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 Carpathians revealed the existence of four different stress fields during Oligocene-Miocene times. The generalized Oligocene-late Middle Miocene maximal horizontal stress axis changes gradually from WNW-ESE to ENE-WSW and the Late Miocene axis trends to NNE-SSW. Except for the late Middle Miocene (late Sarmatian, 11 Ma) weak, transient stress field, the others reflect important deformations and are connected to the formation of the Vienna basin. The integration of microtectonic and stress-field data with other structural observations and sedimentological data resulted in a complete structural analysis. It suggests a four-stage evolution of the Vienna basin and its surroundings. The Late Oligocene and Early Miocene deformations were connected to progressively developed, NE-SW-trending sinistral strike-slip zones. During the earliest Miocene (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 Late Miocene evolution (16-5.4 Ma) was characterized by a combination of extensional and strike-slip faulting (transtension) rather than a pure strike-slip or pure tensional regime. A combination of palaeomagnetic and tectonic data demonstrates that the stress field did not change during the Oligocene and Early Miocene. The compressional axis was oriented 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-Early Miocene sinistral shear zones. These rotations affected mainly small crustal blocks between strike-slip faults. This mechanism permitted the transfer of sinistral slip toward the northeast through the entire junction area. All these deformations explain the mechanism of the tectonic escape of the whole East Alpine-North Pannonian block.