SEDIMENTARY EVOLUTION OF THE NAM CO BASIN,TIBET,SINCE 116ka BP AND QINGHAI-TIBET PLATEAU UPLIFT

U-series isochron dating of lacustrine and lakeshore deposits shows that six lake terraces below 48m above lake level and highstand lacustrine deposits from 48 to 139.2m above lake level have formed along the banks of the Nam Co since 116ka BP.Three sedimentary facies may be distinguished.Their evolution progressed through four stages:① 116 to 72 ka BP:a deep lake environment predominated in the basin,with a paleolake level 140 to 48 m above the level of the present Nam Co; ② 72 to 37 ka BP:the lake environment was of moderate depth,with a paleolake level 48 to 26 m above lake level; ③ 37 to 30 ka BP:a shallow lake environment occurred,with a paleolake level 26 to 19 m above lake level; and ④ 30 ka BP to present:the lake has become progressively shallow and the paleolake level was <19m above lake level.The response of Nam Co basin deposits to the Qinghai-Tibet plateau uplift reveals that the plateau has experienced the following stages since 116 ka BP:the stable stage and progressive rapid uplift stage(116 to 37 ka BP),the abrupt and strong uplift stage(37 to 30 ka BP),and the relatively stable stage(since 30 ka BP).The uplift of the Qinghai-Tibet Plateau is a multi-stage,variable-speed and nonuniform complex process.