Comment on egusphere-2022-1529

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Within the Northern Calcareous Alps fold-and-thrust belt of the Eastern Alps, multiple deformation phases have contributed to the structural grain that localised deformation at later stages. In particular, Jurassic rifting and opening of the Alpine Tethys led to the formation of extensional basins at the northern margin of the Apulian plate. Subsequent Cretaceous shortening within the Northern Calcareous Alps produced the enigmatic Achental structure, which forms a sigmoidal transition zone between two E-W striking major synclines. One of the major complexities of the Achental structure is that all structural elements are oblique to the Cretaceous direction of shortening. It was therefore proposed to be a result of forced folding at the boundaries of the Achental basin. This study analyses the structural evolution of the Achental structure through integrating field observations with crustal-scale physical analogue models, to elucidate the influence of pre-existing crustal heterogeneities on oblique basin inversion and the prerequisites for the formation of a sigmoidal hanging wall that outlines former basin margins. From brittle-ductile models, we infer that shortening oblique to pre-existing extensional faults can lead to the localisation of thrust faults at the existing structure within a single deformation phase. Prerequisites are 1) a weak basal d&eacute;collement that is offset by an existing normal fault, 2) the presence of topography in the hinterland, 3) a thin-skinned deformation style. Consequently, the Achental low-angle thrust and corresponding folds was able to localise exactly at the basin margin, with a vergence opposite to the Jurassic normal fault, creating the characteristic sigmoidal morphology during a single phase of NW-directed shortening.