Early Miocene tectono-sedimentary shift in the eastern North Alpine Foreland Basin and its relation to changes in tectonic style of the Eastern Alps

A striking difference along the Alpine Orogen is the style of collisional tectonics during the Oligo-Miocene, with the onset of escape tectonics in the Eastern Alps. The indentation of the Adriatic Plate into the Eastern Alpine Orogen resulted in the formation of conjugate dextral and sinistral strike-slip faults in the vicinity of the Tauern Window. Moreover, major changes occurred in the foreland of the Eastern and Southern Alps in the Early Miocene, with the cessation of the northern Alpine front propagation and the onset of thrusting along the Southern Alpine Front. In this study, we present new results from structural, stratigraphic and subsidence analyses of the eastern North Alpine Foreland Basin (NAFB).

Our results show an initial phase of foreland sedimentation in the eastern NAFB between ca. 33-28 Ma, followed by a period of strong, tectonically driven subsidence between ca. 28-25 Ma, ending with a phase of erosion and formation of the basin-wide erosional unconformity, the Northern Slope Unconformity (NSU). During this time, the rift-related Mesozoic normal faults of the European platform were reactivated and then capped by the NSU. We interpret this phase as an increase in the flexure of the subducting European Plate under the growing Alpine Orogen. Between 25-19 Ma, the eastern NAFB remained in a deep-marine, underfilled state with a gentle increase in subsidence. A major shift took place around 19-17 Ma with tectonic uplift, ranging from 200 m (absolute minimum) to 1200 m depending on uncertainties on paleo-water depths, and rapid sedimentary infill of the basin. We discuss the possible causes for this major tectono-sedimentary shift in the eastern NAFB in relation to changes in collisional tectonics within the Eastern and Southern Alps, including a potential Early Miocene slab break-off event beneath the Eastern Alps.