ALP 2002 - A seismic 3D-model of the Eastern Alps

The development of the Eastern Alps during the Cenozoic period represents a geodynamically interesting situation. We have a compression in the West (Brenner) and a transition to an extrusion to the Pannonian basin in the East. Not only the formation of the Tertiary basins but also actual seismicity may be interpreted in this sense. However, a thorough geophysical exploration of the lithosphere of the Eastern Alps is indespensable for a deeper understanding of these tectonic processes. Therefore the goal of this project is the preparation of a seismic 3D-model of the Eastern Alps. At the western border of the investigation area the TRANSALP profile elucidates the upper lithosphere from Munich to Verona. In the East a clear image of the lithosphere cannot be derived from the availabale seismic data. Certainly, a substantial improvement of the situation came from CELEBRATION 2000, a refraction/wide-angle reflection seismic experiment carried out by an international working group with Austrian participation. It focused on the lithosphere of Central Europe, the Pannonian basin and the northern part of the Eastern Alps. The investigation of the southern part of the Eastern Alps is still missing. Within the scope of this project it is planned to close this gap down to the Dinarides and the Po plane in cooperation with partners of the CELEBRATION 2000 team and new teams from Croatia, Italy and Slovenia in summer 2002. Approximately 1000 mobile seismic stations will be available without costs owing to international participation. The Austrian Academy of Sciences is ready to cover the costs for the seismic shots. Modern methods of ray tracing and tomographic inversion will be applied for inverting the data. The demand of a 3D-model requires the application of a high performance computer to which access is permitted by Vienna Univerity of Technology. Furthermore, dynamic information of the seismic waves will be used. The result will directly contribute to the solution of geodynamic problems (f.e. actual stress field and neotectonic activity) and will constitute the basis for later investigations with higher resolution (f.e. reflection seismology) or deeper penetration (f.e. passive seismic experiments).

ALP2002 is a large, international 3D seismic refraction and wide-angle reflection experiment, which focuses on the tectonics and geodynamics of the Eastern Alps. It provides new information on crustal structure and geological processes caused by the N-S directed collision between Europe and Africa and the subsequent tectonic escape of parts of the Eastern Alps to the Pannonian basin in the East. ALP2002 was designed to build on the earlier TRANSALP and CELEBRATION 2000 efforts and to provide comprehensive seismic coverage in the Eastern Alps. The ALP2002 data acquisition was undertaken in July 2002 when 947 seismic stations, deployed along 13 lines with a total length of 4,300 km, recorded the seismic waves of 38 seismic shots up to 600 km distance. A cooperation of geophysicists from 11 countries made the realization possible. The interpretation has been a combination of an innovative approach to directly generate a 3D model and traditional, interactive methods to produce detailed 2D cross sections along selected profiles. Data from the previous CELEBRATION 2000 were also used. The P-wave velocity distribution of the crust and the structure of the crust-mantle boundary, the Moho, facilitate to identify the main lithospheric domains, which are the European plate, the Adriatic micro-plate, and the Tisza block. Further, a new fourth unit, the Pannonian fragment, was revealed between the other three tectonic domains. Due to the collision of Europe and Africa and the following compression, the European plate thrusts under the Adriatic micro-plate. The generation Pannonian fragment is in close context with the compression in north-south direction and the subsequent tectonic escape to the east. Ongoing compression between Africa and Europe may be facilitated by thrusting of Adriatic mantle under the Pannonian fragment. Moho depths reach from 45-55 km below the central Eastern Alps to 25-30 km below the Adriatic foreland and the Pannonian basin. Besides the large scale tectonic structures interesting local features have been revealed. Low seismic velocities in the upper and partly middle crust characterize Neogene and Mesozoic extensional zones, granite intrusions and the Tauern window. A rigid Adriatic indenter can be delineated by its high velocities. Terranes can be identified by high velocities in the middle crust. The highest velocities at lower crust level are found below and around the Vienna basin and indicate special mechanisms of crustal thickening. The use of the new 3D seismic model of the Eastern Alpine area is not confined to further geophysical investigations and tectonic interpretations. It benefits current and future tasks comprising improved localization of earthquakes, estimates of the stress field around deep tunnels, interpretation of GPS data, rock density models for geoid research, and geothermal studies.