Crustal structure of the Eastern Alps and their foreland: seismic model beneath the CEL10/Alp04...

Geophys. J. Int. (2009) 177, 279–295 doi: 10.1111/j.1365-246X.2008.04074.x

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Crustal structure of the Eastern Alps and their foreland: seismicmodel beneath the CEL10/Alp04 profile and tectonic implications

M. Grad,1 E. Bruckl,2 M. Majdanski,1,3 M. Behm,2 A. Guterch3

and CELEBRATION 2000 and ALP 2002 Working Groups∗1Institute of Geophysics, University of Warsaw, Pasteura 7, 02-093 Warsaw, Poland. E-mail: [email protected] of Geodesy and Geophysics, Vienna University of Technology, Vienna, Austria3Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452 Warsaw, Poland

Accepted 2008 November 27. Received 2008 October 19; in original form 2007 July 20

S U M M A R YThe subject of this paper concerns the seismic modelling of the crustal structure in thetransition zone from the Bohemian Massif, across the Molasse basin and the Eastern Alps tothe Southern Alps, mainly on the territory of Austria. The CEL10/Alp04 profile crosses thetriple point of the European plate, Adriatic microplate and the recently identified Pannonianfragment. The seismic data along the presented profile originate from two large experiments:CELEBRATION 2000 and ALP 2002. The wavefield observed in the Eastern Alps is morecomplex than in the Bohemian Massif. Strong first arrivals (Pg) are distinct up to 60–90 kmoffset and are characterized by large variations in apparent velocity and amplitude. The contactbetween the Molasse basin and the Eastern Alps represents a barrier for seismic waves. Mid-crustal reflections (Pc) are usually recorded at short distance intervals (20–50 km) and arealso characterized by variations in apparent velocity and amplitude. The Moho reflections areusually strong and well correlated, while Pn arrivals are only fragmentarily recorded. Detailed2-D forward modelling of all refracted, post-critical and reflected phases, identified in thecorrelation process, was undertaken using a ray-tracing technique. The P-wave velocity in thecrystalline upper crust of the Bohemian Massif and Molasse basin is about 6.15 km s−1, whichis slightly higher than in the Alpine area (about 6.0 km s−1). Below the northern accretionarywedge of the Eastern Alps low-velocity sediments penetrate towards the southwest (SW) downto about 10 km depth. In the middle crust of the Alpine part, a reflective zone was modelledby a lamellae structure with alternating high and low velocities and thicknesses of about2–3 km. The lower crust in this part of the model is more homogeneous, with a velocity of about6.9 km s−1. In the Bohemian Massif, a high-velocity (HV) body (VP ∼ 7 km s−1) of a fewkilometres thickness was delineated in the depth interval 18–23 km. The crustal thickness alongthe CEL10/Alp04 profile changes from about 42–44 km in the SW (Alpine part), to around40 km in the central part of the profile (Molasse basin), and 38–40 km in the NE (BohemianMassif). The velocity in the uppermost mantle determined from Pn wave traveltimes is about8 km s−1 along the whole profile. The interpretation of the seismic wavefield is supplementedby an existing 3-D P-wave velocity model of the area. Main features derived by 2-D modelling(low velocities beneath the accretionary wedge, high velocities in the lower crust of theBohemian Massif, Moho topography) well correlate with the 3-D model. Furthermore, the3-D model allows assessing the lateral extent of significant features alongside the CEL10/Alp04profile. This area is affected by both collision and escape tectonics. The high-reflectivity zone

∗CELEBRATION 2000 and ALP 2002 Working Groups comprise: S. Acevedo, K. Aric, I. Asudeh, M. Behm, A.A. Belinsky, F. Bleibinhaus, T. Bodoky,M. Broz, E. Bruckl, W. Chwatal, R. Clowes, W. Czuba, T. Fancsik, M. Fort, E. Gaczynski, H. Gebrande, A. Gosar, M. Grad, H. Grassl, A. Guterch, Z. Hajnal,S. Harder, E. Hegedu′′s, S. Hock, V. Hock, P. Hrubcova, T. Janik, G. Jentzsch, P. Joergensen, A. Kabas, G. Kaip, G.R. Keller, F. Kohlbeck, K. Komminaho, S.L.Kostiuchenko, A. Kovacs, D. Kracke, W. Loderer, K.C. Miller, A.F. Morozov, J. Oreskovic, K. Posgay, E.-M. Rumpfhuber, Ch. Schmid, R. Schmoller, O. Selvi,C. Snelson, A. Spicak, P. Sroda, F. Sumanovac, E. Takacs, H. Thybo, T. Tiira, C. Tomek, Ch. Ullrich, J. Vozar, F. Weber, M. Wilde-Piorko, J. Yliniemi.

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in the middle crust is explained by intermediate to mafic intrusions, rather than by ductileextensional deformation as generally observed in the lower crust.

Key words: Controlled source seismology; Body waves; Continental tectonics: compres-sional; Crustal structure.

I N T RO D U C T I O N

During the last decade, Central Europe has been covered by a se-ries of large seismic refraction and wide-angle reflection experi-ments (POLONAISE’97, CELEBRATION 2000, ALP 2002 andSUDETES 2003) to obtain a better knowledge of the lithosphericstructure of this area. (Guterch et al. 1999, 2000, 2001; Brucklet al. 2003; Grad et al. 2003b; Guterch et al. 2003a,b). This pa-per looks at the two overlapping profiles CEL10 and Alp04, fromthe CELEBRATION 2000 and ALP 2002 experiments. Togetherwith other profiles in this area, the CEL10/Alp04 profile provides acomprehensive seismic coverage along a section through the south-eastern Bohemian Massif, across the Molasse basin, and throughthe Eastern and Southern Alps. Another paper on CEL10/Alp04concentrates on the Bohemian part of this profile and the transi-tion to Trans-European Suture Zone (TESZ; Hrubcova et al. 2008).The first results from the evaluation of particular lines within theALP 2002 project were obtained using 2-D ray-tracing techniques(Bleibinhaus et al. 2006; Bruckl et al. 2007). Similar 2-D inter-pretation was also done for other areas of the Alps (e.g. Luschenet al. 2004; Bleibinhaus & Gebrande 2006), for neighbouring re-gions of the Bohemian Massif (Ru

◦zek et al. 2003; Hrubcova et al.

