3D structural geomodelling in the complex metamorphic units of the Italian Western Alps (Aosta Valley, Italy)

Structural 3D geomodelling of metamorphic units that underwent a sequence of ductile and brittle deformation events is an extremely challenging task. In the context of the ongoing RESERVAQUA project, a 3D geomodel that runs along the Italian-Swiss border is being built. The Italian part of the area has an extension of c. 1,000 km2 and crosses units of the Austroalpine, Penninic and Helvetic domains, showing a great variability of metamorphic environments and deformative style. In such a vast and heterogeneous setting, developing a conceptual model was necessary to define the scale of the problem and what level of detail to give to the 3D model. The first step in our analysis was to define homogeneous tectonic zones through an orientation statistics study. Progressing with the actual geomodelling phase, a first structural study in vertical cross-sections was followed by the usage of implicit algorithms (implemented in the software SKUA/Gocad, for this study). However, we tested different commercial software packages and some open-source research libraries to find that no one is capable of modelling Aosta Valley’s complex structures out-of-the-box. This is not surprising since generally these codes are geared towards modelling gently deformed sedimentary sequences and not complex metamorphic tectonic settings. Nevertheless, it was possible to overcome a large range of obstacles by “fooling” the modelling algorithms to properly represent isoclinal folds, large volume variations of the lithological bodies, tectonic contacts between large-scale tectono-metamorphic units, lenticular bodies and many other complex geological geometries. For instance, we chose (1) to “cheat” on the geological meaning of some entities or (2) to build a “fake” stratigraphy where the same units were repeated several times. In this contribution, some of these modelling solutions are compared in terms of their straightforward implementation and their ability to generate models that properly fit the very detailed geological maps available in our study area (mapped at 1:5.000-1:10.000 with a dense set of structural and petrographic stations).