Kinematic modelling of fold and thrust structures in anisotropic layered rocks in examples from the Northern Calcareous Alps (Eastern Alps, Austria)

The presented study is a part of a broader project, which includes field as well as numerical investigations into the evolution of tectonic structures within fold-and-thrust belts, with a particular focus on the development of fault-related structures in layered rocks. The existing models for deformation in fold-and-thrusts belts predominantly adopt a kinematic approach, wherein layering is considered as passive. The kinematic approach neglects rheological effects such as mechanical anisotropy, which plays an important role in layered rocks commonly found within fold-and-thrusts belts.

To explore the role of mechanical anisotropy we have analysed folding and thrusting in the central Northern Calcareous Alps (NCA), which comprise the Permo-Mesozoic sediments of the Upper Austroalpine unit. The NCA represents a fold and thrust belt, in which folds are formed by processes along overthrusts (e.g. fault-bend folds or fault-propagation folds), but out-of-syncline overthrusts are also present. The tectonic evolution of NCA is strongly influenced by sedimentary facies. Nappes in the NCA were imbricated during Jurassic and Cretaceous thrusting. Our research has focused on the Scythian (Lower Triassic) mixed clastic-carbonate sediments of the Werfen Fm, located to the SW of Hallstatt at the base of the fold-and-thrust system, as well as on the Upper Jurassic limestones of the Oberalm Fm located SE of Bad Ischl that are deformed syndepositionally into thrusts and folds forming the shallowest part of the fold-and-thrust belt during the Jurassic deformation stage.

During fieldwork, documentation was gathered through the acquisition of orientation measurements of tectonic structures: folded bedding, faults with slickensides, fold axes, cleavage, joints, etc. Photographic documentation of tectonic structures was undertaken to produce georeferenced photogrammetric models. Digital outcrop models in the form of georeferenced textured polygon meshes, which allow the integration of spatial data with the results of detailed geological mapping, were created. All field observations, including outcrop images and measurements, were integrated in a 3D environment, which facilitated the collection of additional data from the digital models.

Sequential restoration and kinematic forward modelling of structures performed in Move software confirm the limitations inherent with kinematic modelling to represent real-world strain patterns. Hybrid modes of kinematic models provide more acceptable results and prove that rheological contrasts within the sedimentary pile exert a strong control in the distribution of folding and faulting. These observations made at the meter-scale, relate to structures that represent a scale of strain normally not represented in regional-scale (kilometer-scale) cross-sections and dealt with as ‘internal strain’. Our observations imply that strain distribution within sedimentary units can be strongly anisotropic and its distribution should be contemplated when performing kinematic modelling of regional-scale structures.