The stress and strain state of Yalahe fault in the Kangding segment of the Xianshuihe fault zone and its seismogenic environment

Abstract

鲜水河断裂带位于青藏高原东缘，是中国大陆内部地震活动性最强的大型左行走滑断裂之一，揭示其深部应力状态与应变是认识孕震环境和评估断裂带地震危险性的重要依据。本文选择鲜水河断裂带未来强震危险性较高的康定段作为研究对象，聚焦雅拉河断裂带内的糜棱岩，通过光学显微镜和扫描电镜（SEM）的显微构造分析、粒度统计、石英组构和Ti含量测试，确定石英递进变形过程中差应力的变化和不同阶段的变形温度，构建鲜水河断裂带康定段的地壳强度剖面。观测结果显示，糜棱岩在抬升过程中石英发生颗粒边界迁移（GBM）、亚颗粒旋转（SGR）和膨凸重结晶（BLG）三种类型的变形机制。构建的地壳强度剖面显示，雅拉河断裂带应变速率约为6.08×10^-13~1.62×10^-11s^-1，脆韧性转化带发生在~12.5±2.5km深度，该处岩石极限强度~145.5MPa。实际岩石强度（μ=0.383）小于理论破裂摩尔圆（μ=0.85）的岩石强度发育应变弱化。当韧性变形阶段由位错蠕变形成的应变局部化以及细粒化等机制降低岩石的强度至临界条件时，会导致破裂频次的增强，应力释放发生地震。同时，位错蠕变引起的应变弱化导致岩石强度降低，使高应力无法积累。;The Xianshuihe fault zone, located at the eastern margin of the Tibetan Plateau, is a sinistral strike-slip fault zone with current ~10mm/yr slip rate. Calculation of crustal stress state and strain rates of fault zone is important to understand seismogenic environment and assess the seismic risk along the active segments of fault zone. In this contribution we select the Kangding section of the Xianshuihe fault zone, which has a higher risk of strong earthquakes in the future, as the research object, focusing on the mylonite in the Yalahe fault. According to optical microscope and scanning electron microscope (SEM) microstructure analysis, grain size statistics, quartz fabric and Ti content test to determine the change of differential stress and temperature at different stages during the progressive deformation of quartz, and construct the crustal strength profile of the Kangding section of the Xianshuihe fault zone. Three types of quartz deformation mechanisms can be identified, including grain boundary migration (GBM), sub-grain rotation (SGR), and bulging recrystallization (BLG) during the gradual mylonization. The established crustal strength profile shows that the strain rate of the Yalahe fault is about 6.08×10^-13~1.62×10^-11s^-1, and the ultimate strength of the rock in the brittle-ductile transition zone is ~145.5MPa, which is corresponding ~12.5±2.5km depth. The actual rock strength (μ=0.383) is smaller than the theoretical rupture Mohr circle (μ=0.85), which indicated the strength of the rock is weakened within the fault zone. The strain localization and grain size reduction at the ductile deformation stage can reduce the strength of the rock, which may lead to an increase in the frequency of rupture, stress release and earthquakes. However, the weakening of the mylonite caused by the dislocation creep leads to a decrease in the strength of the rock, which makes it impossible to accumulate high stress earthquake.