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 and 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 in rock salt over geological timescale 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 10 m scale chevron
folds of a tectonically sheared sedimentary layering, with smaller folds
developing on different scales in the hinges. Fold patterns at various
scales clearly indicate that during folding, the sequence was mechanically
stratified. The dark layers contain more impurities and are characterised
by a more regular layer thickness compared to the bright layers and
are thus 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 recrystallisation
and strong evidence for solution–precipitation creep. This provides support
for linear (Newtonian) viscous rheology as a dominating deformation
mechanism during the folding. Deviatoric stress during folding was lower
than during shearing in the detachment at 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. By comparing 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 organise themselves into effectively
single layers with variable thickness, can control deformation on various
scales. Our results show that impurities can significantly change the
viscosity of rock salt deforming at low deviatoric stress and introduce
anisotropic viscosity, even in relatively pure layered rock.
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