Geological Characteristics of the Early-Middle Triassic Volcano-Sedimentary Basin in the Eastern Tibet

There are four northwest-southeast extending Early-Middle Triassic strata belts in eastern Tibet,the Pushuiqiao-Walasi Formations belt,Malasongduo-Jiazhila Formations belt,Xiayacun Formation belt and Zhuka Group belt,which are exposed westwards roughly in parallel.Based on the analysis of the lithologic-sedimentary characters and their spatial variation,the similar tendency of sedimentary strata-increasing and volcanic beds-decreasing from northwest to southeast,and of the regional tectonic features,we extract a conclusion that these four isolated Early-Middle Triassic strata belts formed in a uniform volcano-sedimentary basin,whose transgression and related sedimentation advanced from east to west.The basin reached its largest size with deepest water in Middle Triassic time,when there occurred bathyal flysch along Jiangda-Mangkang line.The volcanism accompanied the whole evolution process of the basin with the intensivest volcanic activity in the northwest and southwest areas.