六盘山东麓泾河上游河流阶地的形成年代及新构造运动意义

Age of Upper Jinghe River Terraces at the Eastern Piedmont of Liupanshan and Its Significance for Neotectonic Movement

六盘山是研究青藏高原东北缘隆升与构造变形的关键部位,其东麓泾河河流阶地是第四纪以来六盘山构造隆升的地貌载体。通过对泾河上游河流阶地的剖面实测,厘定了河流阶地的级序,泾河上游主要发育5级河流阶地T5—T1,不同地段阶地的形态类型不同,柳树沟T5、T4和T3为堆积阶地,T2和T1为基座阶地,高家山均为侵蚀或基座阶地;利用光释光（OSL）、电子自旋共振（ESR）测年并与黄土-古土壤序列进行对比,建立了河流阶地的年代格架,T5—T1年代分别为541 ka B.P.、477 ka B.P.、279 ka B.P.、60 ka B.P.和8 ka B.P。阶地的形态类型、形成年代与气候阶段的对应关系研究表明,泾河上游河流阶地以构造隆升驱动为主,气候变化影响为辅;541-279 ka B.P.期间柳树沟地区为堆积区,堆积速率为0.31 m/ka,而高家山为构造隆升区,河流下切速率为0.37 m/ka,堆积速率与侵蚀速率基本一致,说明T5、T4和T3阶面上较厚的砾石层是六盘山隆升的相关沉积;279 ka B.P.时期由于六盘山快速隆升驱使山前柳树沟地区由堆积区转为侵蚀区,河流侵蚀作用加强造成T2、T1平均下切速率增大为1.13 m/ka,这次地貌转换事件是在青藏高原扩展隆升的背景下六盘山向东挤出隆升的结果。研究结果为青藏高原东北缘以面积和体积扩张的观点提供了新的依据。

Liupanshan is the key region to research the uplift and tectonic deformation of the northeastern Tibet-an Plateau.Jinghe River terraces at the eastern piedmont are geomorphology carriers to record tectonic uplift process of Liupanshan since the Quaternary.The detailed study of profiles of the upper Jinghe River terraces shows that Jinghe River mainly developed 5 river terraces,i.e.T5 -T1.Terraces of T5,T4 and T3 in Liu-shugou are accumulation terraces,and T2 and T1 are pedestal terraces;terraces in Gaojiashan are erosional terraces or pedestal terraces.Trough dating results of OSL and ESR and combined with loess-paleosol sequences the ages of 5-level terraces were defined as 541 ka B.P.,477 ka B.P.,279 ka B.P.,60 ka B.P.and 8 ka B.P.,respectively.The morphological types of terraces and the relationship between age of terraces and cli-mate change indicate that the upper Jinghe River terraces are mainly driven by tectonic uplift while the effects of climate change are complementary.During 541 ka B.P.to 279 ka B.P.,Liushugou region is accumulation area and the accumulation rate is 0.31m /ka,correspondingly,the incision rate of Jinghe River in Gaojiashan is averaged as 0.37 m /ka,which is located in tectonic uplift region,and the accumulation rate is almost equal to the incision rate,indicating the thick gravel layers in T5,T4 and T3 related to Liupanshan rapid uplift.In 279 ka B.P.,Liushugou transforms to an erosion area under sustained uplift in Liupanshan,resulting the incision rate in T1 and T2 increasing to be 1.13 m /ka,indicating eastward extrusion uplifting under the background of expansive uplift of the northeastern Tibetan Plateau.The study provides a new evidence for the cognition that the Tibetan Plateau expands in a way of area and volume.