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Structure and dynamics of the lithosphere-asthenosphere system is one of the key questions for understanding geological processes. Constraining the styles, mechanisms and fabrics evolution in the crust and the upper mantle come from both direct and indirect observations with the use of variety of methods. Seismological studies focusing on anisotropy have successfully improved our knowledge of deformation patterns, acting both at present as well as in the past. When combined with tomographic models, velocity anisotropy can shed light on the geometry, structure, and dynamics of deformation in the lithosphere and the asthenosphere. Sophisticated geodynamic modelling and laboratory experiments enhance our understanding of flow patterns in the upper mantle and their effects on vertical motions of the crust and the lithosphere. Combining with inferences from seismic anisotropy, these methods have the potential to reveal mechanisms that create deformation-induced features such as shape preferred orientation (SPO) and lattice-preferred orientation (LPO), which created in the past or during the last deforming processes. Structural and kinematic characterization of deformation events by geometric and kinematic analyses infer the direction and magnitude of the tectonic forces involved in driving deformation within crust and upper mantle. Additionally, both physical analogue and numerical modelling foster our understanding of complex 3D-plate interaction on various timescales, controlled through the degree of plate coupling and the rheology of the lithosphere.
However, additional work is required to better integrate various experimental and modelling techniques, and to link them with multi-scale observations. The session aims at bringing together inferences from different disciplines that focus on structure and deformation of the lithosphere and the sub-lithospheric upper mantle as well as on the dynamics and nature of the lithosphere-asthenosphere system. The main goal is to demonstrate the potential of different methods, and to share ideas of how we can collaboratively study lithosphere structure, and how the present-day fabrics of the lithosphere relates to the contemporary deformation processes and ongoing dynamics within the asthenospheric mantle. Contributions from studies employing seismic anisotropy observation, geodynamical modelling (analogue and numerical), structural geology, and mineral and rock physics are welcome.

Invited Speakers:
Eric Debayle (Laboratoire de Geologie de Lyon-Terre, Planètes, Environnement, CNRS, France)
Christof Völksen (Bayerische Akademie der Wissenschaften, Germany)

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Co-organized by SM4/TS14
Convener: Ehsan QorbaniECSECS | Co-conveners: Irene Bianchi, Boris Kaus, Jaroslava Plomerova, Ernst Willingshofer
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| Attendance Mon, 04 May, 08:30–10:15 (CEST)

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Chat time: Monday, 4 May 2020, 08:30–10:15

D1503 |
EGU2020-9340
David Schlaphorst, Graça Silveira, and João Mata

Madeira and the Canary Islands, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions. Their tracks have been reconstructed to past locations close to the south-western part of the Iberian Peninsula and north-western Africa, respectively. Furthermore, due to their close proximity, an interconnected origin of these two hotspots has been proposed but details remain unclear. A better understanding of the crust and upper mantle structure beneath these islands is needed to investigate this potential connection.

The subsurface structure has an influence on the stress field, which can be investigated studying seismic anisotropy patterns of the region. Seismic anisotropy leads to variations in the speed of seismic waves as a function of the direction of wave propagation. In the crust an orientation in the direction of maximum stress is observed, commonly being parallel to the alignment of fractures or cracks. In the upper mantle the orientation is influenced by mantle flow. A widely used method to identify anisotropy is the observation of shear-wave splitting of data from teleseismic events. In case of multiple anisotropic layers, including measurements from local events it is possible to distinguish crustal from upper mantle influences.

As part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we carried out the first detailed study of seismic anisotropy beneath both archipelagos, using teleseismic SKS and local shear-wave splitting measurements of data collected from land stations of seismic networks located on Madeira and the Canary Islands.

Significant changes, both in orientation and delay time, can be observed on short length-scales on the order of tens of kilometres, matching major geological features such as, for example, the major rift zone on Madeira island. In a further step, we compare these results to previous studies of crustal and upper mantle anisotropy focusing on north-western Africa and the Iberian Peninsula to investigate the nature of the lithospheric corridor between the present day hotspot positions and the Atlas-Gibraltar region.

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

How to cite: Schlaphorst, D., Silveira, G., and Mata, J.: Investigating Seismic Anisotropy of the Madeira and Canaries Hotspots Using Teleseismic and Local Shear-Wave Splitting with the SIGHT project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9340, https://doi.org/10.5194/egusphere-egu2020-9340, 2020

D1504 |
EGU2020-8071
Ayoub Kaviani, Meysam Mahmoodabadi, Georg Rümpker, Farzam Yamini-Fard, Mohammad Tatar, Ali Moradi, and Faramarz Nilfouroushan

We used more than one decade of core-refracted teleseismic shear (SKS) waveforms recorded at more than 160 broadband seismic stations across the Iranian plateau and Zagros to investigate seismic anisotropy beneath the region. Splitting analysis of SKS waveforms provides two main parameters, i.e., fast polarization direction and split delay time, which serve as proxies for the trend and strength of seismic anisotropy beneath the stations. Our observation revealed a complex pattern of splitting parameters with variations in the trend and strength of anisotropy across the tectonic boundaries. We also verified the presence of multiple layers of anisotropy in conjunction with the lithosphere deformation and mantle flow field. Our observation and modeling imply that a combined system of lithosphere deformation and asthenospheric flow is likely responsible for the observed pattern of anisotropy across the Iranian Plateau and Zagros. The rotational pattern of the fast polarization directions observed locally in Central Zagros may indicate the diversion of mantle flow around a continental keel beneath the Zagros. The correlation between the variation in lithosphere thickness and the trend of anisotropy in the study area implies that the topography of the base of lithosphere is also a determining factor for the pattern of mantle flow inferred from the observations.

How to cite: Kaviani, A., Mahmoodabadi, M., Rümpker, G., Yamini-Fard, F., Tatar, M., Moradi, A., and Nilfouroushan, F.: SKS splitting observations across the Iranian plateau and Zagros: the role of lithosphere deformation and mantle flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8071, https://doi.org/10.5194/egusphere-egu2020-8071, 2020

D1505 |
EGU2020-6919
| solicited
Eric Debayle, Yanick Ricard, Stéphanie Durand, and Thomas Bodin

Massive surface wave datasets constrain upper mantle seismic heterogeneities with horizontal wavelengths larger than 1000 km, allowing us to investigate the large-scale properties and alignment of olivine crystals in the lithosphere and asthenosphere. The azimuthal anisotropy projected onto the direction of present plate motion shows a very specific relation with the plate velocity. Plate-scale present-day deformation is remarkably well and uniformly recorded beneath plates moving faster than ∼4 cm/yr. Recent geodynamic models suggest that cold sinking instabilities tilted in the direction opposite to plate motion below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with our observations. Beneath slower plates, plate-motion aligned anisotropy is only observed locally, which suggests that the lithospheric motion does not control mantle flow below these plates.

Radial anisotropy extends deeper beneath continents than beneath oceans, but we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates moving faster than ∼4 cm/yr.

The same inversion also provides 3D models of seismic velocity and attenuation. The simultaneous interpretation of global 3D shear attenuation and velocity models has a great potential to decipher the effect of temperature, melt and composition on seismic observables. We will discuss our findings from the simultaneous interpretation of our latest models.

How to cite: Debayle, E., Ricard, Y., Durand, S., and Bodin, T.: Global imaging of the lithosphere-asthenosphere system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6919, https://doi.org/10.5194/egusphere-egu2020-6919, 2020

D1506 |
EGU2020-11597
Seth Kruckenberg and Vasileios Chatzaras

Constraining the seismic structure of the West Antarctic mantle is important for understanding its viscosity structure, and thus for accurately predicting the evolution of the West Antarctic Ice Sheet.  Seismic anisotropy, which is the dependence of seismic velocities on the propagation and polarization direction of seismic waves, is a valuable tool for understanding mantle deformation and flow.  We provide petrological and microstructural data from a suite of 44 spinel peridotite xenoliths entrained in Cenozoic (1.4 Ma) basalts of 7 volcanic centers located in Marie Byrd Land, West Antarctica.  Equilibration temperatures obtained from three different calibrations of the two-pyroxene geothermometer and the olivine-spinel Fe-Mg exchange geothermometer range from 780°C to 1200°C, calculated at a pressure of 1500 MPa.  This range of temperatures corresponds to extraction depths between 39 and 72 km, constraining the source of the xenoliths within the lithospheric mantle above the low velocity zone modelled by seismic studies.

