龙门山造山带的早期活动及其对造山作用的启示

2008年汶川地震后,在映秀-北川同震地表破裂带南段虹口乡八角庙地区发现有假玄武玻璃出露于~240m宽的断裂带内,代表了断裂带以往地震和断裂活动的直接产物。这套假玄武玻璃的高温熔融成因得到了元素地球化学和熔融结构的证实。玻璃基质、蚀变矿物和碎屑斑晶的化学分析显示假玄武玻璃继承了碎裂岩/超碎裂岩围岩的主要化学成分,除石英外,主要由长石和云母两种端员组分选择性熔融形成,并呈现出了化学组分分布的不均一性。假玄武玻璃的锆石U-Pb和玻璃基质~(40)Ar/~(39)Ar定年结果证实映秀-北川断裂的古地震发生于229~216Ma的中-晚三叠世,并具有11~14km的震源深度,表明映秀-北川断裂的早期活动始于印支期的造山运动。伴随着印支造山运动的发生,龙门山断裂带形成了其初始构造框架,并对之后的构造演化产生了深远的影响。

Early tectonic activity and its implications for the orogenesis in the Longmen Shan orogenic belt

After the 2008 Wenchuan earthquake, a pseudotachylyte vein, which is the direct product of the ancient earthquake and faulting, was found in a ~240m-wide fault zone along the southern section of the Yingxiu-Beichuan co-seismic surface ruptures in the Bajiaomiao village, Hongkou town. The pseudotachylyte formation by high-temperature melting is corroborated by a detailed petrological investigation of chemical and texture. Chemical analyses on the glass matrix, alteration phases, and clasts indicate that the main chemical compositions of the host rocks of the cataclasite/ultracataclasite are inherited by the pseudotachylyte. Besides quartz, the main compositions of the pseudotachylyte can be described by an approximately binary mixture of plagioclase and biotite, which resulted in a strong chemical heterogeneity. Using zircon U-Pb dating and ⁴⁰Ar/³⁹Ar dating from glass matrix, the age of the ancient earthquake is constrained at 229~216Ma with a focal depth of 11~14km, indicating that the initial faulting of the Yingxiu-Beichuan fault took place during Indosinian orogeny. During the Indosinian orogeny, the initial tectonic framework was established for the Longmen Shan fault belt, which was strongly influenced the tectonic evolution since then.