龙门山断裂带假玄武玻璃特征及断层弱化机制的探讨

Pseudotachylytes in the Longmen Shan fault zone and fault weakening mechanisms

与断层相关的假玄武玻璃被认为是地震化石,记录了地震发生过程和断裂机制的重要信息,对认识断层地震活动性具有重要意义.本文综合地表露头及汶川科钻(WFSD-1、WFSD-2)岩心研究,对映秀—北川断裂带南段彭灌杂岩中发育的假玄武玻璃进行详细的结构、构造及矿物成分分析.这些假玄武玻璃以断层脉和单个/网状贯入脉的形式产出,厚度由几毫米到数十厘米不等,与碎裂岩和超碎裂岩相伴生.显微结构观察发现假玄武玻璃中发育大量熔融特征构造,如港湾状熔蚀边、蜂窝状气孔构造、黏滞性流动构造以及针状、球粒状、羽状、纤维状、粒状等多种形态的微晶及微晶集合体,表明其为断层滑动摩擦熔融的产物,指示摩擦熔融润滑是断层滑动过程中最主要的弱化机制.同时热增压和机械润滑机制可能存在于断层滑动的不同阶段.此外,从野外和显微镜下都观察到假玄武玻璃脉具有浅灰、深灰、浅褐、棕褐色及黑色等不同的颜色,可能代表着不同程度的脱玻化作用,并且不同颜色有相互穿插交切关系,可能指示存在多期假玄武玻璃,也就是说,产生假玄武玻璃的大地震事件沿映秀—北川断裂带重复发生.最近的研究认为这些假玄武玻璃形成于晚三叠世时期地下~10—15km深度.结合须家河组地层中发育的厚层断层泥和角砾岩,~180—280m宽的映秀—北川断裂岩带是断裂长期活动演化的产物.作为龙门山地区最陡峭的地形边界,映秀—北川断裂带长期地震活动造成的地壳缩短加厚可能是龙门山快速隆升的主要原因.

Fault generated pseudotachylytes are considered as fossil earthquakes,preserving important information about faulting mechanisms and earthquake generation processes,which are of great significance to understanding fault behavior and seismicity.Integrating the surface outcrops and study of the Wenchuan earthquake Fault Scientific Drilling（WFSD-1 and WFSD-2）cores,this paper presents detailed analyses on the structures and mineral composition of the pseudotachylytes inthe Longmen Shan fault zone.These pseudotachylytes and related cataclasitic rocks are well preserved in the Neoproterozoic Pengguan Complex along the southern Yingxiu-Beichuan fault.They are black,brown to gray in appearance,with thicknesses ranging from several millimeters to tens of centimeters,appearing as fault veins and injection veins.Multiple lines of microstructural evidence,such as embayed fragments,honeycomb-like vesicles,viscous flow structures and microlites or aggregates with various morphologies of acicular,lath-like,plume-like,fibrous and granulous,indicate that the pseudotachylytes were formed as results of frictional melting during the seismic slip.This implies that melt lubrication played a most important role in the fault weakening process.Thermal pressurization and mechanical lubrication could also occur at different slip stages.Besides,different colored pseudotachylyte veins,observed at both macroscopic and microscopic scales,may indicate different alteration or devitrification degrees,and their overprinting relations suggest they were multiply generated.This means that the pseudotachylyte-generated earthquake events had happened repeatedly in the Yingxiu-Beichuan fault zone.Recent research shows that the pseudotachylytes were produced at^10—15 km-depth in the Late Triassic.Combined with fault gouge and fault breccia in the Xujiahe Formation,the^180—280 m-wide Yingxiu-Beichuan fault rocks zone represents the products of long-term fault activities.Known as the steepest topographic boundary in the Longmen Shan area,the crust shortening caused by the long-term seismic activity of the Yingxiu-Beichuan fault may be responsible for the rapid uplift of the Longmen Shan.