GD3.3

Geodynamics of continental crust and upper mantle, and the nature of mantle discontinuities

We invite, in particular multidisciplinary, contributions which focus on the structure and evolution of the continental crust and upper mantle and on the nature of mantle discontinuities. The latter include, but are not limited to, the mid-lithosphere discontinuity (MLD), the lithosphere-asthenosphere boundary (LAB), and the mantle transition zone, as imaged by various seismological techniques and interpreted within interdisciplinary approaches. Papers with focus on the structure of the crust and the nature of the Moho are also welcome. Methodologically, the contributions will include studies based on seismic, thermal, gravity, petrological, and/or electro-magnetic data interpretations.

Invited presentations by
Irina M. Artemieva (Augustus Love Medal Lecture) and
Shun-Ichiro Karato

Co-organized by SM4, co-sponsored by ILP
Convener: Alexey Shulgin | Co-conveners: Hans Thybo, Lev P. Vinnik
vPICO presentations
| Wed, 28 Apr, 09:00–10:30 (CEST)

vPICO presentations: Wed, 28 Apr

Chairpersons: Alexey Shulgin, Hans Thybo, Lev P. Vinnik
09:00–09:05
09:05–09:07
|
EGU21-3202
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ECS
Felix Bissig, Amir Khan, and Domenico Giardini

The mantle transition zone (MTZ) is bounded by seismic discontinuities at average depths of 410 km and 660 km, which are generally associated with major mantle mineral transformations. A body wave impinging from above on these discontinuities develops a refracted and reflected branch, leading to multiple arrivals of the same wavetype within a short time window. These so-called triplicated body waves are observed at regional epicentral distances (15-30°) and carry information on MTZ structure due to their strong interaction with the 410 km and 660 km discontinuities. Careful data selection and processing as well as the assessment of source parameters are necessary steps in obtaining a high quality triplication data set. In this study, we consider recordings of events in Central America at permanent and transportable USArray stations, which are inverted for mantle structure. Our methodology is based on a joint consideration of mineral physics and seismic data in a probabilistic inversion framework and allows for determination of mantle thermo-chemical and seismic velocity structure. We present constraints on the mantle structure underneath the Gulf of Mexico.

How to cite: Bissig, F., Khan, A., and Giardini, D.: Constraints on the mantle transition zone structure using triplicated body waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3202, https://doi.org/10.5194/egusphere-egu21-3202, 2021.

09:07–09:09
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EGU21-8193
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ECS
Mingqi Liu, Taras Gerya, and Ling Chen

In the last decades, the high heterogeneity of lithospheric mantle in term of its physical properties and chemical compositions is widely documented by geophysical, petrological, and geochemical studies. A sharp discontinuity in seismic velocity (~2-10% reduction over no more than 30-40 km) is detected at 60 – 160 km depth in the continental lithosphere and at an average depth of 70 km in the oceanic lithosphere. Several models have been proposed for the genesis of this mid-lithosphere discontinuity (MLD) that include (1) presence of partial melts or fluids, (2) layered anisotropy, (3) layered composition, and (4) elastically accommodated grain boundary sliding. However, all of these models have some limitations and cannot explain all the characteristics of the MLD. Here we propose a new model for the genesis of the MLD and explore its mechanism through thermomechanical numerical modeling at subduction zones. In the model, the deforming lithospheric mantle is affected by grain size reduction and growth processes. Numerical results show that the lithospheric deformation induced by subduction causes the grain size to sharply decrease within the 10-20 km thick brittle/ductile transition zone over significant regions inside the lithosphere. The depth depends mainly on the age of oceanic lithosphere and the thickness of continental lithosphere and is consistent with the observations. In addition, based on the previous study of dislocation slip-system and related olivine fabrics in the mantle, grain size reduction plays an important role in fluid pumping and phase nucleation through grain boundaries. This may in turn produce an increased intra-lithospheric water content resulting in high electrical conductivities and large seismic velocity drops at the MLD depths.

How to cite: Liu, M., Gerya, T., and Chen, L.: The effect of grain size reduction for the origin of the mid-lithosphere discontinuity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8193, https://doi.org/10.5194/egusphere-egu21-8193, 2021.

