| The Hanjiang River,which originated in the South Qinling Mountains,is the largest tributary of the middle reaches of the Yangtze River.The local uplift and water erosion gradually shaped the multi-level terrace pattern of the Hanjiang River.The Hanjiang River Basin has also become an important area for studying the features and evolution of river geomorphology,regional environmental evolution and other researches.The rich deposits on the terraces provide a basis for exploring the evolution of modern landforms on the Hanjiang River,and the stone products retained on several terraces also provide materials for paleoanthropological research.However,due to limited dating technology and effective samples,the absolute dating and research of some terraces in the river basin are still blank.In order to fill up the absolute chronological sequence of terraces,deduce the development context of terrace landform systems,analyze the features of tectonic landforms and the spatiotemporal laws of ancient human activities,this paper uses the 26Al/10Be burial dating method to analyze four terraces in Hanjiang River Basin(two of which are paleolithic sites)to conduct an absolute dating study.Through magnetic susceptibility and paleomagnetic data comparison,the credibility of the buried age is verified,and the following conclusions are obtained based on the chronological results:(1)The age of the oldest fill terraces in the Hanzhong basin on the upper Hanjiang River is greater than 1.5 million years,and the age of the oldest fill terraces in the Yunxian basin is greater than 2.0 million years.This suggests that the modern Hanjiang River geomorphology began to form about 2.0 million years ago.(2)Up to 6-level river terraces are preserved in the upper basin of the Hanjiang River.The approximate chronological framework of the development is as follows:Class Ⅴand Ⅵ terraces were formed in the Early Pleistocene(～2Ma ago);Class Ⅳ terraces were formed in the Early Pleistocene(～1.4 to 2 Ma ago);Class Ⅲ terraces were formed in the Middle Pleistocene(～0.9 to 1.4 Ma ago);Class Ⅱ terraces were formed in the late Mid-Pleistocene to Late Pleistocene(～0.2 to 0.9 Ma ago);Class Ⅰ terraces were formed in the Late Pleistocene(～0.06 to 0.2 Ma ago).(3)The highest terrace currently preserved by the Jinshui River,a tributary of the Hanjiang River,was formed at least～0.74 Ma ago,then developed and evolved during the continuous uplift of the Qinling Mountains.Since the early Middle Pleistocene,the Jinshui River Basin in the southern foot of the Qinling Mountains was uplifted by about 210 meters due to tectonic movements,and the average uplift rate was about 0.283 mm/a.A comparative study found that the uplift within and between the Qinling subzones during this period showed a markedly non-equilibrium quality.The overall uplift in the southern Qinling area was gentle,and the uplift in some areas(like Jinshui River Basin)was more intense.The uplift is relatively intense,and the average uplift rate in the South Qinling is slightly faster than that in the North(4)The third level terrace of Hanjiang River in the Yunxian section where the Wujiagou site is located was formed at least～0.9Ma years ago.The ages of the point stone and fossil layers in the site were in the early Mid-Pleistocene(～0.6Ma),which was the warmest and most suitable period in the Qinling region.The results also revealed the activities of ancient humans in Yunxian Basin at this stage;The age of Yonggucun paleolithic site is early Middle Pleistocene(～0.8Ma)and the climate south of the Qinling Mountains is more suitable for human survival.This result supports the conclusion that the Qinling Mountains may be the transition zone of ancient human migration from north to south.(5)The earliest humans in China originated in the Qinling region(～2.1Ma,Shangchen site),and then gradually formed settlement centers in the Qinling,Nihewan,Yangtze river,Yunnan-Guangxi and Northeast regions,and spread to other regions.Since the Mid-Pleistocene,the reproduction and expansion of humans have been significantly affected by the climate regulation of the glacial-interglacial period.Around～0.8Ma ago,the northern humans began to migrate across the Qinling Mountains to the south due to the cold climate.At this period,the Qinling mountains may have been the Center and regulatory zone for the migration of ancient humans from north to south.About～0.3Ma years since the late Pleistocene,modern humans who gradually adapted to the environment began to multiply rapidly,and formed a distribution pattern dominated by the second ladder,of which the Qinling area is one of the main gathering areas. |
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