Enter Zoom Meeting

GD7.3

EDI
Structure, deformation and dynamics of the lithosphere-asthenosphere system

The geological processes that we infer from observations of the Earth’s surface, together with the landscape features are direct consequences of the dynamic Earth, and in particular, of the interaction between tectonic plates. Seismological studies are key for unraveling the present structure and fabric of the lithosphere and the asthenosphere. However, interdisciplinary work is required to fully understand the underlying processes and how features such as anisotropies in the crust, lithospheric mantle or the asthenosphere evolved through time and how they are related. Here we want to gather those studies focusing on seismic anisotropy and deformation patterns that can successfully improve our knowledge of the processes, leading to the observed present geometries (of the crust and the upper mantle). The main goal of the session is to establish closer links between seismological observations and process-oriented modelling studies to demonstrate the potential of different methods, and to share ideas of how we can collaboratively study upper mantle 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 observations, tomography and waveform modeling, geodetic data, numerical and analogue modelling are welcome.

Including GD Division Outstanding ECS Award Lecture
Co-organized by SM5
Convener: Ehsan Qorbani | Co-conveners: Ana MG Ferreira, Jaroslava Plomerova, Ernst Willingshofer
Presentations
| Thu, 26 May, 08:30–11:49 (CEST)
 
Room -2.91

Thu, 26 May, 08:30–10:00

Chairpersons: Ehsan Qorbani, Jaroslava Plomerova, Ana MG Ferreira

08:30–08:32
Introduction

08:32–08:42
|
EGU22-1781
|
solicited
|
Virtual presentation
Fiona Darbyshire

The eastern Canadian Shield and its margins represent an excellent natural laboratory to study the formation and evolution of continental lithosphere, as the rocks and structures preserve approximately 4 billion years of geological history. The core of the continent is made up of several large Archean cratonic blocks and continental fragments, welded together by Paleoproterozoic mobile belts. Subsequent Proterozoic orogenesis added to the southern and eastern margins, building the Laurentian landmass, and a series of Wilson cycles established the form of the continent we see today. Laurentian lithosphere is characterized in seismic tomography by a thick, seismically fast continental keel, representing cold temperatures and a depleted composition, whereas the Phanerozoic margins have slower seismic wavespeeds and a thinner lithosphere.

Over the last several decades, numerous seismic anisotropy measurements have been used to investigate lithospheric and sublithospheric fabrics beneath the region. Shear wave splitting shows strong lateral variability in both the strength and fast-polarisation orientation of the anisotropy, and measurements at closely-spaced stations suggest a significant lithospheric component as well as a likely sublithospheric contribution. Recent regional and continental-scale surface wave tomography studies allow for some depth constraint on the azimuthal anisotropy, which appears pervasive, but varying, for different depth ranges within the lithosphere and asthenosphere.

We compare the measurements from shear wave splitting and surface wave tomography with several geological and geophysical observations that could relate to anisotropic fabric, such as surface tectonic boundaries, magnetic anomalies, absolute plate-motion directions and mantle flow patterns from global geodynamic models. We use these comparisons to investigate the relative contributions to the seismic anisotropy observed across the region from lithospheric deformation, basal shear of the North American plate, and active mantle convection.

How to cite: Darbyshire, F.: Evidence for lithospheric and sublithospheric anisotropy of the eastern Canadian Shield and its margins, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1781, https://doi.org/10.5194/egusphere-egu22-1781, 2022.

08:42–08:49
|
EGU22-4897
|
Virtual presentation
Cedric Legendre et al.

Seismic anisotropy beneath Anatolia is complex, with several layers of different anisotropy. 
The average anisotropy is well constrained by shear-wave splitting measurements [Kaviani et al., 2009], suggesting very strong anisotropy (over 1.5s delay time). However, the vertical layering of anisotropy and the contribution of each layer is still an open question. 