The Marie Byrd Land xenoliths are fertile with average clinopyroxene mode that ranges between 15 and 24%.  Based on their modal composition, xenoliths are predominantly classified as lherzolites (n=30), with lesser occurrences of harzburgite (n=4), wehrlite (n=3), dunite (n=3), olivine websterite (n=1), websterite (n=1), and clinopyroxenite (n=2).  Petrological data suggest that the xenoliths have been affected by various degrees of partial melting as well as by reaction with silicate melts or fluids.  For example, clinopyroxenes in the more fertile lherzolites and wehrlites show a constant TiO2 concentration at 0.65 wt% and 0.8 wt% over a range of olivine Mg# values, while TiO2 decreases rapidly with increasing Mg#, down to 0.01 wt% in the more refractory harzburgites and dunites.  The observed trend is interpreted to indicate a refertilization process.  Microstructures also indicate multiple episodes of reactive melt percolation under either static conditions or during the late stages of deformation.  Pyroxenes may enclose rounded olivine grains in crystallographic continuity with neighbouring grains, cross-cut the subgrain boundaries of olivine grains, or show an interstitial habit, either forming cuspate-shaped grains in olivine triple junctions or films along olivine-olivine grain boundaries.  Olivine shows a range of crystallographic preferred orientation (CPO) patterns, including the A-type, axial-[010], axial-[100], and B-type.  Pyroxenes have weaker but not random CPOs with [001] axes having similar orientation to olivine [100] axes in the majority of the xenoliths.  Calculated P and S waves anisotropy is variable (2–12%) and increases with olivine fraction but decreases with both increasing ortho- or clinopyroxene content.  P-wave anisotropy is correlated with the strength of olivine CPO expressed with the M-index and increases with increasing strength of the orthopyroxene CPO, but seems to be less correlated with the strength of the clinopyroxene CPO.

How to cite: Kruckenberg, S. and Chatzaras, V.: Seismic anisotropy of the lithospheric mantle beneath Marie Byrd Land, West Antarctica: Constraints from peridotite xenoliths, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11597, https://doi.org/10.5194/egusphere-egu2020-11597, 2020

D1507 |
EGU2020-3480
| solicited
Christof Völksen, Laura Sánchez, Alexandr Sokolov, Herbert Arenz, and Florian Seitz

structure of the lithosphere has been considered in advance. in the boundary region between Switzerland, Austria and Italy.

How to cite: Völksen, C., Sánchez, L., Sokolov, A., Arenz, H., and Seitz, F.: Recent Crustal Surface Deformation of the Alpine Region Derived from Geodetic Observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3480, https://doi.org/10.5194/egusphere-egu2020-3480, 2020

D1508 |
EGU2020-11957
Agnes Kiraly, Clinton P. Conrad, Lars N. Hansen, and Menno Fraters

Developing an appropriate characterization of upper mantle viscosity structure presents one of the biggest challenges for understanding geodynamic processes in the upper mantle. This is because different creep mechanisms become activated depending on depth, accumulated strain, and applied stress, and other factors such grain size and anisotropic fabric can change as the deformation develops, changing the effective viscosity. Here we focus on the relationship between anisotropic fabric development and viscous anisotropy.

Under applied shear, olivine crystals, which form a large proportion of the asthenosphere, rotate towards the shear direction and accumulate a lattice preferred orientation (LPO) parallel to the macroscopic deformation. On a large scale, LPO can be observed through the propagation of seismic waves because of the anisotropic elastic properties of olivine. As olivine is anisotropic in its viscous properties, this developing texture within the asthenosphere can affect the macro-scale viscosity of the asthenosphere. This behavior has been detected in rock mechanics measurements on pure olivine aggregates, showing more than an order magnitude of viscosity change between shear parallel to the olivine aggregate’s LPO versus shear across this fabric (Hansen et al., EPSL 2016a, JGR 2016b).

Here, we use numerical models developed first in MATLAB and then implemented into the mantle convection code ASPECT. These models incorporate both anisotropic fabric development and anisotropic viscosity, both calibrated according to laboratory measurements on slip system activities of olivine aggregates (Hansen et al., JGR 2016b), to better understand the coupling between the large-scale formation of LPO textures and changes in asthenospheric viscosity.

The modeling results allows us to discuss the role of anisotropic viscosity on the processes of plate tectonics. An asthenosphere with a well-developed LPO becomes weak parallel to its texture, allowing for increasing plate velocity at the surface, for a given applied driving force.  On the other hand, this fabric resists abrupt changes in the direction of plate motion because the effective viscosity is elevated for shear perpendicular to the developed LPO. This increased resistance to fabric-perpendicular shear also decreases strain rates, which slows texture development. This means that asthenospheric fabric can impede changes in plate motion direction for periods of over 10 Myrs. However, the same well-developed texture in the asthenosphere could enhance the initiation of subduction or lithospheric gravitational instabilities as vertical deformation is favored across a sub-lithospheric olivine fabric, and the sheared fabric can quickly rotate into a vertical LPO. These end-member cases examining shear-deformation across a formed asthenospheric fabric illustrate the importance of olivine fabrics, and their associated viscous anisotropy, for a variety of geodynamic processes.

How to cite: Kiraly, A., Conrad, C. P., Hansen, L. N., and Fraters, M.: The formation of viscous anisotropy in the asthenosphere and its effect on plate tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11957, https://doi.org/10.5194/egusphere-egu2020-11957, 2020

D1509 |
EGU2020-5663
Magdala Tesauro, Mikhail Kaban, Alexey Petrunin, and Alan Aitken

The Australian plate is composed of tectonic features showing progression of the age from dominantly Phanerozoic in the east, Proterozoic in the centre, and Archean in the west. These tectonic structures have been investigated in the last three decades using a variety of geophysical methods, but it is still a matter of debates of how temperature and strength are distributed within the lithosphere. We construct a thermal crustal model assuming steady state variations and using surface heat flow data, provided by regional and global database, and heat generation values, calculated from existing empirical relations with seismic velocity variations, which are provided by AusREM seismic tomography model. The lowest crustal temperatures are observed in the eastern part of the WAC and the Officer basin, while Central and South Australia are regions with anomalously elevated heat flow values and temperatures caused by high heat production in the crustal rocks. On the other hand, the mantle temperatures, estimated in a previous study, applying a joint interpretation of the seismic tomography and gravity data, show that the Precambrian West and North Australian Craton (WAC and NAC) are characterized by thick and relatively cold lithosphere that has depleted composition (Mg# > 90). The depletion is stronger in the older WAC than the younger NAC. Substantially hotter and less dense lithosphere is seen fringing the eastern and southeastern margin of the continent. Both crustal and mantle thermal models are used as input for the lithospheric strength calculation. Another input parameter is the crustal rheology, which has been determined based on the seismic velocity distribution, assuming that low (high) velocities reflect more sialic (mafic) compositions and thus weaker (stiffer) rheologies. Furthermore, we use strain rate values obtained from a global mantle flow model constrained by seismic and gravity data. The combination of the values of the different parameters produce a large variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. The sharp lateral strength variations are coincident with intraplate earthquakes location. The strength variations in the crust and upper mantle is also not uniformly distributed: In the Archean WAC most of the strength is concentrated in the mantle, while the Proterozoic Officer basin shows the largest values of the crustal strength. On the other hand, the younger eastern terranes are uniformly weak, due to the high temperatures.

