The eastern Lepontine dome nappe stack 3D model (swissAlps3D)

Over the past few decades, 3D geometrical modelling of the subsurface geological structure has become an essential tool in structural geology, significantly improving the visualization and interpretation of complex geological architectures. However, due to their structural complexity and multi-phase deformation histories, orogenic belts remain a major challenge for 3D modelling. The swissAlps3D project (2024-2030), led by swisstopo, aims to build a consistent, large-scale underground 3D geological model of the main geological and structural boundaries of the Swiss Alps and neighbouring regions. The project is subdivided into eight domain-specific sub-projects, including the Lepontine 3D project (LP3D). Here, we present the datasets, modelling strategy, and preliminary results of a 3D geological model of the eastern Lepontine dome.

The Lepontine dome consists predominantly of amphibolite-facies metamorphic rocks of late Eocene–Oligocene age and covers approximately 4,500 km² at the surface. At depth, it extends from the Simplon Fault Zone to the west and from the Forcola Line to the east. It is further bounded by the Periadriatic Lineament to the south and the Northern Steep Belt to the north. Structurally, it comprises a tectonic window within the Central Alps, which exposes the deepest Alpine nappes in the core of the bell-shaped architecture. It is made up predominantly of polycyclic basement gneisses, intruded by Permo-Carboniferous granitoids and locally bounded by a thin autochthonous or parautochthonous Mesozoic-Cenozoic sedimentary cover. Moreover, ductile isoclinal sheath folding, complex fold interference structures and a strongly debated tectono-stratigraphy (swisstopo 2024) make the Lepontine dome a challenging area for testing and refining its tectonic evolution and 3D modelling techniques capable of representing such units.

The presented 3D geological model, targets the major lithostratigraphic and structural units of the dome and is constructed by combining explicit and implicit 3D modelling approaches that interpolate a heterogeneous input dataset composed of newly acquired and published geological and structural maps, cross-sections, borehole data, existing 3D geological models, and available geophysical constraints from the literature. Preliminary modelling results depict from bottom to top the Leventina-Lucomagno, Simano, Adula, Piz Terri-Lunschania, Vals, Aul, Grava and Tomül nappes. These Lepontic and Lower Penninic units dip eastward at low angle beneath the overlying Pennine and Austroalpine nappe stack. Emphasis is placed on the 3D-architecture of the Lepontic-Penninic boundary, which is dissected by the Forcola extensional shear zone and is interpreted to form an eastward-developing ramp–flat geometry.