Lower Miocene structural evolution of the central Vienna Basin (Austria)

Abstract

The tectonic evolution of the Vienna Basin overlying the Alpine-Carpathian fold and thrust belt includes two stages of distinct basin subsidence and deformation. The earlier phase contemporaneous with thrusting of the Alpine-Carpathian floor thrust is related to the formation of a wedge-top basin (“piggy-back”), which was connected to the evolving foreland basin (Lower Miocene; c. 18.5–16 Ma). This stage is followed by the formation of a pull-apart basin (Middle to Upper Miocene; c. 16–8 Ma). Sediments of the latter unconformably overly wedge-top basin strata and protected them against erosion. Analyses of subcrop data from the central part of the basin focus on the complex stratal geometries and tectonics of the Lower Miocene wedge-top basin. During that period the overriding allochthon is characterised by syntectonic subsidence and sedimentation, out-of-sequence fold-thrusting, normal faulting and sinistral wrenching. All types of faults are dated as Lower Miocene by the occurrence of growth strata and the age of overlying post-tectonic sediments related to the pull-apart basin stage. Faults mapped in 3D seismic are compared to faults along the SW margin of the Vienna Basin. They prove that complex Lower Miocene deformation occurred in large parts of the Austroalpine overriding nappes concurrent with thrusting at the Alpine floor thrust and in the Molasse unit. The structural analysis of 3D seismic data shows that Lower Miocene sediments of the Ottnangian and Karpatian stage unconformable overly the Alpine-Carpathian thrust nappes with a thickness of up to 1500 m. Geometries include onlaps to the basin floor, internal unconformities and toplaps to the overlying Middle Miocene sediments. These toplaps are the results of tilting and subsequent erosion of the sediment pile by out-of-sequence thrusting in the central part at the Karpatian–Badenian transition (c. 16.3 Ma) prior to the onset of pull-apart deformation.