3D model of folding and shearing along nappe boundaries of the Monte Rosa, Zermatt–Saas, and Combin units: No breaking news, but a substantially enhanced picture of exhumation and collision in the classical outcrops near Gressoney (Aosta Valley, Italian NW Alps)

The upper Gressoney (or Lys) Valley, at the foot of the Monte Rosa Massif in the NW Alps, offers one of the most complete and readable exposures of the relationships between the continental Monte Rosa Nappe and the ophiolitic Zermatt–Saas and Combin Units. It represents an ideal natural laboratory for integrating structural and metamorphic data into 3D models that capture not only the geometry of these units, but also their tectonic evolution—particularly during the collisional and exhumation stages. The northwest-verging nappe stack comprises, from top to bottom, ophiolites of the Combin (or Tsatè) and Zermatt-Saas units, followed by the European (or Briançonnais) Monte Rosa Nappe. The Combin Unit recorded a blueschist peak, which is only sparsely preserved due to overprinting by a regionally penetrative greenschist-facies imprint. In contrast, both the Zermatt-Saas and the Monte Rosa exhibit common eclogitic assemblages (up to UHP in a few localities), followed by penetrative blueschist foliations, finally overprinted by greenschist to amphibolite facies assemblages. The PTt paths of the Zermatt-Saas and Monte Rosa show strong similarities, pointing to an early juxtaposition during the initial stages of exhumation from high-pressure conditions. In contrast, the metamorphic evolution of the Combin is markedly different, and its juxtaposition with the other two units is inferred to be more recent and shallower. Although the tectonic boundaries of these units record a multiphase evolution under varying metamorphic conditions, the highly localized strain has mainly preserved only the final deformation stages. As a result, these boundaries are not ideal candidates for reconstructing the complete tectonic history. Here, we present results from new mapping and detailed 3D modelling of cross-cutting and overprinting relationships between mylonites at tectonic boundaries and more diffuse deformation developed within the nappes, mainly represented by multiple generations of folds and foliations. Our analysis confirms the early juxtaposition of Monte Rosa and Zermatt-Saas Units, and further emphasizes the late-stage character of the Combin fault zone (the mylonite at the base of the Combin unit), which in 3D reveals spectacular examples of crosscutting relationships, literally decapitating large folds in both the hanging wall and footwall units.