Abstract. 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.

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盐矿的多层褶皱的形态，微观结构和数值模型

摘要。盐构造和盐工程中的变形分析和预测需要有关岩盐力学特性的信息，时间尺度远比实验室可能长。众所周知，在实验室时间尺度上，具有不同组成和微观结构的岩盐样品在稳态蠕变速率上会显示出差异，但是在低应变速率和相应的偏应力下如何表现出这种差异是未知的。在这里，我们旨在通过分析地质时间尺度上形成的多层褶皱来对此进行量化。我们在罗马尼亚的Ocnele Mari盐矿中研究了层状折叠岩盐的优异暴露情况。地层由超过90％的盐岩组成，而明显的多尺度分层是由杂质比例的变化引起的。区域构造强烈剪切后，区域构造和矿山褶皱结构与剪切带中的变形相一致，形成了构造剪切沉积层的十多米级人字形褶皱，铰链上不同尺度上出现了较小的褶皱。折叠模式在各种尺度上的形态学清楚地表明，在折叠过程中该序列被机械地分层。与亮层相比，暗层包含更多的杂质，并且具有比常规层更规则的层厚度的特征，因此被推断为具有更高的粘度。伽玛修饰的样品的光学显微镜显示出强形状的平行于叶面的盐岩晶粒首选取向，该取向平行于所研究褶皱的轴向平面。微观结构表明位错蠕变，伴随着大量的流体辅助重结晶，以及有力的证据表明溶液-沉淀蠕变表明了折叠过程中线性（牛顿）粘性流变。折叠过程中的偏应力小于分离过程中的剪切过程中的偏应力，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折痕发展，限制了选定露头的相对层厚度，并设计了数值模型。我们探索层之间不同的牛顿粘度比对不同尺度上的演化褶皱的影响。通过现场数据和数值结果的比较，我们估计层之间的有效粘度比大于10且高达20。我们证明，层厚度的显着变化并不是在不同尺度上形成褶皱的关键因素。取而代之的是，薄层的不均匀分布会有效地将自身组织为厚度可变的单层，从而引发各种规模的变形。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低偏应力下岩盐变形的粘度并引入各向异性粘度。