Seismic evidence for whole lithosphere separation between Saxothuringian and Moldanubian tectonic units in central Europe

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L09304, doi: /2006gl029188, 2007 Seismic evidence for whole lithosphere separation between Saxothuringian and Moldanubian tectonic units in central Europe B. Heuer, 1 H. Kämpf, 1 R. Kind, 1 and W. H. Geissler 2 Received 22 December 2006; revised 2 March 2007; accepted 21 March 2007; published 2 May [1] The Bohemian Massif is part of the Variscan belt of central Europe. We carried out a high resolution mapping of lithospheric thickness beneath central Europe by investigating 264 teleseismic events recorded at 80 broad band stations in the western Bohemian Massif with the method of S receiver function analysis. A negative phase beneath the Saxothuringian and northeastern Teplá- Barrandian units at about 9 10 s before the S onset is interpreted as caused by the lithosphere-asthenosphere boundary (LAB) at km depth. In the Moldanubian unit, the negative phase occurs at s before the S onset, corresponding to lithospheric thickness of km. The boundary between the domains is oriented E-W and probably marks the northern extension of Moldanubian lithosphere. The Moho also deepens from the Saxothuringian to the Moldanubian unit. The observed crustal/lithospheric domains could represent two distinct microplates with a relatively sharp boundary cutting through the whole lithosphere. Citation: Heuer, B., H. Kämpf, R. Kind, and W. H. Geissler (2007), Seismic evidence for whole lithosphere separation between Saxothuringian and Moldanubian tectonic units in central Europe, Geophys. Res. Lett., 34, L09304, doi: /2006gl Introduction [2] Lithospheric thickness is a critical parameter regarding the structure and geological evolution of continents. The terms lithosphere and asthenosphere were originally defined with regards to rheology. However, additional, differing usages of the term lithosphere have been introduced such as thermal, seismic, or chemical lithosphere [Anderson, 1995]. The lithosphere is seismologically defined as the high-velocity outer layer of the Earth. The lithosphereasthenosphere boundary (LAB) is expected to have relatively weak contrast in seismic parameters and is therefore difficult to observe. [3] The Bohemian Massif forms the eastern part of the Variscan belt of central Europe which developed approximately between 390 and 300 Ma during a period of large-scale crustal convergence, subduction, and collision of continental plates and microplates [Matte et al., 1990]. The study area, the western Bohemian Massif, pertains to four Variscan structural units (from NW to SE): the Mid-German Crystalline High, the Saxothuringian, the Moldanubian and the small crustal Teplá-Barrandian units (Figure 1). 1 GeoForschungsZentrum Potsdam, Germany. 2 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany. Copyright 2007 by the American Geophysical Union /07/2006GL [4] In this study we present a high resolution mapping of the lithosphere in central Europe. It was carried out using the S receiver function technique [Farra and Vinnik, 2000; Li et al., 2004]. We used data of 80 broad band stations of the international passive seismic experiment BOHEMA [Plomerová et al., 2003] and another experiment described by Geissler et al. [2005] (Figure 1). Apart from the prominent signal of the Moho in the S receiver functions, a second deeper discontinuity producing a signal with negative polarity is observed in this study. Following earlier S receiver function studies [e.g., Li et al.,2004;kumar et al., 2006; Sodoudi et al., 2006], the latter discontinuity is interpreted as the LAB. 2. Method [5] TheS receiver function method [Farra and Vinnik, 2000] was applied in order to isolate S-to-P (Sp) converted phases from the incident S phases. The method comprises component rotation, deconvolution and summation of many traces. A comprehensive overview about the potential of the S receiver function method is given by Yuan et al. [2006]. The S-to-P converted waves arrive at the station earlier than the direct S waves, whereas the multiple reverberations arrive after the S onset. In P receiver functions, primary conversions from the LAB are often masked by the arrival of strong multiple reverberations in the same time window. This problem is avoided in the S receiver function technique. [6] The selected 264 teleseismic events lie within an epicentral distance range of and have magnitudes of 5.7 and higher. [7] The processing of the data, the moveout correction and stacking basically follow the description of e.g., Kumar et al. [2006]. Finally, the time axis and sign of amplitudes are reversed so that the S-to-P converted phases have positive arrival times and can be compared to the results of P receiver function analysis that analyzes P-to-S converted phases. [8] The frequency content of the signals used in the two methods is different: P receiver functions have dominating wave periods around 1 s, while for S receiver functions wave periods around 5 s prevail. Hence the resolution is also different. A thin layer of anomalous velocity might be detectable by P receiver functions and invisible in S receiver function data. A sharp gradient might be better visible in P receiver function data, while a broad gradient might be better to detect in S receiver function data. 3. High Resolution Mapping of Lithospheric Thickness of the Saxothuringian and Moldanubian Units [9] Examples of S receiver functions obtained at two stations are shown in Figure 2. At 3 4 s lead time before L of5