How to cite: Tesauro, M., Kaban, M., Petrunin, A., and Aitken, A.: Temperature, strain rates, and rheology: the key parameters controling strength variations in the Australian lithosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5663, https://doi.org/10.5194/egusphere-egu2020-5663, 2020

D1510 |
EGU2020-2088
Hitoshi Kawakatsu, Hisashi Utada, Sang-Mook Lee, YoungHee Kim, Hajime Shiobara, Nozomu Takeuchi, Kiyoshi Baba, Takehi Isse, Akiko Takeo Takeo, and Hogyum Kim

With a simple crustal structure and short geological history, ocean basins provide an unblemished view into mantle dynamics, including convective flow and melting processes that control deformation and evolution of Earth’s surface. With the full spectrum of plate-boundary processes and abundant mid-plate volcanism sourced deep in the mantle, the Pacific basin provides an outstanding setting to explore connections between shallow dynamics and the deep interior. Exploiting advances in seafloor instrumentation, research groups in Japan, the US, and elsewhere have demonstrated the utility of broadband ocean-bottom seismic and EM arrays for providing new, high-resolution constraints on mantle structure and dynamics. These activities have coalesced into the international collaboration Pacific Array, which seeks to merge individual efforts into a large-scale "array of arrays" that will effectively cover the entire Pacific basin diachronously over a decadal time scale.

    As a part of the Pacific Array initiative, a team comprised of scientists from Japan and South Korea has completed the Oldest Array observation on the oldest seafloor in the western Pacific. Oldest Array consists of 12-seismic and 7-EM array that was deployed in Oct-Nov, 2018, for a duration of 12 months, followed by a successfully recovered in Oct-Nov, 2019. The instruments and vessels are respectively provided by ERI and KIOST. The array covers the northwestern side of the ~170Ma old magnetic lineation triangle aiming to delineate the lithosphere-asthenosphere system beneath the oldest Pacific basin to elucidate the enigma of seafloor flattening, as well as the dynamics of the birth of Pacific plate. The initial look at data indicates beautiful recordings, and we plan to report the first analysis results at the meeting.

How to cite: Kawakatsu, H., Utada, H., Lee, S.-M., Kim, Y., Shiobara, H., Takeuchi, N., Baba, K., Isse, T., Takeo, A. T., and Kim, H.: Oldest Array (Pacific Array on the oldest seafloor), the first result, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2088, https://doi.org/10.5194/egusphere-egu2020-2088, 2020

D1511 |
EGU2020-7757
Luděk Vecsey, Jaroslava Plomerová, Vladislav Babuška, the AlpArray-EASI Working Group, and the AlpArray Working Group

We examine lateral variations of shear-wave splitting evaluated from data recorded during the passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019). The swath about 200 km broad and 540 km long along 13.3° E longitude was selected to study the large-scale anisotropy in the mantle lithosphere beneath the Bohemian Massif (BM) and the Eastern Alps. The region is covered by about 200 broad-band temporary and permanent stations.

The shear-wave splitting evaluation consists of several steps: it starts by automated identification and pre-processing of SKS waveforms, filtering and quality check. Then we analyse and, if needed, also correct seismic waveforms for seismometer mis-orientations of all stations used. To improve results of splitting analysis of signals distorted by noise, we carefully apply two splitting methods (eigenvalue, transverse energy). We stack splitting measurements for waves closely propagating within the upper mantle and include particle motion analysis. The modified version of the splitting methods (Vecsey et al., 2008) enables us to retrieve 3-D orientation of large-scale anisotropic structures in the mantle lithosphere and deformations within the sub-lithospheric part of the upper mantle.

Both the evaluated shear-wave splitting parameters and the particle motions are consistent within sub-regions of the Alpine and BM upper mantle and exhibit significant and often sudden lateral changes across the whole region. We relate such changes to sharply bounded anisotropic domains with uniform fossil fabrics in the mantle lithosphere.

How to cite: Vecsey, L., Plomerová, J., Babuška, V., Working Group, T. A.-E., and Working Group, T. A.: Shear wave splitting as diagnostics of variable tectonic fabrics across the Eastern Alps – Bohemian Massif contact zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7757, https://doi.org/10.5194/egusphere-egu2020-7757, 2020

D1512 |
EGU2020-7450
Eric Löberich and Götz Bokelmann

The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation φ and delay time Δt, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations φ show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters φ and Δt, and in the backazimuthal variation of δφ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.

How to cite: Löberich, E. and Bokelmann, G.: Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7450, https://doi.org/10.5194/egusphere-egu2020-7450, 2020

D1513 |
EGU2020-1019
shivam chandra

Anisotropy in the Earth’s upper mantle is a signature of past and present deformation. Sri Lanka comprises four main lithological units, viz. the Highland Complex (HC), the Wanni Complex (WC), the Vijayan complex (VC) and the Kadugannawa complex (KC). To calculate the upper mantle anisotropy, we have collected the earthquake data from IRIS (Incorporated Research Institutions for Seismology) network. The upper mantle anisotropy beneath Sri Lanka is measured in the frequency band 0.01–0.15 Hz, with magnitude (Mw) of six or more and within the epicentral distance of 90°-140°. We have analyzed (the fast direction and delay time) shear wave splitting of SKS/SKKS phases at 3 stations, namely, MALK (WC), HALK (HC) and PALK (KC) in Sri Lanka. In this study, shear wave splitting measurements were done using high-quality seismograms (~30) of many earthquakes occurring in the region. We have used rotational correlation (RC) , minimum energy (SC) and eigenvalue techniques. The result of the shear-wave splitting measurement shows the presence of two anisotropic layers in the upper mantle. The upper and lower layer’s fast-polarization direction is found to be NE-SW and NW-SE, has the delay time varies from 0.4-0.5s in the upper layer, and 0.6-0.8s in the lower layer. We found two major fast directions in the upper and lower layers, viz. NE-SW in the upper layer of MALK and PALK and NW-SE for the HALK stations, and NNE-SSW in the lower layer beneath MALK and HALK stations and NW-SE in the PALK station. Overall, Fast direction for Sri Lanka region is found to be NE-SW in the lower layer and NW-SE in the upper layer. Our study suggests that fast axis direction of lower layer with an average delay time of 0.6 s depicts a ~67 km thick anisotropic layer with 4% anisotropy (from previous studies) beneath Sri Lanka region. However, if we assume an anisotropy range of 3–5%, then the calculated delay time of 0.6 s would correspond to thickness variation of 89.3 to 53.59 km, respectively, for the inferred anisotropic layer. Comparing from APM (Absolute Plate Motion) direction with our fast directions, we infer that the SAF(Simple Asthenospheric Flow) model prevails in this region and secondly, when Shmax (Maximum Horizontal stress) and the GPS (Global Positioning System) data compared with the fast direction we infer that there is partial contribution from lithospheric mantle. So, we confirmed that anisotropy in the region is mainly governed by asthenospheric flow and partially due to lithospheric mantle.

How to cite: chandra, S.: Upper mantle anisotropy and it's geodynamical implications in Sri Lanka region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1019, https://doi.org/10.5194/egusphere-egu2020-1019, 2019

D1514 |
EGU2020-3850
Julia Rewers, Piotr Środa, and AniMaLS Working Group

The passive seismic experiment AniMaLS was organized in 2017 in the Sudetes in Poland. One of the objectives was to study the anisotropy of the sub-crustal lithosphere and asthenosphere beneath the NE termination of the Bohemian Massif. Temporary seismic network of 23 broadband stations was operating in the area of Sudetes mountains and Fore-Sudetic Block, covering a ~200 x 100 km large area, with ~30 km spacing between stations. Obtained recordings were supplemented with data from permanent stations of Czech and Polish seismological networks located in the study area.

The Sudetes belong to internal zone of Variscan Orogen and are located in the NE part of the Bohemian Massif, between the Elbe Fault in SW and the Odra Fault in NE. The sudetic lithosphere represents a complex mosaic of several units with distinct histories of tectonic evolution and with consolidation ages ranging from the upper Proterozoic to the Quaternary. The aim of the project is to study seismic structure and anisotropy of the lithosphere-asthenosphere system based on broadband seismograms of local, regional and teleseismic events. The obtained data will be analysed using several interpretation methods. The poster presents the results of analysis by shear wave splitting method.

