Fig. 1 Simplified tectonic map of the Eastern Alps showing the location of the deep seismic line "TRANSALP".

"TRANSALP" - THE NEW DEEP SEISMIC LINE ACROSS THE EASTERN ALPS The new reflection deep seismic profile, acquired in the frame of the international TRANSALP Program, runs from close to Munich (Germany) in an approximately N-S orientation to Treviso in Italy (Fig. 1). Tectonic elements imaged through deep seismic profiling comprise (1) the European plate, which carries the Molasse basin and part of the orogenic wedge (pro-wedge), consisting of stacked continental and partly oceanic units of European and Adriatic origin, and (2) the Adriatic plate which underlies the southern part of the orogenic wedge (retro-wedge) and the Molasse sediments of the Po Basin. Pro- and retro-wedge are separated by Periadriatic Fault, a prominent dextral strike-slip fault.

The first results of the reflection seismic line have been published in EOS Trans (TRANSALP Working Group, 2001, v.82, pp 453, 460-461) and in press in AGU's Geophys. Res. Letters, v.29, No. 10. In the latter publication the authors compiled data from the vibro-seis, explosion seismic, and passive seismic experiments to arrive at the first crustal-scale interpretation of the Eastern Alps architecture (Fig. 2).

Fig. 2 Deep structure of the Eastern Alps after TRANSALP Working Group, 2002.

CALCULATION OF LITHOSPHERIC STRENGTH ALONG THE TRANSALP LINE. The results of the deep seismic survey (TRANSALP Working Group, 2002) and the surface geology have been used to constrain the geometry of the orogen and to set up a lithospheric-scale rheological model (Fig. 2). The thermal structure of the Eastern Alps, a crucial input parameter for the strength calculations, has been determined on the basis of 2D numerical modelling.

The kinematic model was calibrated against the well constrained thermal history of the Tauern Window in the central part of the orogen. Finally the yield strength along the TRANSALP sections was calculated based on failure (Byerlee's Law) and creep (power law creep) functions for which experimentally determined rock mechanics data and constant strain rates (10-14s-1)are used. In agreement with the present-day stress regime strength calculations have been performed for compressional deformation. In this study we consider a "strong" and a "weak" case differing in the choice of rock analogues representing upper and lower crustal rheologies. For the strong model we used dry quartzite, dry granite, and mafic granulites for sedimentary cover, upper crust and lower crust compositions, respectively. Whereas in the weak model those layers are made of wet quartzite, wet granite and wet diorite. Dry olivine represents in both cases the mantle lithosphere.

Fig.3 Rheological stratification of the Eastern Alps for strong (left) and weak (right) model configurations. Note the better correspondence of earthquake hypocentres (coloured circles) with the mechanically strong upper crust of the left model. After Willingshofer & Cloetingh (2003).

The modelling results suggest pronounced lateral and vertical variations of lithospheric strength for both, the strong (Fig. 3 left) and the weak (Fig. 3 right) case. In both sections the area underneath the Tauern Window is predicted to be weakest. The most prominent difference between the models lies in the strength of the lower crust, which appears as strong, load-bearing layer in the strong case but is part of a thick low-strength layer in the weak case. As a consequence full crust-mantle decoupling can be expected in the weak case, whereas stronger coupling of the lower crust to the upper mantle is suggested for the strong case.

Plotting the seismic activity along and in the vicinity of the TRANSALP Line onto the model predictions clearly suggests a better correlation between the depth distribution of earthquakes and the mechanical strong parts of the layers for a strong model configuration. Other neotectonic data derived from levelling and GPS measurements document relative uplift of the central part of the Eastern Alps and eastward motion of crustal blocks. In summary, the neotectonic activity in the Eastern Alps is in better agreement with a strong rheology along the TRANSALP transect suggesting that the lateral extrusion of the central Eastern Alps towards the Pannonian basin is still (again) ongoing.