Paleostress Analysis from Calcite Twins at the Longshan Dome (Central Hunan, South China): Mesozoic Mega-Fold Superimposition in the Reworked Continent
Paleostress Analysis from Calcite Twins at the Longshan Dome (Central Hunan, South China): Mesozoic Mega-Fold Superimposition in the Reworked Continent
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DOI：
10.3390/geosciences11110456
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发表时间：
2021-11
期刊：
影响因子：
2.7
通讯作者：
Jian Zheng;Y. Shan;Simin Hu
中科院分区：
文献类型：
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作者：
Jian Zheng;Y. Shan;Simin Hu
人们普遍认为，中生代北北东向褶皱叠印了东西向褶皱，形成了华南大陆中部的龙山穹隆，尽管干涉图并没有告诉褶皱组的相对年龄。为了加深我们对大陆改造过程的理解，本研究使用方解石孪晶进行古应力分析，以验证或证伪该模型。从穹顶侧面的上古生界石灰岩中采集了 10 个石灰岩样品，并使用方解石 e-twin 通用平台进行了测量。样品中的电子孪晶可分为厚（≥1 μm）和薄（<1 μm）两种，分别表明变形温度相对较高和较低。使用 Shan 等人 (2019) 方法的改进版本获得的应力估计分为两个层并行缩短 (LPS) 子集和三个非 LPS 子集。这些子集包括四种构造模式：NWW-SEE 挤压（LPS1 和非 LPS1）、NNE-SSW 挤压（LPS2 和非 LPS2）、NW-SE 伸展（非 LPS3a）和 NNE-SSW 伸展（非 LPS3b）。它们进一步按温度降低的顺序排列，以建立研究区域的复杂变形序列。在序列中，NE−NNE 走向的褶皱的年龄比东西走向的褶皱更老，这与模型有所不同。造成前褶皱的大约南北向区域压缩应该会对强烈变形的大陆产生深远的影响，这一点在早期的研究中被忽略了。
It is generally accepted that during the Mesozoic NE−NNE-trending folds overprinted E−W-trending folds to form the Longshan dome in the central South China continent, although the interference map does not tell the relative ages of the fold sets. In an effort to deepen our understanding of the process of reworking the continent, paleostress analysis using calcite twins was carried out in this study to verify or falsify this model. Ten limestone samples were collected from Upper-Paleozoic limestones on the flanks of the dome and were measured using the universal stage for calcite e-twins. E-twins in the samples are divisible into two kinds, thick (≥1 μm) and thin (<1 μm), indicative of relatively higher and lower deformation temperatures, respectively. Stress estimates obtained using the improved version of Shan et al.’s (2019) method were grouped into two layer-parallel shortening (LPS) subsets and three non-LPS subsets. These subsets comprise four tectonic regimes: NWW−SEE compression (LPS1 and non-LPS1), NNE−SSW compression (LPS2 and non-LPS2), NW−SE extension (non-LPS3a) and NNE−SSW extension (non-LPS3b). They were further arranged in a temperature-decreasing order to establish a complex deformation sequence of the study area. In the sequence NE−NNE-trending folds have an older age than E−W-trending folds, something different from the model. The approximately N−S regional compression responsible for the former folds should have a profound effect on the intensely deformed continent, something ignored in earlier work.