2005; Majdanski et al. 2006), for the TESZ (Janik et al. 2002; Gradet al. 2003a; Janik et al. 2005), as well as for the Carpathians andthe Pannonian Basin (Grad et al. 2006; Sroda et al. 2006).

During each of the above-mentioned experiments, seismic shotswere recorded simultaneously by all receivers. In each experiment,the data volume contains about 20 per cent inline recordings, whilethe rest are crossline recordings. Data of the third CELEBRATION2000 deployment and from the whole ALP 2002 experiment wereused for the generation of a 3-D model by stacking and tomographicmethods (Behm 2006; Behm et al. 2007). 3-D interpretation hasalso been done for bordering regions: for the northern BohemianMassif (Majdanski et al. 2007; Ru

◦zek et al. 2007); for the TESZ and

East European craton (EEC) in central Poland (Czuba et al. 2002;Sroda et al. 2002); for SE Poland at the transition from EEC tothe Carpathians (Malinowski et al. 2008) and for the northeasternpart of the Pannonian basin (Hajnal et al. 2004). In the future,such 3-D approaches will provide an opportunity for consistent 3-Dinterpretation of all new surveys, and data from older profiles for thewhole area between the Baltic and the Adriatic Seas. In this paper,we concentrate on the 2-D data of the CEL10/Alp04 profile, but wealso refer to results of the 3-D model by Behm et al. (2007).

T E C T O N I C S E T T I N G A N DG E O L O G I C A L C RO S S - S E C T I O N

The Eastern Alps result from orogenic processes in the Creta-ceous and early Tertiary, representing a continent–continent colli-sion between the European and Adriatic–Apulian plates. The majorgeological units of the Eastern Alps and their surrounding tec-tonic provinces are shown in Fig. 1 (Oberhauser 1980; Franke &Zelazniewicz 2000; Schmid et al. 2004). The Bohemian Massif inthe north represents the European platform. To the south, Europeancrust dips below the Molasse basin, the foreland of the Alpine

orogen. The accretionary wedge of the Eastern Alps comprisesthe Flysch belt and Austroalpine nappes including the NorthernCalcareous Alps (NCA). European crust has been exhumed in theTauern Window. The Periadriatic Lineament (PAL) separates theEastern Alps from the Southern Alps. The latter are bounded tothe south by the External Dinarides, the Po plain and the IstriaPeninsula. To the northeast (NE), the Eastern Alps continue into theCarpathians, and to the east they border to the Pannonian basin.

Major tectonic processes of the Eastern Alps include orogeniccycles in the Cretaceous (Eoalpine phase) and early Tertiary, repre-senting a continent–continent collision between the European andAdriatic–Apulian plates. Crustal shortening of the Eastern Alps inthe north–south direction followed. The maximum extent of short-ening is assumed to be 100 km, which corresponds to 50 per cent ofthe original width. Since the Late Oligocene and Early Miocene, theongoing north–south oriented compression of the Eastern Alps hasbeen accompanied by vertical and lateral extrusion (in particular ofthe Tauern Window) and tectonic escape of large crustal wedgesto the unconstrained margin represented by the Pannonian basin inthe east. Major fault systems (e.g. PAL, SEMP; see Fig. 1) werereactivated or formed by this tectonic process (Ratschbacher et al.1991).

A geological cross-section along the CEL10/Alp04 profile downto a depth of 5 km is shown in Fig. 1. This cross-section is basedon geological maps and sections by Franke & Zelazniewicz (2000),Oberhauser (1980), Schmid et al. (2004) and Wessely (2006). Inthe southwest (SW), the profile begins at the northwestern cornerof the External Dinarides. After crossing the South-Alpine thrustfault, the profile enters the Southern Alps with their Mesozoic coverand further to the north with their Palaeozoic basement. Around theprofile distance of 60 km, the profile intersects the PAL and reachesthe Eastern Alps. This is also the location of the most southwesternshotpoint SP31140. Within the Eastern Alps, the profile first crossesthe southern Mesozoic cover (MC in Fig. 1) and the crystallinebasement of the Austroalpine nappes (DG—Drautal/Gurktal, OB—Otztal/Bundschuh, KW–Koralpe/Wolz in Fig. 1), then it crossesthe Greywacke Zone (GW), and the NCA. The NCA overthrust theFlysch belt in the north (NCT). The Eastern Alps and the Flysch beltform the accretionary wedge of the Eastern Alpine orogen, whichoverthrusts the Molasse at the North Alpine thrust (NAT). Theextent of the Molasse zone is less than 20 km along CEL10/Alp04.Thereafter, the profile continues within Moldanubian and Moravianunits of the Bohemian Massif.

S E I S M I C DATA

The field layout of seismic experiments within the territory of Aus-tria and its surroundings is shown in Fig. 2. The shots and receiversused in this paper are highlighted. The northern part of profileAlp04 from the ALP 2002 experiment overlaps with the southernpart of the CEL10 profile, recorded during the CELEBRATION2000 experiment. The combined CEL10/Alp04 profile presentedin this paper is 380-km long, with nine shotpoints at an averageinterval of 40 km. The profile extends from the Austrian/Czechborder in the north to upper Italy in the south. Along the CEL10

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Crustal structure of the Eastern Alps and their foreland 281

Figure 1. Tectonic setting of the investigated area (top) and geologic cross-section along profile CEL10/Alp04 (bottom). Map and cross-section are compiledin accordance with Schmid et al. (2004), Oberhauser (1980) and Franke & Zelazniewicz (2000). On the cross-section numbered triangles refer to shotpoints.SP26800 shows common position with another shot fired in this place: SP26801. Crossing points with Alp01, Alp02 and CEL09 are indicated. Geological unitsand major faults are designated by the following abbreviations: in the Bohemian Massif: BA, Barrandian; MD—Moldanubian, MR—Moravian; in the EasternAlps: F—Flysch belt, NCA—Northern Calcareous Alps, GW—Greywacke Zone, MC—Mesozoic cover of Austroalpine basement nappes, ‘∗∗’ designates theAustroalpine basement nappes DG—Drautal/Gurktal, OB—Otztal/Bundschuh, KW—Koralpe/Wolz, TW—Tauern Window, G—Gneiss core of TW; SouthernAlps: ST—post-volcanic and Mesozoic cover, SB—Palaeozoic basement; ED—External Dinarides; ID—Internal Dinarides; AF—Adriatic foreland; TU—Tisza Unit. Thrusts and lineaments: NAT—North Alpine Thrust, NCT—Northern Calcareous Alps Thrust, SEMP—Salzach/Enns/Maria-Zell/Puchberg-Line,PAL—Periadriatic Lineament; SAT—South Alpine Thrust, CT—Cicarija Thrust, MHZ—Mid-Hungarian Zone.

