The Deformation Phases Of Deep Subduction-exhumation Geological Block Of The Yuka Region,the North Qaidam

The north Qaidam orogen has been through Paleoproterozoic basement forming and evolution,Meso-Neoproterozoic amalgamation of the supercontinent,Paleozoic continental collision,Mesozoic and Cenozoic basin mountain coupling and Tibetan plateau uplifting.And the most noticeable of the Qaidam orogen is Paleozoic continental collision system,which reserved HP/UHP metamorphic rocks and ophiolitic mélange.We consider eclogite lenses,gneiss and schist that went through UHP metamorphism as a whole,called the deep subduction-exhumation block.Based on geological section and mapping,we conclude that,(1)eclogites are generally lenticular and stratiod in form and extend in accord with the gneissosity,which indicate that protolith of eclogite have had stratified sheared before UHP metamorphism;(2)The contact relation between the deep subduction-exhumation block and surrounding rocks is contacted by ductile and brittle-ductile shear zone in early phase and overlaid thrust-nappe in later phase;(3)Part of eclogite lenses and Yukahe terrain lies in granitic gneiss as roof pendants,rather than eclogite lenses lies in granitic gneiss separated.This article mainly research on the geometry,kinematics and dynamics of the deep subduction-exhumation block and divide structural evolution into four deformation phases at Yukahe region,north Qaidam.(1)The first phase is subduction structure: this phase is dominated by plastic progressive deformation.It is featured by boudins parallel to the gneissosity,bedding recumbent folds fold by initial foliation,stratiod eclogite lenses(boudins formed by basic volcanic rocks via stratified shear)and foliation reserved in garnet prophyroblast.The eclogite lenses' tail reserved illustrates that is formed before metamorphic peak(2)The second phase is exhumation structure,this phase dominated by ductile and brittle-ductile deformation.It is featured by rootless folds derived from granitic gneiss;thicken hinge folds bended by schist and gneiss,northwest trending ductile shear zones in the deep subduction-exhumation block.These structures formed in early exhumation period.Cleavage paralleled to the gneiss and domino structure derived from eclogite lenses,and the ductile shear zone between deep subduction-exhumation block and Tanjianshan terrain.These structures formed in later exhumation period.(3)The third phase is thrust-nappe structure,this phase dominated by ductile-brittle deformation.It is featured by thrust-nappe zone derived from the contact between deep subduction-exhumation block and Dakendaban terrain,and secondary thrusts formed in deep subduction-exhumation block.Tanjianshan terrain schist bends into fold,which is influenced by thrust-nappe.There is intensive cleavage belt developed in main thrust zone.(4)The fourth phase is gravitational sliding structures,this phase dominated by brittle deformation.It is featured by gravitational sliding structures,such as brittle faults and joints.It is the latest deformation in survey area.