2 Figure 1. Map of the Bohemian Massif with its major tectonometamorphic units and fault zones (modified after Plomerová et al. [2005]): Mid-German Crystalline High (MGCH); Saxothuringian (ST), Moldanubian (MD), and Teplá-Barrandian (TB) unit; Mariánské Lázně Complex (MLC); Central Bohemian Pluton (CBP); Eger Graben (EG); West Bohemian Shear Zone (WBSZ); Central Bohemian Shear Zone (CBSZ); Bavarian Shear Zone (BSZ); Elbe Fault Zone (EFZ); Blanice Graben (BLG); Intra-Sudetic Fault (ISF); and Regensburg-Leipzig-Rostock zone (RLR). KTB is the location of the German continental deep drilling boreholes. Black triangles show the location of stations of the BOHEMA experiment and of the pre-study by Geissler et al. [2005]. (inset) Location of the investigated area within central Europe. Armorican Massif (AM), Massif Central (MC), Rhenish Massif (RM), Bohemian Massif (BM), Saxothuringian unit (ST), Moldanubian unit (MD), and Rhenohercynian unit (RH). the S onset (positive time values in Figure 2 due to reversed time axis, called delay time ), the strong signal of the Moho Sp conversion can be seen. It corresponds very well, i.e. within ±0.2 s, with delay times observed in P receiver functions [Heuer et al., 2006]. At about s lead time, a weaker negative phase is observed. It corresponds to a negative velocity gradient and is attributed to the LAB. [10] For a lithospheric thickness of 80 km (assumed thickness of the lithosphere beneath the western Eger Rift), the piercing points of the Sp rays are located km away from the station. As the stations used in this study are spaced quite densely, the piercing points of different stations strongly overlap. The area was for stacking purposes divided into 28 non-overlapping boxes shown in Figure 3. The width of the boxes is 1 longitude and between 0.4 and 1 latitude. Only box 25 is smaller. The traces were stacked within each box to improve the signal-to-noise ratio. Most boxes show an unambiguous negative conversion signal after the Moho Sp conversion. For some boxes (boxes 2 5, 9, 21 23) the negative signal in the sum trace was either doubled or very broad, so that no clear decision for a certain delay time of the signal could be made (Figure 3). A good approximation of the depth origin of the negative phase can be achieved by multiplying the picked delay time by a factor of 9 based on the IASP91 reference Earth model [Kennett and Engdahl, 1991]. The error introduced by this model and by the wavelength of the Sp phase is about 10 km. The result of the depth estimation is visualized in the map in Figure 3. The corresponding values are given in Table S1 of the auxiliary information. 1 If the negative phase is interpreted as the lithosphere-asthenosphere transition, the map in Figure 3 shows lithospheric thickness of 80 to 90 km beneath the Saxothuringian and the northern part of Teplá- Barrandian unit. Beneath the southern part of the Teplá- Barrandian unit, the thickness seems to increase slightly (boxes 20, 18, 24: 95 km) and increases strongly beneath the Moldanubian (115 to 135 km). Boxes 25, 26 and 28 in the very south are not well covered by data. However, as the obtained depth values of the boxes in the Moldanubian part are coherent, the result of a thick lithosphere in this area is considered to be reliable. Asthenospheric updoming beneath the western Eger Rift as indicated in a P receiver function study by Heuer et al. [2006] could not be observed in the S receiver functions, probably due to the different frequency content of the signals in the two methods. [11] Figure 3 shows data along three N-S profiles. They cross the boundary between the Saxothuringian and Moldanubian units and show a negative phase at about 9 10 s delay time in the Saxothuringian and at s in the Moldanubian. The negative phase in the very north of profiles A and B (boxes 2 and 3, beneath the Mid-German Crystalline High) needs to be investigated by more data yet. 4. Discussion: Geological Implications of Differences in Lithospheric Thickness [12] In the Moldanubian unit, a relatively deep LAB can be observed at about km (Figure 3). In the northeastern part of the Teplá-Barrandian and in the Saxothuringian region, the lithosphere is significantly thinner (80 to 90 km). In the boxes in the transition zone from the Saxothuringian type lithosphere to the thicker Moldanubian lithosphere (boxes 21 to 23), it was difficult to determine