关键词: 宽谷, 构造抬升, 阿尔金北缘断裂, 雁丹图, 长草沟

Abstract: Strath terrace is tectogenetic stream terrace.In equilibrium state,lateral erosion of the stream bevels bedrocks beneath the active channel,giving rise to widening of the channel and the formation of strath.Downcutting of the stream due to tectonic movement causes the rise of strath above the channel,resulting in strath terrace.Strath includes major strath and minor strath,and accordingly strath terrace includes major strath terrace and minor strath terrace.The existence of strath terrace may indicate that stream has once reached the equilibrium state,and that tectonic uplift has occurred.The investigation of strath terrace,therefore,may reconstruct the history of tectonic uplift of the studied area.Yandantu and Changcaogou are located at the middle segment of the Altun Northern Marginal Fault,where multistage strath terraces are well developed.Basing on field investigation of strath terraces and thermoluminescent(TL)dating of the ages of treads of terraces,and combining with the data of paleoclimate of Qinghai-Xizang Plateau,we discuss the tectonic uplift since late Epipleistocene as reflected by strath terraces in these two areas.At Yandantu,three levels of stream terraces(T₁,T₂and T₃)have been developed since16ka BP,among which T₁and T₃are strath terraces,while T₂is fill terrace.A buried major strath is exposed in the area as well.The ages of three treads are dated to be about16.1ka BP,12.8ka BP and 6.2ka BP,respectively.The three terraces reflect three tectonic uplift events,while the three ages of treads represent the occurrence time of these events.The stream is still beveling the bedrock and widening channel at pre-sent,and modern strath is being generated.The uplift rate is 4.8～4.5mm/yr since 16.1ka BP in this area.From 12.8ka BP to 62ka BP,the uplift rate was 6.4mm/yr,while since 6.2ka BP till nowthe uplift rate is 3.1mm/yr. At Changcaogou,four levels of stream terraces(T₁,T₂,T₃and T′₁)have been developed since 7ka BP.All of them are fill terraces,and there are buried straths under deposits of every terrace.The buried straths of T₃and T₂are major strath,and those of T′₁and T₁are minor strath.The ages of treads of three terraces(T₃,T₂and T′₁)are 7ka BP,3ka BP and 2.5ka BP,respectively.The four terraces reflect two uplift events induced by tectonic activities.One of them occurred in about 7ka BP,and the other in 3ka BP.The uplift rate is 5.9mm/yr since 7.0ka BP at Changcaogou.From7ka BP to 3ka BP,the uplift rate was 7.0mm/yr,and since 3ka BP till now the uplift rate is 4.7mm/a.

Key words: strath, tectonic uplift, Altun northern marginal Fault, Yandantu, Changcaogou