Cenozoic Ductile Deformation Characterization And Its Tectonic Significance In The Middle Segment Of The Lancang Tectonic Zone, Western Yunnan

The Lancang tectonic zone is a large strike-slip shearing zones in the southeastern Tibet, which is the boundary between the Lanping-Simao block in the east and the Baoshan-Qiangtang block in the west. There are early oblique stretching lineations with fabrics showing dextral ductile deformation and late horizontal stretching lineations with fabrics showing sinistral brittle-ductile deformation in the middle segment of the Lancang tectonic zone. CPO patterns of quartz indicate the principal slip systems is prism<a> slip and rhomb<a> slip in medium temperature (400～600℃) relating with dextral ductile deformation, and superimposed by basal <a> slip in low temperature (300～400℃) relating with sinistral brittle-ductile deformation. The zircons from syn-kinematic leucocratic vein give U-Pb crystallization age of 41.90±0.42 Ma, which suggests that the deformation time is not later than 42Ma. The biotite 40Ar-39Ar dating is ranging from 15 to 17 Ma, which represent the change age from dextral ductile to sinistral brittle-ductile deformation according to the closure temperature of the biotite, that is the same as the temperature of the sinistral brittle-ductile. The early dextral oblique strike-slip shearing was related with the extrusion of the Lanping-Simiao block during the early Miocene, and the sinistral horizontal brittle-ductile deformation was formed by the shearing of the southern part of the Baoshan block toward SWW along the NE Nandinghe and Wanding sinistral slip faults during exhumation of the ductile shear zone.