鲜水河断裂带乾宁段蠕滑变形及局部粘滑变形的岩石学记录

鲜水河断裂带作为控制青藏高原物质大规模向东侧向挤出的北部边界，是中国乃至世界上大陆内部强震活动最频繁的断裂带之一，但大地测量学研究表明它具有广泛的浅部蠕滑变形行为。认识断裂带蠕滑与粘滑兼具的变形行为及其控制因素是评估鲜水河断裂带地震活动性的关键。本文以鲜水河断裂带乾宁段在四川甘孜州龙灯乡出露的宽约800m的岩石-构造剖面为研究对象，通过野外地质调查、室内光学显微镜及扫描电镜观察、粉末X射线衍射分析和微区X射线荧光光谱分析等方法，对断层岩的结构构造、矿物组成和地球化学性质开展详细研究。研究剖面由约100m宽的砂岩破碎带和约700m宽断层岩组成，广泛发育断层角砾岩和薄层且呈弥散式分布的断层泥，断层岩内普遍具有压溶构造，但局部断层岩具有快速滑动结构特征，表明断裂带在蠕滑变形过程中存在局部地震粘滑变形行为。断裂带内流体作用使断层岩中长石和白云母蚀变成以伊利石为主的粘土矿物（含量5%~11%），新生的粘土矿物在密集的面理中定向排列，促进了断层的弱化和在浅部（4km以上深度）的蠕滑变形。而在断裂带局部裂隙较为发育的位置，流体的注入导致了大量碳酸盐矿物（方解石和白云石，含量12%~33%）的沉淀和Ca元素的富集，使断层岩内强矿物（石英、长石、方解石和白云石）增多，造成了蠕滑断层的局部强化，促进了断层浅部变形行为由非震蠕滑向地震粘滑的转变。该认识可更好地了解蠕滑断层的地震行为并为区域防震减灾提供科学依据。

Petrological records of creeping and locally stick-slip deformation along Qianning segment of Xianshuihe fault

The Xianshuihe fault, as the northern boundary controlling the large-scale eastward extrusion of Tibetan Plateau, is one of the most seismically active faults within the mainland of China and even in the world. However, geodetic studies have shown that it is extensively creeping at shallow depths. Understanding the deformation behavior of both creep- and stick-slip and what controls their occurrence are critical to evaluate the seismic activity of the Xianshuihe fault. This study focuses on a ~800m-wide rock-structure outcrop along Qianning section of Xianshuihe fault near Longdeng Town in western Sichuan. Structural features, mineral composition and geochemical properties of fault rocks in this outcrop were analyzed in detail through field geological investigation, optical and scanning electron microscopy, powder X-ray diffraction and micro area X-ray fluorescence spectroscopy analyses. The study profile is composed of a~100m-wide sandstone damage zone and a~700m-wide fault rock section. The latter contains highly developed fault breccias and interspersed thin fault gouge layers, which generally show pressure solution structures. However, local fault rocks along this profile show rapid-slip characteristics. It suggests that the fault zone experiences local stick-slip during extensive creeping deformation. Fluid-rock interactions within the fault zone alter feldspar and muscovite present in fault rocks into clay minerals (content 5%~11%) dominated by illite. The new formed clays show oriented distribution in dense foliations. These interactions and clay fabric promote weakening and creeping deformation of the fault at shallow depths (above 4km). On the other hand, fluid influx leads to the precipitation of abundant carbonate minerals (content 12%~33%, calcite and dolomite) and the enrichment of Ca elements where fractures are locally developed in the fault zone. These results in an increase content of strong minerals (quartz, feldspar, calcite and dolomite) within fault rocks, locally strengthening the creeping fault and promoting the fault behavior switch from aseismic creeping slip to seismic stick slip. Our study results help to better understand the seismic behavior of creeping faults and provide scientific basis for regional earthquake prevention and disaster reduction.