We construct anisotropic phase-velocity maps of fundamental-mode Rayleigh waves for the Anatolia region using records from several regional seismic stations, using both earthquake and ambient noise data. 
The collision between the Arabia and Eurasia plates leads to the westward extrusion (and EW anisotropy) of the Anatolian crust, consistent with the seismic anisotropy patterns we found in the crust (1%, EW fast axis) and with previous studies [Mutlu et al., 2011; Legendre et al., 2020].
The Aegean/Anatolian subduction system with slab tearing and breakoff induces a complex flow pattern and anisotropy in the upper mantle [van Hinsbergen et al., 2010; Kaviani et al., 2018].
This is in agreement with the anisotropy we image in the lithosphere (1%, N020E and N100E fast axes) and asthenosphere (1%, N120E). However, the anisotropy in these layers display limited amplitudes.
At deeper depth, remnant Bitlis and Tethyan slabs are lying flat above the 660-km discontinuity [Berk Biryol et al., 2011]. 

The uniform pattern of anisotropy from shear-wave splitting observations can not be explained solely by a single anisotropic layer, and is not consistent with the anisotropy observed in the crustl lithospheric and asthenospheric mantle. This suggests that main contribution of the anisotropy likely originates from a deep source around the mantle transition zone [Legendre et al., 2021].

How to cite: Legendre, C., Zhao, L., and Tseng, T.-L.: Large-scale variation in seismic anisotropy in the crust and upper mantle beneath Anatolia, Turkey, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4897, https://doi.org/10.5194/egusphere-egu22-4897, 2022.

08:49–08:56
|
EGU22-11377
|
ECS
|
Virtual presentation
Yajian Gao et al.

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

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

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

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

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

08:56–09:03
|
EGU22-12799
|
ECS
|
On-site presentation
William Buffett et al.

Subduction zone dynamics are important for a better understanding of a broad range of topics ranging from plate tectonics to natural hazards such as earthquakes and volcanoes. New Zealand is a seismically unique place, resting on the Hikurangi Subduction Zone. It experiences a large range of seismic phenomena from evidence of large megathrust events and slow slip activity, to active volcanism within the Taupo Volcanic Zone. Although much seismic imaging has been performed, S-to-P receiver functions can tightly constrain discontinuities and associated dynamics. Here we use S-to-P receiver functions to image lithospheric discontinuities beneath the North Island of New Zealand using IRIS-DMC and Geonet stations. We image the Moho at 15-25 km depth in the south by Wellington, with a second velocity increase with depth imaged just beneath at 40-50 km, possibly corresponding to the Moho of the downgoing plate. On the northern edge of the North Island by Auckland, the Moho is imaged at 20 +/- 5 km depth. Near Napier and Lake Taupo we image 2 positive discontinuities at 10 and 30 km depth, still beneath the upper plate potentially related to crustal layering or the magmatic plumbing system. This is in line with previous studies of the Moho, for example a collation of Moho estimates by Salmon et al. (2013) places the Moho in the region of 20-25 km depth for most of the North Island, except for some deeper phases in the very east and the most southwest. A negative phase corresponding to the lithosphere-asthenosphere boundary (LAB) of the upper plate is imaged at 60-70 km depth across portions of the North Island. The LAB of subducting Pacific Plate is imaged at 70-80 km with the exception of a gap in the LAB phase between 39° and 40° latitude and around 176° longitude corresponding to the mountain ranges of Kaweka Forest Park and Ruahīne Forest Park. We image a velocity increase directly beneath the LAB, potentially related to the base of a melt layer beneath the plate. Furthermore, this is consistent with the estimated thickness of the lithosphere (73 +/- 1 km), for instance from the active source estimates of Stern et al. (2015).

How to cite: Buffett, W., Harmon, N., Rychert, C., and McNeill, L.: S-to-P Receiver Function Analysis of The New Zealand Subduction Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12799, https://doi.org/10.5194/egusphere-egu22-12799, 2022.

09:03–09:10
|
EGU22-13549
|
On-site presentation
Christian Sippl et al.

It is commonly assumed that intermediate-depth seismicity is in some way linked to dehydration reactions inside subducting oceanic lithosphere. There is growing evidence that the hydration state of an oceanic plate is controlled by its structure and degree of faulting at the outer rise, but we do not yet have a quantitative understanding of this relationship.

