The Structural Topology Model: A Topology–Ontology Framework for Assigning Geological Legends in 3D and 2D Geomodelling

Abstract. Three-dimensional geological modelling of orogenic belts and other highly deformed complexes remains challenging, especially in metamorphic complexes, where the obliteration of stratigraphy complicates the definition of units, boundaries, and their relations. In such contexts, approaches inherited from modelling in sedimentary basins often lead to ambiguous definition of the legend (and the units represented therein) and topologically inconsistent 3D models. Here, we present the Structural Topology Model (STM), a conceptual framework designed to formalise the construction of consistent geological legends by making explicit the relations between units, bounding surfaces, and their structural roles, according to the concept of oriented topology (based on the properties of continuity and conformability). Central to STM is the concept of the Structural Level, which integrates absolute ages, relative ages, and structural relations into a scalar field used to define an ordered sequence of geological units and events, even where chronological constraints are incomplete or ambiguous. STM was conceived for 3D modelling, but it could be applied also in 2D to geological maps and cross-section providing useful constraints and tools to validate topological relationships and conceptual models. The method is illustrated with two case studies in the Helvetic units of the Aar Massif (Helvetics, Central Swiss Alps) and in Penninic units of the Gressoney Valley (Western Italian Alps). In both examples, STM enables the construction of topologically consistent volumetric models that preserve internal structures, foliation patterns, and cross-cutting relationships, while providing an unambiguous framework for labelling units and boundaries. STM, already implemented in the open-source PZero 3D modelling platform, is independent of specific software implementations or interpolation techniques and could be potentially applied in a wide range of geological settings and software. In this way, limitations of implicit 3D modelling in complex polydeformed areas are reduced substantially.