09:09–09:11
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EGU21-5214
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ECS
Constanza Rodriguez Piceda, Magdalena Scheck-Wenderoth, Judith Bott, Maria Laura Gomez Dacal, Michaël Pons, Claudia Prezzi, and Manfred Strecker

The Andes represent the modern type area for orogeny at a non-collisional, ocean-continent convergent margin. Subduction geometry, tectonic deformation, and seismicity at this plate boundary are closely related to lithospheric temperature distribution in the upper plate. Despite recent advances in the assessment of the thermal state of the Andean lithosphere and adjacent regions derived from geophysical and geochemical studies, several unknowns remain concerning the 3D temperature configuration at lithospheric scale. In particular, it is not clear how both, the configuration of the continental overriding plate (i.e., its thickness and composition) and the variations of the subduction angle of the oceanic Nazca plate influence thermal processes and deformation in the upper plate. To address this issue, we focus on the southern segment of the Central Andes (SCA, 29°S-39°S), where the Nazca plate changes its subduction angle between 33°S and 35°S from the Chilean-Pampean flat-slab zone (< 5° dip, 27-33°S) in the north to a steeper sector south of 33°S (~30° dip). Additionally, the overriding plate exhibits variations in the crustal geometry and density distribution along- and across-strike of the subduction zone. We derived the 3D lithospheric temperature distribution and the surface heat flow of the SCA from the inversion of S-wave velocity to temperatures and calculations of the steady-state conductive thermal field. The configuration of the region – concerning both, the heterogeneity of the lithosphere and the slab dip – was accounted for by incorporating a 3D data-constrained structural and density model of the SCA into the workflow. We conclude that the generated thermal model allows us to evaluate how mantle thermal anomalies and first-order structural and lithological heterogeneities in the lithosphere, observed across and along-strike of Andean orogen, affect the thermal field of the SCA and thus the propensity of the South American lithosphere to specific styles in deformation. In addition, our results are useful to constrain thermo-mechanical simulations in geodynamic modelling and therefore, contribute to a better understanding of the present-day rheological state of the Andes and adjacent regions.

How to cite: Rodriguez Piceda, C., Scheck-Wenderoth, M., Bott, J., Gomez Dacal, M. L., Pons, M., Prezzi, C., and Strecker, M.: Unravelling the thermal state of the southern Central Andes and its controlling factors, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5214, https://doi.org/10.5194/egusphere-egu21-5214, 2021.

09:11–09:13
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EGU21-7987
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ECS
Isabella Gama, Karen M. Fischer, and Junlin Hua

To resolve the signatures of subduction zone processes in the mantle wedge, and how subduction has interacted with the upper plate, we imaged seismic velocity gradients beneath the US state of Alaska with Sp receiver function common conversion point (CCP) stacking.  Pacific plate lithosphere, and lithosphere bearing the thicker crust of the Yakutat terrane, subduct to the northwest beneath the southern margin of Alaska.   We employed data from hundreds of stations of the US NSF EarthScope Transportable Array, as well as other portable arrays and permanent networks. We calculated waveform components using a free-surface transform with improved estimates of free-surface velocities that were determined from P and SV particle motions. Sp receiver functions were calculated with time-domain deconvolution, and the CCP stack was generated with weighting functions that incorporate the properties of Sp scattering kernels. The CCP stack shows a clear interface between the North American and underthrust Yakutat crust, as well as Yakutat Moho depths of up to 60 km.  Sp phases from the negative velocity gradient at the base of the upper plate are strongest in west-central Alaska, where lithosphere-asthenosphere boundary (LAB) depths lie at 65-100 km.  In west-central Alaska, joint inversions of Sp data at single stations with Rayleigh phase velocities show comparable LAB depths as well as low asthenospheric velocities. This zone includes active magmatism and the upper plate appears to have been thinned by mantle wedge volatiles, melt, and flow.  The LAB phase deepens to the north, reaching depths of ~120 km beneath the northern Arctic Alaska terrane.  This increase in the depth of the LAB phase from the arc to the back-arc is consistent with the sculpting of the upper plate by subduction-related processes. Sp phases also delineate a prominent positive velocity gradient that represents the base of a low-velocity asthenospheric layer at depths of 100-130 km.  The positive velocity gradient is consistent with the onset of partial melting in the asthenosphere.

 

How to cite: Gama, I., M. Fischer, K., and Hua, J.: Imaging the Upper Plate Lithosphere and Asthenosphere beneath Alaska with Sp Converted Waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7987, https://doi.org/10.5194/egusphere-egu21-7987, 2021.

09:13–09:15
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EGU21-45
Mohammad Shehata and Hideki Mizunaga

Long-period magnetotelluric and gravity data were acquired to investigate the US cordillera's crustal structure. The magnetotelluric data are being acquired across the continental USA on a quasi-regular grid of ∼70 km spacing as an electromagnetic component of the National Science Foundation EarthScope/USArray Program. International Gravimetreique Bureau compiled gravity Data at high spatial resolution. Due to the difference in data coverage density, the geostatistical joint integration was utilized to map the subsurface structures with adequate resolution. First, a three-dimensional inversion of each data set was applied separately.