a clear negative signal in the summation trace. Especially for boxes 21 and 22, a doubling and/or broadening of the negative signal over a distance of less than 60 km at surface can be stated which could point to either an abrupt and dramatic increase of lithospheric thickness or a very steep slope or possibly even a remnant of palaeosubduction [Oncken, 1997; Babuška and Plomerová, 2006]. Babuška and Plomerová [2001, 2006] observed in the area of box 21 a mixture of anisotropic characteristics of the Saxothuringian and Moldanubian units within the lower lithosphere south of the surface trace of the Saxothuringian/Moldanubian contact. As an explanation the authors suggested a Auxiliary materials are available at ftp://ftp.agu.org/apend/gl/ 2006gl of5

3 150 km wide transition between the two units with underthrusting of a part of the Saxothuringian subcrustal lithosphere beneath the Moldanubian or a hypothetical remnant of the early Palaeozoic oceanic lithosphere subducted to the south during collision of the Saxothuringian and Moldanubian units. The higher resolution of mapping lithospheric thickness by S receiver function analysis and a closer investigation of the transition between the units along profiles A, B and C (Figure 3) allows us to favour a sharp or very steep boundary separating the Saxothuringian from Moldanubian lithosphere within the eastern part of the study area. The assumed boundary region between the units is marked by a violet area in Figure 3. We think that the Moldanubian lithosphere is reaching as far north as the observed transition in lithospheric thickness, while the small Teplá-Barrandian crustal unit is only superimposed. The E-W orientation of the transition differs from the orientation of Variscan structures at surface and might even represent older (Cadomian) basement. [13] Both the Moho (P receiver function study by Heuer et al. [2006]) and the LAB (this study) show the same relation at depth to the Variscan units: Beneath the Bohemian Massif, the Moho deepens from the Saxothuringian unit (approximately km) to km in the Moldanubian unit, while the LAB deepens across the same boundary from about km to km. [14] Babuška and Plomerová [2001] introduced a model of thickness and anisotropy of the subcrustal lithosphere of the Bohemian Massif which combines variations of P-wave residuals and shear-wave splitting observations. It shows lithospheric thickness of km in the Saxothuringian, a thinning to km beneath the western Eger Rift, and a thicker lithosphere of km beneath the Moldanubian unit. The model corresponds very well to a model of lithospheric thickness derived from heat flow measurements by Čermák [1994]. [15] To explain the present relatively thin lithosphere beneath the Bohemian Massif, Willner et al. [2002] relate the fast exhumation of large volumes of high-pressure rocks from the crustal root after the closure of oceans to delamination and complete detachment of the lithospheric mantle. Zulauf [1997] also uses the concept of lithospheric thickening and delamination in the Moldanubian area to explain the large amounts of plutons (partly derived from mantle melt) along the shear zones in the Moldanubian region. However, Babuška and Plomerová [2001, 2006] argue against large-scale detachment of the crust from the mantle lithosphere on the basis of their observations of the different orientations of the fabrics in the subcrustal lithosphere of the Saxothuringian and the Moldanubian units, as well as the consistency of the fabric within each unit. [16] Figure 4 shows a NNW-SSE section across the western Eger Rift which shows the extent of the Saxothuringian and Moldanubian crust and lithosphere as observed 3of5 Figure 2. Data examples of S receiver functions obtained at (a) permanent station GRA1 and (b) temporary station BG04 (for location of the stations see Figure 1). The delay time of the seismic signals with respect to the time of the S onset is shown. Time axis and amplitudes were multiplied by 1. Each trace shows the S receiver function of a single teleseismic event. A low pass filter of 4 s and moveout correction were applied. Traces are sorted by back azimuth of the corresponding event. Almost no events from southern directions were processed. On top of the single traces, the stacked trace is shown. At 3 4 s delay time, the signal of the Moho Sp conversion can be seen. At about s, a negative signal is observed. Signals occurring at negative delay times are attributed to multiple reflections. Insets at top right show the corresponding sum trace of Q components for assessment of side-lobe magnitudes. At permanent station GRA1, a later arrival of the LAB signal can be observed in the single traces at back azimuths between Most of the piercing points of these rays at 80 km depth lie within box 21 and the southern part of box 16 shown in Figure 3. This corresponds to our observation of a deeper LAB in box 21 (see Figure 3). At temporary station BG04, data amount and quality is not so high.

