Introduction

The TRANSALP project is an international and multidisciplinary research program for investigating orogenic processes driven by the collision of continental lithospheric plates. The target area of the Eastern Alps shows the colliding of the Adriatic-African plate with the southward subducting European plate. Structures and orogenic processes of the Eastern Alps are considered paradigmatic for testing plate-tectonic models of continent-continent collision. The project is also motivated by the insight that problems of great practical interest such as earthquake risk assessment or the hydrocarbon potential of the Alps can only be solved with a well-founded understanding of the geodynamic processes, which built the Alps and which are partially still active.

The project consists of two main parts:

1. Active and passive seismological experiments
2. Accompanying multidisciplinary geoscience projects

The backbone of part (1.) is an almost 340 km long near-vertical reflection profile between Freising in the north and Treviso in the south (s. Fig.1). It covers the Eastern Alps and parts of its northern and southern Molasse Basins. The technique of near-vertical reflection seismics - developed for hydrocarbon exploration in sedimentary basins - has become the most powerful tool in geoscience for investigating lithospheric structure and its history of origins.

Refraction seismics and seismological investigations are further subprojects of (1.) within an integrated framework closely related to the near-vertical reflection seismics.

A large number of accompanying geoscience projects (project part (2.)) are planned in partaking countries and have been partly granted to support TRANSALP by geological, petrological and geochronological investigations.

TRANSALP is a joint venture of researchers in the following national research groups:

• Germany: GFZ Potsdam in cooperation with IfAAG Munich and other partners
• Austria: IfG Leoben and partners
• Italy: CROP (Crosta Profonda; CNR and ENI/AGIP) and partners
• Switzerland: ETH Zürich/NFP20

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Initial scientific objectives

Geology and anticipated seismic key structures as proposed before the start of the TRANSALP project:

Based on the surface geology shown in Fig. 2, two tentative cross sections through the Eastern Alps (following approximately the line marked in Fig. 2) were determined (Fig. 3).

These cross sections were unproven extrapolations from the Western Alps (Lammerer, unpublished). Expected seismic key structures and possible reflecting horizons are marked by red italic numbers , indicating the following features:

1. The European lower crust - often strongly reflective: how far towards the north has it been overprinted due to Alpidic orogeny and basin formation? How far to the south can it be traced beneath the Alps? Are there differences between European and Adriatic lower crustal structures and physical properties?
2. The crust-mantle boundary (Moho) of both lithospheric plates: sometimes visible by reflected seismic signals, better defined by refracted signals; the Moho has to be traced down to unusually great depths, particularly the European Moho eventually beneath the Po plain.
3. Sediments of unknown thickness, which have been overthrust by the Northern Calcareous Alps: they may contain oil and gas resources; similar situations at the northern and southern margins of the Alps.
4. Basement rocks beneath the Northern Calcareous Alps: unproved interpretations proposing palaeozoic crystalline rocks or mesozoic sediments respectively.
5. Steeply dipping structures of the crystalline Tauern Window within the central part of the Eastern Alps, involving substantial differences compared to the Western Alps.
6. A possible Adriatic wedge-shaped indenter, similar to that one proposed in the Western Alps.
7. Tectonic shear zones in the lower crust at the contact of the Adriatic and the European plate.
8. Northward dipping back-thrusting faults, including the Periadriatic line; possibly related to the Adriatic indenter.
9. Back-thrusting at the southern margin of the Alps and its relationship to the earthquake activity.

The European lithosphere is dipping southward with an angle of approx. 3 to 5°. Challenging questions are the following:

• how far does the European plate dip towards the south?
• where is the boundary between the European and Adriatic plates?
• does the collision cause a stacking of lithospheric plates?
• is there any sub-vertical subduction of both lithospheric plates, eventually?
• are both lithospheric plates meshed together in a complicated way?
• which structural differences between western and eastern Alps do exist?