The inversion results of both data sets show a similarity of structure for data structuralizing. The individual result of both data sets is resampled at the same locations using the kriging method by considering each inversion model to estimate the coefficient. Then, the Layer Density Correction (LDC) process's enhanced density distribution was applied to MT data's spatial expansion process. Simple Kriging with varying Local Means (SKLM) was applied to the residual analysis and integration. For this purpose, the varying local means of the resistivity were estimated using the corrected gravity data by the Non-Linear Indicator Transform (NLIT), taking into account the spatial correlation. After that, the spatial expansion analysis of MT data obtained sparsely was attempted using the estimated local mean values and SKLM method at the sections where the MT survey was carried out and for the entire area where density distributions exist. This research presents the integration results and the stand-alone inversion results of three-dimensional gravity and magnetotelluric data.

How to cite: Shehata, M. and Mizunaga, H.: Geostatistical Joint Interpretation of Gravity and Magnetotelluric Data of the US Cordillera, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-45, https://doi.org/10.5194/egusphere-egu21-45, 2021.

09:15–09:17
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EGU21-14003
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ECS
Gaurav Tomar, Srikumar Roy, Christopher J. Bean, Satish C. Singh, Brian O'Reilly, and Manel Prada

The Rockall Trough is an elongate bathymetric depression trending NNE-SSW. It is approximately 1200 km long and up to 300 km wide, extending over the UK and Irish continental margins. The trough is underlain by the Rockall Basin, which forms part of a chain of late Paleozoic-Cenozoic sedimentary basins. The Irish Rockall Basin is vastly unexplored as compared to the UK sector, where extensive flood basalt lava flows, sill complexes and volcanic centers of Late Cretaceous-to-Early Eocene age have been described, which belong to the North Atlantic Igneous Province (NAIP) (Archer et al., 2005). An integrated study of seismic, gravity and magnetic methods elucidates the deeper stratigraphy of the Irish Rockall Basin. More than 10 km of sediments is present in the central part of the basin. We perform first arrival travel time tomography on a downward continued data set of three seismic profiles to model the velocity of the sedimentary structures down to 6 km depth. To better understand the deep structure of the basin we need to estimate the Moho depth from constrained gravity modelling. The modelling results indicate that the Moho depth varies from 12 km to 20 km depth beneath ~10 km thick sediments in the basin. This allows us to measure the crustal stretching factor β. The minimum stretching factor in the basin varies between ~7 in the north to ~6.5 in the south. These values are within the range needed for mantle serpentinisation (O'Reilly et al., 1996; Perez-Gussinye and Reston, 2001). Furthermore, we observe four volcanic ridges in the south part of the basin, which are ~20 km wide and ~ 3 km thick, possibly comprising the Barra Volcanic Ridge System (BVRS) (Scrutton and Bentley, 1988). Results indicate several failed rifting attempts times in late Mesozoic/early Cenozoic times, generating significant basic volcanism, associated with the NAIP. We resolve new volcanic ridges (of late Mesozoic/early Cenozoic age) in the southern part of the Rockall Basin, like many other volcanic ridges/centres observed in other parts of the basin, with correlatable magnetic and gravity anomalies. These may be of late Cretaceous age similar to those found on the conjugate Canadian margin.

How to cite: Tomar, G., Roy, S., Bean, C. J., Singh, S. C., O'Reilly, B., and Prada, M.: Integrated geophysical investigations of deeper stratigraphy of the Irish Rockall Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14003, https://doi.org/10.5194/egusphere-egu21-14003, 2021.

09:17–09:19
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EGU21-2739
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ECS
Irene DeFelipe, Puy Ayarza, Imma Palomeras, Juvenal Andrés, Mario Ruiz, Juan Alcalde, David Martinez Poyatos, Francisco Gonzalez-Lodeiro, Mariano Yenes, Montserrat Torne, and Ramon Carbonell

