GD3.3

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.

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.

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.

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.

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.

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.

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. V_S -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. V_S -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.

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.

              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.