part of the profile 89 single channel Texan–RefTek 125 receiverswere deployed with a 3-km average spacing, which recorded eightshots beginning with SP20010 in the SW and ending with SP29140in the NE. Along Alp04, 68 receivers with a 5.5-km average spac-ing recorded SP31140. These 68 receivers comprised 30 three-component recording units with 1-Hz seismometers, and 38 singlechannel Texan–RefTek 125 type with 4.5-Hz vertical geophones.The sampling rate was 0.01 s and the recording time window was

300 s for each shot. Table 1 summarizes information on the shotsused in the present study. SP31140 is the crossing point of theCEL10/Alp04 profile with two other profiles, Alp01 and Alp02(Bruckl et al. 2007). The northernmost shot used here (SP29140) islocated on profile CEL09 close to its intersection with CEL10. Thestandard shooting procedure in Austria was to employ 200–500 kgof explosives distributed in five to eight boreholes 30–50-m deep.Shots 26800 and 26801 were blasted at approximately the same

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Figure 2. Field layout of seismic experiments 2000–2003 in the area of Austria. Large stars with numbers are shot locations along CEL10 and Alp04 profiles.Table 1 provides detailed information about all used shotpoints. Smaller stars are shot locations along other CELEBRATION 2000, ALP 2002 and SUDETES2003 profiles. Receiver locations are shown by dots. The yellow bar shows the CEL10/Alp04 profile modelled in this paper; the grey diamond in the southwesternend of the profile shows ‘zero’ of profile with geographic coordinates ϕ = 46.1852◦N, λ = 13.0919◦E. Country symbols: A—Austria, CZ—Czech Republic,D—Germany, H—Hungary, HR—Croatia, I—Italy, SLO—Slovenia.

Table 1. Shotpoint data for profile CEL10/Alp04 in Austria; shot 26 800 was in a quarry and other 26 801 from thislocation was recorded.

h a.s.l. Time Offset ChargeShotpoint λ(◦E) ϕ(◦N) (m) (yyyy/mm/dd) hh/mm/ss (km) (kg)

31140 13.5460 46.5540 1170 2002/07/05 02:20:00.660 53.865 30020010 14.0303 46.9706 1500 2000/06/24 02:30:00.441 113.120 30020020 14.3250 47.2160 1410 2000/06/24 02:45:03.070 148.402 30020030 14.5148 47.4262 1448 2000/06/24 03:00:02.327 175.575 30026800 14.9126 47.5320 1140 2000/06/24 03:29:58.824 204.176 20026801 14.9126 47.5320 1140 2000/06/26 02:59:59.035 204.176 50020040 15.0952 47.8837 1035 2000/06/25 02:15:00.458 242.550 30020050 15.5951 48.3404 420 2000/06/25 03:44:59.730 305.417 30020060 15.8447 48.6033 350 2000/06/25 03:00:00.223 339.752 30029140 16.1564 48.6723 417 2000/06/26 02:30:00.396 360.374 300

location in an open pit mine using a production shooting scheme.The timing of the seismic shots was either done by automatic shoot-ing at GPS-controlled time or by recording the ignition current. Thefield records were cut to a length of 100 s starting at 0 s of reducedtime for a reduction velocity of 8 km s−1.

Generally, the P wavefield recorded on the CEL10/Alp04 profilehas a rather good signal-to-noise (S/N) ratio, but there are consid-erable differences between NE (Bohemian Massif) and SW (Alps).In Fig. 3 for shots SP20010 and SP29140 (recorded in Austria),examples of the full wavefield are shown for a reduced time from

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Figure 3. Example of trace-normalized vertical-component seismic record sections of P and S waves for SP20010 (left) and SP29140 (right) along profileCEL10. A band pass filter (2–15 Hz) has been applied. Pg, Sg—waves refracted from the basement; Pc, Sc—waves reflected from mid-crustal discontinuities;P∗, S∗—waves refracted from HV body in the middle crust; PmP, SmS—reflected waves from the Moho; Pcrustal, Scrustal—overcritical crustal waves; Pn,Sn—refracted waves from the Moho; PI —waves from the lower lithosphere. Pg# and Sg# show places of abrupt termination of Pg and Sg traveltime curvesrelated to low-velocity sediments. Reduction velocity = 8.0 km s−1.

0 to 40 s (reduction velocity 8 km s−1). All the data were used toderive the velocity model of the crust presented in Fig. 4; its accu-racy and resolution were tested for different phases (Fig. 5). Thoseparts of the sections documenting features in the crust are shown inFigs 6–10 and are discussed in the next sections.

Wavefield

Waves from the sedimentary cover (Psed) are observed in first ar-rivals only in the close vicinity of a few shotpoints, up to offsetsof about 10–20 km (e.g. SP20060 in Fig. 10). The main crustalphases (Pg, Pc) and Moho reflections (PmP) were correlated forall record sections, while the interpretation of refracted waves fromthe uppermost mantle (Pn) is sometimes hampered by low S/N ra-tio. The Pg phase can be observed up to 60–90 km offset in thesouthwestern part of the profile and up to 150 km in the northeast-ern part. Its apparent velocity ranges from 5.8 to 6.2 km s−1. Aninteresting feature is the abrupt termination of the Pg wave at aprofile distance of 240 km (marked by Pg# in the left section ofFig. 3 and in Figs 6 and 9). Mid-crustal waves (Pc, P∗) are usuallyrecorded along short distance intervals (20–50 km). The exceptionis the record section for the southernmost SP31140, where a strong

mid-crustal reflection (Pc) is followed by a 2–3-s long group of re-flected signals (in Fig. 8c in distance 100–160 km and reduced time4.5–6.5 s). The Moho reflections (PmP) are usually strong and wellcorrelated. Moho refractions (Pn) in the southern part of the profilewere recorded only fragmentarily, with weak arrivals, for example,for SP20010 (Fig. 3) and SP20060 (Fig. 10). In the northern partof the profile, Pn arrivals are very clear, for example, for SP20020(Fig. 9) and SP29140 (Fig. 3). Mid-crustal overcritical reflections(Pcrustal and Scrustal) are well developed in the northern part of theprofile (see e.g. SP29140 in Fig. 3), but are not observed for shotsin the Eastern Alps. As seen from Fig. 3, main P phases have theircounterparts in the observed S wavefield. Particularly, the north-ernmost shot SP29140 shows distinct S phases. Mantle lithosphericwaves (PI ) were observed only fragmentarily (Figs 8–10). The up-per mantle reflector is well resolved only between profile distances150 and 240 km, and its depth is about 70 km.