4 Figure 3. (left) Map of the depth origin of the negative phase, which probably represents the lithosphere-asthenosphere transition beneath the western Bohemian Massif. The area was subdivided into 28 non-overlapping boxes. Red crosses show the data coverage of the boxes. The crosses are piercing points of the rays at 80 km depth. Gray lines show geologic structures corresponding to Figure 1. In the Saxothuringian and northeastern Teplá-Barrandian unit, lithospheric thickness is km, in the southwestern Teplá-Barrandian and the Moldanubian units it is km. In the hatched boxes (2 5, 9), the negative signal of the lithosphere-asthenosphere transition was difficult to interpret. For more information about individual boxes, see Table S1 of the auxiliary material. In boxes 21 23, a transition to deeper lithosphere was observed. The assumed transition region is shaded violet. The location of profiles A, B, and C is marked. (right) N-S profiles (A, B, and C) of stacked S receiver functions. Width of the profiles is 1 longitude. Single traces were stacked according to their piercing points within sliding windows of 0.25 latitude with 0.1 overlap. The positive signal at 3 4 s delay time (gray) is interpreted to originate from the Moho discontinuity, while the following negative signal at 9 15 s (black) is attributed to the LAB. The red dashed line shows the assumed depth of the LAB, which is increasing in the southern parts of the profiles. Violet shading marks the traces that are proposed to show the transition between the two lithospheric domains. in this study and by Heuer et al. [2006]. The transition between the two lithospheric domains is less than 60 km wide. However, the angle of the contact between the two units is not known. [17] Our results could support the thesis of the Saxothuringian and Moldanubian structural units representing distinct microplates formed before the Variscan orogeny as suggested by Babuška and Plomerová [2001, 2006]. However, post-variscan delamination and formation of new crust and lithosphere and different influences of the Alpidic orogeny on the different Variscan structural units can also not be ruled out. 5. Conclusions [18] AnS receiver function investigation in the Bohemian Massif showed the occurrence of a velocity reduction at depths between 80 and 135 km, which is interpreted as the LAB. In the northern part of the investigated area (the Saxothuringian and northeastern Teplá-Barrandian units), lithospheric thickness amounts to 80 to 90 km, while in the southern part (southwestern Teplá-Barrandian and the Moldanubian units) it increases to approximately 120 to 130 km. The strong increase in lithospheric thickness observed between the different Variscan structural units in the north and south points to different lithospheric stratification: beneath the Saxothuringian and northern Teplá- Barrandian, both the Moho and the LAB are shallower than below the Moldanubian unit. This points to distinctly different domains separated by a sharp boundary or steep slope (over less than 60 km distance at surface) crosscutting the whole lithosphere. This boundary is oriented E-W rather than parallel to Variscan structures at surface and can be interpreted as the northern boundary of Moldanubian lithosphere. The small Teplá-Barrandian crustal unit seems to be superimposed on this boundary. [19] The different lithospheric domains might be either interpreted as pre-variscan plate fragments as suggested by Babuška and Plomerová [2001, 2006], or as regions of different post-variscan processes like lithospheric delamination and formation of new lithosphere as suggested by Zulauf [1997] and Willner et al. [2002]. We think that the S receiver function method in future has a high potential to answer questions regarding the evolution of European lithosphere and the tectonic processes involved if larger areas of Europe are mapped and station density is high enough. [20] Acknowledgments. We would like to thank the BOHEMA working group for good cooperation. The German temporary stations were provided by the Geophysical Instrument Pool of the GFZ Potsdam, the data were archived in the GEOFON archive with the help of W. Hanka. Other data were obtained from J. Plomerová (IG CAS, Prague), U. Achauer (Univ. Strasbourg), K. Klinge (SZGRF Erlangen), A. Hemmann (Univ. Jena), and S. Funke (Univ. Leipzig). For data processing we used K. Stammler s program SeismicHandler. The figures were generated using 4of5

