Building a geological legend for 3D geomodelling in metamorphic belts

Geological 3D modelling in metamorphic belts remains a significant challenge in structural geology due to both mathematical and geological complexities. These challenges stem from the need for software capable of interpolating polydeformed surfaces explicitly or implicitly, while at the same time addressing the geological and topological meaning of these surfaces, i.e., the “geological legend” of the 3D model.
Traditional 3D geological modelling uses the boundary representation paradigm, where geological units are represented as hollow volumes bounded by discretized surfaces, typically stratigraphic boundaries or faults. Explicit interpolation methods generate these surfaces individually, possibly leading to inconsistencies. In contrast, implicit methods interpolate entire stratigraphic sequences in a single step, enabling faster workflows and ensuring mathematical consistency. Moreover, implicit methods produce a continuous (locally discontinuous at faults) volumetric “stratigraphic field” that assigns a scalar value representing a geological absolute or relative age, and boundaries are extracted a-posteriori (hence the name of the methods). Extensions of this approach, known as “GeoChron Model” or “time-aware geomodelling,” enable the assignment of ages to depositional, intrusive, or deformative events, linking the mathematical model to a well-defined sequence of geological events.
Here we propose a workflow that combines implicit and explicit modelling to facilitate conceptual interpretation, ensuring topologically and geologically consistent 3D model reconstruction in metamorphic belts. These regions pose particular challenges because time-aware geomodelling is often inapplicable due to the ill-defined or heterogeneous ages of tectonic boundaries, lithologies in tectono-metamorphic units, and deformation-related features like metamorphic foliations.
In our approach, 3D surfaces are analysed and labelled based on their topological relationships with surrounding geological objects in a preliminary conceptual modelling step, where both surface and volume perspectives are considered. Since boundary surfaces can have multiple roles depending on the geological context and might have been reactivated in polyphase deformation, it is essential to implement a systematic classification of volumes, that are distinguished as tectono-metamorphic, tectono-stratigraphic, or intrusive units (implying different boundary surfaces).
A critical strategy is the use of a time-aware legend wherever possible, such as for geological bodies with known absolute or relative ages. When age information is unavailable, as in very old basement complexes, or for coeval but spatially distinct units (e.g., ophiolite sequences emplaced at different crustal levels), a reasonable pseudo-stratigraphy is adopted (e.g. using relative structural levels instead of stratigraphic age).
Our combined workflow provides a structured and replicable methodology for addressing the unique challenges of 3D geological modelling in metamorphic belts. By systematically handling complex geological features, topological relationships, and polydeformed surfaces, it ensures more consistent and reliable geological models. This framework is expected to enhance interpretations in future studies and advance our understanding of metamorphic belts.