The analysis was done based on SKS and SKKS phases recorded during a ~2 years observation period. For analysis, three single-station methods were used: cross-correlation, eigenvalue minimization and transverse energy minimization. The dependence of resulting splitting parameters on the backazimuth of the event was also analysed. The results show that time delays between slow and fast S-wave components are typically in the range of ~0.5-1.6 sec, with average 1.2 sec. The splitting is interpreted as a result of lattice-preferred orientation (LPO) of mantle olivine. The azimuths of fast velocity axis are mostly consistent and showed largely WNW-ESE direction. They correlate well with trends of tectonic units observed at the surface and with strike directions of major fault zones. This suggests vertically coherent deformation throughout the lithosphere and frozen-in LPO, reflecting last tectonic episode which shaped Sudetic area. Obtained results were also compared with previous seismic studies of the upper mantle anisotropy in the neighboring areas by various methods.

How to cite: Rewers, J., Środa, P., and Working Group, A.: Upper mantle anisotropy beneath Sudetes from shear wave splitting - passive seismic experiment AniMaLS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3850, https://doi.org/10.5194/egusphere-egu2020-3850, 2020

D1515 |
EGU2020-6480
Takehi Isse, Daisuke Suetsugu, Akira Ishikawa, Hajime Shiobara, Hiroko Sugioka, Aki Ito, Yuki Kawano, Kazunori Yoshizawa, Yasushi Ishihara, Satoru Tanaka, Masayuki Obayashi, Takashi Tonegawa, and Junko Yoshimitsu

The Ontong Java Plateau (OJP), one of the largest oceanic plateaus located in the western Pacific Ocean, was first formed by a massive volcanism at 122 Ma, which had a major effect on the Earth's environments, including global climate change, oceanic anoxic events, and mass extinction of marine life. However, the cause of the volcanism remainscontroversial since the underground structure beneath the OJP has been poorly understood due to limited geophysical and geochemical data. To improve such situation, we conducted about 1.6-year long-term seafloor observation on the OJP and its vicinity. Using seismograms obtained by this observation as well as those from existing seismic stations, we obtained three dimensional radially anisotropic shear wave velocity structure beneath the OJP at depths down to 300 km.

Obtained structure shows the following new features:

(1) Beneath the Caroline Islands, in the north of the OJP, 1 % slow anomalies exist, which may be associated with the Caroline hotspot activity;

(2) In the center of the OJP at depths between 70–130 km, about 2% fast anomalies, whose shear wave speed is about 4.45-4.55 km/s, exists.

(3) The seismic structure clearly shows that the lithosphere–asthenosphere boundary (LAB) beneath the center of the OJP is located about 40 km deeper than that beneath the surrounding normal oceanic seafloor.

Judging from our results and petrological/rheological constraints given by previous studies, we interpret that the LAB is deepened by dehydrated residual material from hot mantle plume underplating a pre-existing lithosphere during a formation of OJP.

How to cite: Isse, T., Suetsugu, D., Ishikawa, A., Shiobara, H., Sugioka, H., Ito, A., Kawano, Y., Yoshizawa, K., Ishihara, Y., Tanaka, S., Obayashi, M., Tonegawa, T., and Yoshimitsu, J.: Seismic evidence for residual mantle underplating the lithosphere beneath the Ontong Java Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6480, https://doi.org/10.5194/egusphere-egu2020-6480, 2020

D1516 |
EGU2020-7581
yifang chen and jiuhui chen

The deformation of Qilian Orogenic Belt, which is the uplifting front of the northeastern Tibet Plateau, plays a decisive role in understanding the dynamic process of the area uplift. Many of the tectonic processes models of the Tibetan Plateau growth, which are based on geophysical and geological studies, have been conducted in recent years. However, the deformation mode of northeastern Tibetan Plateau (NETP) remains controversial for lack of sufficient proofs. We used teleseismic waveform data collected from the China Array seismic experiment during 2013-2015 and QL temporary stations during 2016-2017. In this study, we used the 3-D Common Conversion Point (CCP) technique (with the P/S receiver functions) to obtain detailed seismic velocity discontinuities structure of lithosphere beneath the NETP and Alxa block. Our preliminary results can be summarized as follows: 1) The Lithosphere asthenosphere boundary (LAB) lies at a depth pf 110-140 km in Alxa platform, deepens below the North Qilian mountain (160-170 km ) which has been inserted by lithosphere of Central Qilian, between the South Qilian suture zone (SQL) and the north of the Songpan-Ganzi Terranes (160-170 km). 2) The main features in the crust include offset of Moho beneath NQLF, shallower crust thickness below between the NQLF and LSSF and a continuous positive interface over the Moho in the north of the LSSF. 3) According to our observation and previous studies, we suppose that lithosphere had been passive underthrust and localized crust had been shortened and thickened in the NETP.

How to cite: chen, Y. and chen, J.: Deep deformation process of the NE Tibetan Plateau: evidence from receiver function imaging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7581, https://doi.org/10.5194/egusphere-egu2020-7581, 2020

D1517 |
EGU2020-11320
Irene Bianchi, Claudio Chiarabba, Pasquale De Gori, and Nicola Piana Agostinetti

The focus of this study is the mantle structure beneath the Apennines, and aims to understanding how deep processes are connected to shallow deformations. We present new observations from a rich receiver function data set from stations located along the North and Central Apennine chain, and use it for comparison and to strengthen the observations of previous seismic tomography images. The two methodologies define a low shear wave velocity zone (decrease of Vs in the order of 5%) and an increase of Vp/Vs (about 3%) in the shallow mantle between 50 and 90 km depth beneath the orogenic belt. The low Vs melt zone is not restricted to the mantle beneath the Quaternary volcanic areas, as previously thought, but is detected under the whole central Apennines suggesting future broad effects on a large scale.  Our interpretation of the teleseismic RFs and tomography, reveals consistently a diffuse mantle upwelling beneath the Apennines, and we hypothesize that slab-derived fluids might interact with the sub-lithospheric mantle generating melts that accumulate at the top of the mantle feeding post-collisional extension. This mechanism can be potentially applied to other cases of extension that spread over wide continental regions.

How to cite: Bianchi, I., Chiarabba, C., De Gori, P., and Piana Agostinetti, N.: Diffuse mantle upwelling and melts accumulation beneath the Italian Apennines., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11320, https://doi.org/10.5194/egusphere-egu2020-11320, 2020

D1518 |
EGU2020-6842
Ehsan Qorbani, Irene Bianchi, Petr Kolínský, Dimitri Zigone, and Götz Bokelmann

In this study, we show results from ambient noise tomography at the KTB drilling site, Germany. The Continental Deep Drilling Project, or ‘Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland’ (KTB) is at the northwestern edge of the Bohemian Massif and is located on the Variscan belt of Europe. During the KTB project crustal rocks have been drilled down to 9 km depth and several active seismic studies have been performed in the surrounding. The KTB area therefore presents an ideal test area for testing and verifying the potential resolution of passive seismic techniques. The aim of this study is to present a new shear-wave velocity model of the area while comparing the results to the previous velocity models and hints for anisotropy depicted by former passive and active seismological studies. We use a unique data set composed of two years of continuous data recorded at nine 3-component temporary stations installed from July 2012 to July 2014 located on top and vicinity of the drilling site. Moreover, we included a number of permanent stations in the region in order to improve the path coverage and density. Cross correlations of ambient noise are computed between the station pairs using all possible combination of three-component data. Dispersion curves of surface waves are extracted and are then inverted to obtain group velocity maps. We present here a new velocity model of the upper crust of the area, which shows velocity variations at short scales that correlate well with geology in the region.

How to cite: Qorbani, E., Bianchi, I., Kolínský, P., Zigone, D., and Bokelmann, G.: Upper crustal structure at the KTB drilling site from ambient noise tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6842, https://doi.org/10.5194/egusphere-egu2020-6842, 2020

D1519 |
EGU2020-12595
William Hawley and James Gaherty

Detailed knowledge of the seismic structure, fabric, and dynamics that surround the oceanic LAB continue to be refined through offshore seismic studies. Previous high-resolution studies in the Pacific basin far from plate boundaries show asthenospheric fabric that aligns neither with the lithospheric fabric (the paleo-spreading direction) nor with absolute plate motion, but rather in between. Here we present preliminary results from the Blanco Transform and Cascadia Initiative experiments, investigating the structure of the Juan de Fuca and Pacific plates on either side of the Blanco Transform. We measure ambient-noise and teleseismic Rayleigh-wave phase velocities, and solve for the period-dependent azimuthal anisotropy on either side of the transform. We will contextualize and interpret the fabrics based on mantle flow inferred from these previous Pacific basin studies. 

