西藏甲玛-驱龙地区叶巴岩组构造学特征

摘要

通过野外地质调查和定向薄片显微构造研究，对甲玛—驱龙地区叶巴岩组的变形、运动学和动力学进行了初步研究，指出：①原叶巴组实为构造岩，应称“叶巴岩组”，区内可分为四个岩段，构成两个变火山-沉积旋回；②叶巴岩组在宏观上表现为自北而南的轴面总体北倾的复背斜和复向斜；③叶巴岩组经历过两期韧性变形，多期韧脆性变形，且邻甲玛—卡军果推-滑覆构造系主推覆面的 C 岩段还多经历了三期较浅层次的面理置换和一期膝折变形；④叶巴岩组两期韧性变形动向相反，第一期为上层面自南向北的剪切，第二期为上层面自北向南的剪切，第二期韧性剪切与随后的韧脆性变形具多阶段递进特点，在向南推覆褶皱至较浅层次后还经历多期脆-韧性变形，尤其是邻甲玛—卡军果推-滑覆构造系主推覆面部位；⑤叶巴岩组第一期韧性变形发生于94—85 Ma±的晚白垩世土伦期—康尼亚克期，与雅鲁藏布江洋壳向北俯冲导致的弧后裂谷伸展有关；第二期韧性变形及其后主要的韧脆性变形发生于50 Ma±的始新世，与印度-欧亚板块碰撞事件有关，碰撞后的构造作用则导致多期脆-韧性变形的叠加。%Based on field geological survey and oriented thin section observation, the authors studied deformation, kinematics and dynamics of the Yeba tectonite group, and reached the following conclusions: ① the previously named “Yeba group” is actually a tectonite group and should be called “Yeba tectonite group”, which consists of four lithologic sections and can be divided into two volcano-sedimentary cycles; ② macroscopically, from north to south, Yeba tectonite group shows an inclined anticlinorium and an inclined synclinorium whose axis trends northward; ③ Yeba tectonite group experienced two-stages of ductile deformation and multi-stage brittle-ductile deformation, and the C lithologic section experienced three-stages of foliation replacement and one stage of kink deformation at the superficial level, lying near the main napping plane of Jiama(Gyama)-Kajunguo thrust - gliding nappe tectonic system; ④ in Yeba tectonite group, kinematic direction of two-stages of ductile deformation is on the opposite: in the first stage, the upper layer shearing was from south to the north; in the second stage, the upper layer shearing was from north to south. The second ductile deformation and later multi-stage ductile-brittle deformation had multi-stage progressive deformation characteristics, and after southward nappe-fold lifting to the superficial level, Yeba tectonite group also experienced multi-stage brittle-ductile deformation, especially at the position near the main napping plane of Jiama(Gyama)-Kajunguo thrust-gliding nappe tectonic system; ⑤ the first stage ductile deformation of Yeba tectonite group occurred around 94–85 Ma, belonging to Late Cretaceous Turonian-Coniacian period and related to back-arc rift stretching that resulted from northward subduction of the Brahmaputra oceanic crust; the second stage ductile deformation and the later main brittle-ductile deformation occurred around 50 Ma in Eocene, related to Indo-Asia collision, and the post collision tectonism resulted in the superposition of multi-stage brittle-ductile deformation.