D E R I VAT I O N O F C RU S TA L M O D E LU S I N G A R AY- T R A C I N G T E C H N I Q U E

Detailed 2-D forward modelling of all refracted, reflected and post-critical phases identified in the correlation process was undertaken

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Figure 4. Compilation of seismic models along CEL10/Alp04 profile. At the top, topography along the profile is shown. Numbered triangles re-fer to shotpoints; shot with underlined number 26800 shows common position with shot SP26801. Crossing point with Alp01 and Alp02 pro-files is at SP31140. Vertical exaggeration Y /X is 4:1 for seimsic models, and 25:1 for topography. Depth in models is relative to sea level.(a) 2-D velocity model for CEL10/Alp04 profile obtained by forward ray-tracing modelling using the SEIS83 package (Cerveny & Psencık 1983b). Thethick solid lines are well-documented layer boundaries and dashed lines are continuations of boundaries needed in ray-tracing model; thin lines are P-waveiso-velocity contours in km s−1. Areas of missing ray coverage are marked by grey overlay. (b) Vertical section through the 3-D seismic model (by Behmet al. 2007). The thick black line indicates the Moho where it is constrained by data, a dashed line indicates the Moho obtained by interpolation from nearbydata values. Areas of missing coverage are white.

using a ray-tracing technique. The sometimes-difficult identifica-tion and correlation of seismic phases were done manually on acomputer screen using the ZPLOT software which allows scaling,filtering and reduction velocity (Zelt 1994; Sroda 1999). Computa-tions of traveltimes, ray paths and synthetic seismograms were per-formed using the ray-theory package SEIS83 (Cerveny & Psencık1983a,b) enhanced by employing the interactive graphical inter-faces MODEL (Komminaho 1997). The 2-D velocity model alongthe profile CEL10/Alp04 was successively altered by trial and error,and traveltimes were recalculated many times until agreement was

obtained between observed and model-derived traveltimes. A misfitof the order of 0.1–0.2 s was typical.

Apart of P waves, also S-wave modelling was performed forsome shotpoints along the CEL10/Alp04 by determining theVP/VS ratio for each layer. As S-wave traveltime data were notsufficient for independent modelling of VS structure, we kept thegeometry of the P-wave model (Fig. 4a) and fitted theoreticalS-wave traveltimes to the observed ones by adapting the VP/VS ratio.In the uppermost 2 km, we assumed VP/VS = 1.80. In the crystallinebasement with VP = 6.0–6.2 km s−1, the shear wave velocities are

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Figure 5. An assessment of model uncertainties from traveltimes calculated using the SEIS83 ray-tracing technique for SP20010. Thick lines marked by Pg,PmP and Sg are traveltimes calculated for the final ray-tracing model of the structure shown in Fig. 4a. The arrival times of the Pg phase with the velocityperturbed by +0.2 km s−1 and –0.2 km s−1 shown by lighter lines are clearly too early and too late in relation to recorded first breaks. This shows that thevelocity of the upper crust could be determined from the Pg wave with an uncertainty ±0.1 km s−1. The arrival time of PmP phase reflected from the Moho inthe final model (∼42 km depth) is shown together with arrival times with the depth perturbed by ±2 km marked by thinner lines. The sensitivity for estimationof the S-wave velocity in the upper crust was tested using Sg phase with VP/VS perturbed by ± 0.03. Early and late arrivals of the Sg phase are shown by lighterlines. See the text for further discussion. For this section, a bandpass filter (2–10 Hz) has been applied. Reduction velocity is 4.5 km s−1. In the inset, enlarged15 right side traces are shown with 1-s time bar.

relatively higher, and the modelled VP/VS is about 1.68. These valueswere well determined from good quality Pg and Sg wave traveltimes.For the deeper part of the model, a standard value VP/VS value of1.73 fits the observed S-wave traveltimes quite well. Particularly inthe range of the HV body in the middle crust, a VP/VS ratio of 1.73explains the observed P∗ and S∗ phases very well.

In addition to kinematic modelling, synthetic seismograms (in-cluding both P and S waves) were calculated to control velocitygradients within the layers and velocity contrasts across seismicboundaries. Finally, the synthetic seismograms show good qualita-tive agreement with the relative amplitudes of observed refractedand reflected waves.

The model derived for the structure along the profileCEL10/Alp04 is shown in Fig. 4a. The model shows large vari-

ations in the crustal structure, while the Moho topography changesin a relatively narrow depth interval of 38–44 km. P-wave velocitiesin the CEL10/Alp04 model agree with the models at cross-pointswith other profiles: Alp01 and Alp02 profiles in the south, beneathSP31140 (Bruckl et al. 2007) and with CEL09 profile in the north,beneath SP29140 (Hrubcova et al. 2005).