5 Figure 4. (left) Overview map (see Figure 1) to show the location of the section P-P. The observed transition between Saxothuringian and Moldanubian lithosphere beneath the Teplá-Barrandian crustal unit is shaded violet as in Figure 3. (right) Cartoon illustrating lithospheric thickness beneath the western Bohemian Massif as observed in this study. The section represents a NNW-SSE profile across the western Eger Graben (EG) with true proportions. TB is the Teplá- Barrandian unit, CBSZ is the Central Bohemian Shear Zone (see Figure 1). Moho depths are according to Heuer et al. [2006]. The Moho is deepening from the Saxothuringian (27 31 km) to the Moldanubian unit (35 39 km). The LAB shows the same trend: it is strongly deepening from km beneath the Saxothuringian to km beneath the Moldanubian unit. However, a clear image of the nature of the contact between the domains could not be obtained. the Generic Mapping Tools (GMT). For valuable comments on improving this manuscript we are very grateful to P. and E. Bankwitz, F. Sodoudi, and two anonymous reviewers. We thank the Deutsche Forschungsgemeinschaft (DFG) for the support of the BOHEMA experiment (grant KI 314/15), the EU for support within the NERIES project and GFZ Potsdam for further funding of B.H. References Anderson, D. L. (1995), Lithosphere, asthenosphere, and perisphere, Rev. Geophys., 33(1), Babuška, V., and J. Plomerová (2001), Subcrustal lithosphere around the Saxothuringian-Moldanubian suture zone A model derived from anisotropy of seismic wave velocities, Tectonophysics, 332, Babuška, V., and J. Plomerová (2006), European mantle lithosphere assembled from rigid microplates with inherited seismic anisotropy, Phys. Earth Planet. Inter., 158, Čermák, V. (1994), Results of heat flow studies in Czechoslovakia, in Crustal Structure of the Bohemian Massif and the West Carpathians, edited by V. Bucha and M. Blíkovský, pp , Springer, New York. Farra, V., and L. Vinnik (2000), Upper mantle stratification by P and S receiver functions, Geophys. J. Int., 141, Geissler, W. H., H. Kämpf, R. Kind, K. Bräuer, K. Klinge, T. Plenefisch, J. Horálek, J. Zedník, and V. Nehybka (2005), Seismic structure and location of a CO 2 source in the upper mantle of the western Eger (Ohře) Rift, central Europe, Tectonics, 24, TC5001, doi: /2004tc Heuer, B., W. H. Geissler, R. Kind, and H. Kämpf (2006), Seismic evidence for asthenospheric updoming beneath the western Bohemian Massif, central Europe, Geophys. Res. Lett., 33, L05311, doi: / 2005GL Kennett, B. L. N., and E. R. Engdahl (1991), Travel times for global earthquake location and phase identification, Geophys. J. Int., 105, Kumar, P., X. Yuan, R. Kind, and J. Ni (2006), Imaging the colliding Indian and Asian lithospheric plates beneath Tibet, J. Geophys. Res., 111, B06308, doi: /2005jb Li, X., R. Kind, X. Yuan, I. Wölbern, and W. Hanka (2004), Rejuvenation of the lithosphere by the Hawaiian plume, Nature, 427, Matte, P., H. Maluski, P. Rajlich, and W. Franke (1990), Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing, Tectonophysics, 177, Oncken, O. (1997), Transformation of a magmatic arc and an orogenic root during obligue collision and its consequences for the evolution of the European Variscides (Mid-German Crystalline Rise), Geol. Rundsch., 86, Plomerová, J., U. Achauer, V. Babuška, M. Granet, and BOHEMA working group (2003), BOHEMA : Passive seismic experiment to study lithosphere-asthenosphere system in the western part of the Bohemian Massif, Stud. Geophys. Geod., 47, Plomerová, J., L. Vecsey, V. Babuška, M. Granet, and U. Achauer (2005), Passive seismic experiment MOSAIC A pilot study of mantle lithosphere anisotropy of the Bohemian Massif, Stud. Geophys. Geod., 49, Sodoudi, F., X. Yuan, Q. Liu, R. Kind, and J. Chen (2006), Lithospheric thickness beneath the Dabie Shan, central eastern China, from S receiver functions, Geophys. J. Int., 166, Willner, A. P., E. Sebazungu, T. V. Gerya, W. V. Maresch, and A. Krohe (2002), Numerical modelling of PT-paths related to rapid exhumation of high-pressure rocks from the crustal root in the Variscan Erzgebirge Dome (Saxony/Germany), J. Geodyn., 33, Yuan, X., R. Kind, X. Li, and R. Wang (2006), The S receiver functions: Synthetics and data example, Geophys. J. Int., 165, , doi: /j x x. Zulauf, G. (1997), Rheological collapse of a Bohemian Tibetan plateau: The Teplá-Barrandian unit (central European Variscides), J. Czech Geol. Soc., 42(3), W. H. Geissler, Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, D Bremerhaven, Germany. B. Heuer, H. Kämpf, and R. Kind, GeoForschungsZentrum Potsdam, Telegrafenberg, D Potsdam, Germany. (heuer@gfz-potsdam.de) 5of5