Cenozoic Structural Deformation Analysis And Its Tectonic Significance Of The Lushui-nujiang Fault Zone In The Western Yunnan

The Lushui-Nu River located in the middle segment of gaoligong tectonic zone,forms the boundary between the Tengchong and Baoshan blocks and is one of the main Cenozoic strike-slip fault systems of the Eastern Himalayan Syntaxis(EHS),which is therefore one of the keys to study the accommodation mechanism of the intra-continental deformation in an obliquely convergent regime of southeast Asia.In associated with the analysis of structural geology,microtectonics,kinematic vorticity(Wm),petrology,geochemistry and the U-Pb geochronology,this paper proposes that the provenance of the deformed material of this fault zone possesses the same characteristics with the S-type granite of Gangdese Batholith and the zone is characterized by three stages of deformation(D1-D3).D1 structures record early compressive deformation during the Eocene-Oligocene,which was dominated by the pure shear and formed tight/isoclinals F1 folds of bedding with S1 foliation and NS-trending lineation;during the late Oligocene-Miocene,the dextral ductile strike-slip shearing during D2 overprinted the most of D1 structures and developed strong sub-vertical mylonite folination and sub-horizontal stretching lineation with NS trending;Later ductile-brittle deformations(D3)during Miocene characterized by the normal stike-slip shearing with sinistral shearing indicators.The P-T pseudosection and CPO pattern of quartz support the three-stages deformation by identifying three different P-T conditions during deformations.The first stage was deformed in the high-amphibolite faces with medium-high temperature and medium-high pressure(580?-720?,8.5-10.8Kbar);subsequently,the high temperature and medium pressure(730?-820?,5.8-7.7Kbar)deformation(D2)followed by a deformation(D3)with relative low temperature and pressure(440-680?,3-5.5Kbar).The zircons from F1 deformed granitic vein give U-Pb crystallization age of 42.9±2.1Ma,which suggests that the D1 deformation time is no later than 42.9 Ma;the zircons from the syn-kinematic leucocratic vein produce U-Pb crystallization age of 30.86±0.53 Ma and 20.62±0.57 Ma,which indicates that the D2 deformed no later than 30.86 Ma and at least continued to 20.62Ma;The last sinistral brittle-ductile deformation happened no earlier than ~11Ma.Based on the previous papers of Three River fault system,this study suggests that the previously compressive deformation(F1-folds)was intimately related to the crustal thickening and subsequently deformed under transpression shearing resulted from the strain partitioning with simple shearing dominated zones.During the late Miocene,the sinistral stike-slip shearing characterized by brittle-ductile to brittle deformation during the vertical exhumation of crustal material reflect that this fault zone played an inevitable role in absorbing the vast majority of internal deformation of crustal fragments during Oblique collision.