Wide-angle observations of ALP 2002 shots on the TRANSALP profile: Linking the two DSS projects

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1 Tectonophysics 414 (2006) Wide-angle observations of ALP 2002 shots on the TRANSALP profile: Linking the two DSS projects Florian Bleibinhaus a, *, Ewald Brückl b ALP 2002 Working Group a Department of Earth and Environmental Sciences, Geophysics Section, University of Munich, Germany b Institute of Geodesy and Geophysics, Vienna University of Technology, Austria Received 10 September 2004; received in revised form 10 June 2005; accepted 4 October 2005 Available online 10 January 2006 Abstract Dynamite shots of the crustal-scale refraction seismic project ALP 2002 were recorded by an array of 40 seismological threecomponent stations on the TRANSALP profile. These observations provide a direct link between the two deep seismic projects. We report preliminary results obtained from these data. In a first approach, we verified the TRANSALP refraction seismic velocity model computing travel times for several shots and comparing them to the new observations. The results generally confirm this model. Significant first-break travel time differences in and near the Tauern Window are explained by anisotropy. Large-scale features of the model, particularly the Moho structure, seem to be continuous towards the east. Travel time residuals of wide-angle reflections indicate a slight eastward dip component of the Adriatic Moho. D 2005 Elsevier B.V. All rights reserved. Keywords: Crustal structure; Eastern Alps; Tauern Window; TRANSALP; ALP 2002; P-wave velocities; Refraction; Anisotropy; Moho 1. Introduction For the purpose of connecting the two Deep Seismic Sounding (DSS) projects, we deployed 40 stations on the TRANSALP profile (TRANSALP Working Group, 2002) to observe the ALP 2002 refraction seismic shots. These stations constitute the line ALP 12 at the western border of the ALP 2002 investigation area, which is covered by a network of 13 passive lines * Corresponding author. Present address: Department of Earth and Environmental Sciences, Geophysics Section, Theresienstr. 41, D Munich, Germany. Tel.: ; fax: address: bleibi@geophysik.uni-muenchen.de (F. Bleibinhaus). several hundred kilometres in length (Fig. 1) (Brückl et al., 2003). TRANSALP results may therefore provide important boundary conditions for ALP 2002 models. Also, ALP 2002 may provide valuable constraints with respect to lateral variations east of TRANSALP. Seismic images and models of the crustal structure along TRANSALP revealed large bi-vergent intracrustal shear zones related to collision, and a Moho geometry related to southward subduction of Penninic oceanic crust. These results resemble the structures found in the Central Alps in many aspects. For a discussion of these deep seismic studies see e.g. Pfiffner (1992), Schmid et al. (1996), TRANSALP Working Group (2002), Lippitsch et al. (2003), Kummerow et al. (2004), Bleibinhaus and Gebrande (2005 this issue) and references therein /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.tecto

2 72 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) Fig. 1. Map of ALP 2002 seismic sources (1) used in this investigation and ALP12/TRANSALP three-component stations (z). The inset displays all ALP 2002 receiver lines. FB Foreland Basin, NCA Northern Calcareous Alps, QPZ Quartzphyllite Zone, TW Tauern Window, UAG Upper Austroalpine gneisses, D dolomites, TC Tertiary clastics, PL Periadriatic Lineament.

3 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) One major question is how far the crustal structure imaged by TRANSALP continues towards the east. In order to find a preliminary answer to this question, we computed travel times for several shots located between 20 km west and 120 km east of the profile (Fig. 1) using the TRANSALP refraction seismic model (Bleibinhaus and Gebrande, 2005 this issue). This model is 2D, and the velocities are extrapolated in the E W direction. Ray parameters were computed with a pseudobending, two-point ray tracer integrated into the inversion scheme (Um and Thurber, 1987; Bleibinhaus, 2003), which was applied to derive the TRANSALP model. The comparison of measured and computed travel times allows a verification of the model and provides information about lateral continuity. 2. Anisotropy Computed first-breaks match the data well (Fig. 2), except for the wider Tauern Window (TW) area, where the deviations amount to 0.3 s (SP 201, 202) and 0.8 s (SP 113, 203), respectively. Lateral heterogeneity could Fig. 2. Observations and modelled ray paths for selected shots recorded on the TRANSALP/ALP12 line. SP 201 (a) and 202 (b) are within 20 km distance from the profile. SP 203 (c), 113 (d) and 114 (e) are located 80, 110 and 120 km, respectively, towards the east. Seismograms are plotted vs. profile coordinate. Ray paths are displayed in a N S section (middle) and in a top view (bottom). EM European Moho, AM Adriatic Moho. Reflection points of the computed Moho reflections are indicated by black dots in the maps. Black dots in the seismic sections denote travel times computed from the TRANSALP model (Bleibinhaus and Gebrande, 2005 this issue) and hollow dots mark travel times computed in a model with a steeper dip of the AM. The model, ray paths and reflection points displayed for SP 113 and 114 correspond to this steeper dip. Phase correlations of Moho reflections are relatively clear for the EM, but ambiguous for the AM in some sections.