An assessment of the resolution and uncertainty of 2-Dray trace modelling

The resolution achieved by refraction and wide-angle reflectiontechniques was investigated when previous experiments such asPOLONAISE’97 and CELEBRATION 2000 were interpreted. Thecorresponding profiles were characterized by similar methodology,

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Figure 6. Amplitude-normalized seismic record sections for SP20020, SP20030 and SP26801 with theoretical traveltimes of P waves calculated for the crustalmodel derived using the SEIS83 ray-tracing technique: (a) synthetic seismograms, (b) record sections, (c) model and diagrams with chosen rays. Phases labelledas in Fig. 3. Numbers are P-wave velocities in km s−1.

source and receiver density and comparable data quality (Janiket al. 2002; Grad et al. 2003a, 2006). For the CEL10/Alp04 pro-file, an assessment of model uncertainties for calculated traveltimesusing the SEIS83 ray-tracing technique is illustrated in Fig. 5. Trav-eltimes of Pg, PmP and Sg phases, calculated for the 2-D model,are marked by thick lines. The arrival times of the Pg phase withthe velocity perturbed by +0.2 and –0.2 km s−1 shown by thin-ner lines are noteably too early and too late in relation to recordedfirst breaks. The arrival times of PmP phases reflected from theMoho in the final model (∼42 km depth) are shown together withtraveltimes calculated for the Moho being perturbed by ±2 kmdepth (marked by lighter lines). The sensitivity for estimation of theS-wave velocity in the upper crust was tested using Sg phaseswith VP/VS perturbed by ±0.03. Early and late arrivals of theSg phase are shown by thinner lines. This indicates that for the up-per crust (consolidated basement), where the coverage by Pg wavesis at its highest, uncertainty of velocity in the basement could be±0.1 km s−1. We expect a similar uncertainty in the uppermostmantle (for areas where Pn waves are well recorded). Althoughwaves refracted from the lower crust are very rarely observed inthe first arrivals, in many cases the situation improves because ofwell-recorded overcritical crustal waves (Pcrustal) that penetratethe lower crust (the uncertainty of velocity determination here isabout ±0.2 km s−1). The depths of mid-crustal boundaries are usu-ally determined with a depth uncertainty of ±2–3 km, and theMoho boundary with an even lower uncertainty of ±1–2 km, in thecase of good ray coverage. The misfit in Moho topography betweenthe new model 2-D (Fig. 4a) and the 3-D model (Fig. 4b) alongCEL10/Alp04 profile is smaller than ±2 km.

The accuracy of S-wave velocity, or rather VP/VS ratio, could beestimated as ±0.02 for the upper crystalline crust (determined fromPg and Sg waves), and ±0.03 or more for deeper parts of the modelwhere only envelopes of waves were correlated. In the case of low-quality data for shotpoints (record sections) with big noise, someareas could be of lower accuracy and resolution.

In the process of modelling, however, the limitations of the raytheory must be kept in mind. In addition, 2-D modelling does nottake into account out-of-plane refracted and reflected arrivals, whichmust have occurred particularly in such structurally complex areaas the Alps.

V E RT I C A L S E C T I O N T H RO U G H 3 - DS E I S M I C M O D E L A L O N G C E L 1 0 / A l p 0 4

Behm et al. (2007) used a 3-D approach to resolve the P-wavevelocity structure of the crust, Moho topography and sub-Moho ve-locity in the Eastern Alps and their surroundings based on the ALP2002 and CELEBRATION 2000 data. Their data set comprises ap-proximately 79 000 traces of which about 80 per cent are crosslinedata (Fig. 2). The CEL10/Alp04 profile is located at the centre oftheir investigated area and is well covered by rays with different az-imuths. The method is based on stacking and inversion techniquesdesigned for 3-D wide-angle reflection and refraction data. Stackingwas required since the S/N ratio was sometimes low in the EasternAlps (Behm 2006; Behm et al. 2007). The applied methods makeno difference between inline and crossline data and therefore ensurethat the whole data set is used. Stacking of diving waves through thecrust (Psed, Pg, Pcrustal) yields local 1-D traveltime curves, which

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Figure 7. Traveltimes of Pg wave for shotpoint SP31140 recorded in six directions along profiles Alp01, Alp02 and Alp04. Seismic record sections forSP31140 show slightly lower apparent velocities towards NE and ESE, and slightly higher towards S. In the traveltime plot, the vertical size of ellipsescorresponds to time error of first arrival picking: ±0.05 s (offsets 20–30 km), ±0.10 s (offsets > 30 km) and ±0.15 s (for very low S/N ratio).

are inverted for 1-D velocity models. These local 1-D models arecombined to give a 3-D velocity model. Stacking of Pn waves pro-vides velocities of the uppermost mantle and the Moho topographyexpressed as delay times. These delay times are combined with two-way traveltimes obtained from stacking of PmP waves. All modelsbased on stacking are supplemented by traditional methods basedon traveltime picks from the (single-fold) record sections whereverthe S/N ratio permits accurate picking.

The crust is represented by a smooth 3-D velocity distributionand the Moho is modelled as a first-order discontinuity. Comparedto standard procedures (e.g. 3-D tomography based solely on travel-time picks from record sections), the application of stacking resultsin models with large coverage but low resolution. In the following,

we describe the most important features of the 3-D model along avertical section following the CEL10/Alp04 profile (Fig. 4b).

Velocities in the upper crust (down to 10 km depth) of the South-ern Alps and around the PAL (profile distance 0–80 km) rangebetween 5.5 and 6.1 km s−1. The upper crust of crystalline unitsin the Eastern Alps (∼90–150 km) shows higher velocities (5.7–6.2 km s−1). Below the NCA and Flysch and Molasse basins(∼190–280 km), relatively low velocities (5.7–6.0 km s−1) arefound at a depth between 5 and 10 km, whereas the upper crustof the Bohemian Massif has moderate to high velocities again(5.6–6.2 km s−1). The middle crust (10–25 km depth) shows veloc-ities between 6.1 and 6.6 km s−1, and no significant lateral velocitycontrasts are found. However, the model also covers the lower crust

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Figure 8. Comparison of modelling results for SP31140. (a) Synthetic seismograms calculated using reflectivity method (Fuchs & Muller 1971) for 1-D,(b) synthetic seismograms of P waves calculated for the crustal model derived using the SEIS83 ray-tracing technique, (c) amplitude-normalized seismic recordsections with theoretical traveltimes for Pg, PmP and Pn waves and (d) model with chosen rays. Phases labelled as in Fig. 3. Numbers are P-wave velocities inkm s−1.

in the northern part of the profile where relatively high velocities(6.7–6.9 km s−1) are determined in the 25–35 km depth range. Theadjacent area in the vicinity of the Vienna Basin (to the east of thenorthern end of profile CEL01/Alp04) shows even higher velocitiesin these depths (7.0–7.2 km s−1).

The Moho has a depth of 35 km in the northernmost part of theprofile. To the south, it remains relatively flat below the BohemianMassif and starts to slope down below the NCA and reaches adepth of 43 km. Between profile distance 160 and 40 km, the Mohocannot be resolved along CEL10/Alp04, but is interpolated from

nearby data values. Only at the southernmost end, a relatively deepMoho (45 km) is observed.

