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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.