The Study Of Deformation Characteristics And Geochronology In Southeast Section Of Xianshuihe Fault

Xianshuihe fault is a large sinistral strike-slip faults which is located in Western Sichuan Plateau, it has been active since the Indosinian and the activity of this fault is strong nowadays. The total length of Xianshuihe fault is about350kilometers, north from the Ganzi East Valley through Luhuo, Daofu, Bamei, Kangding extends southward to south Moxi of Luding. In general spread form northwest to southeast. In the regional constitute the northeast boundary of Sichuan-Yunnan rhombus block and the southwest boundary of Bayan Har block. As the restriction fault of Tibetan Plateau substance and the northeast boundary of Sichuan-Yunnan rhombus block, it is no doubt that the researching,which includes tectonic evolution and geochronology analysis in this area,stands on a significant point to understand the rule of tectonic evolution system and related stress system in Eastern Qinghai-Tibet Plateau and Sichuan-Yunnan rhombus block. In this lecture, the author has finished the researching of tectonic data, microstructure feature which from filed thin section and geochronology analysis in such observation point in Southeast of Xianshuihe fault. By such researching the author summarizes the characteristics of tectonic period, tectonic stress sources and specific time of tectonic movement in this area. Using the researching results of motion features and microstructure feature in fault zone in this area, and combining with the result of Quartz dissolution micro morphology dating and ESR dating. We can summarize such conclusion below:1. Xianshuihe fault is a significant fault in southeastern Qinghai-Tibet Plateau and it also is the northeast boundary of Sichuan-Yunnan rhombus block. The southeast section of the fault locates in the border area between Western Sichuan Plateau and Sichuan Basin. In this area the character of the fault is strike-slip in left side. The tectonic period of this area can be divided into3parts:Firstly this fault was tensioned by the main stress al, which distributed along the East-West path. Secondly this fault was squeezed by the main stress σ1, which distributed along the Northeast-Southwest path. At last the path of the squeeze stress has changed during a period of geologic time, and it distributed along the Northwest-Southeast path. The earliest tension stress happened in interim of Late Triassic, about200Ma ago. In the next period this area was squeezed by Northeast-Southwest main stress and formed such rifts which feature was left-cut. This main stress maybe was born by the collision between Eurasian plate and Indian plate. At last the determinants of the changing of the main stress from Northeast-Southwest to Northwest-Southeast maybe depended by Substance in Qinghai-Tibet Plateau moved into south-east,which cased by the collision between Eurasian plate and Indian plate.2. In the lecture the author has finished the researching which includes Quartz dissolution micro morphology dating and ESR dating, which used by field sample from all observation points in the fault. By the analysis of the dating result. The author deems that the earliest activity of the southeast section of Xianshuihe fault began at about late period, Late Triassic. After this period, the fault’s activity tended to decline in a such long period of geological history. However, at about20Ma ago, at Early Miocene, this fault’s activity turned into strong strength again and reached the top data at about5Ma ago,at Late Miocene to Pliocene. After Miocene, Excessive activity had also happened in this area.3. Combining the researching result of tectonic period, tectonic stress sources and dating result from all observation points in the fault. The author deems that the southeast section of Xianshuihe fault was tensioned by main stress in interim of Late Triassic, during this time tension joint and shear joint had been formed. After this period, the fault’s activity tended to decline in such a long period of geological history. However, at about at about20Ma ago, at Early Miocene, this fault was squeezed by Northeast-Southwest main stress and its activity turned into strong strength again. This time coincided with the time of the collision between Eurasian plate and Indian plate and the main stress maybe was born by the collision. At last this fault’s activity turned into strong strength again and reached the top data at about5Ma ago,at Late Miocene to Pliocene. This area was squeezed by the main stress, which had changed into Northwest-Southeast and may depended by Substance in Qinghai-Tibet Plateau moved into south-east.