The Iberian Central System represents an outstanding topographic feature in the central Iberian Peninsula. It is an intraplate mountain range formed by igneous and metasedimentary rocks of the Variscan Iberian Massif that has been exhumed since the Eocene in the context of the Alpine orogeny. The Iberian Central System has been conventionally interpreted as a thick-skinned pop-up mountain range thrust over the Duero and Tajo foreland basins. However, its lithospheric structure and the P-wave velocity distribution are not yet fully resolved. In order to place geophysical constraints on this relevant topographic feature, to identify lithospheric discontinuities, and to unravel the crustal deformation mechanisms, a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. It is a NNW-SSE oriented 360-km long profile that runs through the Duero basin, the Iberian Central System and the Tajo basin. First results based on forward modeling by raytracing show an irregularly layered lithosphere and allow to infer the depth extent of the northern Iberian Central System batholith. The crust is ~ 31 km thick under the Duero and Tajo basins and thickens to ~ 39 km under the Iberian Central System. A conspicuous thinning of the lower crust towards the south of the Iberian Central System is also modeled. Along this transect, a continuous and high amplitude upper mantle feature is observed and modeled as the reflection of an interface dipping from 58 to 62 km depth featuring a P-wave velocity contrast of 8.2 to 8.3 km/s. Our preliminary results complement previous models based on global-phase seismic and noise interferometry and gravity data, provide new constraints to validate the accuracy of passive seismic methods at lithospheric scale, and contribute with a resolute P-wave velocity model of the study area to unravel the effect of the Alpine reactivation on the central Iberian Massif.
This project has been funded by the EIT-RawMaterials 17024 (SIT4ME) and the MINECO projects: CGL2016-81964-REDE, CGL2014-56548-P.

How to cite: DeFelipe, I., Ayarza, P., Palomeras, I., Andrés, J., Ruiz, M., Alcalde, J., Martinez Poyatos, D., Gonzalez-Lodeiro, F., Yenes, M., Torne, M., and Carbonell, R.: Lithospheric structure of the Iberian Central System (Iberian Massif) imaged by the wide-angle seismic reflection/refraction CIMDEF experiment , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2739, https://doi.org/10.5194/egusphere-egu21-2739, 2021.

09:19–09:21
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EGU21-3265
Montserrat Torne, Wentao Zhang, Ivone Jimenez-Munt, Ana Negredo, Estefania Bravo, Jaume Vergés, and Daniel García-Castellanos

The present-day structure of the lithosphere and uppermost mantle of Northern Apennines and Dinarides region results from a complex tectonic scenario mainly driven by subduction of Tethyan oceanic domains. The study area and surrounding regions have been the goal of a large number of geophysical studies that have provided information on the velocity, density and temperature distribution in the lithosphere and uppermost mantle. However, the majority of them do not consider the contribution of the chemical composition and phase transitions on the physical properties in the lithospheric mantle. By applying and integrated petrological-geophysical approach -LitMod2D_2.0- we aim at constraining and characterizing the present-day lithosphere and mantle structure along a NE-SW trending 730 km long geo-transect crossing the Northern Tyrrhenian Sea, the Northern Apennines, the Adriatic Sea, the Dinarides fold belt and the Pannonian back-arc basin. Along the modelled geotransect, we infer the spatial distribution of density, thermal conductivity and seismic velocities based on the variations of gravity, geoid, elevation and heat flow consistently with the thermochemical conditions and with isostatic equilibrium. Our results show significant lateral variations in the lithospheric structure, affecting crustal and lithospheric mantle thickness, temperature, density distribution, and mantle composition that reveals the imprint of the complex geodynamic evolution of the area. This is a GeoCAM contribution (PGC2018-095154-B-I00)

Keywords: Alpine Mediterranean orogeny, geoid and gravity anomalies, elevation, integrated petrological-geophysical modelling, mantle seismic P and S-wave velocity.

How to cite: Torne, M., Zhang, W., Jimenez-Munt, I., Negredo, A., Bravo, E., Vergés, J., and García-Castellanos, D.: An integrated geophysical-petrological view of the lithosphere of the northern Apennines, Dinarides and Pannonian Basin., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3265, https://doi.org/10.5194/egusphere-egu21-3265, 2021.

09:21–09:23
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EGU21-15058
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ECS
Vahid Teknik, Irina Artemieva, and Hans Thybo

We interpret the paleotectonic evolution and structure in the Tethyan belt by analyzing magnetic data sensitive to the presence of iron-rich minerals in oceanic fragments and mafic intrusions, hidden beneath sedimentary sequences or overprinted by younger tectono-magmatic events. By comparing the depth to magnetic basement (DMB) as a proxy for sedimentary thickness with average crustal magnetic susceptibility (ACMS), we conclude:

 (1) Major ocean and platform basins have DMB >10 km. Trapped ocean relics may be present below Central Anatolian micro-basins with DMB at 6-8 km and high ACSM.  In intra-orogenic basins, we identify magmatic material within the sedimentary cover by significantly smaller DMB than depth to seismic basement.