D I S C U S S I O N O F S I G N I F I C A N TS T RU C T U R E S D E R I V E DF RO M P WAV E S

In the following section, we show seismic data, synthetic seismo-grams and ray diagrams indicating noteworthy structures of the

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Figure 9. Interpretation and modelling of record section for SP20020: (a) synthetic seismograms, (b) trace-normalized seismic record sections with theoreticaltraveltimes calculated for the 2-D ray-tracing model, (c) diagram with chosen rays. P∗—wave from the HV body in the middle crust; other phases labelled asin Fig. 3. Note termination of Pg phase at a distance of about 240 km and a strong Pn wave. Numbers are P-wave velocities in km s−1.

seismic model of the CEL10/Alp04 profile (Fig. 4a). We furtherrelate these structures to corresponding features of the 3-D model(Fig. 4b) to support the 2-D interpretation and to gain informationon the lateral extent alongside the profile.

Low-velocity zone dipping under NCAand Greywacke Zone

Waves from the sedimentary cover or other near surface strata withP-wave velocities ranging from 5.0 to 5.5 km s−1 are observed in theclose vicinity of a few shotpoints (e.g. SP29140). The corresponding

layer, about 2-km thick, can be followed along most of the profile.Clear arrivals of refracted and reflected P waves from the crystallinecrust are typically observed along the whole profile. Velocities in thecrystalline upper crust of the Bohemian Massif and Molasse basinare about 6.15 km s−1, slightly higher than in the Alpine area (about6.0 km s−1). An exception is the record sections in the central part ofthe profile, where Pg phases travelling from SW to NE (SP20020,SP20030, SP26801), terminate at a profile distance of 240 km.Their termination is abrupt and in the record sections marked byPg# (Figs 3 and 6). The corresponding shot-receiver distances are90 km for SP20020, 60 km for SP20030 and 40 km for SP26801.

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Figure 10. Interpretation and modelling of record section for SP20060: (a) synthetic seismograms, (b) trace-normalized seismic record sections with theoreticaltraveltimes calculated for the 2-D ray-tracing model, (c) diagram with chosen rays. P∗—wave from the HV body in the middle crust; other phases labelled asin Fig. 3. Numbers are P-wave velocities in km s−1.

We modelled this abrupt termination by a low-velocity zone (5.3–5.6 km s−1) dipping under NCA and Greywacke Zone down to adepth of about 8 km. It is difficult to constrain velocity and thicknessof this ‘low-velocity body’, which is surrounded by basement withhigher velocities. However, its top is well constrained by reflectedwaves, and particularly from termination points of refracted wavesin the crystalline basement Pg#. Synthetic seismograms (Fig. 6a)and ray diagrams (Fig. 6c) of shots SP20020, SP20030, SP26801prove the consistency of our model with seismic data. Further tothe SW, we introduce a nearly horizontal reflector at about a 10 kmdepth to explain crustal reflections observed in the distance interval

of 120–160 km. A similar shape of the south dipping low-velocityzone was interpreted along the profile CEL10/Alp04 by Hrubcovaet al. (2008) and along profile Alp01 (about 150 km to the west) byBruckl et al. (2007). Corresponding low velocities are also obtainedfrom the 3-D model where they extend at least 150 km to the west.

Basement and middle crust around PAL

Shotpoint 31140 in the southwestern part of the profile wasa common point for three profiles: Alp01, Alp02 and Alp04.It gives the opportunity to compare Pg wave arrivals in six

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different directions, with an azimuth step of about 60◦ (Fig. 7).To enhance Pg first arrivals, the record sections are shown withvery big amplification, even if later arrivals are overamplified. Thedifference between the traveltimes is not large, but traveltimes ofPg waves show slightly lower apparent velocities towards NE andESE, and slightly higher towards south. The maximum arrival timedifference at distances 50–80 km is only about 0.3–0.4 s (less than3–4 per cent). This observation indicates that anisotropy in the up-per crust is not significant. In the neighbouring areas of the TauernWindow, the Bohemian Massif in the Czech Republic and in themarginal zone of the EEC in SE Poland, the anisotropy of the uppercrust can reach 5–10 per cent (Ru

◦zek et al. 2003; Luschen et al.

2004; Vavrycuk et al. 2004; Bleibinhaus & Gebrande 2006; Sroda2006).

In the southwestern part of the CEL10/Alp04 profile, a strongmid-crustal reflection (Pc) marks the beginning of a 2–3-s longzone of ‘ringing’ reflections (SP31140 in Fig. 8c). Within this zone,amplitudes are higher than for Pg waves and a few individual ‘re-flections’ could be correlated. Between the end of reflections andPmP arrivals, no strong arrivals are observed, which could be anindication of a more homogeneous lower crust in this part of theprofile. The reflective zone observed in the record section was mod-elling using 2-D ray-tracing technique (Cerveny & Psencık 1983a,b;Fig. 8b) as a laminated middle crust with alternating high (6.4 and6.6 km s−1) and low velocities (6.3 and 6.4 km s−1), and a thick-ness of about 2–3 km for each lamella. However, in the ray-tracingsynthetic seismograms (Fig. 8b), we do not observe some reflectedphases with their characteristic coda. Therefore, we used 1-D reflec-tivity method (Fuchs & Muller 1971; Fig. 8a) to simulate variabilityof the middle crust and transition to the Moho discontinuity. The1-D velocity model used for the reflectivity method represents theaverage of the 2-D ray-tracing model between the distances of 50and 100 km. Both synthetic sections in Fig. 8 reproduce the ringingreflections as observed in the record, and reflectivity method showscomplexity of the wavefield not visible in the ray-tracing synthetics.The lower crust in this part of the model seems to be more homo-geneous, and a velocity of about 6.9 km s−1 is derived from PmPwaves.

Further indications for increased crustal reflectivity in thispart has also been reported from receiver function analyses on30 passive monitoring stations deployed along the Austrian partof the CEL10/Alp04 profile during the ALP 2002 experiment(Rumpfhuber & ALP 2002 Working Group 2004).

