Composition And Analysis Of The Tectonic Deformation Of Mélange Belt In Zhongba Area,Tibet

As special geological bodies of convergent plate boundaries, mélange recordslarge amount of geological information of the ocean basin evolution andcollissional processes,and is always a hotspot in the national and internationalresearch,especially the mélanges of the Yarlung zangbo river suture zone in Tibet.Due to the low research degree of mélanges of Zhongba area, which exerts influenceon profound understanding the evolution of Yarlung zangbo river suture zone, thisthesis mainly carries on restoration of the relationships of mélange formation andtethyan evolution, through analysis of composition, age, forming environment insouthern and northern melange belt of Zhong ba region and of Tectonicdeformation.According to research, following conclusions are reached:on basis of thedistribution of mélanges,they could be dividied into northern Gerong songduomélange belt and southern Jiang muna mélange belt;in the light of research of thebasic composition and time of two mélange belts, Ge rongsongduo mélange beltfrom north to south can be divided into four rock groups: Qu bu group, Ba nalegroup, Zhuole group, Riagong group; the southern Jiang muna mélange belt can bedivided into six Tectonic rock groups: Ma quanhe group, Tan wa group, Erba group,Yi Sang group, Tanong group and Ba bazhadong group.Through the analysis of tectonic deformation,it shows that the melanges mainlyundergo two stages of tectonic deformation. The first-phase structural deformation ischaracterized by tight homoclinal folds which form under bedded shear mechanismof medium deep levels,and the shear deformation presents early tight homoclinalfolds and ductile shear zones developing within every kind of rock units.Thesecond-stage deformation which is featured by thrust faults and fold structurescaused by mid-deep squeezing mechanism constitutes the main structural style ofthe mélanges nowadays,it establishs the structural framework of different rockgroups,and has coaxial superposition with the first-period folds.According to the analysis of mélange assemblage and tectonic deformation, thetectonic evolutions of this research area are put forward as follows: the splitting ofGondwana may take place during Late Permian-Middle Triassic;the northern marginof Indian plate produced initial ocean basin under extensional mechanism duringEarly Triassic-Middle Triassic;Tethyan oceanic crust further expanded during LateTriassic-Middle Jurassic;during Late Jurassic-Paleocene, the Gangdise magmaticarc and Shigatse fore-arc basin formed with the subduction of the Tethys sea;duringthe end of Paleocene to early Eocene,Zhongba micro-terrane collided with Lhasablock at about50Ma,however,the existence of deep-water basin between Indianplate and Zhongba micro-terrane at that time evidences southern oceanic crust is notclosed,and the collision may occur after the early Eocene;during Eocene toQuaternary, Indian plate and Zhongba micro-terrane go into intracontinentalorogeny and plateau uplift stage with full collision.