Abstract: How to survey fault activity and determine seismogenic structures in a relatively stable and bedrock-distributed region is a challenging research work. Ruichang-Tonggu Fault and Yifeng-Jingdezhen Fault, distributed mainly at the pre-Cenozoic bedrock region, show the geological evidences of activity in the Quaternary and they are two important tectonic belts for the occurrence of moderate earthquakes in the central-northern Jiangxi Province. Fault gouge stripes can be found on the outcrop sections of the two faults. The imageries of the microstructures of fault gouge show abundant phenomena about the structural deformation, and it is clear that the fault gouge was formed by structural activity. As to the deformation modes, there are not only the Y-shears and R1-shears, which represent the localized-brittle deformation, but also the P-foliations, which reflect the ductile deformation in the microstructures of fault gouge. These features demonstrate that the micro-scale rapid deformation can exist in the seismogenic structure at the near-surface in the occurrence process of moderate earthquakes very possibly. The microstructures of soft material from the fault outcrop section at the southern segment of Hukou-Xingan Fault, which is inactive in the Quaternary, reflects that the soft material from the fault zone can also be the products of rainwater leaching and filling, or weathering in the later periods. Based on the macroscopic observation in the field, it is sometimes difficult to distinguish the differences of structurally-formed or non-structurally formed soft gunk in the fault zones, however, their differences in the microstructure on the slices grinded indoor are obvious. The relatively stable bedrock areas in South China often are not only favorable for the siting of major projects, such as nuclear power plant in China, but also the economically-developed, densely populated, urban agglomeration areas. The study of microstructure of fault gouge provides a technical reference approach for the identification of fault activity and the determination of seismogenic structure of moderate-strong earthquakes in assessing the seismotectonic environment in these regions.

Key words: central-northern Jiangxi Province, fault at bedrock region, fault gouge, microstructure, moderate earthquake

摘要： 如何在相对稳定的基岩区开展断裂活动性调查与发震构造判定是一项具有挑战性的研究工作。江西中北部瑞昌-铜鼓断裂和宜丰-景德镇断裂主要发育在前新生代基岩区，但存在第四纪有过活动的地质和年代学证据，是2条重要的中强地震构造带。在这2条断裂露头剖面上均发育断层泥条带，断层泥显微构造图像揭示了丰富的构造变形现象，构造成因机制明确。在变形方式上，断层泥显微构造中既发育Y剪切、R剪切以及棱角状、次棱角状碎斑等等局部化脆性变形特征，又有P叶理和碎屑颗粒拖尾构造等韧性变形特征。在中强地震发生过程中，沿着发震构造的近地表滑动面很可能存在微观尺度的构造变形。在缺少第四纪活动证据的湖口-新干断裂南段露头剖面上采集的松软物质的显微构造研究结果，反映了断裂构造带上泥状松软物质也可以是后期雨水淋滤充填或风化的产物。在野外现场观察中，断裂滑动面上构造成因与非构造成因的泥状松软物质有时很难进行区分；而在室内磨制的薄片显微构造观察中，两者之间的显微构造差异明显。华南相对稳定的基岩区常常是中国重大工程（如核电厂）选址中优先考虑的地区，同时也是中国经济发达、人口密集的城市群主要分布区；在这些地区的地震构造环境评价中，断层泥显微构造研究为鉴定断裂活动性、判定中强地震发震构造提供了可以借鉴的技术途径。

关键词: 江西中北部, 基岩区断裂, 断层泥, 显微构造, 中强地震