Architecture and controlling factors of intra-salt deformation in diapiric structures: A numerical modelling approach

Salt tectonics is often simplified with a homogeneous halite rheology, but natural evaporite sequences are heterogeneous, including frictional-plastic anhydrite and low-viscosity K-Mg salts, that can alter the architecture and controlling factors of intra-salt deformations in diapiric structures. We use 2D high-resolution finite-element simulations (FANTOM) to investigate how the vertical position of intra-salt layers controls the formation, geometry, and internal architecture of salt diapirs. The models simulate diapirism driven by sedimentary loading (with varying sedimentation rates and no basal tectonics) and explore different intra-salt stratigraphies. Our results shows that layer position have a first-order control on diapir evolution. When an anhydrite layer is placed at the top of the salt sequence, it acts as a stiff caprock that limits salt flow, resulting in a broad, low-relief salt structure with minimal surface deformation. In contrast, a mid-level anhydrite induces flow partitioning and a bimodal deformation pattern: it decouples movements above and below anhydrite, producing sharp diapir margins and localized folding and disruption of the internal layers. This leads to contrasted intra-diapir complexity. If the strong layer is located near the base of the salt, it initially shows high diapirism from the upper salt but eventually forces the lower salt to flow inside this first diapirs. These tall diapirs are associated with intense rotation of the minibasins and the development of welds where the intra-salt layer breaks and salt flows upward. The presence of low-viscosity K-Mg salt layers further amplifies internal deformation: these weak units flow fast and undergo drastic thinning, creating additional shear zones and irregular internal geometries without significantly impeding diapir growth. Our high-resolution models demonstrate that even thin intra-salt layers significantly influence the localization of deformation, thereby shaping both the external form and internal structure of diapirs. These results are applicable to layered evaporite sequences (LES, e.g. Zechstein Basin) and offer a new way for interpreting complex intra-salt features observed at the seismic scale. These insights have important implications for structural interpretation, resource exploration, and the development of salt formations as effective caprock for CO₂ and for hydrogen storage in salt caverns.