FROM TRANSPRESSION TO TRANSTENSION - OLIGOCENE MIOCENE STRUCTURAL EVOLUTION OF THE VIENNA BASIN AND THE EAST ALPINE WESTERN CARPATHIAN JUNCTION

Palaeostress analysis in the easternmost Alps and westernmost Carpathi ans revealed the existence of four different stress fields during Olig ocene-Miocene times. The generalized Oligocene-late Middle Miocene max imal horizontal stress axis changes gradually from WNW-ESE to ENE-WSW and the Late Miocene axis trends to NNE-SSW. Except for the late Middl e Miocene (late Sarmatian, 11 Ma) weak, transient stress field, the ot hers reflect important deformations and are connected to the formation of the Vienna basin. The integration of microtectonic and stress-fiel d data with other structural observations and sedimentological data re sulted in a complete structural analysis. It suggests a four-stage evo lution of the Vienna basin and its surroundings. The Late Oligocene an d Early Miocene deformations were connected to progressively developed , NE-SW-trending sinistral strike-slip zones. During the earliest Mioc ene (Eggenburgian, 21-18.5 Ma), the deformations were characterized by transpression. At the end of the Early Miocene (17-16 Ma), the Vienna basin became a pull-apart structure for a short period. Middle and La te Miocene evolution (16-5.4 Ma) was characterized by a combination of extensional and strike-slip faulting (transtension) rather than a pur e strike-slip or pure tensional regime. A combination of palaeomagneti c and tectonic data demonstrates that the stress field did not change during the Oligocene and Early Miocene. The compressional axis was ori ented originally N-S, and than turned to the actual WNW-ESE or NNW-SSE orientation. The gradual counterclockwise rotation of the structures took place simultaneously with the development of the Late Oligocene-E arly Miocene sinistral shear zones. These rotations affected mainly sm all crustal blocks between strike-slip faults. This mechanism permitte d the transfer of sinistral slip toward the northeast through the enti re junction area. All these deformations explain the mechanism of the tectonic escape of the whole East Alpine-North Pannonian block.