渭河三阳川盆地最高级阶地的年代厘定及其对河谷发育的指示意义

青藏高原东缘水系的演化历史长期存在着重大争议,鉴于任一水系的形成演化都是通过主要河谷的发育及其不断延展与整合完成的,因此确定河谷发育的起始时代是研究水系演化的关键。本文针对渭河上游三阳川盆地最高级阶地形成时代的研究,发现李家小湾河流阶地砾石层的ESR年代为1.26±0.15 Ma和1.32±0.19 Ma,26Al/10Be埋藏年代为1.45±0.70 Ma和1.04±0.43 Ma,说明该段河谷形成于早更新世晚期。综合青藏高原东缘夷平面、剥蚀面与河流阶地的研究成果,推断该区现代河谷系列主要形成于1.2 Ma以后,河流平均下切速率较高,为0.1~0.32 m/ka,指示了中更新世以来该区快速的地表抬升与河谷发育过程;而其前少数地段的先成河谷下切速率介于0.04~0.29 m/ka之间,说明区域地势总体低平,地表过程以剥蚀夷平为主,即高原东缘的现今水系格局主要是第四纪期间构造和气候共同作用下河流侵蚀的产物。

Dating of the Highest Terrace of the Weihe River in the Sanyangchuan Basin and Its Implications to Valley Evolution

There has been a long-term debate on the evolution of the great river systems of the eastern Qinghai-Tibet Plateau.The formation and development of any drainage system is completed by the origin of the valley,with its continuous extension and integration,under the regional tectonic and climatic change.Therefore,it is the key to study of the issue in the beginning of the valley development.In this paper,the electron spin resonance(ESR)and the cosmogenic nuclide 26Al/10Be burial dating techniques were used to test and analyze the accumulation age of the gravel layer in the highest terrace of the upper Weihe River in the Sanyangchuan Basin.The ESR and 26Al/10Be burial ages of the gravel layer were 1.26±0.15 Ma,1.32±0.19 Ma and 1.45±0.70 Ma,1.04±0.43 Ma,respectively,indicating that the modern Weihe River valley was formed in the late Early Pleistocene.Combined with the researches of the planation,erosion surfaces and other fluvial landforms in central China,it could be found that a few valleys have been formed before 1.2 Ma,and the incision rate is between 0.04~0.29 m/ka,but the overall regional geomorphic process is still dominated by planation or partial planation.The modern river valley of the trunk stream was mainly formed after 1.2 Ma,and the average incision rate increase to 0.1~0.32 m/ka,indicating the rapid surface uplift process in the region since the Middle Pleistocene.