Buckle folding in the Northern Calcareous Alps - Field observations and numeric experiments

Abstract We report the results of a numerical modelling study based on folds in the western Northern Calcareous Alps (NCA) fold-and-thrust belt. We study boundary conditions of folding based on the mechanical properties of the rocks involved. One key control on the model is stratigraphy that can be simplified to three layers: (1) an incompetent base along the decollement of the thrust sheet, and (2) an up to 3 km thick competent layer, which is overlain by (3) an incompetent top layer. The incompetent base layer needs to be very weak to facilitate folding and is a salt-bearing evaporitic unit. Another key is erosion prior to folding. It needs to remove half of the competent layer to allow the creation of folds with limb lengths comparable to field observations. The results of the numeric model contribute to the understanding of the structural development in the western NCA. Folding within the upper thrust sheet was only possible above a salt-bearing decollement and after a decrease in lithostatic pressure related to Upper Cretaceous and/or Paleogene erosion.