Previous Next Tübinger Geowissenschaftliche Arbeiten, Series A, Vol. 52, p. 113.
Abstracts of the 4th Workshop on Alpine Geological Studies, Tübingen 21-24 Sept. 1999

N054

Upper crustal structure of the Austroalpine thrust complex of westernmost Austria

Thomas May* 1, Gerhard Eisbacher 1


 1 

Institut für Regionale Geologie, Universität Karlsruhe, Germany

 * 

Correspondence:  Kaiserstr. 12, D-76131 Karlsruhe, Germany (de40@rz.uni-karlsruhe.de)

 

Two NW-SE oriented cross-sections across the Alpine orogen of western Austria have been constructed by extrapolation of structures mapped at the surface, by the interpretation of reflectors on seismic lines obtained by ÖMV industry and by on-strike correlations with published borehole data. The results of NRP 20 in Switzerland (Pfiffner et al. 1997) have been used as a source of information on the deep structure of easternmost Switzerland. The Alpine chain of western Austria is made up from bottom to top (i.e. from north to south) of the European Variscan basement, an autochthonous Mesozoic platform cover and the Molasse foreland basin, the folded and thrust faulted Helvetic nappes, intensely deformed Penninic units and the thrust sheets of the Austroalpine complex. The most important difference between the sections "Lake Constance" and "Reutte" is the role of the Helvetic nappes which decrease in thickness from more than 4.2 kilometres in the west to about 2.8 kilometres in the east. This probably results not only from the stacking of several thrust sheets in the west but also from a thickness change of the formations. Polyphase deformation structures along the plane of the two sections indicate that NW-directed thrusting of the Austroalpine units was followed by north to north-northeast directed long-distance movement of the Austroalpine complex onto the Penninic realm in latest Cretaceous - Paleogene time. Subsequent emplacement of Penninic and Austroalpine units onto the Helvetic shelf produced NNW-SSE shortening in Late Oligocene to Late Miocene. Because of the polyphase heteroaxial contraction history the cross sections could only be tentatively restored. In a first step the main structural units were placed back in their assumed original paleogeographical position. Although the northwest-southeast extent of the Penninic realm is estimated from between 100 to 500 km (e.g. Pfiffner 1992), for the restoration of the cross-sections a conservative minimum value of 100 km was chosen. In a second step the internal shortening of the European basement and the Helvetic cover on one hand and the Austroalpine complex on the other were retrodeformed. The restored western section shows a minimum line length of 307 km which was shortened to the present 69 km. Together with about 10 km of out-of-sequence thrusting of the Silvretta complex onto the trailing portion of the Northern Calcareous Alps the minimum shortening thus amounts to about 248 km. The restored eastern section shows a minimum line length of 338 km and was shortened to 60 km. Minimum out-of-sequence thrusting of the basement complexes onto the trailing Northern Calcareous Alps is about 12 km. Total minimum shortening along the trace of eastern section thus is about 290 km.

 

 

Pfiffner, O.A. , 1992,  Alpine Orogeny. in Blundell, D. ,   Freeman, R. ,   Mueller, S., A continent revealed: The European Geotraverse, Cambridge, University press, p.180-199

Pfiffner, O.A. ,   Lehner, P. ,   Heitzmann, P. ,   Mueller, S. ,   Steck, A. , 1997,  Deep structure of the Swiss Alps., Birkhäuser, Basel