The investigations of the deformation of the Great Counter Thrust (GCT) in Zedang area, South Tibet: A case study of magnetic fabric and EBSD analysis

Abstract

仁布-泽当逆冲断层是喜马拉雅大反向逆冲断层（GCT）在藏南地区的重要延伸部分，也是喜马拉雅造山带北部边界新生代最为活动的构造单元之一。新生代以来特提斯喜马拉雅的构造变形组构特征的研究对于深入理解碰撞造山带演化与高原隆升具有重要构造意义。本文综合GCT泽当-琼结段断层的宏观与微观变形特征，对断裂带石英脉、围岩中石英和云母矿物的电子背散射（EBSD）组构及断层两侧岩石磁组构（AMS）特征进行对比分析。结果表明对AMS主要贡献来自顺磁性云母、绿泥石等，磁化率各向异性椭球体以压扁状为主，磁面理与构造面理（劈理、断层面）基本重合，显示较强的构造变形磁组构特征；磁线理优选方向近南北向，且与观测北向逆冲断层方向一致，揭示剪切作用在变形过程中的持续作用。研究发现泽当地区GCT附近石英微观结构从围岩至断层区，石英至少呈现3种不同类型的微观变形机制：围岩区溶解蠕变、断裂带石英以膨凸重结晶和亚颗粒旋转重结晶作用为主。断裂带石英的c轴EBSD组构指示变形为低温（300~400℃）环境，其中黑云母的结晶学优选（CPO）与磁组构主轴优选方向存在高度的一致性，进一步证实了顺磁性矿物黑云母对AMS的主要贡献。综合研究表明泽当地区GCT的韧性变形是断层处在中上地壳韧性带的活动阶段变形的结果，也代表了特提斯喜马拉雅在碰撞、高原隆升期的变形主要特征。;Renbu-Zedang thrust fault is one of the most important parts of the Great Counter Thrust (GCT) in South Tibet, as well as one of the most active faults of the Himalaya Orogeny in Cenozoic. The investigation of the deformed fabric has great importance for deep understanding of the orogen evolution and plateau uplift. In this paper, we combined the mesoscopic and micro-structures of the fault rocks, especially, the comparison analysis of the Electron Backscattered Diffraction (EBSD) fabrics of quartz, mica and the anisotropy of magnetic susceptibility (AMS) of samples around the fault zone. The AMS results suggest an oblate magnetic ellipsoid predominant at most sample sites, and magnetic foliations nearly coincide with the tectonic foliations (cleavage or fault plane), both of which suggest strong tectonic fabric dominant of the area. S-N directed magnetic lineation in line with the fault moving direction indicated the continuing thrusting happened in the fault zone. Quartz microstructure suggests that this fault zone at least underwent three different levels of deformation mechanism, namely, the dissolution creep static recrystallization (quartz grains shows polygon seams), the bulging (BLG) recrystallization and the subgrain rotation (SGR) recrystallization, respectively. Low deformation temperature (300~400℃) is suggested from the c-axes of quartz EBSD fabric. The Crystallographic Preferred Orientation (CPO) of biotite shows strong correspondence with the magnetic fabrics which further confirmed the dominance of paramagnetic to the AMS. The ductile deformation of GCT in the Zedang area is the result of the fault in ductile zone of the middle and upper crust uplift movement stage, also represent the plateau uplift and deformation of the Tethyan Himalayan since Cenozoic.