How to cite: Hawley, W. and Gaherty, J.: High-resolution constraints on LAB structure at the Blanco transform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12595, https://doi.org/10.5194/egusphere-egu2020-12595, 2020

D1520 |
EGU2020-6416
Jia-ji xi, Guo-ming jiang, Gui-bin zhang, and Xiao-long he

    There exists an important polymetallic ore belt in Nanling of the southeastern China. Previous studies suggest that the mineralization of Nanling is probably related to the bottom intrusion of magmatic rocks in the late Mesozoic. In this study, a natural seismic section was installed by using 81 portable stations with an interval of 5 km from July 2017 to August 2019, which runs across the Nanling belt in the south of Fujian and Jiangxi provinces. As a result, we have picked up 3,818 relative residual data from 215 teleseismic events with magnitude greater than 5.5. And we have applied the teleseismic full-waveform tomography and the teleseismic travel-time tomography to study the crust and the mantle velocity structure beneath the Nanling metallogenic belt, respectively. Our preliminary results show that: (1) a clear low-velocity anomaly exists in the crust beneath the Zhenghe-Dapu fault and its east side, which might be related to the rich ore deposits in Nanling; (2) some high-velocity anomalies in the uppermost mantle beneath the Wuyi metallogenic belt may be relevant to the igneous rock cooling and the lithospheric thickening; (3) there are obvious low-velocity anomalies at the upper mantle beneath the Wuyi and Nanling metallogenic belts, which are speculated to be hot materials from asthenosphere upwelling into the bottom of the lithosphere. Our results provide a new insight for investigating the deep structures and deep dynamic processes of Nanling tectonic belt.

How to cite: xi, J., jiang, G., zhang, G., and he, X.: 2D crust-upper mantle velocity structure along a seismic section in Nanling, South China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6416, https://doi.org/10.5194/egusphere-egu2020-6416, 2020

D1521 |
EGU2020-7371
Olga Kuchay, Natalia Bushenkova, Victor Chervov, and Andrey Jakovlev

The study is devoted to the analysis of seismotectonic deformations (STD) main axes directions distribution according to the mechanisms of earthquake foci and their complex comparison with the structure of the lithosphere based on the results of seismotomography and numerical modeling of the structure of convective flows in the upper mantle.

The International Seismological Center (ISC) catalog for 570 seismic events with M=5.0–8.0 was used to calculate the STD [http://www.isc.ac.uk/iscbulletin/search/fmechanisms/] that occurred between 1976 and May 2019 with the addition of materials on 154 foci 1905-1975 from [Radziminovich et al, 2016, Geodynamics & Tectonophysics; Imaev et al., 2000; Kuchay, 2013]. 

The STD field reconstruction was carried out for the region 38°-80° N and 63o-156o E using the technique described in [Bushenkova et al, 2018, Geodynamics & Tectonophysics; Kuchai, Kozina, 2015, Russian Geology and Geophysics]. The reconstructed STD field for each elementary volume of averaging shows that the predominant direction of the STD axes changes from West to East. The submeridional horizontal shortening, characteristic for the Tien Shan and Altai, turns to the NE, at ~ 93 мeridian and persist up to 105 meridian, where the shortening in the Baikal rift zone  occurs in the near-vertical direction and then again takes the NE orientation in Yakutia. The northern part of the study area is characterized by a near-vertical shortening. The predominant subhorizontal elongation appears in the Earth's crust in the eastern part of the study region.

The 3D seismotomographic model of the upper mantle velocity anomalies is based on ISC catalog data since 1964. When specifying boundary conditions in the 3D thermal convection numerical simulation, variations in the thickness of the lithosphere are taken into account (from geological and geophysical data, including seismotomographic data, specify the boundaries of the thickened lithosphere of plates and cratons surrounded by the thinned lithosphere of the northern Asia fold belts), according to the conclusions of our previous studies on the really significant effect of changes in lithosphere thickness on the structure of convective flows in the upper mantle [Bushenkova et al, 2018, Geodynamics & Tectonophysics; Chervov, Chernykh, 2014, Journal of Engineering Thermophysics].

Comparing the  orientations distribution of the STD main axes with the seismotomographic model of the region, we observe the areas of the STD axes directions turning coincide with the sharp boundaries of the seismic velocities anomalies sign change in the upper mantle.

Comparing the numerical model of thermal convection with the distribution of the STD main axes orientations we observe an obvious correlation of the STD main axes directions with extended downflows in the upper mantle (elongations are aligned along the strike of the downflow in the plan and shortenings across it). The orientation change occurs mainly above the convection upflows. The most clear correlation is observed in the southern half of the study region, because the lithosphere here has a smaller thickness and block size and the crust is less consolidated, which makes it more exposed to mantle processes.

How to cite: Kuchay, O., Bushenkova, N., Chervov, V., and Jakovlev, A.: The seismotectonic deformations main axes directions distribution in northern Asia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7371, https://doi.org/10.5194/egusphere-egu2020-7371, 2020

D1522 |
EGU2020-16409
Ángela María Gómez-García, Álvaro González, Magdalena Scheck-Wenderoth, Denis Anikiev, Gaspar Monsalve, and Gladys Bernal

Active continental margins are potentially exposed to geohazards of different nature, including earthquakes and gas hydrate destabilisation, which may result in submarine landslides and devastating tsunamis. The northern margin of the South American plate is characterised by two flat-slab subductions: the Nazca plate from the west, and the Caribbean plate from the north. This defines a complex and poorly understood tectonic setting which poses a risk for the inhabitants of the region.

Gaining insight into the physical conditions (such as rock strength and temperature) at which earthquakes nucleate in this region requires building an improved lithospheric model, and determining the thermal and rheological states of the tectonic plates involved in this subduction system.

Combining 3D lithospheric-scale thermal and rheological modelling is a novel approach to establish the spatial variation of seismogenic zones, both at shallow and intermediate depths, thus providing crucial information about the range of conditions at which earthquakes may occur. This method is especially useful in regions like the South Caribbean where more classical approaches are limited because seismic records do not extend far back in time and the frequency of megathrust earthquakes is low.

Furthermore, in river-dominated continental margins, such as the South Caribbean, the destabilisation of gas hydrates deposits has been recently recognised as one of the most important triggering factors of submarine landslides. Gas hydrates are stable in low-temperature and high-pressure environments, normally found in marine sediments within continental slopes, with dominant temperatures ranging from 5°C to 10°C, at depths greater than 400 m. However, the gas hydrate stability zone is mainly controlled by the local geothermal gradient and the bottom water temperature, being both parameters influenced by the particular setting of each region.

Our research aims to evaluate the physical state of the seismogenic zones in the northern margin of the South American plate and Panama microplate, and to identify the locations of potential gas hydrates accumulation in the South Caribbean margin.

Here we present the complete workflow of this analysis, starting from the definition of an up-to-date 3D lithospheric-scale model which has been validated with the forward modelling of gravity anomalies. This model is the main input for calculating the 3D steady-state thermal field and the 3D pressure field, using the software LYNX. Based on our modelled results, we evaluate the rheological behaviour of the present-day lithospheric configuration, considering the locations of the earthquakes from the Bulletin of the International Seismological Centre. Finally, by modelling the temperature and pressure within the marine sediments, we constrain the spatial distribution of the potential gas hydrate stability zone.

With this work we exemplify how 3D lithospheric-scale thermal and rheological models may contribute to the assessment of geohazards in a region such as the Caribbean Sea, where hundreds of thousands of coastal inhabitants, tourists and infrastructures are potentially at risk.

