| 龙门山灌县—安县断裂带断层泥低磁化率的矿物、化学响应和蠕滑作用环境 |
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| 引用本文： | 何祥丽,李海兵,张蕾,王焕,葛成隆,曹勇,白明坤,李成龙,叶小舟,韩帅.龙门山灌县—安县断裂带断层泥低磁化率的矿物、化学响应和蠕滑作用环境[J].地球物理学报,2018,61(5):1782-1796. |
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| 作者姓名： | 何祥丽 李海兵 张蕾 王焕 葛成隆 曹勇 白明坤 李成龙 叶小舟 韩帅 |
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| 作者单位： | 1. 地球科学与资源学院, 中国地质大学(北京), 北京 100083;2. 自然资源部深地动力学重点实验室, 中国地质科学院地质研究所, 北京 100037;3. 自然资源部古地磁与古构造重建重点实验室, 中国地质科学院地质力学研究所, 北京 100081 |
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| 基金项目： | 国家自然科学基金项目（41520104006，41330211，41602226）和中国地质科学院基本科研业务费项目（YYWF201601）联合资助. |
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| 摘 要： | 断层带内的流体不仅可以通过水岩反应改变断裂岩的矿物组成和化学成分，从而导致化学性质和物理性质的变化，而且可以影响或控制断裂带的变形行为.断裂带中岩石磁学特征是由特定化学环境下磁性矿物的种类和含量所决定的，因此，从矿物学和地球化学角度探讨断裂岩的磁性变化，对揭示断层的变形行为和环境具有一定的指示作用.本文以汶川科钻WFSD-3P钻孔中龙门山灌县—安县断裂带断裂岩为研究对象，运用高分辨率磁化率测试、XRD矿物成分半定量分析、XRF元素扫描以及不同价态Fe元素含量分析等多种方法开展断层磁学变化和变形环境的研究.磁化率测试结果表明灌县—安县断裂带断层泥的磁化率值普遍低于对应的围岩磁化率平均值.结合前人研究成果表明造成该断层泥低磁化率异常的原因是在间震期的长期流体作用下，铁磁性矿物（例如磁铁矿）转变成顺磁性矿物（铁硫化物、菱铁矿或含铁的黏土矿物）.新生铁硫化物和含铁黏土矿物是在间震期缓慢形成的，而黏土矿物含量的增加弱化了断层强度，促进断层蠕滑，这说明断层泥低磁化率异常可能指示了该断裂在间震期长期缓慢活动，即为蠕滑变形.断层泥中黄铁矿的发育和高Fe2+和S元素、低Fe3+的特征显示灌县—安县断裂作用环境通常是在低温、还原环境中进行的.这些结果与低磁化率值的相关性暗示断层泥低磁化率异常可能对活动断层的低温还原环境具有指示意义.
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| 关 键 词： | 灌县—安县断裂带 断层泥 磁化率 蠕滑 WFSD-3P |
| 收稿时间： | 2018-03-19 |
Mineral and chemical response to low magnetic susceptibility of the fault gouge from the Guanxian-Anxian fault zone and fault creep setting in the Longmen Shan |
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| HE XiangLi,LI HaiBing,ZHANG Lei,WANG Huan,GE ChengLong,CAO Yong,BAI MingKun,LI ChengLong,YE XiaoZhou,HAN Shuai.Mineral and chemical response to low magnetic susceptibility of the fault gouge from the Guanxian-Anxian fault zone and fault creep setting in the Longmen Shan[J].Chinese Journal of Geophysics,2018,61(5):1782-1796. |
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| Authors: | HE XiangLi LI HaiBing ZHANG Lei WANG Huan GE ChengLong CAO Yong BAI MingKun LI ChengLong YE XiaoZhou HAN Shuai |
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| Institution: | 1. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;2. Key Laboratory of Deep-Earth Dynamics of Natural Resources Ministry, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;3. Key Laboratory of Paleomagnetism and Tectonic reconstruction of Natural Resources Ministry, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China |
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| Abstract: | Fluid infiltration within fault zones can not only alter the mineral and chemical composition of fault rocks by fluid-rock interactions and thus causes variation in chemical and physical properties, but can also influence or control the deformational behavior of a fault zone. Rock magnetic property in the fault zone is controlled by the type and content of magnetic minerals in a particular chemical environment. Therefore, studying the magnetic variation of fault rocks by mineralogy and geochemistry can reveal the deformational behavior and environment of an active fault. In this study, high-resolution magnetic susceptibility measurements, X-Ray Diffraction (XRD) minerals analyses, X-Ray Fluorescence (XRF) element scanning, and Fe element geochemical analyses were implemented on some representative cores from the Wenchuan earthquake Fault Scientific Drilling Hole-3P (WFSD-3P) located on the Guanxian-Anxian Fault zone (GAF) to study the magnetic variation and deformational environment of the fault. The measurement results show that the magnetic susceptibility values of the fault gouge in the GAF are commonly less than the average of the corresponding host rocks. The low magnetic susceptibility of the fault gouge results from the transformation from ferromagnetic minerals (such as magnetite) to paramagnetic minerals (Fe-sulfides, siderite, or iron-bearing clay minerals) under long-term fluid infiltration during interseismic periods based on Scanning Electron Microscope (SEM) observation and previous results. New Fe-sulfides and iron-bearing clay minerals were formed slowly during interseismic periods. An increase in clay minerals content weakens the fault and facilitates the creeping of the GAF, which suggests that low magnetic susceptibility of the fault gouge may be an indicator of a slowly slipping active fault during interseismic periods (i.e. creep deformation). The development of pyrite, relatively high contents of Fe2+ and sulfur element, and the low content of Fe3+ in the fault gouge implies that the faulting of the GAF is commonly at a low temperature and reducing environment. The relationship between the above observations and the low magnetic susceptibility of the fault gouge shows that the latter may attest to the interseismic low-temperature and reducing environment along an active fault. |
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| Keywords: | Guanxian-Anxian fault zone Fault gouge Magnetic susceptibility Creeping fault WFSD-3P |
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