4 74 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) Fig. 2 (continued). account for these differences. However, based on observations of shear wave splitting and azimuthal variations of average first-break velocities, Bleibinhaus and Gebrande (2005 this issue) found an anisotropy value of 10% in the upper 2 3 km of the western TW with the fast axis oriented E W. This anisotropy can explain the deviations we observe because it was not taken into account when deriving the refraction seismic TRANS- ALP velocity model. As this model is constrained by mainly N S-oriented rays, modelled travel times corresponding to E W-oriented paths are too large. Absolute travel times for recordings of SP 201 (Fig. 2a) by stations in the TW and at its northern rim vary between 2.5 s and 3.5 s, thus the deviations are roughly in agreement with 10% anisotropy. Absolute travel times of SP 202 (Fig. 2b) recordings in the TW vary between 7 s and 8 s. However, as the shot is located in the Upper Austroalpine and corresponding ray paths run through the TW in parts only, the observed deviations of 0.3 s are still consistent with 10% anisotropy. For SP 113 in the eastern TW (Fig. 2d), deviations of modelled and observed first-break travel times in the TW amount to 4% of the absolute travel time only, although rays run almost entirely in the E W direction through the TW. Ignoring the possible influence of unresolved heterogeneities within the TW this indicates that anisotropy is either restricted to the western TW or distinctly decreases with depth. Future investigations

5 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) Fig. 2 (continued). including other ALP 2002 lines will provide further insights with respect to this ambiguity. Strong intracrustal reflection observations at profile coordinate km from SP 201 (Fig. 2a) emphasize the importance of a reflector below and south of the Northern Calcareous Alps (NCA). It was previously interpreted as the basal Austroalpine thrust fault, compensating N S convergence (Bleibinhaus and Gebrande, 2005 this issue). 3. Lower crustal structure Lower crustal structure is constrained only by reflections from the crust mantle boundary. The

6 76 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) Fig. 2 (continued).

7 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) Fig. 2 (continued).

8 78 F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) recordings of SP 203 and 113 (Fig. 2c, d) provide clear observations of the European Moho (EM), and modelled reflection travel times match them quite well. Corresponding reflection points are located km east of TRANSALP. This indicates that the structure of the EM does not change significantly between 128E and 12.78E. Reflections from the Adriatic Moho (AM) are observed for all shots, but they are ambiguous for SP 202 and 203, whereas SP 113 and 114 provide better phase correlations. Corresponding modelled travel times approximately match the observations, but the apparent velocities deviate. This can be compensated by a steeper inclined AM (Fig. 2d, e). Such modification of the model transforms the N-directed dip of the AM of 48 into a NNE-ward dip of 108 with a maximum depth of 46 km at its northern edge (ca N 12.78E). A deviation of computed and observed Moho reflection travel times from SP 201 (Fig. 2a, at profile coordinate 200 km) could be matched by a shallower, less inclined AM slightly west of the profile. However, this observation is not very clear and might also be related to reflectivity from within the Adriatic lower crust, which is very complex in the vicinity of the Periadriatic Lineament (see Fig. 17 in Lüschen et al., 2005 this issue). 4. Discussion The data used in this study are sparse with respect to the area of coverage and independently cannot resolve the structures under discussion. However, forward computation in the extrapolated TRANSALP velocity model yields preliminary results regarding lateral continuity. In the first instance, observed lateral variations are small and suggest continuity toward the east of the gross crustal structures observed on TRANSALP (TRANSALP Working Group, 2002; Kummerow et al., 2004; Bleibinhaus and Gebrande, 2005 this issue). However, from tomographic studies of the upper mantle, Lippitsch et al. (2003) suggest a reversal of subduction direction from S-directed in the Central Alps to N-directed in the Eastern Alps. In terms of crustal structure, this implies that the AM should reach deeper than the EM, which is not supported by the observations presented here. The suggested lateral variation of the dip of the AM is certainly not unique. However, it is the simplest model modification sufficient to explain the observed differences, because only one model parameter the dip of the AM was changed. A presumed eastward dip component could indicate a SE-ward bending of the orogenic root related to the NE directed Dinaric subduction of Adriatic crust (Bada et al., 1999). Future investigations of ALP 2002 data will shed new light on these questions. Acknowledgements The ALP 2002 programme is jointly financed by governmental and academic institutions of Austria, Canada, Croatia, the Czech Republic, Denmark, Finland, Germany, Hungary, Poland, Slovenia and the USA. Seismological stations for the line ALP 12 were provided by the Geophysical Instrument Pool Potsdam. References Bada, G., Horváth, F., Gerner, P., Fejes, I., Review of the present-day geodynamics of the Pannonian basin: progress and problems. J. Geodyn. 27, Bleibinhaus, F., Entwicklung einer simultanen refraktions- und reflexionsseismischen 3D-Laufzeittomographie mit Anwendung auf tiefenseismische TRANSALP-Weitwinkeldaten aus den Ostalpen. PhD thesis, 171 p. Bleibinhaus, F., Gebrande, H., Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography. Tectonophysics. 414, doi: /j.tecto (this issue). Brückl, E., Bodoky, T., Hegedüs, E., Hrubcová, P., Gosar, A., Grad, M., Guterch, A., Hajnal, Z., Keller, G.R., Špičák, A., Sumanovac, F., Thybo, H., ALP2002 Working Group, ALP 2002 seismic experiment. Stud. Geophys. Geod. 47, Kummerow, J., Kind, R., Oncken, O., Giese, P., Ryberg, T., Wylegalla, K., TRANSALP Working Group, A natural and controlled source seismic profile through the Eastern Alps: TRANSALP. Earth Planet. Sci. Lett. 225, doi: /j.epsl Lippitsch, R., Kissling, K., Ansorge, J., Upper mantle structure beneath the Alpine Orogen from high-resolution teleseismic tomography. J. Geophys. Res. 108 (B8), doi: / 2002JB Lüschen, E., Borrini, D., Gebrande, H., Millahn, K., Nicolich, R., TRANSALP Working Group, TRANSALP deep seismic Vibroseis and explosive seismic profiling in the Eastern Alps, Tectonophysics. 414, 9 38 doi: /j.tecto (this issue). Pfiffner, A., Alpine orogeny. In: Blundell, D., Freeman, R., Mueller, S. (Eds.), A Continent Revealed: The European Geotraverse. 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