How to cite: Gómez-García, Á. M., González, Á., Scheck-Wenderoth, M., Anikiev, D., Monsalve, G., and Bernal, G.: Characterisation of seismogenic zones and gas hydrates accumulation regions in the South Caribbean margin using 3D lithospheric-scale thermal and rheological models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16409, https://doi.org/10.5194/egusphere-egu2020-16409, 2020

D1523 |
EGU2020-2181
Bing Xia, Irina Artemieva, and Hans Thybo

We calculated the thermal lithosphere structure of Tibet and adjacent regions based on the new thermal isostasy method. Moho depth is constrained by the published receiver function results. The calculated surface heat flow in the surrounded Tarim, North China, and Yangtze cratons have a good match with the real measurements of surface heat flow. We recognize the northern Tibet anomaly where has a relatively thin lithosphere with a thermal thickness of <80 km and surface heat flow of >80 - 100 mW/m 2 may cause by the removal of lithospheric mantle and upwelling of asthenosphere. In Lhasa Block, the cold and thick lithosphere (>200 km) with a surface heat flow of 40 - 50 mW/m 2. In the east Tibet, the heterogeneous thermal lithosphere does not follow the widely spread large scale strike-slip faults and suggested that the faults do not cut down to the lithosphere. The surrounding cratons have different thermal lithosphere features. The Tarim and Yangtze cratons show typical cold and thick lithosphere with a lithosphere of >200km and surface heat flow of <50 mW/m2. The western North China Craton has an intermated lithosphere with a thickness of 120-200km and surface heat flow of 45-60 mW/m2. Our result suggested that high and flat Tibet has different isostatic compensation in different blocks. The heterogeneous lithosphere thermal structure of the Tibet suggested that the uplife force drive are difference in Tibet.

 

How to cite: Xia, B., Artemieva, I., and Thybo, H.: The Tibet lithosphere is not all hot, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2181, https://doi.org/10.5194/egusphere-egu2020-2181, 2020

D1524 |
EGU2020-9668
Edgar Santos and Victor Sacek

In this work, we studied the mantle flow around cratonic keels using numerical models to simulate the thermochemical convection in the terrestrial mantle taking into account the relative displacement between the lithosphere and asthenosphere. The numerical simulations were performed using the finite element code developed by Sacek (2017) to solve the Stokes Flow for an incompressible Newtonian fluid. Several synthetic models in 2D and 3D were constructed considering different keel geometries and different regimes of relative displacement between the lithosphere and asthenosphere. In the present numerical experiments, we adopted a rheology in which the viscosity of the mantle is controlled by temperature, pressure and composition, assuming that the cratonic keel is compositionally more viscous than the surrounding asthenosphere, using a factor f to rescale the lithospheric viscosity compared to the asthenospheric one. We tested different f values, reference viscosity for the asthenosphere, and relative velocity between the lithosphere and the base of the upper mantle, quantifying the amount of deformation of the cratonic keel in each scenario. In general, we conclude that for a relatively low compositional factor (f < 20), the lithospheric keel can be significantly deformed in a time interval of few tens of million years when the lithosphere is moving horizontally relative to the base of the upper mantle, does not preserving its initial geometry. The synthetic models can be helpful for a better understanding of the interaction in the lithosphere-asthenosphere interface such as the deformation and flow patterns in the mantle around the keels, the rate of erosion of the root of the continental lithosphere due to the convection in the upper mantle and how it affects the thermal flow to the surface.

Sacek, V. (2017). Post-rift influence of small-scale convection on the landscape evolution at divergent continental margins. Earth and Planetary Science Letters, 459, 48-57.

How to cite: Santos, E. and Sacek, V.: Numerical simulation of asthenospheric flow around cratonic keels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9668, https://doi.org/10.5194/egusphere-egu2020-9668, 2020

D1525 |
EGU2020-704
Jyotirmoy Paul and Attreyee Ghosh

The recent discovery of mid-lithospheric discontinuity (MLD) within most cratons has added a new dimension in the understanding of cratonic survival. The MLD shows up as a seismic discontinuity at ~80-160 km depth. However, there is controversy regarding the strength of this layer. While some studies suggest that this layer is as strong as the craton itself, others advocate that under some special conditions (e.g. metasomatism) MLD can become weak and aid in the delamination of cratons. In this study, we develop 3-D full spherical mantle convection models to understand the effect of MLD in the survival of cratons. In our models, we incorporate MLDs of variable strength, depth and thickness. Along with varying the strength of MLDs, we use different combinations of craton and asthenosphere viscosity to quantitatively estimate how deformation pattern varies. Results obtained from the models suggest that in the presence of a weak MLD stress magnitudes decrease but strain-rates increase  ~2-3 times. This could potentially lead to delamination of cratons. To constrain the present-day strength of MLDs, we predict deviatoric stresses from these different models and compare them to the observed SHmax directions obtained from the World Stress Map. The deviatoric stress pattern changes as the viscosity, depth and thickness of MLD changes.

How to cite: Paul, J. and Ghosh, A.: Understanding deformation of cratons in presence of mid-lithospheric discontinuity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-704, https://doi.org/10.5194/egusphere-egu2020-704, 2019

D1526 |
EGU2020-18448
Chung-Liang Lo, Wen-Bin Doo, and Shu-Kun Hsu

The subduction zone is a convergent plate boundary, and where most seismic activity is concentrated and megathrust may occur. To evaluate the potential hazard in subduction zones always relates to the plate coupling status. From previous studies, the status of plate coupling between plates can be reflected by the vibration of the buoyancy of mantle lithosphere (Hm). As far as the respective plate coupling states are concerned, more than a dozen Hm profiles across different subduction zones have been successfully verified. It is normally to determine the coupling status depending on the Hm vibration without manifest definition. We therefore propose a method to estimate the plate coupling factor (pcf) quantitatively. The pcf is defined as the difference of the Hm caused by the respective subduction and overriding plates between the distances where Hm deviated from the normal lithospheric Hm value across the plate boundary. The collected Hm profiles are calculated by the proposed method, the results show that the pcf value is corresponding well to the plate coupling status in the respective subduction zone. The small pcf is for strong plate coupling, such as the northern Sumatra and the southern central Andes subduction zones, while the large pcf is for weak coupling, such as the Calabria and the northern Manila subduction zones. The calculation of pcf is a feasible solution for determination of plate coupling status, but more Hm profiles across subduction zones will help the estimation more reliable.

How to cite: Lo, C.-L., Doo, W.-B., and Hsu, S.-K.: The rule of thumb for inferring plate coupling status based on the mantle lithospheric buoyancy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18448, https://doi.org/10.5194/egusphere-egu2020-18448, 2020

D1527 |
EGU2020-12041
Qunfan Zheng and Huai Zhang

East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western Indo-Asian continental collision and the eastern oceanic subduction mainly from Pacific plate. Till now, mantle dynamics beneath this area is not well understood due to its complex mantle structure, especially in the framework of global spherical mantle convection. Hence, a series of numerical models are conducted in this study to reveal the key controlling parameters in shaping the present-day observed mantle structure beneath East Asia under 3-D global mantle flow models. Global mantle flow models with coarse mesh are firstly applied to give a rough constraint on global mantle convection. The detailed description of upper mantle dynamics of East Asia is left with regional refined mesh. A power-law rheology and absolute plate field are applied subsequently to get a better constraint on the related regional mantle rheological structure and surficial boundary conditions. Thus, the refined and reasonable velocity and stress distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations. The derived large shallow mantle flow beneath the Tibetan Plateau causes significant lithospheric shear drag and dynamic topography that result in prominent tectonic evolution of this area. And the Indo–Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo–Asian collision has occurred for 50 Ma, and this collision can still continue to accelerate uplift in the Tibetan plateau. Finally, we also consider the possible implementations of 3-D numerical simulations combined with global lithosphere and deep mantle dynamics so as to discuss the relevant influences.

How to cite: Zheng, Q. and Zhang, H.: Mantle convection beneath East Asia under global mantle convection spherical framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12041, https://doi.org/10.5194/egusphere-egu2020-12041, 2020

D1528 |
EGU2020-9861
Sergei Medvedev and Alexander Minakov

It is well-accepted that stresses and deformation are controlled by active forces, such as tractions applied along lateral boundaries and base of the lithosphere and body forces raised from density heterogeneities within or below the lithosphere. Here we analyze how structure, geometry and strength distribution, of the Earth crust and upper mantle can affect the pattern of stresses and deformation. As an application example, we use the North Atlantic realm which characterized by strong topography and rheological variations and subjected to active forces from, e.g., the Iceland hot spot. We conduct a series of numerical experiments modelling the lithosphere as an elastic shell of altering geometries influenced by various mechanisms. The first set of experiments demonstrates that lithosphere, as a part of the spherical Earth, is structurally stronger than the flat lithosphere if boundary moments applied. An application of more realistic, topography derived, geometry of the lithospheric shell in the second set of experiments demonstrates the importance of strong topography changes, for example along continent-ocean transition, as a concentrator of bending stresses and deformations. In the third set, we show how viscous properties of the sub-lithospheric asthenosphere may control the lateral extent of the membrane stresses in the lithosphere.