Middle and lower crust of the European plate

In the central and NE parts of the profile (distance interval ∼170–320 km), an HV body of a few km thickness is delineated (VP ∼7 km s−1) in the depth interval 18–23 km. It is constrained usingreflected (Pc) and refracted (P∗) waves from several shotpoints(e.g. SP20020 and SP20060; Figs 9 and 10). At the northern endof the profile, an HV lower crust is found, with velocities of 7.2–7.4 km s−1 in the depth interval 26–40 km. The estimation of thesehigh velocities is based on refracted waves (secondary arrivals) inthe offset range 200–250 km (e.g. SP 26801). The HV lower crustinferred from 2-D modelling at the northern end of the profile is alsoclearly visible in the 3-D model where it extends to the southeast(Fig. 11a).

Fragmentation of the Moho

PmP reflections from SP31140 with depth points around a pro-file distance of 100 km are observed as a strong wave (SP31140;

Fig. 8c). Further to the NE, the Moho reflections, for example,from SP20020 (Fig. 9b) and SP20060 (Fig. 10b), are clear and formabout 2-s long wave groups. Their midpoints are situated in thedistance range of about 150–250 km. In this part of the profile andfurther to NE, the Moho is also well constrained by Pn phases.The crustal thickness along CEL10/Alp04 changes from about 44–42 km in the SW (Alpine part) to about 40 km in the centralpart of the profile (Molasse basin) and to 38–40 km in the NE(Bohemian Massif). The velocity in the uppermost mantle deter-mined from Pn wave traveltimes is about 8 km s−1 along the wholeprofile length.

The southward directed dip of the Moho is interrupted by a bulgeat a profile distance of 100 km which was modelled using reflectedwaves from SP31140 (Fig. 8). A projection of the profile onto theMoho map (Fig. 11b; Behm et al. 2007) shows that CEL10/Alp04crosses three Moho fragments that are the European, Pannonian andAdriatic Moho. Thus, the bulge in the 2-D model is interpreted asthe westernmost tip of the Pannonian Moho fragment. Along theEuropean Moho, where both the 2-D and the 3-D models provideMoho depths, the difference (3-D subtracted by 2-D) is –1.1 ±0.8 km.

T E C T O N I C I N T E R P R E TAT I O N

The area of the CEL10/Alp04 profile is characterized by a com-plex tectonic setting. The profile begins in the Po plain near thenorthwestern corner of the External Dinarides and continues north-eastwards over the Southern and Eastern Alps to the Flysch beltand the Molasse basin. It passes the central range of the EasternAlps about 20 km east of the Tauern Window. Finally, it reaches thesoutheastern border of the Bohemian Massif near the Moldanubian–Moravian thrust. According to the Moho map constructed by Behmet al. (2007), the profile crosses the triple point of the Europeanplate, Adriatic microplate and Pannonian fragment (Fig. 11b). Thetectonic interpretation of the seismic model emphasizes structuresand processes relevant for the Alpine orogenesis. Because of theorientation and location of the profile, crustal structures mainly cor-respond to a classic collision orogen (e.g. Moores & Twiss 1995;Hatcher & Williams 1986), but are also affected by extrusion andescape tectonics (e.g. Ratschbacher et al. 1991). Down to a depthof 5 km, the tectonic model was constrained by the geological pro-file (Fig. 1), while the interpretation of deeper structures relies onfirst-order discontinuities and significant velocity anomalies of boththe 2-D and 3-D seismic models. The intersection of CEL10/Alp04with the profiles Alp01 and Alp02 at the PAL (Bruckl et al. 2007)and with profile CEL09 (Hrubcova et at. 2005) in the BohemianMassif provides further constraints.

What follows is a detailed description of the tectonic model(Fig. 12), starting with the upper crust. We discriminate betweennew information from CEL10/Alp04, already existing informationfrom the 3-D seismic model and intersecting profiles, and ratherspeculative interpretive features. A low-velocity zone dipping un-der NCA and Greywacke Zone is resolved by the new seismic data(Fig. 6). A comparison with the geological profile shows that thelower boundary of the low-velocity zone coincides with the crys-talline basement of the Bohemian Massif, dipping below Molasseand overthrusted by the Alpine accretionary wedge. The horizontalreflector around a profile distance of 150 km at 10 km depth furtherdelineates the decollement of Austroalpine nappes. We constrainthe extent of the upper European crust to the south by a long andcontinuous velocity structure dipping from 7 km depth below the

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Figure 11. (a) Comparison of average P-wave velocities between 28- and 32 km depth from 2-D model (bar along CEL10/Alp04), and a depth slice at 30 kmthrough the 3-D velocity model by Behm et al. (2007). (b) Moho surface and superimposed depth contours from the 3-D model by Behm et al. (2007). Boldyellow lines separate European, Adriatic and Pannonian plates. Bars indicated with (1) and (2) show the Moho depths obtained from 2-D ray tracing: (1) isassociated with the European Moho and (2) is the protuberance (see the text) and correlates with the Pannonian Moho.

Figure 12. Tectonic interpretation of the CEL10/Alp04 profile based on the seismic model (Fig. 4a) with geological cross-section from Fig. 1 (down to 5 kmdepth). Solid lines show seismic boundary elements or significant velocity structures mapped by reflected and/or refracted waves; chessboard areas are HR orHV zones. Bold dashed lines separate European and Adriatic plates. PAN marks Pannonian fragment. S-TR—Sub-Tauern ramp.

Bohemian Massif at a profile distance of 300 km to 15 km belowthe Eastern Alps. Back-thrusting of the upper Adriatic crust hastaken place along the SAT. We may hypothetically extend the thrustdown to the southern termination of upper European crust, thusfollowing the scheme of a bivergent orogen (Hatcher & Williams1986). The high-reflectivity (HR) zone in the middle crust aroundPAL, an area of crustal thickening, has been correlated with theindenting Adriatic crust. The genesis of this reflective zone willbe discussed later in more detail. The elongated HV body at about20 km depth within the European crust may either be an ophiolite,a felsic–mafic, or a mafic intrusion. The Moldanubian–Moravian

thrust is very close to and subparallel to the CEL10/Alp04 profilein this area and ophiolite emplacement or magmatic activity is doc-umented close to this suture. An HV middle crust was also foundwithin the Bohemian Massif along profiles Alp01 (Bruckl et al.2007) and CEL09 (Hrubcova et al. 2005).

