Abstract
Abstract. In the frame of the AlpArray project we analyse teleseismic data from permanent and temporary stations of the Alpine region to study seismic discontinuities down to about 140 km depth. We average broadband teleseismic S-waveform data to retrieve S-to-P converted signals from below the seismic stations. In order to avoid processing artefacts, no deconvolution or filtering is applied, and S arrival times are used as reference for stacking. We show a number of north–south and east-west profiles through the Alpine area. The Moho signals are always seen very clearly, and negative velocity gradients below the Moho depth are also visible in a number of profiles. A Moho depression is visible along larger parts of the Alpine chain. It reaches its largest depth of 60 km beneath the Tauern Window. However, the Moho depression ends abruptly near about 13∘ E below the eastern Tauern Window. This Moho depression may represent the crustal trench, where the Eurasian lithosphere is subducted below the Adriatic lithosphere. East of 13∘ E an important along-strike change occurs; the image of the Moho changes completely. No Moho deepening is found in this easterly region; instead the Moho bends up along the contact between the European and the Adriatic lithosphere all the way to the Pannonian Basin. An important along-strike change was also detected in the upper mantle structure at about 14∘ E. There, the lateral disappearance of a zone of negative velocity gradient in the uppermost mantle indicates that the S-dipping European slab laterally terminates east of the Tauern Window in the axial zone of the Alps. The area east of about 13∘ E is known to have been affected by severe late-stage modifications of the structure of crust and uppermost mantle during the Miocene when the ALCAPA (Alpine, Carpathian, Pannonian) block was subject to E-directed lateral extrusion.
Highlights
The complicated tectonic structure of the Alpine mountain belt is a result of the collision of the African and European plates
This marked difference is due to the fact that in our data the Moho in the profile of Fig. 20b is located in the European lithosphere, which bends down across the SW–NE-striking front of the Western Carpathians, while the profile of Fig. 20a runs within the Pannonian realm characterised by the eastwardshallowing Moho depth shown in E–W profiles 10, 11 and 12, caused by Miocene backarc extension in the Pannonian Basin, along the MHZ and in the internal parts of the Western Carpathians region (Horváth et al, 2015)
In the shallow mantle we observe, extending from 10 to 14◦ E along an E–W section along 47◦ N combined with N–S sections, steeply SE-dipping areas of downward velocity reductions referred to as negative velocity gradients (NVGs) that extend to at least 140 km depth at 14◦ E and disappear east of there
Summary
The complicated tectonic structure of the Alpine mountain belt is a result of the collision of the African and European plates. R. Kind et al.: Moho and uppermost mantle structure in the Alpine area from S-to-P converted waves tle tomography results in the Eastern Alps either indicate a change of subduction polarity by revealing a north-dipping slab or alternatively cannot resolve the polarity clearly (Lippitsch et al, 2003; Molinari et al, 2015; Guidarelli et al, 2017; Kästle et al, 2018, 2020). In the last few years large-scale international seismic experiments have been carried out (CELEBRATION: Hrubcová et al, 2005; AlpArray and EASI: Hetényi et al, 2018a, b; and SWATHD: Heit et al, 2017, 2021) and have provided large amounts of new data. We processed the seismic records using a relatively novel method This new method uses S-to-P conversions, similar to the S-receiver function method, except that raw untouched broadband data are stacked without any filtering or deconvolution. The differences from the S-receiver function method are that the waveforms are not modified by any filtering (including deconvolution) and that the first arrival times of the SV signal (approximately radial component) are used for stacking rather than the maxima of the modified SV waveforms
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