How to cite: Medvedev, S. and Minakov, A.: Structural controls on stresses and deformations in a large-scale lithospheric shell, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9861, https://doi.org/10.5194/egusphere-egu2020-9861, 2020

D1529 |
EGU2020-13630
Francesca Maddaloni, Damien Delvaux, Magdala Tesauro, Taras Gerya, and Carla Braitenberg

The Congo basin (CB) is an intracratonic basin that occupies a large part of the Congo Craton (1.2 million km2) covering approximately 10% of the continent [1]. It contains up to 9 km of sedimentary rocks from the Mesoproterozoic until Cenozoic age. The formation of the CB started with a rifting phase during Mesoproterozoic with the amalgamation of the Rodinia supercontinent (1.2 Gyr). Afterwards, the main episodes of subsidence occurred during the subsequent Neoproterozoic post-rift phases, which were followed by phases of compression at the end of the Permian and during the Early Jurassic age and other sedimentation episodes during Upper Cretaceous and Cenozoic [2].

 We reconstruct the stratigraphy and tectonic evolution of the basin by analyzing seismic reflection profiles. Furthermore, we estimated the velocity, density, and thickness of the sedimentary layers in order to calculate their gravity effect. Afterwards, we calculate the gravity disturbance and Bouguer anomalies using a combined satellite and terrestrial data gravity model. The gravity disturbance obtained from the EIGEN-6C4 gravity model [3] shows two types of anomalies. One with a long wavelength (~50 mGal) that covers the entire area of the Congo basin and a second one with a short wavelength (~130 mGal), having a NW-SE trend, which corresponds to the main depocenters of sediments detected by the interpretation of seismic reflection profiles. These results have been used as input parameters for 3D numerical simulations to test the main mechanisms of formation and evolution of the CB. For this aim, we used the thermomechanical I3ELVIS code [4] to simulate the initial rift phase. The numerical tests have been conducted considering a sub-circular weak zone in the central part of the cratonic lithosphere [2] and applying a velocity of 2.5 cm/yr in two orthogonal directions (NS and EW), to test the hypothesis of the formation of a multi extensional rift in a cratonic area. We repeated these numerical tests by increasing the size of the weak zone and varying its lithospheric thickness. The results of these first numerical experiments show the formation of a circular basin in the central part of the cratonic lithosphere, in response to extensional stress, inducing the uplift of the asthenosphere.

[1] Kadima, et al. (2011), Structure and geological history of the Congo Basin: an integrated interpretation of gravity, magnetic and reflection seismic data, doi:10.1111/j.1365-2117.2011.00500.x.
[2] De Wit, et al. (2008), Restoring Pan-African-Brasiliano connections: more Gondwana control, less Trans-Atlantic corruption, doi:10.1144/SP294.20
[3] Förste et al. (2014) EIGEN-6C4 The latest combined global gravity field model including GOCE data up to degree and order 2190 of GFZ Potsdam and GRGS Toulouse; doi: 10.5880/ICGEM.2015.1, 2014
[4] Gerya (2009), Introduction to numerical geodynamic modelling, Cambridge University Press

How to cite: Maddaloni, F., Delvaux, D., Tesauro, M., Gerya, T., and Braitenberg, C.: Tectonic evolution of the Congo Basin using geophysical data and 3D numerical simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13630, https://doi.org/10.5194/egusphere-egu2020-13630, 2020

D1530 |
EGU2020-12137
Huai Zhang, Qunfan Zheng, and Zhen Zhang

East Asia is a tectonically active area on earth and has a complicated lithospheric deformation due to the western continental collision from the cratonic Indian plate and the eastern oceanic subduction mainly from Pacific plate. Studies have suggested that the Indo–Asian continental collision may have driven significant lateral mantle flow, but the velocity, range and effect of the mantle flow remain uncertain. Hence, a series of 3-D numerical models are conducted in this study to reveal the impacts of the Indo–Asian collision on mantle dynamics beneath the East Asia, especially on the asthenospheric mantle. Global model domain encompasses the lithosphere, upper mantle and the lower mantle with different viscosity for each layer. A global temperature structure built from seismic tomography and absolute plate field are applied subsequently to get a better constraint of the initial temperature condition and surficial velocity boundary condition. Thus, the reasonable velocity and temperature distributions of upper mantle beneath East Asia at different depths are retrieved based on our 3-D global mantle flow simulations, and the key controlling parameters in shaping the present-day observed mantle structure are investigated. The results show different scales of convection beneath East Asia.

Our results suggest that Indo–Asian collision may have induced mantle flow beneath the Indian plate and the different velocity structures between the asthenosphere and lithosphere indicate the shear drag of asthenospheric mantle. That may explain the reason that Indo–Asian collision has occurred since 50 Ma, and this collision can still continue to accelerate in the Tibetan Plateau. The simulation results also show the lithospheric delamination and the induced mantle upwelling, which is consistent with the general understanding from previous observations. The Indian lithosphere and its asthenosphere move northward, while the Yunnan lithosphere and its asthenosphere move southward, that may reflect the differences in deep mantle dynamics between the eastern and western Himalayan Syntaxis.

How to cite: Zhang, H., Zheng, Q., and Zhang, Z.: Deep Mantle Dynamics in East Asia: Numerical Simulation of Mantle Convection Based on Seismic Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12137, https://doi.org/10.5194/egusphere-egu2020-12137, 2020

D1531 |
EGU2020-2305
Agnes Wansing, Jörg Ebbing, and Eva Bredow

We present an integrated geophysical-petrological model of the Eifel region. The Eifel is a volcanic active region in West Germany that exhibits Tertiary as well as Quaternary volcanism. One suggestion for the source of this volcanism is a small-scale upper mantle plume.

The 3D model includes the crust and upper mantle and was generated by combined modelling of topography and the gravity field with constraints from seismology and geochemistry. In the best-fit model, the subcontinental lithospheric mantle is associated with a Phanerozoic-type composition, resulting in a depth of 80 km for the lithosphere-asthenosphere boundary (LAB) beneath the Eifel and in comparison 110 - 130 km beneath the Paris basin. A Proterozoic-type composition in contrast results in a LAB depth of 120 km in the Eifel. While the model fits the geophysical observables and features a thin lithosphere, it does not lead to a plume-like structure and does not feature a seismic low-velocity anomaly.

The measured low-velocity anomaly can be reproduced by introducing (1) an even thinner lithosphere or (2) a plume-like body above the thermal LAB with a composition based on data from Eifel xenoliths, which have a mainly basanitic composition. This additional structure results in a thermal anomaly and has an effect on the isostatic elevation of c. 360 m, but it does not result in a significant signal in the gravity anomalies. Further modelling showed how crustal intrusions could additionally mask the gravitational effect from such a small-scale upper mantle plume.

The model does not conclusively explain the source of the Eifel volcanism, but the models and the calculation of synthetic dispersion curves help to assess the possibility to resolve a small-scale upper mantle plume with joint inversion in future analysis.

