Time Limit of Tectonic Activity in Southern Section of Sanjiang Tethys Orogenic Belt, SE Tibet Plateau—New Evidence of Apatite Fission Track Age

The Sanjiang Tethys orogenic belt located on the southeast of the Tibetan Plateau, has undergone PaleozoicNeoproterozoic accretionary orogeny and convergent orogeny, and has highly interested in the academic community, but its tectonic activity has been little studied since the Cenozoic. The apatite fission track chronology is sensitive to lowtemperature thermal history, so it was adopted to analyze the tectonic activity of the Sanjiang Tethys orogenic belt since the Cenozoic in this paper. The ages of 14 apatite fission track range from about 1336 Ma. 13 samples have P(χ2)>5%, and the grain age histogram shows singlepeaked distribution, which belongs to single age. The YN16 sample has P(χ2)<5%, and the grain age histogram shows a doublepeaked distribution, which belongs to mixed age. Applying Binomfit software to decompose the age of YN16 sample, the results show that the decomposition ages are 19.4 Ma and 35.4 Ma, indicating that it has experienced two thermal events. The poor correlation between sample age and elevation and scattered distribution indicate that the study area is influenced by tectonic activity. The apatite fission track length is about 10.413.1 μm, and the sample age values show a negative correlation with the track length, indicating that the apatite stays in the annealing zone for a long time. HeFTy software was applied to model the thermal history. The results show that the tectonic activity of the Sanjiang Tethys orogenic belt since the Cenozoic is classified into three stages. Stage 1 is a rapid uplift stage, dating from the EoceneOligocene (4530 Ma), and the dynamic mechanism is the collision between Indian and Eurasian plates, resulting in partial melting of crustal material near the JinshaAilao suture zone and the formation of largescale potassic magmatic belts and ultrapotassic magmatic rocks. Stage 2 is the slow uplift phase, which is from the Oligocene to the Miocene (308 Ma), and is attributed to the subduction of the Indian continent to the Lhasa Block and the Western Myanmar Block, and the subduction of the South China Plate to the Indochina Block, resulting in the slow uplift of the Zhongzan Block. Stage 3 is the rapid uplift stage, from the Late Miocenepresent (8 Mapresent), which is the rapid uplift of the Zhongzan Block in an intraland extrusion tectonic environment following the subduction of the Indian continent, which caused the rifting of the Sanjiang area. The overall uplift of the Zhongzan Block since 45 Ma is 3.37 km, with an average uplift rate of 0.10 mm/a.