Thrust-related folding in early burial: an example from the Upper Jurassic Oberalm Formation, Northern Calcareous Alps, Eastern Alps, Austria

Large-scale fault-related folds are typically studied using reflection seismic or scattered, limited quality, outcrop data. The nature of both data types tends to obscure the information from the structurally most complex areas, particularly fold forelimbs. Additionally, in fold and thrust belts, syn-tectonic deposits are often deformed shortly after deposition, resulting in the co-occurrence of both tectonic and mass-transport-related deformation structures. This is the case in the Northern Calcareous Alps, where the Alpine orogeny has produced complex systems of folds and thrusts along with associated syn-tectonic deposits. Here, we focus on a Late Jurassic siliceous to calciturbiditic succession of Tauglboden Fm and Oberalm Fm, which infill a basin developed in front of a salt-fed thrust: the Grabenbach thrust. Our aim is to characterize deformation structures observed in these formations in the immediacy and in areas surrounding the outcrop of the thrust, to discriminate between tectonic and mass-transport-related deformation structures. We conducted detailed structural mapping, generated photogrammetric models and collected samples, analyzing geometries at scales ranging from millimeters to decameters to assess the deformation conditions and mechanisms. We identify segments of strongly localized deformation, including complex folding patterns, within broad domains of a mostly undeformed, flat-lying succession. We interpret these segments as parts of large scale anticlines developed in the hanging walls of thrusts. Our observations highlight the importance of a multiscale approach to strain analysis in compressional settings. Moreover, our study has implications for the rates of consolidation in carbonate-rich rocks and for deformation mechanisms in such systems.