Detection of a new sub-lithospheric discontinuity in Central Europe with S-receiver functions
Introduction
The basic geology of central Europe north of the Alps is highly complex and determined by the Caledonian and Variscan orogenies which resulted from the collision of the plates of Gondwana and Laurussia, and numerous Peri-Gondwanan related microterranes which lay in between. Especially the closing of the Rheic Ocean in the Paleozoic (e.g. Linnemann, 2007, Nance and Linnemann, 2008, Zeh and Gerdes, 2010, Kroner and Romer, 2013) caused subduction, volcanisms and accretion of a number of terrains (e.g. Bohemian Massif or Rhenish Massif). The geology of the Mediterranean area is determined by the Alpine orogeny which is caused by the collision of the African plate with the European plate and several microplates (Adria, Iberia, Anatolia) since the late Mesozoic (e.g. Faccenna et al., 2014). The Alps, the Apennines, the Dinarides and Carpathians are expressions of this collision (see Fig. 1 for location of tectonic boundaries). The Tornquist-Teisseyre Zone (TTZ) is the most significant structure in Europe which separates the East European Craton (EEC) from Phanerozoic Europe. Finding the cause of this dynamics of the lithospheric plates requires integrating images of deep structures with surface geology which preserves the records of motion back in time. Here we are studying the deep structure. There are numerous seismic techniques used for studying discontinuities in the upper mantle. The oldest technique is wide angle seismics where the horizontal ray path is much longer than the vertical one. Gutenberg (1926) found with this technique the downward velocity reduction in the oceanic upper mantle at 60–80 km depth which bears his name. He concluded that the mantle was crystalized to that depth. There are many wide and steep angle controlled source profiles which sample the structure of the continental mantle below the Moho in northern Europe, North America and other regions. North of the Alps and beneath Paleozoic Europe, the Moho is relatively flat at a depth of about 30 km and shows no significant lateral variations (Grad et al., 2009). Geology indicates a complex history of accretion and subduction, followed by late- to post-Variscan magmatism and oblique-slip tectonics (e.g., Matte, 1998, Franke, 2000, Franke, 2014). Accordingly, the overall laterally continuity of the European Moho is attributed to this post-Variscan thermal and magmatic equilibration (Oncken, 1998). In contrast, in Proterozoic and Archean cratons many inclined structures below the Moho have been observed and interpreted as remnants of subduction zones (e.g., Babel Working Group, 1990; see also Steer et al., 1998 for a summary). Bostock (1998) confirmed in the Slave craton in Canada the existence of such structures with receiver function data down to about 200 km depth. Balling (2000) interpreted in controlled source data north-dipping structures in the Baltic sea and south-dipping structures in the North Sea as remnants of fossil subduction zones. Thybo and Perchuc (1997) concluded mainly from wide angle controlled data the global existence of a low velocity zone in cratons near 100 km depth (the 8° discontinuity). At lower crustal depths the location of the tectonic boundaries in northern central Europe is under discussion (see e.g. Rabbel et al., 1995). Likely it is of Avalonian origin but altered by post-Caledonian processes. Gossler et al. (1999) and Bayer et al. (2002) concluded that the lower crust of the East European Craton extends to the Elbe Line below the Caledonian upper crust. Steep angle reflection projects in Germany (DEKORP) found only isolated events of inclined structures below the Moho which have been interpreted as remnants of fossil subduction (e.g. Meissner and Rabbel, 1999).
Seismic tomography is probably the most-used method to study velocity anomalies in the upper mantle. Gradual velocity changes are resolved by this technique but it is less sensitive to seismic discontinuities (velocity jumps of c. 5–10% over c. 0–30 km depth range). Several groups have produced tomographic images of the European and Mediterranean area (Zielhuis and Nolet, 1994, Marquering and Snieder, 1996, Villaseñor et al., 2001, Koulakov et al., 2009, Bijwaard and Spakman, 2000, Legendre et al., 2012, Zhu et al., 2012, Zhu et al., 2015, Zhu and Tromp, 2013, Auer et al., 2014, Meier et al., 2016). The results show that under the East European Craton the velocities, in particular the shear-wave velocity, increase in the mantle lithosphere by up to 6%. According to these studies, the cratonic mantle lithosphere is about 250 km to 300 km thick. In contrast, shear-wave velocities are low in the upper mantle beneath central Europe and the Pannonian Basin and the thickness of the lithosphere decreases locally to about 50 km to 60 km. Intermediate lithospheric thickness is observed between the Tornquist–Teisseyre Zone and the Elbe Line (e.g., Legendre et al., 2012).
