The importance of structural data in constraining 3D implicit structural models: the Northwestern Alps case study, Italy

The Northwestern Alps are one of the most studied and well-known orogenic areas worldwide, exposing a variety of challenging geological structures. however, no updated 3D model of the area exists, due to difficulties in representing the complexity given by polyphase ductile and brittle structures. In the context of the Italian-Swiss Interreg RESERVAQUA project, a new 3D structural model of ca. 1300 km2 is being built, allowing many important implications on the large-scale tectonic interpretation of the area, and other applications such as studies of circulation and storage of deep-water resources hosted in the bedrock. Input data is represented by a high-resolution geological database and one key point in our modelling strategy was aimed at taking full advantage from the abundant structural data. This is not straightforward since commercial 3D modelling software is mainly aimed at hydrocarbon reservoirs and do not allow exploiting this kind of data. Our analysis started defining homogenous structural domains, characterized by roughly homogenous foliations (in terms of metamorphic assemblages and orientation) and/or fold axes. Structural data within these domains have been spatially averaged over grids with variable resolution, applying statistical analysis tools allowing to assess the cylindricity in different structural domains. A more or less traditional, but quantitative, conceptual interpretation has been carried out by projecting onto vertical cross-sections the structural field data and map traces of stratigraphic and tectonic elements. This projection was performed along vectors defined for each homogeneous domain in the orientation analysis. Interpolation of 3D surfaces using advanced implicit algorithms follows, with both field data and the conceptual interpretation in vertical cross-sections as constraints. Ten structurally homogeneous domains have been defined, leading to a clear and objective distinction of sections of the orogenic wedge that are characterized by different tectonic styles. These fall in three macro-domains: (i) an internal Austroalpine-Upper Penninic domain with sub-horizontal nappe boundaries, greenschist to eclogitic peak metamorphism and diffuse collisional greenschist re-equilibration, and both Oligocene and Miocene brittle normal faults striking NE-SW and NW-SE, (ii) the intermediate domain represented by the Grand St-Bernard nappe system, with blueschist peak metamorphism, diffuse greenschist re-equilibration and mainly Miocene brittle faults striking NW-SE, and (iii) an external system with low-T greenschist peak metamorphism, foliations consistently SE-dipping, relatively young semi-brittle thrusts and no Miocene or Oligocene normal faults. In the whole area, fold axes are sub-horizontal and oriented NE-SW or NW-SE. In this contribution, we focus on examples that are explicative of the geomodelling workflow that can be applied in this tectonic area, and we show how the abundancy of structural data, together with the rugged topography, can compensate for the absolute lack of subsurface data.