(2) Known magmatic arcs (Pontides and Urima-Dokhtar) have high-intensity heterogeneous ACMS. We identify a 450 km-long buried (DMB >6 km) magmatic arc or trapped oceanic crust along the western margin of the Kirşehır massif from a strong ACMS anomaly. Large, partially buried magmatic bodies form the Caucasus LIP at the Transcaucasus and Lesser Caucasus and in NW Iran.

(3) Terranes of Gondwana affinity in the Arabian plate, S Anatolia and SW Iran have low-intensity homogenous ACMS.

(4) Local poor correlation between known ophiolites and ACMS anomalies indicate a small volume of presently magnetized material in the Tethyan ophiolites, which we explain by demagnetization during recent magmatism.

(5) ACMS anomalies are weak at tectonic boundaries and faults. However, the Cyprus subduction zone has a strong magnetic signature which extends ca. 500 km into the Arabian plate.

How to cite: Teknik, V., Artemieva, I., and Thybo, H.: Tethys Belt in the Anatolia-Caucasus-Black Sea Region: Basins, Magmatic Arcs, Ophiolites, and LIPs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15058, https://doi.org/10.5194/egusphere-egu21-15058, 2021.

09:23–09:25
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EGU21-15825
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ECS
Anna Makushkina, Benoit Tauzin, Meghan Miller, Hrvoje Tkalcic, and Hans Thybo

Large-scale topography is thought to be mainly controlled by active tectonic processes. Fennoscandia is located far from any active tectonic setting and yet includes a mountain range along its passive North Atlantic margin. Models proposed to explain the origin of these enigmatic mountains are based on glacial isostatic adjustments, delamination, long-term isostatic equilibration, and dynamic support from the mantle, yet no consensus has been reached. We show that topography along the continental margin of Fennoscandia may be influenced by its deep structure. Fennoscandia formed by amalgamation of Proterozoic and Archean continental blocks; using both S- and P-receiver functions, we discovered that the Fennoscandian lithosphere still retains the original structural heterogeneity and its western margin is composed of three distinct blocks. The southern and northern blocks have relatively thin crust (~40-45 km), while the central block has thick crust (~60 km) that most likely was formed by crustal stacking during the Proterozoic amalgamation. The boundaries of the blocks continue into the oceanic crust as two major structural zones of the North-East Atlantic, suggesting that the Fennoscandian amalgamation structures determined the geometry of the ocean opening.  We found no evidence for mountain root support or delamination in the areas of high topography that could be related with mountain formation. Instead, our results suggest that both crustal and lithospheric heterogeneity of Fennoscandia along the continental margin might have a control on geodynamic forces that support the rise of Scandinavian mountains. 

How to cite: Makushkina, A., Tauzin, B., Miller, M., Tkalcic, H., and Thybo, H.: Opening of the north-eastern Atlantic and onshore mountain rise controlled by Fennoscandian deep structure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15825, https://doi.org/10.5194/egusphere-egu21-15825, 2021.

09:25–09:27
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EGU21-15798
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ECS
Nevra Bulut, Valerie Maupin, and Hans Thybo

We present a seismic tomographic image of Fennoscandia based on data from the ScanArray project in Norway, Sweden, and Finland, which operated during 2012-2017, together with data from earlier projects and stationary stations. We use relative traveltime residuals of P- and S- waves in high- and low-frequency bands and apply the frequency-dependent crustal correction. We use seismic signals from earthquakes at epicentral distances between 30° and 104° and magnitudes larger than 5.5. The general purpose of this study is to understand the possible causes of the high topography in Scandinavia along the passive continental margins in the North Atlantic as well as the interrelation between structure at the surface and in the lithospheric mantle.

We present an upper-mantle velocity structure for most Fennoscandia derived for the depth range 50-800 km with a 3D multiscale parameterization for an inversion mesh-grid with dimensions dx=dy=17.38 km and dz=23.44 km. In all body-wave tomography methods, smearing of anomalies is expected. Therefore resolution tests are critical for assessing the resolution of the parameters determined in the velocity models. The resolution of the models depends on several factors, including the noise level and general quality of data, the density of observations, the distance and back-azimuthal distribution of sources, the damping applied, and the model parameterization. We use checkerboard and model-driven (block and cylindrical) tests for assessing the resolution of our models.

Seismic models derived in this study are compared to existing and past topography to contribute to understanding mechanisms responsible for the topographic changes in the Fennoscandian region. The models also provide a basis for deriving high-resolution models of temperature and compositional anomalies that may contribute to understanding the observed, enigmatic topography.

How to cite: Bulut, N., Maupin, V., and Thybo, H.: Seismic Body-Wave Tomography in Fennoscandia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15798, https://doi.org/10.5194/egusphere-egu21-15798, 2021.

