The Study On River Capture Of Dadu River Based On Detrital Ziron U-Pb Ages Analysis Method

The flow direction of the Dadu River turns from southward to eastward at Shimian,Sichuan Province.This abrupt change has been suggested to be result of river capture.One hypothesis is that the Dadu River once flowed southward throw the valley of Liziping River,a present northward flowing tributary of the Dadu River,into the Anning River.A suite of fluvial sediments near Liziping,about 40 km south to Shimian,adjacent to the divide(at altitude of 2600 m)separating the headwater of the Anning River and the Liziping River,have been suggested to be wind gap deposits of the palco-Dadu River.However,whether the Liziping sediments were laid down by the Dadu River has not been proved yet.Here we use detrital zircon U-Pb chronology method to trace the possible sources of the fluvial sediments at Liziping.For this purpose,we analyzed the detrital zircon ages using LA-ICP-MS technique for the samples from the fluvial sediments in Liziping,and from modern Dudu River and its tributary(Luding-Jinkouhe segment)for comparison.Four samples from the trunk stream of Dadu River(Guzan town of Luding country,Jiajundaqiao of Luding,Anqun village of Shimian,Huanguoping of Jinkouhe District)are measured.The result shows that detrital zircon ages of all the four samples display four age peaks at 200?300 Ma,400?500 Ma,700?1000 Ma,1.8?2.0 Ga.This age spectra of the detrital zircon of Dadu River indicate they are in good consistency.The tributary sample(collected from the Zheduo-Gonggar massif)located at the confluence,where the tributary Hailuogou,sourced from Gonggar Mountain,joins the Dadu River.Its detrital zircon ages show a single peak age at about 200 Ma.Moreover,the age cluster of 200 Ma is obviously observed in the sample of Anquan village as well(sampling site adjacent to Zheduo-Gonggar massif).This single peak is consistent with the age of the Gonggar massif(216-204 Ma).While the samples of the fluvial sediments in Liziping and the modern sand bar from Liziping River only display an age peak at about 800 Ma.The detrital zircon U-Pb ages of the two samples coincide with the ages of the bedrock distributed in upstream area,indicating a rather local provenance.The results show that the age spectra of Dadu River are consistent and reliable;in addition,the zircon grains with age centered at 200 Ma are able to be transferred downstream more than 100 kilometers.Assuming that the Dadu River had once flew southward into Anning River through Shimian,and laid down the fluvial sediments at Liziping,the sediments should contain detrital zircon aged 200 Ma,and should have preserved similar age spectra with the samples in main stream of Dadu River.However,this is not the case because samples of Liziping sediments and the modern sand bar of the Liziping River show unique age peak at about 800 Ma.The results indicate that the fluvial sediments of Liziping can not be the wind gap sediments of the paleo-Dadu River.So that the Liziping fluvial sediment couldn't be the evidence of the south flowing Dadu/Anning River.The hypothesis of south flowing paleo-Dadu/Anning River need to seek other evidence.