3D geomodelling of multiphase ductile and brittle deformations: a unique tool for quantifying structural relationships and tectonic evolution (Penninic units of the NW Alps)

The North-Western Alps represent the better-known orogenic playground worldwide, exposing the stack of the Western Austroalpine, Penninic, and Helvetic metamorphic nappes, separated by ophiolitic sutures. However a modern and detailed 3D structural model, including the complexity of polyphase ductile and brittle structures, does not exist, and the reference 3D model is still the Argand’s (1911) block diagram. Here we present preliminary results of a new 3D structural model of a large area (1300 km2) running along the Italian-Swiss boundary ridge, from the Helvetic Mont Blanc massif to the Penninic Monte Rosa nappe, including all main Penninic and Austroalpine units. Input data are represented by structural surveys and detailed geological mapping, representing a truly 3D dataset thanks to a difference in elevation, from valley floors to mountain summits, of 3-4 km. 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 surface algorithms: advanced geomathematical tools allowing to model, under some conditions, complex structures such as those arising from multiphase ductile and brittle deformations. This project aims at improving our understanding and our capacity to quantify some fundamental processes of Alpine tectonics. In addition to better representing and quantifying structures that were already qualitatively known, we have finally solved some problems that could not be solved in 2D. For instance, we will present the solution to a long-lasting debate on a structure, known as the “Accident Col de Bard-Saint Nicolas”, that has been discussed for 25 years. Supported by field work, we demonstrated that the “Accident” is a brittle normal fault that represents the Miocene western continuation of the Aosta-Ranzola normal fault. This also solves problems of correlation within the Grand St-Bernard, since the “Accident” juxtaposes the highest nappe of the system (Mont Fort, to the N) to the lowermost (Ruitor) with tectonic elision of intermediate units. A similar debate has been solved about the Aouillette ophiolitic unit, a portion of the Combin Nappe, from which it is separated by a graben limited by Oligocene and Miocene normal faults. Another important outcome of the 3D model is the clear distinction between sections of the orogenic wedge characterized by different tectonic styles, namely (i) an inner Austroalpine-Upper Penninic domain with sub-horizontal nappes, eclogitic and greenschist peak metamorphism, and both Oligocene and Miocene brittle normal faults; (ii) an intermediate sector represented by the Grand St-Bernard nappe system, with blueschist peak metamorphism and prevailing Miocene brittle faults; and (iii) an outer system with low-T greenschist peak metamorphism, younger thrusts and no Miocene or Oligocene normal faults. In addition, a quantitative and detailed 3D model is invaluable as a basis for applications, such as those related to the circulation and storage of deep water resources hosted in the bedrock, including geothermal fluids. We feel confident that this kind of application could result in a renewed interest for fundamental studies in tectonics and structural geology in the circum-Mediterranean Variscan and Alpine belt.