Moho and uppermost mantle structure in the greater Alpine area from S-to-P converted waves

In the frame of the AlpArray project we analyze teleseismic data from permanent and temporary stations of the greater 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. We show a number of north-south and eastwest profiles through the greater Alpine area. The Moho signals are always seen very clearly, and also negative velocity gradients below the Moho are 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. The Moho depression ends however abruptly near about 13°E below the eastern Tauern Window. The Moho depression may represent the mantle 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 is updoming along the contact between the European and the Adriatic lithosphere all the way into 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 P-wave velocity gradient 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.

Moho interface) is also contributing to the signal form. Below the Moho in the right-hand panels we see, for most of the profiles, concentrations of red signals. These are caused by sharp or gradual downward velocity decreases. These signals below the Moho appear scattered, and lack clear lateral correlations, when compared to the Moho signals, that align much better. For this reason, we refer to the red regions below the Moho as zones of Negative Velocity Gradients (NVG). In some profiles we marked the bottom of the NVGs by a broad grey line that approximately indicates the earliest arrival times from the deepest scatterers. A negative seismic velocity gradient is often expected to mark the lithosphere asthenosphere boundary. But its location depends on the nature of the geophysical measurements used to locate this boundary. Apart from temperature effects this negative gradient may also be caused by changes in, for example, chemical composition or anisotropy (e.g. Eaton et al., 2009). Therefore, we prefer to use the neutral term NVG here.

North-south profiles
We first present a series of north-south oriented profiles shown in Figs. 3-11. In the entire region between 8.0 to 13.5°E (profiles 2-6 in Figs. 4A-8A) we see a south dipping European Moho marked by black dotted lines. The Moho reaches its largest depth in all profiles 2-6 at about 46.5° N. This is near the latitude at which Spada et al. (2013) draw the boundary between European and Adriatic Moho, respectively (see black line in Fig. 2) that roughly coincides with the location of the E-W striking part of the Periadriatic line west of longitude 13°E. This implies that, according to geological evidence (Schmid et al. 2004;Kissling et al. 2006), the Moho south of about 46.5° N, depicted in the profiles 2-6, is the Adriatic Moho. In profiles 2-6 this Adriatic Moho rises towards a culmination located near 45.5°N before descending southward beneath the Ligurian Moho at the front of the Apennines. This is shown by the onsets of the Adriatic Moho, which is also marked by dotted black lines (Figs. 4A-8A). At first sight, this geometry suggests a wedge-shaped trough below the Alpine chain. In fact, the two Moho's are separated from each other by a subduction corridor, whereby the European Moho dips undisputedly southward beneath the Adriatic Moho according to geological and geophysical evidence (Schmid et al. 2004;Kissling et al. 2006), at least west of 11°E. A far-reaching European Moho below the Adriatic Moho is, however, not observed in our data. This is different to the continental collision observed between India and Asia in Tibet where a double Moho is observed and is interpreted as Indian crust reaching several hundred kilometers below the Tibetan crust (Yuan et al. 1997, Nabelek et al. 2009). In the Alps, east of 11°E opinions about the subduction polarity have remained controversial so far (see discussion in Kästle et al. 2020). The maximum depth of the Moho reaches about 50 km near 8.5°E (Fig. 4A) and increases to about 60 km near 13°E (Fig. 8A). A very similar maximum Moho depth is also observed in the west near 7.5°E. Here the Moho depth reaches about 65 km at about 45.5°N (see profile 1 Fig. 3A). This Moho interface is hence located further south, which is caused by the south bending of the Alpine chain in the west (Fig. 2). On the other hand, a so far unknown substantial Moho depression similar to that observed at the plate interface west of 13°E is observed further in the north (at 48-49°N) in the profiles east of about 13°E (see profiles 7-9 in Figs. 9A-11A). In the case of profiles 7 and 8 this Moho depression is located well north of the Alpine front (marked with red arrows in Figs. 9A and 10A; see also Fig. 2) and beneath the southern Bohemian Massif belonging to the European plate. In profile 9 the deepest point is below the external Western Carpathians. In this very wide (between 17-19°E) profile 9 a big jump in the Moho onset time is seen near 47°N marked with a black arrow in Fig. 11A. This occurs at the location of the Mid-Hungarian Fault Zone (MHZ in Fig. 2, e.g. Hetényi et al. 2015). North of this zone the Moho is at about 30 km and deepens to 50 km underneath the frontal West Carpathians. South of the MHZ the Moho suddenly rises to only some 20 km depth beneath the Pannonian Basin south of Lake Balaton. The influence of potentially different velocity models is not considered in the Moho depth estimates discussed above. Fig. 15 summarizes locations and maximum depths of the Moho along the Alpine chain in map view.

