金沙江何时开始向东流

When the Jinsha River started to flow east

青藏高原的东南缘具有独特的地貌特征,不像其它边缘,这里坡面非常平缓,这上面发育的主要河流有金沙江,雅砻江和大渡河。金沙江的主支流河谷保存着大量的湖相沉积——昔格达层,我们通过宇宙成因埋藏年龄法定出湖相沉积的年龄为1.58～1.34 Ma。对金沙江上游的碎屑锆石U-Pb年龄分析表明其物源主要体现了所流经的羌塘地块的冈瓦纳地体特征,并加入了华北地体的特征,明显有别于雅砻江流域的物源。对昔格达及其下伏河流砂砾的U-Pb年龄分布特征研究表明昔格达古湖形成之前攀枝花至涛源河流流向为自东而西,与现流向相反;伴随着昔格达古湖的形成与切穿,攀枝花至涛源一带的水流开始由西向东,金沙江从此开始向东流去。

The southeastern Tibetan Plateau possesses unique low and continuous relief, where the Jinsha River, Yalong River and Dadu River are developed. Large quantities of lacustrine deposits,called Xigeda, are well preserved in the trunk and tributaries of the Jinsha River.Using cosmogenic nuclide burial age dating we determined the deposition age of the lacustrine deposits： 1.58 - 1.34 Ma. Studies of detrital zircon U-Pb age distribution related to the upstream of the Jinsha River show that the age spectrum is mainly derived from its course in the Qiangtang sedimentary block together with some characteristics of north China craton, which differs obviously from the provenance of the Yalong River. U-Pb age distributions related to Xigeda and underlying sand suggest westward stream flow from Panzhihua to Taoyuan, which was in the opposite direction of the current stream flow. The results further suggest changes of stream flow direction from Taoyuan to Panshihua accompanying with the drainage of the Xigeda paleo-lake,and the Jinsha River started to flow east.