Inversion Tectonics in the Alpine Foreland, Eastern Alps (Austria)

Abstract

In this thesis, the 3D structure and kinematics of the locally and mildly inverted Lower Austria Mesozoic Basin beneath the Alpine-Carpathian fold-and-thrust belt is described. This study has been carried out by the integrative interpretation of 2D and 3D seismic surveys, well and geophysical logs data and gravity maps. A basin-scale, 3D structural model has been carried out, focused on the sub-thrust and foreland zones. The Late Eocene to Early Miocene Alpine–Carpathian fold-and-thrust belt resulted from the subduction of the European plate beneath the Adriatic one, and the subsequent continental collision between both plates. The Alpine–Carpathian foredeep and fold-and-thrust belt recorded the long-lasting involvement of the European crystalline basement in several deformation events: from late Variscan transtension, to Jurassic rifting, and Cretaceous to Neogene shortening. In this thesis, two additional basement fault reactivation events have been defined in relation to the Alpine-Carpathian Cenozoic shortening: an extensional reactivation event related to the bending of the European plate coeval with Egerian to Karpatian (ca. 28–16 Ma) thin-skinned thrusting; followed by the selective positive inversion of the basement faults in the sub-thrust and in the foreland during Karpatian to Badenian times (ca. 16-12.5 Ma). The flexural bending of the European plate and the associated extensional fault reactivation were promoted by high lateral gradients of lithospheric strength in addition to the slab pull forces associated with subduction. Delamination of the European lithosphere during the final stages of collision around Karpatian times (ca. 16 Ma) promoted a large-wavelength uplift and an excessive topographic load. This topographic load was compensated by broadening the orogenic wedge through the compressional reactivation of the inherited fault array in the Euroepan plate beneath and ahead of the thin-skinned thrust system. Ultimately, collapse and deep burial of the Alpine-Carpathian tectonic wedge took place by the formation of the Pannonian basins system. To gain further insights in the deformational processes in sub-thrust and foreland settings, sandbox analogue models of brittle and brittle-viscous sand wedges have been carried out. The models aimed testing the influence of different topographic loads (i.e., thrust wedges) on the sub-thrust inversion of extensional basins, as well as the influence of the initial orientation of the extensional basins, and the presence or absence of weak detachment layers. Segmented half-graben basins -striking at 90º, 45º and 15º to the extension direction- were created first, and then shortened using different angles for the basal detachment and topographic slope. A shallow layer of viscous polymer over the half- graben basin was included in one of the models. The experiments were analysed using time-lapse photography, topography laser scans and image-based 3D voxels. The modelling results indicate a deformation sequence characterised by layer-parallel compaction, fault reactivation, thrust propagation and related folding. Fault reactivation and basin inversion were associated with layer-parallel compaction accomplished by slip along the basal detachment, prior to and in between pulses of thrusting. The results of the sandbox analogue models reveal a fundamental control imposed by the vertical load of the tectonic wedge and its integrated strength profile in the inversion of sub-thrust basins. Small vertical loads or strong gradients of vertical load have revealed as fundamental factors aiding in the inversion of buried, sub-thrust basins. The integrated strength profile resulted from the combination of inherited, strain-softened fault zones, as well as the presence or absence and distribution of weak, viscous horizons. The results of the sandbox models carried out indicate that the vertical load, its gradient over the sub-thrust basins and the inherited, strain-softened faults, are more important than the obliquity between the direction of shortening and the orientation of pre-existing fault systems. As indicated by the results of sandbox analogue models, the recurrent and long-lasting frictional reactivation of the Lower Austria basement fault array may have been favoured by fault-weakening mechanisms, as well as by steep gradients of vertical loads generated by thin-skinned out- of-sequence stacking of the Rhenodanubian Flysch located south of the inverted basement fault array.