P-wave tomographic studies of the Alps (Lippitsch et al., 2003, Mitterbauer et al., 2011) and of the greater Alpine region (Piromallo and Morelli, 2003, Spakman and Wortel, 2004, Giacomuzzi et al., 2011) indicate a south-east dipping anomaly beneath the central Alps, interpreted as subducted European lithosphere. More detailed studies of the area suggest along-strike changes in slab orientation beneath the Central and Eastern Alps. Lippitsch et al. (2003) interpreted a subvertical to NNE-dipping anomaly as Adriatic lithosphere (Schmid et al., 2004) which has since been supported by additional tomography (Karousova et al., 2013). Other studies conducted at somewhat lower resolution (Spakman and Wortel, 2004, Mitterbauer et al., 2011) show ambiguous polarity, with the latter authors arguing for a continuous European slab beneath the Alps which becomes subvertical to steeply N-dipping beneath the eastern Alps. P-wave tomography models indicate a slab gap below the northern Dinarides (Bijwaard and Spakman, 2000, Piromallo and Morelli, 2003, Spakman and Wortel, 2004, Koulakov et al., 2009, Serretti and Morelli, 2011, Legendre et al., 2012, Zhu et al., 2012, Zhu et al., 2015). However, Sumanovac and Dudjak (2016) do not see a gap but a continuous slab in the northern Dinarides.
A more recent technique is the receiver function method which is sensitive to seismic discontinuities, but less to gradual velocity changes (e.g., Kind et al., 2012). The discontinuities usually observed in the upper mantle with receiver functions are the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), a relatively recently found Mid-Lithospheric Discontinuity (MLD, Yuan and Romanowicz, 2010) beneath cratons, the discontinuities bordering the upper mantle transition zone at 410 and 660 km depth, occasionally the Lehmann discontinuity (Lehmann, 1959) and perhaps other additional discontinuities (see for the central and northern European areas: Gossler et al., 1999—Moho in northern Germany, Denmark and Sweden; Alinaghi et al., 2003—Moho, 410 and 660 in northern Germany, Denmark, Sweden and Finland; Kummerow et al., 2004—Moho, 410 and 660 below the eastern Alps; Wilde-Piorko et al., 2010—Moho, LAB, 410 and 660 across the TTZ; Heuer et al., 2006, Heuer et al., 2007, Heuer et al., 2011—Moho, LAB, 410 and 660 below Bohemia; Sodoudi et al., 2006, Sodoudi et al., 2015—Moho and LAB below the Aegean; Geissler et al., 2008, Geissler et al., 2010, Geissler et al., 2012—Moho, LAB, 410 and 660 below Bohemia, Europe; Lombardi et al., 2008, Lombardi et al., 2009—Moho, 410 and 660 below the Alps; Jones et al., 2010—LAB in Europe; Hrubcova and Geissler, 2009—Moho below Bohemia; Plomerova and Babuska, 2010—LAB in Europe; Miller and Piana Agostinetti, 2012—Moho and LAB below Italy; Knapmeyer-Endrun et al., 2013, Knapmeyer-Endrun et al., 2017—LAB, MLD, 410 and 660 across the TTZ; Bianchi et al., 2014—Moho and LAB below the eastern Alps). In receiver function processing we usually assume the discontinuities are horizontal. Strongly inclined discontinuities generate less converted waves. Schneider et al. (2013) and Kind et al. (2015b) discussed P-receiver functions generated at inclined discontinuities and found that good results are obtained for inclinations up to about 30°. S-receiver functions at inclined subduction zones are commonly observed (e.g., Sodoudi et al., 2011). However, it should be mentioned that waves converted in the heterogeneous Earth are scattered waves and they are sensitive to any kind of sharp local heterogeneity.
The LAB and MLD of the EEC are so far not very well imaged with the exception of the neighboring Scandinavia where more data are available (Kind et al., 2013) and very recently across the TTZ (Knapmeyer-Endrun et al., 2017). The question if the LAB of cratons is observed in receiver functions is a far reaching problem because it is an indication of the sharpness of this discontinuity. Such observations would indicate that the cratonic LAB could not be caused by temperature changes alone (see e.g., Kind et al., 2015a, Hopper and Fischer, 2015). Another still open question in craton formation is the nature of the MLD. There are several suggested explanations (Mierdel et al., 2007, Karato et al., 2015, Selway et al., 2015, Rader et al., 2015). Hopper and Fischer (2015) discussed these suggested solutions. Their preferred model of the MLD is a layer of frozen-in volatile-rich melt. The shear velocity reduction may be caused by the presence of phlogopite or amphibole. Rader et al. (2015) conclude that the MLD could be interpreted as a remnant of the LAB of the emerging craton. Seismic observations may support this view, since receiver function signals from the LAB of tectonically active margins of cratons are undistinguishable from the MLD signals within cratons (e.g., Kind et al., 2013, Knapmeyer-Endrun et al., 2017). Our intention at present is to use the largely untapped data base of teleseismic S waves in the European open data archives to derive higher resolution S receiver functions and thus to contribute to the detection of upper mantle discontinuities and their topography beneath the North German-Polish Plain (NGPP), the Bohemian Massif (BM), the Alps, the Pannonian Basin (PB) and the SW part of the East European Craton (EEC).