09:27–09:29
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EGU21-15666
Metin Kahraman, Hans Thybo, Irina Artemieva, Alexey Shulgin, Alireza Malehmir, and Rolf Mjelde

The Baltic Shield is located in the northern part of Europe, which formed by amalgamation of a series of terranes and microcontinents during the Archean to the Paleoproterozoic, followed by significant modification in Neoproterozoic to Paleozoic time. The Baltic Shield includes an up-to 2500 m high mountain range, the Scandes , along the western North Atlantic coast, despite being a stable craton located far from any active plate boundary.

We study a crustal scale seismic profile experiment in northern Scandinavia between 63oN and 71oN. Our Silverroad seismic profile extends perpendicular to the coastline around Lofoten and extends ~300km in a northwest direction across the shelf into the Atlantic Ocean and ~300km in a southeastern direction across the Baltic Shield. The seismic data were acquired with 5 explosive sources and 270 receivers onshore; 16 ocean bottom seismometers and air gun shooting from the vessel Hakon Mosby were used to collect both offshore and onshore.

We present the results from raytracing modelling of the seismic velocity structure along the profile. The outputs of this experiment will help to solve high onshore topography and anomalous and heterogeneous bathymetry of the continental lithosphere around the North Atlantic Ocean. The results show crustal thinning from the shield onto the continental shelf and further into the oceanic part. Of particular interest is the velocity below the high topography of the Scandes, which will be discussed in relation to isostatic equilibrium along the profile.

How to cite: Kahraman, M., Thybo, H., Artemieva, I., Shulgin, A., Malehmir, A., and Mjelde, R.: Crustal Structure across Central Scandinavian Peninsula along Silver Road refraction profile, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15666, https://doi.org/10.5194/egusphere-egu21-15666, 2021.

09:29–09:31
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EGU21-9246
Alexey Shulgin, Jan Erik Lie, and Sverre Planke

The Barents Sea shelf has been coverded by a numerous wide-angle seismic profiles, aiming to resolve the crustal structure of the shelf. However, the overall structural arcitecture of the crystaline crust is still not fully understood, due to limited and sparse distribution of deep-sampling seismic profiles. 

The petroleum related seismic exploration in Norwegian waters has been ongoing for decades. The recent increase of the seismic broadband stations onshore (including temporal deployments) provokes the idea to use these stations and the active seismic sources from the regional seismic reflection surveys, including academic and industry seismic projects, to reveal the crustal scale structure of the western Barents Sea.

We have analyzed seismic records from 8 permanent seismic stations from Norway, Sweden and Finland, and 12 temporally deployed broadband seismic stations from the ScanArray seismic network, which recorded more than 100’000 marine airgun shots from academic and oil industry campaigns in the south-western quarter of the Barents Sea.

The overall quality of the seismic records is exceptionally good. We clearly identify phases recorded from the offsets reaching 750 km. The identified phases include refracted crustal and mantle arrivals as well as Moho reflections, including both P and S waves. The overall quantity, quality, and the geometry of the seismic data makes it perfect for the application of the 3D joint refraction/reflection travel time seismic tomography to study the crustal structure of the Barents Sea.

The preliminary results show very complex and laterally inhomogeneous crustal structure of the Barents Sea, which has been known before. However, with the help of 3D seismic tomography we are able to cover the gaps in between isolated deep-sampling seismic profiles and cross-correlate structures identified on them. In this work we would like to present our up-to-date results from the 3D seismic tomography.

How to cite: Shulgin, A., Lie, J. E., and Planke, S.: 3D active seismic tomography of the Barents Sea., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9246, https://doi.org/10.5194/egusphere-egu21-9246, 2021.

09:31–09:33
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EGU21-7815
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ECS
Igor Ognev, Jörg Ebbing, and Peter Haas

A new crustal model of the Volga-Uralian subcraton was built. The compilation of the model was subdivided in two steps: (1) inverse gravity modeling followed by (2) thorough forward gravity modeling.

For inverse gravity modeling GOCE gravity gradients were used. The effect of the Earth sphericity was taken into account by using tesseroids. Density contrasts between crust and mantle were varied laterally according to the tectonic units present in the region.  The model is constrained by the available seismic data including receiver function studies, and deep reflection and refraction profiles.