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Subprojects

The scientific objectives required high structural resolution in the upper crust, similar to industrial exploration, including steeply dipping and complicated 3D structures. Furthermore, penetration of seismic signals deeper than 70 km was required to test existing geodynamic models. These challenging tasks were addressed in several subprojects:

Near-vertical seismic reflection profiling

This was the backbone of TRANSALP: an almost 340 km long near-vertical reflection profile (main line) was measured in combination of high-fold vibroseis for imaging the upper crust and low-fold high-energy explosive sources to get images down to the upper mantle. The design followed closely very successful examples of former deep seismic profiling in the Western Alps (Pfiffner et al., 1997).
The survey was commissioned to a contractor company and was realised in two field campaigns in 1998 and 1999, both in late summer to autumn. The alpine summit (Zillertaler Alpen) cannot be traversed directly. This gap (approx. 7-10 km) had to be filled in an appropriate way by undershooting from both sides. The acquired data belongs to the international TRANSALP Working Group and is processed by their member institutions.

In addition, the near-vertical incidence seismic reflection measurements were combined with several seismological supplementary experiments conducted by personnel and instruments of universities and research institutions. These experiments used the energy sources of the contractor and were to achieve 3-dimensional control on key structures along the main line, better control on seismic velocities at great depths and knowledge of the recent tectonic stress regime. Further details on the subprojects:

Stationary cross lines

The TRANSALP line had been designed to fulfill 2D-conditions as far as possible; important faults were crossed perpendicular. In view of the complicated structure of the Eastern Alps, however, precautions for 3D-control by cross line observations had been taken. Several stationary cross line surveys were conducted, each with a spread of 20 km. These spreads were operated in slave mode to record all dynamite and as much vibrator points as possible from the main line. Two cross lines were active at the same time. Thereby it was possible to obtain single-fold coverage in a continuous 10 km wide strip around the main line (cf. Fig. 4a).

In addition, two shot points were located at the ends of each cross line. They were fired three times each and recorded by the main line spread to provide single fold CMP lines parallel to the main line (cf. Fig. 5).

Mapping of the Moho

The main line's explosion sources were recorded by a mobile array of 3-component stations operating at 80 to 130 km offset from the actual shotpoint. This provided a continuous constant-offset mapping of the Moho and the lower crust by wide-angle reflections and refractions suitable for velocity determinations and imaging complementary to near-vertical reflection profiling.

Passive monitoring of local seismicity and earthquake tomography

Numerous 3-component stations were deployed for several months in a roughly 20 km wide strip around the TRANSALP profile for monitoring the local and regional seismicity, for analyzing the seismotectonic stress field and for teleseismic purposes.

Controlled source tomography

During the phases of active seismic experiments the station network was extended and reinforced by additional 3-component stations at low-noise sites to record the seismic sources (vibrations and explosions) to as large distances as possible. The data was used for tomographic reconstruction of the crustal p-wave velocity field.

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Acquisition parameters

The East-Alpine Reflection Seismic Traverse

Download detailed acquisition parameters as .pdf files:

• Field Campaign 1998 (TRANSALP'98-N & TRANSALP'98-S)
• Field Campaign 1999

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Final Conference 2003

Selected extended abstracts from the Conference Proceedings:

Helmut GEBRANDE and TRANSALP Steering & Technical Committees
The TRANSALP Project: Concept and Main Goals

Ewald LUESCHEN, Helmut GEBRANDE, Karl MILLAHN and Rinaldo NICOLICH
Seismic profiling by the TRANSALP working group: Deep crustal Vibroseis and explosive seismic profiling

Karl MILLAHN, Ewald LUESCHEN and Helmut GEBRANDE
Seismic profiling by the TRANSALP working group: Cross-line recording for 3-D control

Florian BLEIBINHAUS
Seismic profiling by the TRANSALP working group: Refraction and wide-angle reflection seismic traveltime tomography

Joern KUMMEROW, Rainer KIND, Onno ONCKEN, Kurt WYLEGALLA, Frank SCHERBAUM
Seismic profiling by the TRANSALP working group: Receiver functions image and Upper Mantle anisotropy

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Final Volume 2006:
TRANSALP - A Transect Through a Young Collisional Orogen
(Tectonophysics, Special Issue, Vol.414, Iss.1-4, p.1-282)

Contents

Editorial

TRANSALP - A transect through a young collisional orogen: Introduction
H. Gebrande, A. Castellarin, E. Lüschen, K. Millahn, F. Neubauer and R. Nicolich (p.1)