East-west profiles
In a second step we discuss the Moho depth shown along three east-west profiles presented in Figs. 12A-14A. Fig. 12A shows profile 10 centered at 47°N and entirely located on the European plate east of longitude 11.5°E but straddling along the plate boundary with Adria further to the east. This profile reaches the maximum Moho depth of around 60 km at about 13°E, i.e. in the area of the Tauern Window (see Fig. 2). Eastward, the Moho depth of this profile steadily rises to less than 20 km beneath the Pannonian Basin. West of its deepest point beneath the Tauern Window the Moho also rises, but exhibits a secondary depression between 7-8°E located in the European plate near the Alpine Front (Fig. 2). For easier comparison the dotted black line marking the Moho trend in Fig. 12A is copied into the neighboring profiles 11 and 12 located north and south of profile 10, respectively (Figs. 13A and 14A). Profile 11 (Fig. 13A) located further north at 48°N, entirely runs within the European plate and is similar to the one at 47°N (Fig. 12A). However, in its western part the Moho is generally shallower in profile 11, probably due to the proximity to the Rhine Graben. Conversely, in the eastern part of this profile 11 the Moho is generally deeper than that of profile 10 at 47°N. This confirms the existence of a substantial Moho depression beneath the Bohemian Massif, which was found in the N-S profiles 7 and 8 (Figs. 9A and 10A). This profile 11 (Fig. 13A) also shows that the shallowing of the Moho towards the east is by far more moderate compared to that seen in profile 10. The southernmost E-W profile 12 at 46°N (Fig. 14A) runs within the Adriatic plate east of longitude 8.5° and shows a Moho trend that is very similar to the one at 47°N (Fig. 12A). Only in its western part, running within the European part, the Moho is somewhat deeper.

Structure within the uppermost Mantle
The left and right panels of Figs. 3-14 show profiles of summed traces, which have their piercing points within the same geographical bins. However, the piercing points are computed for different depths, namely 50 km for the left hand panels and 100 km for the right hand panels. Therefore, the summed traces in the bins are not identical. We now focus on the right panels computed by choosing a piercing point of 100 km, which is optimal when searching for structures in the shallow mantle below the Moho. Almost all Figs. 3B-14B show a relatively large number of negative signals (marked in red) indicating downward velocity reductions at some depth below the Moho. However, there are significant differences between these red signals compared to the blue signals marking the Moho. The amplitude of the Moho signals is nearly 10% of the incident SV wave, whereas the amplitude of the negative signals from below the Moho is in most cases below 4%.
Moreover, in most cases the Moho signals mark a clear discontinuity, which can be correlated over the entire length of the profiles, whereas the negative signals from below the Moho do not mark a clear discontinuity but appear much more scattered. However, in parts of some of the profiles we were able mark the lower boundary of regions with exceptionally high concentrations of negative signals with a scattered grey line. We refer to such red regions as Negative Velocity Gradient zones (NVG). Similar to the Moho, the bottom of such NVG zones marked by the broad grey line indicates the arrival times of the deepest scatterers.

North-south profiles
An NVG could be mapped in profile 1 (Fig. 3B), south dipping down to about 110 km. In profiles 2-4 ( Fig. 4B-6B, between 8-11°E) the NVG is horizontal or slightly inclined to the south. The base of the NVGs is found at depths of 80-90 km, and they end towards the south at around latitude 47° to 48°, i.e. at or near the front of the Alps (see Fig. 2). This shows that they are definitely embedded in the European plate of the Alpine foreland dipping southwards underneath the front of the Alps.
In profiles 5-7 (Figs. 7B-9B, between 11-14.5°E), the NVGs appear more pronounced. They are again south dipping and their base reaches down to 115-130 km depth. They also end southward at around latitude 47°, which is somewhat further inside the Alps given the SSW-ENE strike of the Alpine front but still within the European plate (see Fig. 1). However, there is practically no significant indication of an NVG in profile 8 (Fig. 10B, 14.5-17°E), i.e. in the area located well east of the Tauern Window spanning the Alps-Carpathians transition area (see Fig. 1). Note that profile 8, together with profile 9, was chosen relatively wide because of the scarcity of stations in the area. In the easternmost N-S profile 9 east of 17°E (