The Moho discontinuity obtained during the gravity inversion was consequently modified, and complemented by the sedimentary cover, upper crust, lower crust, and lithospheric mantle layers in the process of forward gravity modeling. Obtained model showed crustal thickness variation from 34 to more than 55 km in some areas. The thinnest crust with the thickness below 40 km appeared on the Pericaspian basin with the thickest sedimentary column. A relatively thin crust was found along the central Russia rift system, while the thickest crust is located underneath Ural Mountains as well as in the center of the Volga-Uralian subcraton. In both areas the crustal thickness exceeds 50 km. At the same time, the gravity misfit of ca. 95 mGal between the measured Bouguer gravity anomaly and forward calculated gravity field was revealed in the central area of the Volga-Uralian subcraton. This misfit was interpreted and modeled as high-density lower crust which can possibly represent an underplated material.

In the end, the new crustal model of Volga-Uralian subcraton respects the gravity and seismic constraints, and reflects the main geological features of the region. This model will be used for further geothermal analysis of the area.

How to cite: Ognev, I., Ebbing, J., and Haas, P.: Inverse and forward gravity modeling for revealing the crustal structure of Volga-Uralian subcraton, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7815, https://doi.org/10.5194/egusphere-egu21-7815, 2021.

09:33–09:35
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EGU21-16160
Valentina Mordvinova, Maria Khritova, Elena Kobeleva, Mikhail Kobelev, Irina Chuvashova, and Alexandr Treussov

The results of teleseismic wave modeling show that the south-southwestern boundary of the Siberian craton is close to vertical to a depth of 120 km and corresponds to the southern margin of the Siberian platform (fig. 1). The deepest part of the craton (in the depth interval of 150–250 km) passes under the Tunkinsky rift, and then under the foot of the Khamar-Daban ridge. The edge of the craton is a wedge moving at an angle of 45° under Baikal, and wedges out completely to the east of the lake at a depth of about 150 km.

The distribution of velocity heterogeneities shows a logical connection with the existing tectonic structures. The wedge-shaped form of the southeastern margin of the craton exists possibility along all of the Baikal rift. It is this oblique shape of the craton that could have contributed to the accretion-collisional processes that formed the uplift at the edge of the craton.

At the end of the Mesozoic – Cenozoic, compression ceased and did not prevent the accumulated heat from rising from under the Siberian craton, due to which the collision uplift on the southeastern edge of the craton was destroyed, and the thrust faults were transformed into gentle faults, which led to the formation of rifts and to the exhumation of metamorphic cores. In addition to the inclined edge of the craton, the expansion of the Baikal rift depression is facilitated by the thinned margin of the craton, which is prone to faults, and the heated volume under the wedge (“canopy”), where convection traps are formed, which appear on tomography as intense negative anomalies of seismic wave velocities. Such conditions can lead to decompression magmatism of varying intensity. These conclusions are supported by our more detailed model (fig. 1D), constructed by the method of the longitudinal receiver function (according  to Vinnik, 1977).

Fig. 1. VS -section and topography along profile 9206–ZAK (Bratsk reservoir – Zakamensk).

P - tomography MOBAL_2003 (A - the triangles mark, B - the position of seismic stations ). Surface-wave tomography (C).  The red vertical line near the Tunka rift is a correlation reference mark for all the models. The models are shown in the same scale, except the depth-stretched meridional section D. Velocity isolines are drawn from 2.4 to 4.6 km/s  with a step of 0.1 km/s.  The red line shows the profile route, red boxes mark the termination of profile A* and the point where the profile crosses the Tunka rift.

Fig. 2. VS -section and topography along profile 9206–ZAK.

A, B, C  --  P - tomography MOBAL_2003 . D - Surface-wave tomography. The red vertical line near the Tunka rift is a correlation reference mark for all the models. The models are shown in the same scale, except the depth-stretched meridional section D. Velocity isolines are drawn from 2.4 to 4.6 km/s  with a step of 0.1 km/s.  

 This work is supported by the RSF grant 18-77-10027.

How to cite: Mordvinova, V., Khritova, M., Kobeleva, E., Kobelev, M., Chuvashova, I., and Treussov, A.: Deep structure of the southern margin of the Siberian craton and its role the formation of modern geodinamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16160, https://doi.org/10.5194/egusphere-egu21-16160, 2021.

