Three-dimensional retro-deformation of the Lechtal-Nappe, northern Calcareous Alps

We determine the amount of shortening, depth of detachment and the style of deformation of the Lechtal Nappe, the largest nappe within the Northern Calcareous Alps (German/Austrian border), by three-dimensional retro-deformation analysis of ca. 40 km2 area. A three-dimensional model was constructed by splining lines from eight N-S cross-sections, spaced E-W at ca. 4 km intervals, to define fault geometrics and seven stratigraphic layers within the Permo-Triassic to Cretaceous strata. The modelled volume has four main thrusts which link to a common detachment at 2-5 km depth. Three-dimensional fault displacements and heaves were determined using Allan diagrams. The algorithms fault-parallel flow and flexual-slip unfolding were used to restore northwards movement on the thrusts and folding of beds over thrust planes, respectively. Minimum shortening estimates vary, from east to west, from 25 to 42% (typical error 6%), but additional shortening in the west is due only to folding.