Late Carboniferous Schlingen in the Gotthard nappe (Central Alps) and their relation to the Variscan evolution

Abstract

Pre‐Mesozoic basements of the Alpine belt commonly contain kilometre‐scale folds with steeply inclined axial planes and fold axes, which are named “Schlingen” folds. The structural evolution of Schlingen folds and their geodynamic significance for the Variscan evolution are unclear. To close this gap, this study investigates a well-preserved Schlingen structure in the Gotthard nappe (Central Swiss Alps). This Schlingen fold evolved by a combination of shearing and folding under amphibolite-facies conditions. Detailed digital field mapping, coupled with petrographical and structural investigations, reveal local synkinematic migmatisation in the fold hinges parallel to axial planes. Zircon crystals from leucosomes in the fold hinges have magmatic cores that yield an age of 449 ± 3 Ma, and rims with a range of dates from ~ 270 to 330 Ma (main cluster at 315 ± 4 Ma). We ascribe this late Carboniferous age to peak metamorphic conditions of the late Variscan Schlingen phase. Moreover, we describe for the first time post-Schlingen, but pre-Alpine transpressional deformation. The investigated Schlingen fold is discussed with respect to comparable structures of the wider Alpine realm, located in the most southern part of the Variscan belt. We propose that Schlingen formation concurred with the crustal-scale transpressional tectonics. This scenario separates, at least in a structural sense, the Southern Variscides from more northern parts (also Gondwana derived), where Schlingen folds are absent.