A metamorphic belt modelling approach: the Structural Topology model

3D geological modelling in polydeformed metamorphic belts remains a significant challenge due to the structural complexity and the limited applicability of time-aware methods. In such settings, the difficulty lies not only in surface interpolation techniques but also in the geological and topological meaning assigned to each boundary. We propose a modelling strategy that combines conceptual surface and volume labelling and topological analysis to reconstruct robust geological legends and to ensure consistent 3D structural models. Our approach can integrate explicit and implicit modelling techniques, using conceptual surface and volume labelling. Geological units (i.e. volumes) are classified as stratigraphic (S), tectonostratigraphic (TSU), tectonometamorphic (TMU), intrusive (IU) or shear zone (SZ), depending on their origin and deformation history. Geological boundaries (i.e. surfaces) are characterized in relation to their spatial and functional role within larger tectonic domains. A structural framework based on polarity vectors and tectonic position is adopted in the absence of absolute or relative age constraints, enabling the application of time-aware interpolation even in complex metamorphic settings. While it is being translated into a consistent 3D framework, our methodology has been designed so that the geological reasoning typically used in 2D mapping is preserved, such as stratigraphic ordering, interpreted boundaries, cross-cutting relationships and domain correlation. In this context, a Structural Topology model (STm) is developed, emphasizing the hierarchical role of surfaces in space and time. This preserves topological consistency, allowing seamless translation into volume meshes and integration into volume-based modelling environments. The workflow presented here enhances interpretability, reproducibility and conceptual robustness by ensuring that geological and topological information is coherently embedded, particularly in metamorphic settings. The proposed strategy supports a more reliable 3D geological modelling practice, enabling subsequent geoscientific analyses and applications in structurally complex regions, highlighting the benefits of combining surface- and volume-based modelling.