Double seismic zones offer the possibility of investigating changes in oceanic-plate hydration not only along strike but also with depth beneath the slab surface. To quantify the impact of oceanic-​plate structure and faulting on slab hydration and intermediate-depth seismicity, with a focus on the genesis of double seismic zones, we correlate high-resolution earthquake catalogs and seafloor maps of ship-based bathymetry for the northern Chilean and Japan Trench subduction zones. The correlations show only a weak influence of oceanic-plate structure and faulting on seismicity in the upper plane of the double seismic zone, which may imply that hydration is limited by slow reaction kinetics at low temperatures in the oceanic crust 5–7 km below the seafloor and by the finite amount of exposed wall rock in the outer-rise region. These factors seem to limit hydration even if abundant water is available.

Seismicity in the lower plane is, in contrast, substantially enhanced where deformation of the oceanic plate is high and distributed across intersecting faults. This likely leads to an increase in the volume of damaged wall rock around the faults, thereby promoting the circulation of water to mantle depths where serpentinization is faster due to elevated temperatures. Increased lower-plane seismicity around the projection of subducting oceanic features such as seamounts or fracture zones to depth may also be caused by enhanced faulting around these features. Our results provide a possible explanation for the globally observed presence of rather homogeneous upper-plane seismicity in double seismic zones as well as for the commonly patchy and inhomogeneous distribution of lower-plane seismicity.

How to cite: Sippl, C., Geersen, J., and Harmon, N.: Inferring the hydration of downgoing oceanic crust and lithospheric mantle from intermediate-depth earthquakes and outer rise faulting patterns, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13549, https://doi.org/10.5194/egusphere-egu22-13549, 2022.

09:10–09:17
|
EGU22-3250
|
ECS
|
On-site presentation
Thomas P. Ferrand et al.

Vrancea, Eastern Romania, presents a significant intermediate-depth seismicity, between 60 and 170 km depth, i.e. pressures from 2 to 6.5 GPa. A debate has been lasting for decades regarding the nature of the seismic volume, which could correspond to the remnant of a subducted slab of Tethyan lithosphere or a delamination of the Carpathians lithosphere. We present P-T diagrams showing to what extent these hypocentral conditions match the thermodynamic stability limits for minerals typical of the uppermost mantle, oceanic crust and lower continental crust.

Most triggering conditions match relatively well antigorite dehydration between 2 and 4.5 GPa; at higher pressures, the dehydration of the 10-Å phase provides the best fit. This demonstrates that the Vrancea intermediate-depth seismicity is evidence of the current dehydration of an oceanic slab beneath Romania. Our results are consistent with a recent rollback of a W-dipping oceanic slab, whose current location is explained by limited delamination of the continental Moesian lithosphere between the Tethyan suture zone and Vrancea.

In addition, we investigate the potential link between the triggering mechanisms and the retrieved focal mechanisms of 940 earthquakes, which allows interpreting the stress field distribution with depth. We observe a switch from collision to vertical extension between 100 and 130 km depth, where the Clapeyron slope of serpentine dehydration is negative. The negative volume change within dehydrating subhorizontal serpentinized faults (verticalized slab) likely explains the vertical extension recorded by the intermediate-depth seismicity. This apparent slab pull is accompanied with a rotation of the main compressive stress, which could favour slab detachments in actively subducting slabs.

How to cite: Ferrand, T. P., Manea, E., Craiu, A., Vrijmoed, J. C., and Marmureanu, A.: Dehydration-induced earthquakes and apparent slab pull in a subducted oceanic slab beneath Vrancea, Romania, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3250, https://doi.org/10.5194/egusphere-egu22-3250, 2022.

