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.

中文翻译：

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长期蠕变过程中不纯岩盐的流变分层：罗马尼亚 Ocnele Mari 盐矿多层褶皱的形态、微观结构和数值模型

在实验室时间尺度上，具有不同成分和微观结构的岩盐样品在稳态蠕变速率方面表现出差异，但尚不清楚这种差异是否以及如何在低应变率和相应的偏应力下表现出来。在这里，我们的目标是通过分析罗马尼亚 Ocnele Mari 盐矿在地质时间尺度上在岩盐中形成的多层褶皱来量化这一点。地层由超过 90% 的岩盐组成，而明显的多尺度分层是由杂质比例的变化引起的。区域构造和矿山尺度褶皱结构与区域拆离强烈剪切后的剪切带变形一致，形成了10 m以上尺度的构造剪切沉积层状人字形褶皱，铰链处发育不同尺度的较小褶皱。不同尺度的折叠模式清楚地表明，在折叠过程中，序列被机械分层。与亮层相比，暗层含有更多杂质，其特征在于层厚度更规则，因此推断具有更高的粘度。伽马装饰样品的光学显微镜显示，岩盐颗粒具有强烈的形状偏好取向，平行于叶理，其重新定向平行于所研究的褶皱的轴向平面。微观结构表明位错蠕变，以及广泛的流体辅助再结晶和溶液-沉淀蠕变的有力证据。这为作为折叠过程中主要变形机制的线性（牛顿）粘性流变提供了支持。折叠过程中的偏应力比分离过程中的剪切过程低，约为 1 MPa。我们使用有限元数值模型研究了具有代表性的多层包装中各种尺度的褶皱发展，限制了选定露头的相对层厚度，并设计了一个数值模型。我们探索了层之间不同的牛顿粘度比对不同尺度上不断演化的褶皱的影响。通过比较现场数据和数值结果，我们估计层之间的有效粘度比大于 10 并高达 20。此外，我们证明层厚度的相当大的变化并不是在不同材料上形成褶皱的关键因素。秤。相反，薄层分布不均，它们将自身组织成具有可变厚度的有效单层，可以控制各种尺度的变形。我们的结果表明，即使在相对纯净的层状岩石中，杂质也可以显着改变在低偏应力下变形的岩盐的粘度并引入各向异性粘度。