Folds in evaporites. What can we learn about the rock-salt rheology?

Due to the presence of low-viscosity rock-salt, evaporite sequences show a remarkable susceptibility to deformation across diverse geological settings. These sequences often exhibit intercalations of rock-salt with siliciclastic rocks, anhydrite, and sometimes various bittern salts like carnallite and bischofite. Their distinct layering serves as invaluable markers, facilitating a comprehensive analysis of internal salt deformation. The extensive deformation of the evaporites often gives rise to complex internal architectures within the salt body, characterized by commonly observed fold structures. The geometries of these structures are highly sensitive to the mechanical properties of the layers, thus offering profound insights into rock-salt rheology. Unravelling the rheological behaviour of rock-salt holds significant implications, particularly in salt mining, salt cavern operation, and advancing our understanding of salt tectonics. In this project, we focus on specific outcrops within salt mines located in Romania, Austria, and Poland, where prominently exposed fold structures offer unique field laboratories. These sites hold significant potential for deciphering the mechanical behaviour of rocks during their long-term deformation. In our study, we combined field observations, detailed mapping and microstructural analysis of various single and multilayer folds complemented by numerical models of fold evolution. In our numerical simulations, we use the Carreau model for rock-salt, which captures two primary deformation mechanisms: pressure solution and dislocation creep. The mechanisms correspondingly result in the linear (Newtonian) and non-linear (power-law) rheological regimes, influenced by rock grain size and differential stress. Varying the rock-salt grain size enabled us to analyse fold evolution in both regimes as well as in the transitional domain. By systematically comparing our numerical analysis with field observations, we refine our understanding of the mechanical properties of evaporites, contributing to advancements in the study of rock deformation.