Active out-of-sequence thrusting in the Molasse Basin constrained by a multidisciplinary approach (Eastern Alps, Austria)

Abstract

The current tectonic activity at the frontal part of the Eastern Alps is so far barely assessed due to the lack of geologic record and the scarcity of earthquakes in the external units of the fold-thrust belt and its foreland. This paper presents the results of a multidisciplinary study of the Austrian sector of the Imbricated Molasse Basin, near the Alpine floor thrust front, where independent geologic data proves active thrusting at different out-of-sequence thrusts.The integration of structural borehole image interpretation, in-situ stress analysis, geomechanical modeling, 3D seismic interpretation and surface geology shows that out-of-sequence thrusting had a significant impact on the local structural evolution of the frontal part of the Alpine belt. Stress data from boreholes penetrating two of the out-of-sequence thrusts in the Imbricated Molasse revealed stress perturbations indicating that these thrusts are currently active. This finding is supported by 1D geomechanical modeling that proved a thrust regime in the Molasse units and the overlying Rheno-Danubian Flysch.The observation of active thrusting of the imbricated Molasse units is in line with Pleistocene out-of-sequence thrusts in the external tectonic units of the Eastern Alps (Helvetic units and Rheno-Danubian Flysch). There, Pleistocene fluvial terraces of the Mindel glaciation (MIS 12) are vertically displaced by two different out-of-sequence thrusts, while the younger terraces appear undeformed. One of these thrusts is imaged in 3D seismic allowing to calculate a Pleistocene slip rate of 0.33 to 0.43 mm/a from fault dip, terrace ages and vertical offset.In sum, our data proves that Quaternary shortening activated several pre-existing out-of-sequence thrusts in the frontal part of the Eastern Alps. Data, however, does not allow deciding whether Quaternary thrust shortening is a continuous process or occurs episodically, with each shortening episode reactivating different pre-existing structures.