How to cite: Wansing, A., Ebbing, J., and Bredow, E.: Integrated geophysical-petrological modelling of the Eifel region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2305, https://doi.org/10.5194/egusphere-egu2020-2305, 2020

D1532 |
EGU2020-1644
Leonardo Mairink Barão, Barbara Trzaskos, Rodolfo José Angulo, and Maria Cristina de Souza

The exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile-brittle deformations until the complete emplacement in the oceanic lithosphere. São Pedro and São Paulo Archipelago, is located at 1° N latitude, 1000km from the Brazilian mainlad. Ten isles compose the archipelago with a total exposed area of 17 km². Those isles record the deformational products of ductile, brittle and the rocks/fluid interaction generating specific structures in each domain. The deformational stages are related to the transpressional and transtensional geodynamics of São Paulo Transform Fault (SPTF). The ductile-brittle fabrics were observed in a multiscale context, using drone images, geological mapping, fault analysis, and microstructural studies. Using all these tools to define the tectonic tensions and structures associated with a transition between ductile to the brittle deformational settings. Firstly during the transpressional context, the exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700° - 800°C. Leading to olivine and orthopyroxene recrystallization forming such as well-marked mylonitic foliation and rotated porphyroclast with left-lateral kinematic. The interaction with fluids initially originated from the mantle, generates fragmented crystals of amphibole and oxide-rich levels, marking the transition to semi-brittle deformation. The continuous and rapid uplift led to the superposition of deformation mechanisms, with reactivation of pre-existing structures and predominance of brittle deformation mechanisms. The tectonics associated with an NW-SE shortening in the transpressional tectonics context led to greater availability of hydrothermal fluids. Consequently, the formation of four serpentinization episodes, which are associated with semi-brittle to brittle transition, with temperatures between 300 - 400° C. The presence of serpentine marks the transition between semi-brittle to brittle regimes, whose dextral kinematics is marked by the domino faults, microfaults and gash veins. The kinematics at the brittle moment is compatible with the current movement of the SPTF. Finally, the complete exhumation and establishment of brittle mechanisms led to the carbonatation phase near the surface, with temperatures ranging from 150 - 300°C. The active NW-SE tectonic stress generated an E-W strike-slip faults that filled by carbonates, symbolizing the final exhumation stage.

How to cite: Barão, L. M., Trzaskos, B., Angulo, R. J., and Souza, M. C. D.: Evolution of mantle peridotite rocks - structures generated in a transition from the ductile-brittle regime in the Equatorial Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1644, https://doi.org/10.5194/egusphere-egu2020-1644, 2019

D1533 |
EGU2020-4458
Sejin Jung, Haemyeong Jung, and Håkon Austrheim

The microstructures of amphibole peridotites from the Åheim, Norway were studied to understand the evolution of microstructures of olivine through the Scandian Orogeny and the subsequent exhumation process. The Western Gneiss Region, Norway had undergone UHP metamorphism and subsequent retrogression associated with the Scandian Orogeny. The Åheim amphibole peridotite shows clear porphyroclastic texture, abundant hydrous minerals such as tremolite or chlorite, and much evidence of localized deformation. LPOs of olivine and amphibole were determined by using electron back-scattered diffraction (EBSD) system attached to the scanning electron microscope (SEM).

Detailed microstructural analysis on the Åheim amphibole peridotites revealed the evidence of the multiple stages of deformation during the Scandian Orogeny. The coarse grains of olivine including porphyroclasts showed the A-type LPO of olivine (Jung & Karato, 2001), which is interpreted as an initial stage of deformation. The recrystallized-fine grains of olivine showed the B-type LPO of olivine (Jung & Karato, 2001), which is interpreted as a late-stage deformation in amphibolite facies condition. Observation of abundant hydrous minerals, hydrous inclusions in olivine, as well as high dislocation density of olivine in the fine-grained olivines suggest that fabric transition of olivine from the A-type to B-type LPO was resulted from the deformation in a water-rich condition during the exhumation process. The B-type LPO of olivine is important because it is the one of the possible mechanisms for causing the trench-parallel seismic anisotropy in the mantle wedge. A partial fabric transition from the A-type to the B-type LPO of olivine associated with the localized deformation in a water-rich condition might explain a weak seismic anisotropy observed in NE Japan or Mexico. Amphiboles in the amphibole-rich layer showed the Type-III LPO of amphibole (Ko & Jung, 2015). It is found that strong fabric strength and the resultant seismic anisotropy of amphibole can perform a similar role as other hydrous minerals such as serpentine or chlorite on the trench-parallel seismic anisotropy with the flow dipping along the subducting slab in the mantle wedge.

 

Jung, H., Karato, S., 2001, Science, 293, 1460-1463.

Ko, B., Jung, H., 2015, Nature Communications, 6: 6586.

How to cite: Jung, S., Jung, H., and Austrheim, H.: Deformation microstructure of amphibole peridotite from Aheim, Norway and its implication for the seismic anisotropy of the mantle wedge, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4458, https://doi.org/10.5194/egusphere-egu2020-4458, 2020

D1534 |
EGU2020-4440
Jungjin Lee, Haemyeong Jung, Reiner Klemd, Matthew Tarling, and Dmitry Konopelko

Strong seismic anisotropy is generally observed in subduction zones. Lattice preferred orientation (LPO) of olivine and elastically anisotropic hydrous minerals has been considered to be an important factor causing anomalous seismic anisotropy. For the first time, we report on measured LPOs of polycrystalline talc. The study comprises subduction-related ultra-high-pressure metamorphic schists from the Makbal Complex in Kyrgyzstan-Kazakhstan and amphibolite-facies metasomatic schists from the Valla Field Block in Unst, Scotland. The here studied talc revealed a strong alignment of [001] axes (sub)normal to the foliation and a girdle distribution of [100] axes and (010) poles (sub)parallel to the foliation. The LPOs of polycrystalline talc produced a significant P–wave anisotropy (AVp = 72%) and a high S–wave anisotropy (AVs = 24%). The results imply that the LPO of talc influence both the strong trench-parallel azimuthal anisotropy and positive/negative radial anisotropy of P–waves, and the trench-parallel seismic anisotropy of S–waves in subduction zones.

How to cite: Lee, J., Jung, H., Klemd, R., Tarling, M., and Konopelko, D.: Lattice preferred orientation of talc and implications for seismic anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4440, https://doi.org/10.5194/egusphere-egu2020-4440, 2020

D1535 |
EGU2020-4448
Junha Kim and Haemyeong Jung

The lattice preferred orientation(LPO) of amphibole has a large effect on seismic anisotropy in the crust. Previous studies have reported four LPO types (I–IV) of amphibole, but the genesis of type IV LPO, which is characterized by [100] axes aligned in a girdle subnormal to the shear direction, is unknown. In this study, shear deformation experiments on amphibolite were conducted to find the genesis of type IV LPO at high pressure (0.5 GPa) and temperature (500–700 °C). The type IV LPO was found under high shear strain (γ > 3.0) and the sample exhibited grains in a range of sizes but generally smaller than the grain size of samples with lower shear strain. The seismic anisotropy of type IV LPO is lower than in types I-III. The weak seismic anisotropy of highly deformed amphibole could explain weak seismic anisotropy observed in the middle crust.

How to cite: Kim, J. and Jung, H.: New lattice preferred orientation(LPO) of amphibole experimentally found in simple shear, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4448, https://doi.org/10.5194/egusphere-egu2020-4448, 2020

D1536 |
EGU2020-12579
Magali Billen and Menno Fraters

When modeling subduction processes, the results are usually constrained by looking at the geological surface expressions, geochemistry and geophysical observations such as tomography and seismic anisotropy. Of these observations, seismic anisotropy is the only type of observation that can potentially be directly linked to the spatial flow pattern in the mantle. Seismic anisotropy in the mantle is due to lattice-preferred orientation (LPO) of olivine minerals. In subduction environments, which can have complex and changing flow patterns, it is not expected that the LPO necessarily aligns with the flow pattern. This is partly due to the fact that it takes time to realign the LPO and partly because the olivine fast axis alignment depends on the water content and the magnitude of stress. To overcome this problem, the LPO must be computed for realistic and end member subduction zones in order to be able to relate seismic anisotropy to mantle flow and thereby slab dynamics.

There are many ways to compute LPO. For this study we have used DREX (Kaminski et al., 2004), because the underlying method is accurate and fast enough for use in geodynamic models. To achieve a good and native integration with ASPECT (Kronbichler et al., 2012; Heister et al., 2017; Bangerth et al,. 2019), we have rewritten DREX in CPP as a plugin for ASPECT. In this presentation we will show how it was implemented and what the limitations and possibilities are. Furthermore, we will show initial results from 3D subduction models to study the link between seismic anisotropy and mantle flow.

How to cite: Billen, M. and Fraters, M.: Computing LPO for Geodynamic Models in ASPECT, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12579, https://doi.org/10.5194/egusphere-egu2020-12579, 2020