Tectonic Deformation History And Tectonic Significance Of Muggagangri Melange In Southern Rongma Township,Tibet

The Bangong-Nujiang suture zone is characterized by Jurassic flysch,melange and ophiolite belts stretching over 2,000 km,representing the remnants of the Central Tethys oceanic crust,recording the Late Triassic-Cretaceous The birth and extinction of the Central Tethys Ocean and the subsequent collision process between the Qiangtang block and the Lhasa block.The magmatism and tectonic deformation associated with its evolution are the collage of Gondwana-derived terranes to Asia and the formation of the Tibetan Plateau.And development provides important record data.The Mugagangri melange is distributed in the Bangong-Nujiang suture zone,consisting of a mid-early Jurassic clastic matrix and an oceanic ophiolite composed of multiple stages of toughness deformation.The overall disorder and displacement in time and space is considered to be a proliferating complex in the southern margin of Qiangtang.Correct understanding of the structural characteristics of the Mugagangri melange will be of great significance for understanding the history of the Bangong-Nujiang oceanic crust and the process of the suture zone.Under the guidance of structural analytic theory,this paper takes Mugagangri melange as the research object,through field structure observation,large-scale structural section drawing,dividing and determining the stratigraphic period,and combining zircon U-Pb isotopic age study.The constrained significance of the tectonic deformation of the Mugagangri melange in the evolution history of the Bangong-Nujiang suture zone is discussed.The fourth stage of tectonic deformation is mainly developed in the Mugagangri melange.The three-stage transmissive surface,the sandstone interlayer can be seen in two stages of longitudinal bending folds.The first stage(D1)is a moderate structural toughness deformation,which is a strong contraction dominated by the north-south extrusion stress,and a moderately inclined northwardly inclined isotropic fold on the area;the second stage(D2)is in the EW direction.The deformation under the dominance of ductile shear stress forms a flat fold and a shear-stretch structure.Both the left and right shears on the horizontal section are very common.It is speculated that it can be further divided into two secondary deformation stages;(D3)The longitudinal bending fold and the thrust nappe structure are formed under the NS compression and contraction;the fourth stage(D4)is the EW direction extrusion forming the NS direction fold,and the third stage EW is added to the fold to form the fold shape.Combined with the regional geological background and isotope dating results,it is speculated that the first phase(D1)regional penetrating surface S1 is associated with the northward subduction of the Bangong-Nujiang ocean Early Cretaceous;the second phase(D2)ductile shear Stretching,Bangong-Nujiang suture zone absorbs the results of the new generation NS squeezing and contraction;the third stage(D3)EW to the longitudinal bending fold is inferred to be the intracontinental deformation of the Cenozoic NS to the Tibet Plateau uplift;Period(D4)NS is a kind of derivative deformation of the strike-slip fault system,which is related to the squeezing structure of the Tibet Plateau.