East-west profiles
The east-west profile 10 centered along 47°N (Fig. 12B) shows a very clear NVG that steeply dips eastward and reaches about 130 km depth at 13°E where it turns horizontal and disappears near 14E. Comparing this easterly dip of the NVG in an E-W profile with its southerly dip in the N-S profiles at 11°-14. [5][6][7][8][9] shows that the NVG is in fact dipping towards the southeast. The east-west profile 11 along 48°N (Fig. 13B) along the northern Alpine front also shows an apparently east dipping NVG, however only reaching about 110 km depth at around 12°E. Profile 12 along 46°N (Fig. 14B) below the Po valley and located within the Adria plate in its eastern part, however, lacks a similar east dipping NVG. It seems astonishing that relatively steep dipping structures can be imaged by steeply incident converted waves. Our interpretation is that the NVG area may contain a sufficient number of horizontal scatterers, which can be observed. The dip angle of the NVG might be biased (Schneider et al., 2013).

Comparison with Earlier Results
The comparisons of our results with those obtained by previous studies are discussed here and are displayed in Figs. 17 to 20.
In Fig. 17A we compare our Moho data obtained along the north south profile 6 centered at 13°E (Fig. 8)  (2020) is also reasonably good, except near 47°N, where they have no data. It should also be noted, that the "Moho gap" mapped in white by Spada et al. (2013) at 47°N (Fig. 2) does not seem to exist according to our data, and those of Mroczek et al. (2020) and Hetényi et al. (2018b).
In Fig. 17B we compare our data in the mantle below the Moho with results from the teleseismic body wave tomography by Paffrath et al. (2021) by superimposing the P-wave velocity contours that these authors used for calculating velocity anomalies along our E-W profile 10. The E-dipping base of the red NVG zone roughly coincides with the top of an area associated with abnormally low P-wave velocities below 8.0 km sec-1 marked in yellow in the western part of this profile, but only until 12° longitude. At this point it has to be mentioned that this E-W profile 10 runs at an acute angle to the strike of the Alpine front running SW-NE to WSW-ENE between the arc of the Western Alps and the Tauern Window (see Fig. 2), and hence it records a distorted profile across the SSE-dipping European plate. The same NVG zone was also traversed by the N-S profiles that showed the NVG zone to be hosted within the SSE-dipping European plate. The termination of the base of the east dipping NVG zone turning flat at 13° longitude (green line in Fig. 12B) and terminating at around 14°E hence strongly suggests that the European slab is not reaching the area covered by this E-W profile east of 14°E, i.e. near the eastern margin of the Tauern Window. East of 12°E at a depth of 110 km the P-wave velocity contours from Paffrath et al.
(2021) actually indicate a low velocity zone that rises to a shallower depth towards the east, together with the rise of the Moho seen at the base of the blue Moho amplitudes in this figure (and marked also in Fig.10A focusing on the Moho depth).
This again points towards an important along strike change in the area of the Tauern Window in the axial zone of the Alpine orogen that will be discussed later. In the context of this comparison it should however be noted that our method of using converted waves is sensitive to velocity gradients and not absolute velocities. This means that smaller scale relative velocity reductions could well be embedded in larger high or low velocity regions. Fig.17B demonstrates that our observations of the NVG reaching down to ca. 140 km depth is in agreement with the tomographic low velocity anomaly documented for the area west of longitude 11°E. Between 11°E and 14°E, a downward positive velocity gradient is found by Paffrath et al. (2021) where our data indicate a NVG (see map in Fig. 16 and profile 10 in Fig. 17B). Further studies and high-resolution Swave velocity models are needed to resolve this discrepancy. The maximum recorded depths of the NVG anomaly given in the map in Fig. 16  In Fig. 20A we compare our Moho depth profile along the east west profile 10 at 47°N (Fig. 12A) with early results of seismic refraction experiments (Yan and Mechie, 1989). The agreement in the general trend of eastward shallowing starting in western Tauern Window at around 12°E is good, although there are some differences in details. Fig. 20B presents an additional profile 13 located north of profile 11 at 49-50°N in the European lithosphere crossing the front of the Western Carpathians at 17.5°E (Fig. 2), added for comparison with the Moho map of Grad et al. (2009a)

