

How to cite: Davison, F., Lebedev, S., Xu, Y., Fullea, J., and Gibson, S.: The Case of the Missing Diamonds: New global and regional thermo-compositional models of cratonic lithosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6733, https://doi.org/10.5194/egusphere-egu23-6733, 2023.

The continental crust is formed by the mantle’s successive crystallization differentiation and then aggregation, which is the result of the continuous energy acquisition and evolution of the mantle. This process has been objectively recorded in the growth of zircons which are widely present in the continental crust, owing to the close relationship between the zircon Th/U ratio and the crystallization temperature of zircons. As shown by theoretical calculations, phenomenon statistics, and/or crystallization simulations, higher zircon Th/U generally indicates higher zircon (re)crystallization temperature in metamorphic and magmatic systems. Here, we compiled ~600,000 zircon Th/U data from the global continental crust and obtained the time series of zircon Th/U ratios. The average level of the Th/U ratio in global zircons has a slow growth trend from old to new and fluctuates quasi-periodically around 0.5. There are two significant cycles of zircon Th/U ratios, ca. 600 and 120 Myr, which are associated with the supercontinent cycle and whole-mantle convection, respectively. It is inferred that the zircon Th/U periodicity is related to the periodic thermal state changes in the mantle, which might be regulated by tidal energy dissipation.

How to cite: Wu, Y., Fang, X., and Ji, J.: Implications of zircon Th/U for global continental crustal evolution and geodynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2901, https://doi.org/10.5194/egusphere-egu23-2901, 2023.

We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear-wave velocity (Vs) structure of sub-continental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (P_hs) is a positive amplitude arrival at ∼10–12 s and has positive move out with increasing ray-parameter. From joint inversion, the H-D is modeled at a depth range of 90–106 km, with 9–12% Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–102 km, with 6–9% Vs increase. An intra-lithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude P_ILDs phase, arriving at 8–8.5 s. This is modeled at a depth range of 65–75 km with Vs increase of ∼7±4%. We construct 2D profiles of depth-migrated common conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleo-subducted oceanic-slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.

How to cite: Chaudhury, J. and Mitra, S.: Sub-Continental Lithospheric Mantle Discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4829, https://doi.org/10.5194/egusphere-egu23-4829, 2023.

We analyzed 228 earthquake data of 5≤Mw≤6.9 to estimate the Love wave group velocity tomographic image and investigate the lithosphere structure of NE-India. These events of 2001-2015 were recorded by 26 seismic stations of IMD, India, and IRIS. Multiple Filtering Technique is used to estimate fundamental mode Love wave group velocity dispersion curves between 4s and 70s for 846 paths. Then, we constructed Love wave group velocity maps at different periods from 6 s to 60 s through inversion over a 1°×1° grid indicating group velocity variations between ~2 km/s and 4.6 km/s in this part of the India-Eurasia and India-Burma collision zones. Tomographic maps at lower periods show good correlations with surface features. Group velocities at 6s to 16s are sensitive to the uppermost crust. They show high variation related to local geological features like sedimentary basins, basement rocks, Precambrian, and metamorphic rocks. Bengal-Basin and Indo-Burma Ranges have lower group velocities at periods ≤16s compared to those located at Shillong Plateau, Mikir Hills, and the Eastern Himalayan ranges. Low-velocity zone systematically shifts eastward towards the southern part of the Indo-Burma Range for periods from 16 to 38s. A prominent increase in group velocity from 38s is observed along a line trending in the NE direction through the Shillong Plateau, Mikir Hills, and Assam syntaxis. At periods >50s, low velocity is observed in the Tibetan plateau. Inversion of Love wave group velocity was carried out and a tomographic image of SH velocity variation was obtained for the study area. It shows a significant variation in the SH velocity for the crust and upper mantle region of the study area. Based on the estimated Love wave group velocity and SH velocity tomograms we came to the following conclusions. The sedimentary basins like the Bengal Basin, and Brahmaputra River Basin show up as low-velocity zones in both group and SH velocity tomograms. In the Bengal Basin, sedimentary layer thickness varies from 5km in the western part to 15km in the eastern part. Maximum thickness was observed in the SE part of the basin near the Indo-Burma Ranges. The Moho depth below the Bengal Basin varies between 28 km and 32km and 35km and 45km below NE India. The NE trending region showing high group and SH velocity values passing through the Shillong Plateau, Mikir Hills, and Assam syntaxis represent a zone where the Indian plate has buckled upward. This is caused by it being in a vice-like grip between the Eastern Himalayas towards its north and the Indo-Burma Ranges towards its east. The crust below the Tibet and Lasha block is much thicker (up to ~85 km) compared to other parts of the study area. A low-velocity zone is observed in the mid-to-lower crust beneath southern Tibet. This is caused by partial melting in this zone. Mostly the Love wave inversion result matches with previously observed Rayleigh wave inversion and discrepancies in some sections highlight the existence of radial anisotropy.

How to cite: Chanu, N. M., Kumar, N., Mukhopadhyay, S., and Kumar, A.: Investigation of Lithospheric Structure in NE India Based on Love Wave Data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-526, https://doi.org/10.5194/egusphere-egu23-526, 2023.