The Structural Topology model: a framework for 3D modelling of metamorphic belts

Three-dimensional geological modelling in polydeformed metamorphic belts remains a major challenge, argely due to the inherent structural complexity and the limited suitability of time-constrained or kinematic pproaches. In such geological contexts, the main difficulties arise not only from the methods employed for surface interpolation, but also from the geological and topological significance assigned to each boundary. To address these issues, we propose a modelling strategy that integrates conceptual surface classification ith topological analysis, with the aim of constructing internally consistent geological legends and ensuring the robustness and coherence of 3D structural models. Such an approach is crucial for the shift from 2D base cartography to 3D modelling (Arienti et al., 2024; Musso Piantelli et al., 2022; Pizzella et al., 2024), broadening the scope of classical 3D modelling beyond relatively simple large scale systems (e.g., edimentary basins) or small but intricate ones (e.g., ore bodies) to orogenic belts, which are both vast and highly complex. Our approach can integrate explicit and implicit modelling techniques, relying on conceptual surface labelling and domain-based volume definitions. Geological units are classified as tectonometamorphic (TMU), tectonostratigraphic (TSU), stratigraphic (SU), intrusive (IU), or shear zone (SZ), according to their origin and deformation history. Boundaries are treated not only as surfaces separating adjacent units, but also in terms of their spatial role within broader tectonic domains. In line with both stratigraphic principles and implicit modelling, a unit is defined by its boundaries and its temporal position. Yet, in polydeformed metamorphic belts, neither absolute nor relative time alone is sufficient to establish consistent legends or temporal hierarchies. To overcome this limitation, we adopt the concept of polarity (Parquer et al., 2025): a synthetic temporal descriptor that integrates absolute, relative, and structural information into a coherent sequence. Polarity allows the definition of simplified successions within domains that share a comparable evolutionary history, ensuring temporal consistency and preserving functional topologies in 3D models. While being translated into a consistent 3D framework, the methodology is designed to preserve the geological reasoning typical of 2D mapping, including stratigraphic ordering, interpreted boundaries, cross-cutting relationships, and domain correlation. Within this context, a Structural Topology model (STm) is developed, highlighting the hierarchical role of surfaces in both space and time. This ensures topological consistency and enables seamless translation into volume meshes and integration within volume-based modelling environments. The presented workflow enhances interpretability, reproducibility, and conceptual robustness by coherently embedding geological and topological information, particularly in data-scarce metamorphic settings. Overall, the proposed strategy promotes more reliable 3D geological modelling, facilitating subsequent geoscientific analyses and applications in structurally complex regions, and demonstrating the advantages of combining surface- and volume based approaches.