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Tectonophysics 414

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, Germanyb 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 three-

component 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

0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2005.10.027

* Corresponding author. Present address: Department of Earth and

Environmental Sciences, Geophysics Section, Theresienstr. 41, D-

80333 Munich, Germany. Tel.: +49 89 2180 4202; fax: +49 89

2180 4205.

E-mail 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 intracrus-

tal shear zones related to collision, and a Moho geom-

etry 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. Pfiff-

ner (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.

(2006) 71–78

Fig. 1. Map of ALP 2002 seismic sources (1) used in this investigation and ALP12/TRANSALP three-component stations (z). The inset displaysall 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.

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–7872

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–78 73

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 direc-tion. Ray parameters were computed with a pseudo-

bending, two-point ray tracer integrated into the inver-

sion scheme (Um and Thurber, 1987; Bleibinhaus,

Fig. 2. Observations and modelled ray paths for selected shots recorded on t

distance from the profile. SP 203 (c), 113 (d) and 114 (e) are located 80, 110 a

profile coordinate. Ray paths are displayed in a N�S section (middle) andReflection points of the computed Moho reflections are indicated by black d

computed from the TRANSALP model (Bleibinhaus and Gebrande, 2005—t

a steeper dip of the AM. The model, ray paths and reflection points displayed

of Moho reflections are relatively clear for the EM, but ambiguous for the

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

he TRANSALP/ALP12 line. SP 201 (a) and 202 (b) are within 20 km

nd 120 km, respectively, towards the east. Seismograms are plotted vs.

in a top view (bottom). EM—European Moho, AM—Adriatic Moho.

ots in the maps. Black dots in the seismic sections denote travel times

his issue) and hollow dots mark travel times computed in a model with

for SP 113 and 114 correspond to this steeper dip. Phase correlations

AM in some sections.

Fig. 2 (continued).

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–7874

account for these differences. However, based on obser-

vations of shear wave splitting and azimuthal variations

of average first-break velocities, Bleibinhaus and Geb-

rande (2005—this issue) found an anisotropy value of

10% in the upper 2�3 km of the western TW with thefast axis oriented E�W. This anisotropy can explain thedeviations 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 timescorresponding 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 devia-

tions 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 directionthrough 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

Fig. 2 (continued).

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–78 75

including other ALP 2002 lines will provide further

insights with respect to this ambiguity.

Strong intracrustal reflection observations at profile

coordinate 30�70 km from SP 201 (Fig. 2a) emphasizethe importance of a reflector below and south of the

Northern Calcareous Alps (NCA). It was previously

interpreted as the basal Austroalpine thrust fault, com-

pensating N�S convergence (Bleibinhaus and Geb-rande, 2005—this issue).

3. Lower crustal structure

Lower crustal structure is constrained only by

reflections from the crust–mantle boundary. The

Fig. 2 (continued).

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–7876

Fig. 2 (continued).

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–78 77

F. Bleibinhaus, E. Brückl / Tectonophysics 414 (2006) 71–7878

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

40�60 km east of TRANSALP. This indicates thatthe structure of the EM does not change significantly

between 128E and 12.78E.Reflections from the Adriatic Moho (AM) are ob-

served 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 ap-

proximately 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 maximumdepth of 46 km at its northern edge (ca. 46.68N–12.78E). A deviation of computed and observedMoho 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 pro-

file. 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 com-

putation in the extrapolated TRANSALP velocity

model yields preliminary results regarding lateral con-

tinuity. 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 sub-

duction 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, Fin-

land, Germany, Hungary, Poland, Slovenia and the

USA. Seismological stations for the line ALP 12 were

provided by the Geophysical Instrument Pool Potsdam.

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