Section snippets
Data and data processing
The seismic data used in this study are recorded by about 580 permanent and 120 temporary broadband stations in Europe. A disadvantage of the permanent station is their sparse distribution in some regions. However, it is an advantage of S-receiver functions that they sample much larger regions than P-receiver functions. Therefore most regions are sampled with data from several stations recording events from different epicentral distances and backazimuths. The data are archived in several data
Computation of theoretical seismograms and comparison with observed data
In this section we compare upper mantle structures obtained from surface wave tomography with those obtained from S-receiver functions. We use a tomography model, compute the theoretical seismograms and compare with the observed S-receiver functions. Surface waves are sensitive to shear-wave velocities down to about 300 km depth. One-dimensional models of absolute shear-wave velocity (see Fig. 5D) in central Europe are obtained by inversion of local dispersion curves, which are determined by
Profiles across the European mantle
We have produced two suites of east-west and north-south profiles across the European mantle which are shown in their entirety in Supplementary material (Supplementary Fig. S1, Supplementary Fig. S2). Here we show only several informative examples.
In Fig. 6 are shown four north-south profiles. The two profiles in Fig. 6A and B range from eastern Alps (marked eAlps) across the Bohemian Massif to the North German-Polish Plain entirely through Phanerozoic area. They show a number of seismic phases
North German-Polish Plain
An important observation in our study is the LAB of the East European Craton near 200 km depth. The question if the cratonic LAB can be observed with converted waves is still controversial. It is relevant for the question if this discontinuity is caused by temperature alone or if an additional or a different mechanism is required. Converted waves can only be generated at a relatively sharp discontinuity which would argue against a pure temperature effect. For example, Hopper and Fischer (2015)
Acknowledgements
We appreciate of the open data policy of the international seismology organizations which permits easy mining in data archives. We also appreciate the high quality of the permanent observatory broadband stations. By far most data used in our study came from permanent stations. We would like to thank all the data providers who made their data openly available. These data providers are: Seismic Network of the Republic of Slovenia-SL; MEDNET Project, Rome, Italy-MN; Czech Regional Seismological
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2021, Journal of Asian Earth SciencesCitation Excerpt :It searches for S-to-P converted waves (Sp) in LAB, which arrive earlier than the direct S waves so are not mixed by the crustal multiples. This method has been widely used to study lithospheric structures in subduction and collision zones and beneath mantle plumes (e.g., Kind et al., 2017; 2002; Li et al., 2004; Zhao et al., 2010), which provides reliable evidence for understanding the tectonic evolution in these regions. As described in detail in Kind et al. (2012) and Yuan et al. (2006), the routine procedures of the S wave receiver function include removing noises, rotation, deconvolution, migration, and stacking to produce the final images.
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2020, Physics of the Earth and Planetary InteriorsCitation Excerpt :Sp and Ps phases generated at negative velocity gradients have been observed on a widespread if intermittent basis, often at depths of 60-110 km. The evidence for negative MLDs within the cratonic lithosphere includes detections in North America, Australia, southern Africa, Tanzania, India, China, and Europe (e.g. Rychert and Shearer, 2009; Savage and Silver, 2008; Hansen et al., 2009; Miller and Eaton, 2010; Ford et al., 2010; Geissler et al., 2010; Wölbern et al., 2012; Kind et al., 2012; Bodin et al., 2013; Sodoudi et al., 2013; Foster et al., 2014b; Wirth and Long, 2014; Hopper and Fischer, 2015; Porritt et al., 2015; Hansen et al., 2015; Kind et al., 2017; Sun and Kennett, 2017; Hopper and Fischer, 2018; Kind and Yuan, 2019) although the existence of widespread MLDs in the U.S. has recently been challenged (Kind et al., 2020). While their long-wavelength nature precludes a detection of a sharp boundary, surface-wave phase velocities in cratonic areas also support the presence of a layer of reduced velocities in the depth range of 60-100 Km (Lekic and Romanowicz, 2011; Dalton et al., 2017; Eeken et al., 2018).
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2019, TectonophysicsCitation Excerpt :Besides the marked phases, there are no other significant signals visible. In particular, no deep LAB signals (around 200 km) are observed in almost the entire cratonic China in contrast to some other cratons (e.g. Fischer et al., 2010; Eaton et al., 2009; Miller and Eaton, 2010; Kind et al., 2013, 2015, 2017, 2019; Sodoudi et al., 2013; Foster et al., 2014; Hansen et al., 2015). Under the eastern part of China (Fig. 4A), the Moho is at ~30 km depth and the LAB at ~80 km depth.
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2017, Science BulletinCitation Excerpt :Moreover, much less information has been gained about the structure of the low velocity layer below the MLD, including its thickness, the magnitude of seismic velocity decrease within this layer, and the sharpness of its bottom boundary compared to the MLD. Additional velocity discontinuities or layers were also recently found within or immediately below the lithosphere in some continental (e.g., [10,31,42]) and oceanic regions (e.g., [19–21]) (Fig. 2). Whether these discontinuities or layers are global phenomena or not is still a subject of speculation.
The Pannon LitH<inf>2</inf>Oscope magnetotelluric array in the Pannonian Basin
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