Salt-influenced fold-and-thrust belts: insights from experimental analog models

In this work, we experimentally explore the role played by salt in contractional systems: from simple layer-cake, salt-floored settings to inverted salt-bearing rifted-margins, the inherent low-strength rheology of salt and its distribution determine particular structural styles and kinematics. Salt-detached contractional systems exhibit low taper angles, with no dominant structural vergence, and are comparatively wider in cross section than their non-salt-involved equivalents. The salt-sediment thickness controls structural spacing while the presence of pre-existing isolated salt bodies breaks the mechanical homogeneity. If salt bodies are connected through a framework of salt walls, contractional deformation concentrates in them and structural trends are no longer perpendicular to shortening but determined by the inherited orientation of salt bodies.

Finally, we assess the inversion of a salt-bearing rifted margin, where late syn-rift to early post-rift salt undergoes differential loading and gravity gliding leading to a salt-sediment architecture consisting of a salt wall-minibasin province, and a distal raft system. Upon shortening, inherited salt bodies localize contractional deformation and the salt-sediment architecture determines the structural configuration of the contractional system. In the experiments, a large-transport thrust detached along allochthonous salt accumulated in the distal raft system, and the squeezing of salt walls together with the tilting of minibasins, accounted for most of the shortening in the salt wall-minibasin province. During shortening, about 75% of the original salt evacuates and is eroded. Due to the low salt-sediment ratio found in fold-and-thrust belts, the role played by salt tectonics could be underestimated. So, the best practice approach for understanding structural style and kinematics of salt-influenced fold-and-thrust belt resides in the rifted margin stratigraphy involved, and not in the salt itself, since salt is generally poorly preserved and strongly deformed.