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www.elsevier.com/locate/tecto
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: [email protected]
(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