The Composition And Deformation Characteristics Of Metamorphic Core Complex Zone In The Zhaxikang Area,Southern Tibet

The Zhaxikang area is located between the South Tibetan Detachment System and Yarlung Zangbo Suture Zone.The metamorphic core complexes in Zhaxikang Kuqu,Cuonadong,and Kongbugang are found during 2014 and 2016,they form a large-scale metamorphic core complex zone with the metamorphic core complex found in the western part of the study area,however,there are different understandings for their formation mechanism.Through field geological survey,geochemistry and isotopic chronology analysis,combined with remote sensing interpretation,this paper focuses on the analysis of the material composition,structural style and structural characteristics of the three newly discovered metamorphic core complexes.The formation process of metamorphic core complex is remolded and the formation mechanism of metamorphic core complex is discussed.The results show that the three sets of metamorphic core complex have similar three-layer structure.The metamorphic core complex consists of three part:crystalline basement(nucleus),decollement system and caprock from the inner to the outside or from lower to upper,all of which are in contact with the detachment fault.The detachment fault between the crystalline basement and the slip system is called the basement(lower)detachment fault,and the detachment fault between the slip system and the caprock is called the main(upper)detachment fault.The basement(lower)detachment fault is characterized by ductile shear deformation,and the main(upper)detachment fault is characterized by brittle deformation.The difference of structural assemblage styles of different metamorphic core complex is analyzed in depth.The Kuqu metamorphic core complex is exposed to the edge of the main detachment system,the fault dips northward and develops a single northward trending structural line,the structural form is east-west zonation,the matching fault is a northward detachment fault,and the core magmatic age is 25.11 Ma.A series of circular outward incline normal faults are developed in the Cuonadong metamorphic core complex.The tectonic outline is annular and extroverted,the associated fault is annular extroversion normal fault,and the age of the core magmatic rock is 21.19 Ma.The Kongbugang metamorphic core complex inherits the linear characteristics of the northward dip,the structural shape is distributed in the north-south direction.The occurrence is near-horizontal,and matching faults are normal faults in the south and north direction.The latest age of magmatic rocks in the core is 15.20 Ma.There are two types of extensional processes in the Himalayas,one is the north-dip Tibetan south detachment system(STDS),which is formed by the south-north extensional process,and the other is the south-north rift valley formed by the east-west extensional process The metamorphic core complex in Zaxikang area is closely related to these two extensional structures,which were formed in different stages of extensional structures and preserved some deformation characteristics of early thrust nappe structures in different degrees.The results show that the formation of Kuqu and Cuonadong metamorphic core complex are closely related to the east-west direction of the detachment fault(the same time as the southern Tibetan detachment system).The development of the Kuqu metamorphic core complex is controlled by north-dipping detachment fault,and the outcrop of the leucogianite in the core is related to the detachment process.In addition to the northward extensional faults,mechanism of the metamorphic core complex in Cuonadong is also superimposed by thermal-tectonic diapir,which plays an important role in the formation of the metamorphic core complex during the emplacement of the rock mass.The age of formation of core magmatic rocks(21.19 Ma)basically represents the age of formation of metamorphic nuclear complex rocks.The Kongbugang metamorphic core complex is associated with the north-south rift extensional regime(later than the South Tibetan detachment system),and the core magmatic rock formation(15.20 Ma)is slightly earlier than the north-south extensional regime(starting time is about 14 Ma).The north-south fault has a destructive effect on the core leucogianite;the mechanism of its outcrop is formed by the uplifting processes of the north-south faults(horst)in the later period.