EGU2020-18543
https://doi.org/10.5194/egusphere-egu2020-18543
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

The Alpine Deep Structure from Surface Wave Tomography

Thomas Meier1, Amr El-Sharkawy1,2, and Sergei Lebedev3
Thomas Meier et al.
  • 1University Kiel, Institute for Geosciences, Kiel, Germany (thomas.meier@ifg.uni-kiel.de)
  • 2National Research Institute of Astronomy and Geophysics (NRIAG), 11421, Helwan, Cairo, Egypt
  • 3School of Cosmic Physics, Geophysics Section, Dublin Institute for Advanced Studies, Dublin, Ireland

Collisional tectonics of the Alps is driven by several slab segments. A detailed imaging of the lithosphere-asthenosphere system beneath the Alps is, however, challenging due to the relatively small size of the slab segments and the highly curved geometry of the Alps. Surface waves, due to their high sensitivity to variations in seismic velocities at lower crustal and upper mantle depth, are well suited to study the Alpine deep structure. New azimuthally anisotropic Rayleigh wave phase velocity maps are calculated from automated inter-station phase velocity measurements in a very broad period range (8 – 350 s). The constructed local dispersion curves are then inverted individually for 1-D shear-wave velocity models using a new implementation of the stochastic Particle Swarm Optimization (PSO) inversion algorithm that enables the calculation of a high-resolution 3-D shear-wave velocity model from the crust down to 300 km beneath the Alps. In the Central Alps, a nearly vertical high velocity anomaly down to depth of 250 km is imaged and interpreted as subducting Eurasian mantle lithosphere. In contrast, low velocities in the Western Alps at depth of approximately 100 km and downwards are supporting the shallow slab break-off model. In the Eastern Alps, the presence of a vertically continuous high-velocity anomaly from 75 km to about 200 km depth beneath the northern Eurasian foreland and the almost continuous extension of a high-velocity anomaly from the Dinarides towards the Eastern Alps hint at a bivergent slab geometry beneath the Eastern Alps caused by subducting mantle lithosphere of both Eurasian and Adriatic origin. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a depth of about 150 km. Beneath the northern Apennines, the model indicates an attached Adriatic slab, whereas a slab window is found beneath the central Apennines. The results show that surface wave tomography can contribute to the imaging of complex slab geometries and slab segmentation in the Alpine region.

How to cite: Meier, T., El-Sharkawy, A., and Lebedev, S.: The Alpine Deep Structure from Surface Wave Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18543, https://doi.org/10.5194/egusphere-egu2020-18543, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 05 May 2020
  • CC1: Comment on EGU2020-18543, Ernst Willingshofer, 07 May 2020

    Dear Amr,

    Coming back to Claudios comment earlier this morning concerning no slab under the Tauern Window: that part likely coincides with the transition from the European plate dominated subduction domain (central Alps) to the Dinaric dominated subduction system; please see Fig. 8 in Luth et al., 2013, Tectonophysics for how that could work geometrically.

    Cheers,

    Ernst

  • CC2: Comment on EGU2020-18543, Anne Paul, 07 May 2020

    Hi Amr,

    A question on your display: in slide 10, CA stands for Central-Alpine slab while the anomaly is located North of the Alps, and NA stands for N-Apenninic slab while, for me, it is the CA slab beneath the Po basin. If NA becomes CA (most probable interpretation for me), then what is CA in the North?

    Thanks for your reply

    Regards. Anne

    • AC1: Reply to CC2, Amr El-Sharkawy, 08 May 2020

      Dear Anne,

      thank you for your comment. Beneath the Northern Apennines Adriatic mantle lithosphere is subducting approximately southwards. This can be seen e.g. from the Moho topography or also from a few intermediate-depth earthquakes. On slide 7 there is a high velocity anomaly beneath the Po plain Basin that is consequently interpreted as nearly vertically subducting Adriatic mantle lithosphere. The top of the slab is found approximately beneath the coast line. The bottom of the slab beneath the Po-Basin. This is also seen on slide 10 beneath the Po-Basin. The lateral resolution of surface wave tomography is limited in this region to about 75 km 100 km but the slab is visible – rather the bottom of the slab than the top of the slab. The detailed geometry remains to be determined however. Beneath the northern foreland, Eurasian mantle lithosphere is subducting nearly vertically southwards beneath the Molasse Basin as indicated by the high velocity anomaly.   

      Best regards,

      Amr

      • CC3: Reply to AC1, Anne Paul, 11 May 2020

        Dear Amr,

        Thank you for your reply.

        The high-velocity bodies interpreted as slabs are shifted toward the N in your tomo with respect to the high-velocity bodies imaged by body-wave tomography. Emanuel Kästle's tomo (his 2018 JGR paper + his recent 2020 Solid Earth paper) has exactly the same shift toward the N, while he is using exactly the same method as you, and probably a similar, pre-AlpArray dataset.

        This N-ward shift is particularly clear in Fig. S1 of the supplementary material of Emanuel's Solid Earth paper. At 100 km depth, the high-velocity anomalies are located in the N beneath Switzerland and in the S beneath the Po plain to the S of the frontal thrust of the Apennines (red line). In all body-wave tomographies, the 2 high-vel anomalies are located ~100 km further south, beneath the Southern border of Switzerland or south of it for the northern one (central Alps slab) and along the SW coast of Italy, right beneath the Apennines for the southern one (Apenninic slab).

        Emanuel also compares his results with body-wave tomo results in the Fig. 3 of his 2020 paper. It is very clear in cross-section B that his central Alps high-velocity anomaly is right beneath the Alps while it is beneath the southern flank of the Alps in all body-wave tomographies. In your cross-section of slide 10, the shift to the north is even stronger as the section crosses the most northern high-vel. anomaly at 100 km which is beneath the S of Germany, that is north of the Alps.

        This northward shift is what motivated my comment. I have no explanation for these strong differences between surface-wave tomo and body-wave tomo, in particular for this northward shift. As surface-wave tomo has lower horizontal resolution, I would rather trust body-wave tomographies for the imaging of the slabs beneath the Alpine region. However, it would be nice to understand the origin for these differences.

        Regards

        Anne

        • AC2: Reply to CC3, Amr El-Sharkawy, 20 May 2020

          Dear Anne,

          the presence of the slab segments can be detected and identified but their 3D geometry remains to be resolved. The northwards shift might be apparent because surface waves are mainly sensitive to the higher velocities at the lower part of the nearly vertically dipping slab segments. A standard thickness of a slab is about 100 km and a lateral resolution of about 75 km may already explain the apparently shifted anomaly. To quantify a possible northward shift or better a shift towards the foreland more quantitatively, numerical forward modelling is required. By the way, it would be good to start a discussion on the detailed geometry of the slab segments taking the resolution limits of the applied methods into account. This could also result in a 3D slab model for the area that can be used for forward modelling. 

          Best regards

          Amr and Thomas