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

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