Thomas Scholz^* 1, Joachim Kuhlemann ¹, John Reinecker ¹, Wolfgang Frisch ¹

Lateral extrusion of the Eastern Alps is in general observed in late Early to Middle Miocene times. In contrast, extension in the eastern Southern Alps as well as in the southeastern Eastern Alps continued into Pliocene times, as recorded by young basin sediments. Faulting is evident in the southeastern sector of the Eastern Alps, defining a Late Miocene/ Pliocene active zone. This zone extends from the Karawanken chain in the south to the northern margin of the Klagenfurt basin, which has been filled by Sarmatian to ?Pliocene sediments. At the northern margin of the Klagenfurt basin open pits in Triassic carbonate and the Middle Miocene basalt pipe of Kollnitz have been investigated in order to reconstruct the Miocene to recent structural evolution.

Eight deformation events have been separated, using computer programs of Ratschbacher et al. (1994). The first event D₁ matches the pre-extrusion (until ca. 22 Ma) NW-SE compression, D₂ shows NW-SE extension and NE-SW compression, reflecting a regional variation of the well known pattern of the lateral extrusion period between 22 Ma and 12 Ma. D₃ shows W-E compression and N-S extension, which has been attributed to the "soft collision" of the Eastern Carpathians (Peresson and Decker 1997) and a subsequent short lived compressional period 12 Ma, found by various authors in the last decade. The next two events D₄ and D₅ display a repetition of the events D₂ and D₃, clearly separated from them by overprinting relationships. A combination of strike-slip and normal faults indicate a more extensional character of D₅ as compared to D₃. Whereas D₄ is typically observed as the last structural event in the Eastern Alps, the D₅ orientation of the stress field has not been found so far. We attribute it to the Late Miocene/Pliocene extension in the eastern Southern Alps, where wedge-shaped blocks extruded towards the ESE. The events D₆ and D₈ record late extension either in E-W and N-S or NW-SE directions, separated by an ill-defined weak local E-W to NW-SE compression (D₇). These extensions appear to be of late- to post-Pliocene age and are preliminarily interpreted to result from bulging of the flexed Austroalpine upper crust in front of the Karawanken chain. North to northwestward thrusting of the Karawanken chain in Pleistocene times caused flexural subsidence (Nemes et al. 1997), but relevant accumulation of Pleistocene debris is suppressed by regional uplift and the low base level of the Drau River.