3D structural geomodelling of complexly deformed basement units: the Aosta Valley case study (Western Alps, Italy)

The North-Western Alps represent one of the best-known orogenic playgrounds worldwide, exposing a complexity of polyphase ductile and brittle structures that makes this area also one of the richest in terms of deformation styles. Here we present preliminary results of a new 3D structural model of a large area (1300 km2). Input data are represented by structural surveys and detailed geological mapping, defining a truly 3D dataset that compensates for the absence of subsurface data thanks to important differences in elevation of 3-4 km, from valley floors to mountain summits. Our modelling workflow is based on a first step of conceptual modelling in vertical cross-sections, based on classical and sound structural concepts, followed by interpolation with implicit and explicit surface algorithms. In our area, geological complexity is given by several finite faults, internal to the model, that intersect each other as the result of multiple brittle deformation events. The basement units are also interested by polyphase ductile tectonics: isoclinal and cuspate-lobate folds, polyphasic folds, and other structures that produce great thickness variations of the lithological bodies outcrop. In this setting, the geological legend needs to be conceived as a complex conceptualization of surfaces organized in a hierarchical structure. The most consistent contacts (e.g. between different Alpine domains) are first order surfaces, second order is given to surfaces that divide different tectonic units and following down to the lithological boundaries (the least order of importance). In this contribution we focus our attention on areas that are very explicative of the complex metamorphic nature of the basement. We present the adopted geomodelling solutions while performing a comparison on how different software packages (both licensed and open-source) perform when modelling this challenging structural setting, in order to obtain a model that properly fits the geological interpretation of the area.