Rock characteristics, internal structure and physical-chemical properties of Qianning segment in Xianshuihe fault zone

Abstract

断裂带的变形行为和断层滑移机制是目前地震研究关注的热点，断裂带岩石特征、内部结构与物理化学性质是确定断层蠕滑或粘滑行为以及断层滑移机制的基础和关键。本文以鲜水河断裂带乾宁段地表出露的断裂岩为研究对象，通过野外地质调查、室内光学显微镜、扫描电镜、粒度统计、粉末X射线衍射分析（XRD）和薄片X射线荧光光谱分析（XRF）等多种研究方法，对鲜水河断裂带岩石特征、结构构造、物性、矿物成分及化学元素分布开展了详细的分析，并探讨了相关变形行为和滑移机制。分析表明：（1）断裂带核部主要由黑色断层泥、浅黄色及黄色断层角砾岩和灰色碎裂岩、灰色断层角砾岩组成，呈单核对称结构；（2）黑色断层泥厚3~5cm，具有快速滑动结构特征，表现为断层粘滑行为。断层泥可划分出13个滑移带，最窄滑移带厚约40μm，至少代表13期古地震事件；（3）断层泥主要由伊利石、高岭石和石英等矿物组成，其中边部伊蒙混层含量异常高，为最新一次古地震的主滑移带。由于伊利石和伊蒙混层（或蒙脱石）为主要黏土矿物的断层泥渗透率低、孔隙流体压力大，以及发现断层泥楔入脉，表明地震过程中断层滑动存在热增压弱化机制；（4）从断层泥不同滑动带中碎块蚀变程度和矿物分布特征来看，地震主滑动带有向碎裂岩方向迁移的趋势。推测断层在滑动过程中，更趋向于向弱矿物含量高（如伊利石、伊蒙混层）、强矿物含量低（如方解石、高岭石）的围岩一侧迁移。;The deformation behavior and slipping mechanism of fault zone are the focus of current seismic research. Rock characteristics, internal structure and physical-chemical properties of fault zone are the basis and key to determine creep- or stick-slip behavior and fault slipping mechanism. This study focuses on fault rocks from Qianning segment in Xianshuihe fault zone. Rock characteristics, structure, physical properties, mineral composition and chemical element distribution of the Xianshuihe fault zone have been analyzed in detail through field geological survey, observation of optical microscope and scanning electron microscope, particle-size statistics, powder X-ray diffraction analysis (XRD) and thin section X-ray fluorescence spectrum analysis (XRF). Related deformation behaviors and slipping mechanisms have been discussed. The results show that: (1) the core of fault zone is mainly composed of black fault gouge, light yellow and yellow fault breccia, gray cataclasite, and gray fault breccia, presenting a single-core-symmetrical structure; (2) the black fault gouge is 3~5cm thick and shows rapid-slip structure, indicating stick-slip behavior of the fault zone. The fault gouge can be divided into 13 slip zones, and the narrowest slip zone is about 40μm thick, representing at least 13 paleoearthquake events; (3) the fault gouge is mainly composed of illite, kaolinite, and quartz. The content of illite-smectite (I/S) mixed layer at the edge of fault gouge is abnormally high, which presents the primary slip zone of the latest paleoearthquake. The low permeability and high pore fluid pressure caused by main clay minerals of illite and I/S mixed layer (or smectite) in fault gouge and the discovery of fault gouge injection veins indicate that there is a thermal pressurization weakening mechanism in the fault zone sliding during the earthquake process; (4) according to the degree of fragment alteration and mineral distribution characteristics in different slip zones of fault gouge, the main seismic slip zone has a tendency to migrate towards cataclasite. It is speculated that during the sliding process, the fault tends to migrate to the side of the surrounding rock with high content of weak minerals (such as illite and I/S mixed layer) and low content of strong minerals (such as calcite and kaolinite).