Rheological stratification in impure rocksalt during long-term creep: morphology, microstructure and numerical models of multilayer folds in the Ocnele Mari salt mine, Romania

Analysis and prediction of deformations in salt tectonics and salt engineering require information about the mechanical properties of rocksalt at time scales far longer than possible in the laboratory. It is known that at laboratory time scales, rocksalt samples with different composition and microstructure show a variance in steady-state creep rates, but it is not known how this variance is manifested at low strain rates and corresponding deviatoric stresses. Here, we aim to quantify this from the analysis of multilayer folds that developed over geological time scale. We studied excellent exposures of layered, folded rocksalt in the Ocnele Mari salt mine in Romania. The formation is composed of over 90 % of halite, while distinct multiscale layering is caused by variation in the fraction of impurities. Regional tectonics and mine-scale fold structure are consistent with deformation in a shear zone, after strong shearing in a regional detachment, forming over ten meter-scale chevron folds of a tectonically sheared sedimentary layering, with smaller folds developing on different scales in the hinges. Morphology of the fold pattern at various scales clearly indicates that during folding the sequence was mechanically stratified. The dark layers contain more impurities and are characterized with a more regular layer thickness as compared to the bright layers and, thus, are inferred to have higher viscosities. Optical microscopy of Gamma-decorated samples shows a strong shape preferred orientation of halite grains parallel to the foliation, which is reoriented parallel to the axial plane of the folds studied. Microstructures indicate dislocation creep, together with extensive fluid-assisted recrystallization and strong evidence for solution-precipitation creep indicative for linear (Newtonian) viscous rheology during folding. Deviatoric stress during folding was lower than during shearing in the detachment, around 1 MPa. We investigate fold development on various scales in a representative multilayer package using finite element numerical models, constrain the relative layer thicknesses in a selected outcrop and design a numerical model. We explore the effect of different Newtonian viscosity ratios between the layers on the evolving folds on different scales. Through the comparison of the field data and numerical results, we estimate that the effective viscosity ratio between the layers was larger than 10 and up to 20. Additionally, we demonstrate that the considerable variation of the layer thicknesses is not a crucial factor to develop folds on different scales. Instead, unequal distribution of the thin layers, which organize themselves into effectively single layers with variable thickness can trigger deformation at various scales. Our results show that impurities can significantly change the viscosity of rocksalt deforming at low deviatoric stress and introduce anisotropic viscosity, even in relatively pure, layered rock.