Abstract: It has been found from field observation and laboratory study that the fault rocks on the Beishicheng fault zone in Miyun region, Beijing can be classified into four main types: cataclasite series, mylonite series, pseudotachylite and fault gouge. They have distinct features in mineralogy, deformation and microfabric, representing the different mechanisms of their origin. It is shown by the features and occurrences of the fault rocks in this region that the fault zone had experinced two stage of movement of different mechanical behavior: the ductile shearing in the early stage and the brittle fracturing in the late stage accompanied by seismic events.The existences of pseudotachylite and cataclasite types of fault rocks can be regarded as the potential evidences for seismic events. The former is produced by preferred melting of minerals in country rocks due to the frictional heating on the fault plane during seismic faulting, while the later is a product of brittle fracturing due to rapid release of strain energy. During rapid frictional heating process of seismic faulting, the mafic minerals are the first to be melted, but the felsic minerals are melted at the temperature much higher than the mafic ones, so that the SiO2 contents in the pseudotachylite are much lower than that in the country rocks. That is why the crush breccia and breccia in pseudotachylite consist mainly of felsic materials, and this characteristics are completely different from that of magmatic origin.

摘要： 通过对北京密云北石城断裂带的野外和室内工作,该断裂带的断层岩可分为四种类型:碎裂岩类、糜棱岩类、假熔岩和断层泥。它们具有不同的矿物学、形变和组构特征,分别代表不同的成因机制。假熔岩和碎裂岩类断层岩的存在,可作为古地震事件的可能证据。前者是地震断层运动在断层面上摩擦增温,引起围岩中矿物的选择性熔化形成的。在该断裂带中可能发生不止一次的古地震事件。断层岩特征和产状表明,该断裂带经历了两种不同力学性质的运动,早期为韧性剪切;晚期为脆性破裂,并伴有地震发生。