Research Papers

TRANSALP - deep crustal Vibroseis and explosive seismic profiling in the Eastern Alps
E. Lüschen, D. Borrini, H. Gebrande, B. Lammerer, K. Millahn, F. Neubauer, R. Nicolich and TRANSALP Working Group (p.9)

TRANSALP - Cross-line recording during the seismic reflection transect in the Eastern Alps
K. Millahn, E. Lüschen, H. Gebrande and TRANSALP Working Group (p.39)

Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography
F. Bleibinhaus and H. Gebrande (p.51)

Wide-angle observations of ALP 2002 shots on the TRANSALP profile: Linking the two DSS projects
F. Bleibinhaus, E. Brückl and ALP 2002 Working Group (p.71)

Reviewing pre-TRANSALP DSS models
R. Cassinis (p.79)

Shallow high-resolution seismics and reprocessing of industry profiles in southern Bavaria: The Molasse and the northern Alpine front
R. Thomas, K. Schwerd, K. Bram and J. Fertig (p.87)

Experimental and texture-derived P-wave anisotropy of principal rocks from the TRANSALP traverse: An aid for the interpretation of seismic field data
K. Ullemeyer, S. Siegesmund, P.N.J. Rasolofosaon and J.H. Behrmann (p.97)

Shear wave splitting in the Eastern Alps observed at the TRANSALP network
J. Kummerow, R. Kind and TRANSALP Working Group (p.117)

New gravity maps of the Eastern Alps and significance for the crustal structures
C. Zanolla, C. Braitenberg, J. Ebbing, M. Bernabini, K. Bram, G. Gabriel, H.-J. Götze, S. Giammetti, B. Meurers, R. Nicolich and F. Palmieri (p.127)

The lithospheric density structure of the Eastern Alps
J. Ebbing, C. Braitenberg and H.-J. Götze (p.145)

A review of the thermal regime of the Eastern Alps with respect to the effects of paleoclimate and exhumation
H.-D. Vosteen, V. Rath, C. Clauser and B. Lammerer (p.157)

Paleomagnetic evidence for large en-bloc rotations in the Eastern Alps during Neogene orogeny
W. Thöny, H. Ortner and R. Scholger (p.169)

From Middle Jurassic heating to Neogene cooling: The thermochronological evolution of the southern Alps
M. Zattin, A. Cuman, R. Fantoni, S. Martin, P. Scotti and C. Stefani (p.191)

The Alpine evolution of the Southern Alps around the Giudicarie faults: A Late Cretaceous to Early Eocene transfer zone
A. Castellarin, G.B. Vai and L. Cantelli (p.203)

Structural synthesis of the Northern Calcareous Alps, TRANSALP segment
J.H. Behrmann and D.C. Tanner (p.225)

Kinematics of the Inntal shear zone-sub-Tauern ramp fault system and the interpretation of the TRANSALP seismic section, Eastern Alps, Austria
H. Ortner, F. Reiter and R. Brandner (p.241)

Structure of the lithosphere beneath the Eastern Alps (southern sector of the TRANSALP transect)
A. Castellarin, R. Nicolich, R. Fantoni, L. Cantelli, M. Sella and L. Selli (p.259)

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Selected publications

Kummerow, J., Kind, R., Oncken, O., Giese, P., Ryberg, T., Wylegalla, K., Scherbaum, F. and TRANSALP Working Group, 2004. A natural and controlled source seismic profile through the Eastern Alps: TRANSALP. Earth Planet Sci. Lett. 225, 115-129, doi:10.1016/j.epsl.2004.05.040.

Lippitsch, R.,Kissling, K., and Ansorge, J., 2003. Upper mantle structure beneath the Alpine Orogen from high-resolution teleseismic tomography. J. Geophys. Res. 108 (B8), 5.1-5.15, doi:10.1029/2002JB002016.

Lüschen, E., Lammerer, B., Gebrande, H., Millahn, K., Nicolich, R. and TRANSALP Working Group, 2004. Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling. Tectonophysics 388, 85-102.

TRANSALP Working Group, 2001. European Orogenic Processes Research Transects the Eastern Alps. Eos 82 (40), 453, 460-461.

TRANSALP Working Group, 2002. First deep seismic reflection images of the Eastern Alps reveal giant crustal wedges and transcrustal ramps. Geophys. Res. Lett. 29 (10), 92.1-92.4, doi:10.1029/2002GL014911.