Fission Track Thermochronological Constraints On Tectonic Activity In The Kangding-Xinlong Region Of Western Sichuan Province,Eastern Qinghai-Tibet Plateau

The Kangding-Xinlong area in western Sichuan province belongs to the Songpan-Ganzi block,the eastern part of the Qinghai-Tibet Plateau.The authors take 4 typical areas of Danba,Xinlong,Jiuzhaigou and Wenchuan as the main research objects.Through the field geological survey,indoor basic geological research,focus on the fission trail chronology analysis of rock samples,analyze the tectonic-geological events and tectonic activity periods in the area,reveal the geological-thermal evolution history,provide new chronological evidence for the geological-tectonic activities in the Kangding-Xinlong area in western Sichuan,and enrich the research results in the Kangding-Xinlong area.A total of 28 pieces of apatite fission track(FT)data were obtained,with age variations ranging from 50–1.6 Ma.After decomposition of the age with P(?~2)<5%,the variation range was108–0 Ma,with a total of 5 age groups of 108 Ma,78–76 Ma,64–52 Ma,44–22 Ma,and 14–0Ma,of which the age group 44–22 Ma is divided into 44–34 Ma and 30–22 Ma subgroups,and the age group14–0 Ma is subdivided into 14–8 Ma and 8–0 Ma subgroups.The 108 Ma recorded the closure of the Bangong Lake-Nujiang Ocean,and the collision and merging of the Lhasa block and the Qiangtang block.The age group 78–76 Ma confirms the intracontinental extension event in the Sanjiang area.The age group 64–52 Ma happened to record the closure of the Yarlung Zangbo River.The age group 44–34 Ma recorded that the Yarlung-Zangbo River was completely closed,the Indian-Asian continent completed the collision,and the Tibetan Plateau uplifted.The two age groups of 30–22 Ma and 14–8 Ma are the reflection of uplift and strong tectonic activities in the western Sichuan Plateau.The 8–0 Ma recorded the rapid uplift event in the western Sichuan region in the eastern part of the Qinghai-Tibet Plateau.The simulation of thermal evolution history in the study area has three stages,namely 140–55 Ma,55–8 Ma and 8–0 Ma,which are consistent with the fission track age group,and accurately reveals the tectonic activities in Kangding-Xinlong area of western Sichuan.The first stage of the Early Cretaceous-Eocene(140–55 Ma),the concrete reflect the Bangong-Nujiang Ocean double-sided subduction,the Lhasa block collided with the Qiangtang block,and the Yarlung Zangbo Neo-Tethys ocean subduction decreased,the uplift of the Qinghai-Tibet Plateau and the subduction and collision of the Songpan-Ganzi orogenic belt.The second phase of the Eocene-Miocene(55-8 Ma),under the continuous influence of the collision between the Indian and Eurasian plates,experienced the complete closure of the Neo-Tethys Ocean of the Yarlung-Zangbo River,the northward retrograde thrusting of the West Kunlun,the uplift of the Lhasa Massif,the thickening of the Himalayan crust,and the uplift of the Tibetan Plateau.The third stage since the Miocene(8-0 Ma),the Himalaya has been uplifted overall,the Qinghai-Tibet Plateau has been uplifted strongly,and the western Sichuan region has been uplifted rapidly.The age and sampling elevation show a good linear correlation trend,and the denudation starts at 38 Ma,6 Ma and 3 Ma,which records the events of the uplift of the Qinghai-Tibet Plateau and fault tectonic activity due to the impact of the India-Asia continental collision.The denudation rate of the whole area is about 0.02 mm/a.