06 Feb 2023
 | 06 Feb 2023
Status: this preprint is open for discussion.

Oblique basin inversion leads to fold localisation at bounding faults: Analogue modelling of the Achental structure, Northern Calcareous Alps, Austria

Willemijn S. M. T. van Kooten, Hugo Ortner, Ernst Willingshofer, Dimitrios Sokoutis, Alfred Gruber, and Thomas Sausgruber

Abstract. 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 décollement 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.

Willemijn S. M. T. van Kooten et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2022-1529', Anonymous Referee #1, 21 Mar 2023 reply

Willemijn S. M. T. van Kooten et al.

Willemijn S. M. T. van Kooten et al.


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Short summary
Extensional deformation creates structures that may be reactivated during subsequent shortening. The Achental structure within the Northern Calcareous Alps fold-and-thrust belt is a natural example of a basin margin that was inverted during Alpine orogeny. Based on this example, we have studied the influence of such inherited inhomogeneities in-field and as analogue model. We find that oblique shortening can create structures outlining pre-existing faults within a single deformation event.