The European Moho has been resolved by Pn and PmP phasesfrom profile distance 150 km to about 320 km. It dips slightly tothe south from 39 km at a profile distance of 300 km to 43 kmat a profile distance of 150 km. The overthrusting Eastern Alpinenappe stack causes this overall crustal thickening. However, theEuropean crust, represented by the crystalline basement, thins out

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from 40 km in the north (profile distance 300 km) to 32 km in thesouth (profile distance 170 km). A similar observation was madealong Alp01 (Bruckl et al. 2007) and we again attribute it to anextensional phase of the European platform prior to the Alpineorogenesis (Roeder 1977).

Post-collisional escape and crustal extension generated the Pan-nonian fragment and lead to a Moho uplift (Behm et al. 2007;Bruckl et al. 2007). Along CEL10/Alp04 at profile distance100 km, the westernmost part of the Pannonian Moho is sampled byPmP reflections from the southwestern side. The Adriatic Moho isnot revealed by seismic data along profile CEL10/Alp04. However,this area is well resolved by profiles Alp01 and Alp02 (Brucklet al. 2007). We introduce the Moho topography and the Sub-Tauern ramp (S-TR; TRANSALP Working Group 2002; Luschenet al. 2006) from this tectonic interpretation into our model. Inour interpretation of CEL10/Alp04, the S-TR does not continueup to the surface, because the SW–NE striking profile leavesthe domain of the Tauern Window at a profile distance of about100 km.

We mapped a laminated zone with HR in the middle crust andinterpreted it as the indenting wedge of the Adriatic plate. The com-parison of the recordings of SP31140 along profiles Alp01, Alp02and Alp04 (Fig. 7) shows that a reflective zone is only observedalong the Alp04 profile to NE. Along the profiles Alp01 and Alp02,only single floating reflectors were interpreted (Bruckl et al. 2007).The HR zone is located in a very specific tectonic setting betweenthe Tauern Window in the west and the emerging Pannonian frag-ment in the east, a circumstance which may be in related to itsorigin.

One cause of HR could be lamination due to ductile exten-sional deformation as generally observed in the lower crust bysteep-angle reflection seismic profiles (Meissner & Kusznir 1987;Meissner et al. 2006). However, in our case the HR zone is restrictedto the middle crust and was not observed along profile Alp02 furtherto the east, where the crust had been strongly influenced by exten-sion due to tectonic escape (Bruckl et al. 2007). Another explanationcould be intermediate to mafic intrusions. These intrusions couldhave been triggered by a temperature pulse caused by the roughlycontemporaneous vertical extrusion of the Tauern Window by morethan 25 km (Fugenschuh et al. 1977) in the west, or the Moho upliftof the Pannonian fragment relative to the European and Adriaticplates in the east.

The suture between Adria and Europe has been drawn inFig. 12 partly on the basis of the geologic profile and our newseismic data, partly hypothetically or by the integration of evidencefrom intersecting profiles, especially Alp01. The Molasse and Fly-sch belts separate Adria and Europe at the North Alpine front. Wecannot decide from our data if overhrusted Flysch wedges out underthe NCA, or if ophiolites (Penninicum) exist on top of the decolle-ment at about 10 km depth and around profile distance 150 km.These units are exposed in the TW which has its eastern termina-tion about 50–80 km west of profile CEL10/Alp04. The suture atmid-crustal level represents a crocodile structure with the HR zone(HR in Fig. 12) interpreted as the indenting Adriatic wedge. In thelower crust, we are very near to the intersecting Alp01 profile andsouth of the eastern termination of the TW. Here, we introducedthe S-TR representing the suture zone in the lower crust. The S-TR is a major structure over which upfolding and extrusion of theTW took place (e.g. TRANSALP Working Group 2002; Luschenet al. 2004). However, neither this structure nor the Adriatic Moho isconstrained by the CEL10/Alp01 data. Therefore, we put a questionmark to zone of our tectonic model in Fig. 12.

C O N C LU S I O N S

In general, the interpretation of the seismic data along profileCEL10/Alp04 agrees well with structures revealed by a 3-Dmodel (Behm et al. 2007). The Moho depths derived at profileCEL10/Alp04 using 2-D ray-tracing technique are a good matchto the Moho depths of the 3-D model (–1.1 ± 0.8 km) and arein accordance with the conception of a Pannonian fragment. Itbenefits from the information gained by the profiles Alp01 andAlp02 at their crossing points with Alp04 near PAL. Adriatic Mohocould not be modelled by CEL/Alp04 data. Therefore, our newmodel cannot contribute to a clarification, if in the area of the TWEuropean lithosphere was subducted below Adriatic lithosphere, orif the reverse is true (e.g. Kissling et al. 2006). The decollement ofthe Austroalpine nappe stack over Flysch, Molasse and Europeanbasement has been revealed down to a maximum depth of 10 km.Thinning of European basement crust from the Bohemian Massif tothe south below Austroalpine nappes is in accordance with similarfindings along Alp01 (Bruckl et al. 2007). The suture between theEuropean plate and the Adriatic microplate was interpreted on thebasis of a 170-km long, coherent upper crustal velocity structure inthe European crust and an HR zone in the Adriatic crust. An HVbody in the middle European crust and an HV lower crust under theBohemian massif correlate with an HV zone derived from the 3-Dseismic model. The reflector in the middle crust may be related toMoldanubian–Moravian overthrusting during the Variscian orogen-esis. The HV lower crust extends from CEL10/Alp04 to the Viennabasin and the northwestern Pannonian basin. The most likely anexplanation of its generation has to be given within the context ofthe roll back of the Carpathian arc and the generation of the Viennaand Pannonian basins. However, this is beyond the scope of thisstudy.

A C K N OW L E D G M E N T S

The CELEBRATION 2000 and ALP 2002 experiments were madepossible by the scientific and financial contribution from 15 coun-tries: Austria, Belarus, Canada, Croatia, Czech Republic, Denmark,Finland, Germany, Hungary, Poland, Russia, Slovakia, Slovenia,Turkey, USA. It is only through the participation of almost 1000individuals that the preparation, field work, data acquisition andprocessing in both experiments could be successfully completed.We further extend our gratitude to Johanna Bruckl for compiling ageological map and cross-sections. The authors are grateful to twoanonymous reviewers for helpful and exhaustive comments.

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Crustal structure of the Eastern Alps and their foreland: seismic model beneath the CEL10/Alp04...

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