Summary of Observational Results
The comparison with results from other studies using other methods shows that by using broadband S-to-P converted signals we are able to obtain clear images of the Moho topography along most of the Alpine chain and also reveal the presence of some features that were not detected before. Furthermore, this method allows highlighting an important along-strike change regarding Moho topography that takes place at around 13°E, i.e. in the eastern Tauern Window. The Moho-depression beneath the Central Alps along the Europe-Adria lithosphere boundary along 47°N deepens from some 50 km at 8°E to 60 km at around 13°E. In the N-S profiles the Moho depression very abruptly makes place to an up-doming of the Moho along the axial zone of the Alpine orogen at around 13°E, persisting all the way into the Pannonian Basin, as the depth of the Moho generally decreases to < 30 km also N and S of 47°N. East of 17°E rapid Moho deepening from about 20 to 30 km is observed from south to north across the Mid-Hungarian Fault Zone. Our data also indicate the existence of a substantial Moho depression east of 13°E in front of the Alpine frontal thrust extending eastward into the area of the frontal Western Carpathians whose extent remained so far undetected (see Fig. 15).
In the shallow mantle we also observe, extending from 10°E to14°E, along an E-W section along 47°N combined with N-S sections, steeply SSE-dipping areas of downward velocity reductions referred to as NVGs that extend to at least 140 km depth at 14°E and disappear east of there. The base of the NVG, which roughly coincides with the top of a low velocity zone revealed by mantle tomography west of 12°E ) is related to the European slab. The termination of the NVG at 14°E suggests an important along strike change also in terms of the shallow mantle structure taking place at 14°E, i.e. about 1° east of the along strike change observed for the Moho-topography. Below the frontal West Carpathians at about 49°N we observed a rapid jump of the base of the subhorizontal NVG from about 60 km in the south to 90 km in the north.

Discussion and Conclusions
From a technical point of view the improved images of the present paper are, besides the obvious reason of using much larger amounts of data, also due to the modified data processing technique. In the receiver function technique seismic traces are sometimes filtered without paying sufficient attention to sidelobes or acausality. Another problem is the waveform compression caused by deconvolution and summation along the maximum of the compressed signals, which also produces sidelobes that tend to be misinterpreted. We therefore avoided all filtering and lined up the traces along the onsets of the S signals for summation. We found good agreements of our results with earlier results obtained in many regions. This emphasizes the reliability of our results. We also think that using additional seismic phases could help to increase the uniqueness and resolution of the seismic images and in consequence of the tectonic interpretation. A good example is the usage of PKP multiples below the stations as shown by Bianchi et al. (2020).
From the geological point of view the Moho topography reflects deeper subduction of the European lithosphere only west of and in the Tauern Window area. This is no more the case east of 13° E, i.e. east of the eastern Tauern Window, where the Moho is up domed in the contact area between the European and the Adriatic lithosphere. According to geological evidence (Ratschbacher et al. 1991) and according to the geological interpretation of recent mantle tomographic data , Handy et al., 2021 this updoming is likely due to a late stage modification of the Moho topography in the Eastern Alps, the Western Carpathians and the Pannonian Basin that occurred during the last ca. 20 Ma. Such updoming goes together with crustal thinning associated with lateral extrusion of the so-called ALCAPA mega-unit associated with the Ndirected indentation of the Dolomites indenter and roll back of the Eastern Carpathians (Ratschbacher et al. 1991 Fig. 8. Note the abrupt along strike change in terms of the Moho topography near 13° E longitude, where this section shows the exact opposite, namely a culmination rather than a through at this same latitude (compare with Fig. 8). This location coincides with the white area in the map of Spada et al. (2013) denoting an area where the Moho is ill constrained by their data (" Moho gap", Hetényi et al. 2018b). The red arrow indicates the location of the frontal thrust of the Alpine orogen.        Spada et al. (2013) indicating the maximum recorded depth of the signals at the base of the NVG as marked in profiles 1-12 B. Small white bins in the background of the numbers indicate steeply dipping NVG. Larger backgrounds indicate nearly horizontal NVG. Cyan quadrant marks the region with relatively good NVG observations, coinciding with the area of the " Moho gap" (white).