09:35–09:37
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EGU21-3903
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ECS
Biao Yang, Zhoupeng Wang, and Yanbin Wang

       Under the northward push of the Tibetan Plateau, Qaidam basin is the intersection of Traim block, Bayan Har block and Qilian orogen. Inversion and interpretation of the crustal structure under Qaidam basin are helpful to understand the evolution of the plateau. On November 10, 2008, an Mw6.3 earthquake occurred in the northern margin of Qaidam basin and was recorded by 17 broadband temporary stations installed by the INDEPTH IV Project. We performed inversion of the recorded regional seismic waveforms combining niche genetic algorithm and reflectivity method and obtained the crustal velocity structure of the eastern, western and northwestern part of Qaidam basin.The inversion results show that the structures of the eastern and western basin are similar, where both exist a very thin low velocity layer at about 26km in the middle crust. The thicker lower crust of the west basin results in thicker crust than that of the east basin, which reveals decoupling of the upper and lower crust of the basin. The structure of the northwestern basin is quite different from other regions with much thicker crust, lower velocity of the lower crust and upper mantle, indicating strong deformation and partial melting.

How to cite: Yang, B., Wang, Z., and Wang, Y.: Regional full seismic waveform inversion of crustal velocity structure in Qaidam Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3903, https://doi.org/10.5194/egusphere-egu21-3903, 2021.

09:37–09:39
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EGU21-13984
Gaochun Wang, Thybo Hans, and Irina M. Artemieva

All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. It has been suggested that the lower crust does not need to be basaltic, but until now all seismic observations show high P-wave velocity, which requires that the bulk composition of the lower crust must include at least 20-40% of mafic rocks. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp>7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp<6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. The calculated typical velocity versus depth curves for different crustal lithologies and temperature regimes imply the composition of the lower crust is felsic. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.

 

How to cite: Wang, G., Hans, T., and Artemieva, I. M.: No mafic layer in 80 km thick Tibetan crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13984, https://doi.org/10.5194/egusphere-egu21-13984, 2021.

09:39–09:49
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EGU21-13799
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solicited
Shun-ichiro Karato, Lidong Dai, Gary Egbert, Jennifer Girard, Benjamin Murphy, Tolulope Olugboji, Jeffrey Park, Reynold Silber, and Zhongtian Zhang

              The mid-lithosphere discontinuity (MLD) and the lithosphere-asthenosphere-boundary (LAB) are two well-known seismic discontinuities in the continental upper mantle. Both MLD and LAB are present in most of the continents but at different depths and with different magnitude of velocity change and sharpness. Understanding the causes for these discontinuities including their regional variations is critical in inferring the evolution of the continents from geophysical observations on these discontinuities.

              Among various models, we focus on the elastically-accommodated grain-boundary sliding (EAGBS) model that provides plausible and unified explanations for the MLD and the LAB (Karato and Park, 2019). This model has a few testable predictions, and the main purpose of this talk is to review the current status of these tests.

  • (i) One assumption of the EAGBS model is that EAGBS is enhanced by water. A recent paper by Cline et al. (2018) challenges this hypothesis by showing that water has no effects on attenuation in Ti-doped hydrated olivine. However, the relevance of the results on highly Ti-doped olivine to Ti-poor real upper mantle is unclear.
  • (ii) A clear and unique prediction of the EAGBS is the presence of a peak in seismic attenuation at/near the MLD. However, inferring an attenuation peak in a narrow depth range is challenging and this hypothesis has not been tested.
  • (iii) Another prediction of the “dry” version of the EAGBS model for the MLD is that although seismic wave velocity drops and there is a peak in attenuation, electrical conductivity does not change.
  • (iv) If the MLD is caused by EAGBS, then materials below are in the “relaxed” state. This would explain the lack of large velocity drop at the LAB. However, the validity of this explanation depends on the pressure dependence of grain-boundary sliding. If pressure dependence of EAGBS is large, then the un-relaxed state will re-establish itself at a relatively shallow depth within the lithosphere. In this case, a deeper thermal transition to the relaxed state should produce stronger LAB than reported. 

We have conducted an interdisciplinary study to address these issues including mineral physics and seismology. We found that the addition of Ti modifies the defect-related properties of olivine and complicates the application of Cline et al. (2018) to actual upper-mantle conditions. We determined the pressure dependence of olivine grain-growth, from which we infer that the pressure dependence of grain-boundary sliding is small. Regarding the seismological test of attenuation peak, we forward-modeled surface-wave dispersion in a dispersive medium. Calculations show that the over-tones of Love waves are a key to detecting an attenuation peak near the GBS transition. Combined with a comparison of seismological studies (on velocity and attenuation) and MT estimates of electrical conductivity, we will have better constraints on the validity of the EAGBS model for the origin of the MLD.

How to cite: Karato, S., Dai, L., Egbert, G., Girard, J., Murphy, B., Olugboji, T., Park, J., Silber, R., and Zhang, Z.: Origin and significance of seismic discontinuities in the continental upper mantle: An interdisciplinary study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13799, https://doi.org/10.5194/egusphere-egu21-13799, 2021.

09:49–10:30