09:17–09:24
|
EGU22-13550
Luděk Vecsey and Jaroslava Plomerová

Teleseismic body-wave tomography represents a powerful tool to study regional velocity structure of the upper mantle. Particularly, a need of retrieving anisotropic signal calls for processing of a huge amount of P-wave travel times (Munzarová et al., GJI 2018).  Therefore, automatic picking procedures are needed to supply tomography codes with a large amount of highly accurate absolute arrival times and/or travel-time residuals of body-wave propagation. We present and test a fully automated tool - TimePicker 2017 (Vecsey et al., 2021) for measuring P-wave arrival times on array recordings of passive experiments. The TimePicker 2017 is developed in the ObsPy/Python platform (Krischer et al., 2015) which combines picking, waveform cross-correlation and beamforming. The picker is based on two-step signal cross-correlations and allows us to measure absolute arrival times. Instead of a subjective selection of a reference trace, it cross-correlates all pairs of traces and forms a reference low-noise beam trace as a stack of the shifted traces at all stations. The picker cross-correlates all signals to the reference beam, automatically identifies outliers, and complements all picked absolute arrival times by their error estimates.

We applied the TimePicker 2017 on a set of seismograms from 1920 earthquakes from epicentral distances greater than 30° recorded at 240 temporary and permanent stations involved in the AlpArray experiments. We show uncertainties of measured P-wave arrivals, means and medians of uncertainties for both the complete dataset as well as for subset selected for tomography, and test effects of the standard selection of a reference trace vs. the low-noise beam trace as the reference trace in the TimePicker 2017.

How to cite: Vecsey, L. and Plomerová, J.: TimePicker 2017 – a fully automatic tool to extract P-wave arrivals for high-resolution unravelling structure and fabric of the lithosphere-asthenosphere system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13550, https://doi.org/10.5194/egusphere-egu22-13550, 2022.

09:24–09:31
|
EGU22-8282
|
ECS
|
Virtual presentation
Amr El-Sharkawy et al.

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

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

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

09:31–09:38
|
EGU22-2420
|
ECS
|
On-site presentation
Florence Ramirez et al.

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

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

09:38–09:45
|
EGU22-4306
|
On-site presentation
Thomas Bodin et al.

The seismic low-velocity zone (LVZ) of the upper mantle is generally associated with a low-viscosity asthenosphere that has a key role in decoupling tectonic plates from the mantle. However, the origin of the LVZ remains unclear. Some studies attribute its low seismic velocities to a small amount of partial melt of minerals in the mantle, whereas others attribute them to solid-state mechanisms near the solidus or the effect of its volatile contents. Observations of shear attenuation provide additional constraints on the origin of the LVZ. On the basis of the interpretation of global three-dimensional shear attenuation and velocity models, here we report partial melt occurring within the LVZ. We observe that partial melting down to 150–200 kilometres beneath mid-ocean ridges, major hotspots and back-arc regions feeds the asthenosphere. A small part of this melt (less than 0.3 per cent) remains trapped within the oceanic LVZ. Melt is mostly absent under continental regions. The amount of melt increases with plate velocity, increasing substantially for plate velocities of between 3 centimetres per year and 5 centimetres per year. This finding is consistent with previous observations of mantle crystal alignment underneath tectonic plates. Our observations suggest that by reducing viscosity melt facilitates plate motion and large-scale crystal alignment in the asthenosphere.

How to cite: Bodin, T., Debayle, E., Durand, S., and Ricard, Y.: Seismic Evidence for Partial Melt Below Tectonic Plates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4306, https://doi.org/10.5194/egusphere-egu22-4306, 2022.

09:45–09:52
Discussion1

Thu, 26 May, 10:20–11:50

Chairpersons: Ernst Willingshofer, Ehsan Qorbani

10:20–10:30
|
EGU22-6274
|
solicited
|
Virtual presentation
Fanny Garel and Catherine Thoraval

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

 

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

 

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

 

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

 

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

10:30–10:37
|
EGU22-7462
|
ECS
|
On-site presentation
Adeline Clutier et al.

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

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

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

10:37–10:44
|
EGU22-4614
|
ECS
|
On-site presentation
Adina E. Pusok et al.

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

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

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

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

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

References

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

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

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

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

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

Parsons and McKenzie, 1978, DOI:10.1029/JB083iB09p04485

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

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

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

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

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

10:44–10:51
|
EGU22-11306
|
Virtual presentation
Xuewei Bao et al.

The dominant driving forces for the east-west extension of the Tibetan Plateau since the mid-late Miocene remain vigorously debated. Proposed hypotheses encounter difficulties in reconciling the geological observations of more developed north-trending rifts in southern Tibet as well as the discrepant extension magnitudes among them. With seismic recordings collected from our recently deployed and existing seismic arrays, we locate a mid-crustal simple shear zone characterized by convergence parallel anisotropy beneath the southern plateau, which is likely caused by the underthrusting of the Indian Plate. Furthermore, a zone of reduced S-wave velocity is also resolved between the two rifts with highest extension rate, indicative of the convective removal of the lower Indian mantle lithosphere. Taken together, our results suggest that the enhanced extension occurring in southern Tibet are controlled by both the shear tractions induced by the advancing Indian Plate and the increased buoyancy due to asthenospheric upwelling.

How to cite: Bao, X., Zhang, B., Xu, Y., and Yang, W.: Southern Tibetan rifting controlled by basal shear and heterogeneities of the underthrusting Indian lithosphere , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11306, https://doi.org/10.5194/egusphere-egu22-11306, 2022.

10:51–10:58
|
EGU22-6989
|
On-site presentation
Olivier de Viron et al.

We propose a quantitative approach to search for mantle plumes in global seismic tomography models without prior assumptions on the associated mantle velocity anomalies. We design detection tests with a reasonable detection threshold while keeping false detections at a level lower than 5%. This is based on naive Bayesian clustering analysis, which is possible thanks to the varimax principal component analysis that provides components that are much more independent than the original number of depths slices in the models. We find that using such independent components greatly reduces detection errors compared to using an arbitrary number of depth slices due to correlations between the different slices.

We detect a wide range of behaviour of the seismic velocity profiles underneath the hotspots investigated in this study. Moreover, we retrieve locations away from hotspots that have a similar seismic velocity profile signature to that underneath some hotspots. Hence, it is not possible to obtain a unique definition of seismic velocity anomalies that are associated with mantle plumes and thus care needs to be taken when searching for mantle plumes using prior assumptions about the velocity anomalies that might be associated with them. On the other hand, we identify a criterion that allows establishing a probability distribution of the seismic velocity profiles that is specific to a sub-list of hotspots and we show that this distribution does not occur significantly elsewhere. Overall, the mantle plume zones identified in our analysis do not appear to surround the Africa and Pacific large low shear velocity provinces (LLSVPs) but are rather within them. This supports the idea that LLSVPs may correspond to bundles of thermochemical mantle plumes rather than to compact, dense piles.

How to cite: de Viron, O., Van Camp, M., Ferreira, A. M. G., and Verhoeven, O.: A naive Bayesian method to chase mantle plumes in global tomography models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6989, https://doi.org/10.5194/egusphere-egu22-6989, 2022.

10:58–11:05
|
EGU22-3554
Nobuaki Fuji et al.

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

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

11:05–11:12
|
EGU22-8734
|
ECS
|
|
On-site presentation
Federica Restelli et al.

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

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

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

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

11:12–11:17
Discussion2

11:17–11:24
|
EGU22-13373
|
Highlight
|
GD Division Outstanding ECS Award Lecture
|
On-site presentation
Tim Craig

Earthquakes provide a crucial way of probing the deformation style, strength, and stress state of the lithosphere.  In this talk, I will outline ways in which we can use careful analysis and precise seismological observations of earthquakes, particularly those at moderate magnitudes (M ~5-6), to map out how stress is supported in the lithosphere, and how the rheology of the lithosphere can vary in both space and time, summarising our current understanding of the controls on the distribution of earthquakes.  I will draw on examples from a range of regional studies, and outline what conclusions we can draw about the geological and geodynamic controls on the distribution of earthquakes in each region, and the variation on the style of deformation within the lithosphere.  I will also discuss areas in which our current understanding of the distribution of earthquakes remains unable to explain some observations, and challenges for the future.

How to cite: Craig, T.: Probing the rheology of the lithosphere using earthquake seismology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13373, https://doi.org/10.5194/egusphere-egu22-13373, 2022.

11:24–11:49
GD Division Outstanding ECS Award