| The Qinghai-Tibet Plateau is the plateau with the highest elevation and largest area in the world,and is known as the roof of the world.The uplift of the Tibetan Plateau is one of the most important geological events in the history of the earth.The collision between India and the Eurasian plate was considered to be the main factor that led to the strong uplift of the plateau.Influenced by the collision between India and the Eurasian Plate,the northern margin of the Tibetan Plateau continues to expand northwards,and the Neo-Tethys Ocean retreats from the Tarim Basin from east to west.This not only causes major structural changes in the Asian region,but also has a significant impact on global and regional climate change.The formation and uplift of the Qinghai-Tibet Plateau and its impact on the enviro?ent have always been the focus of international geoscience research.In the interior and periphery of the Tibetan Plateau,a large number of sedimentary basins have been formed due to the impact of the India-Eurasia plate collision and its “remote effect”.The Xunhua Basin studied in this paper belongs to one of the numerous basins on the northeastern edge of the Qinghai-Tibet Plateau.It has accumulated a thick layer of Oligocene-Miocene(27~6 Ma)sediments,which records the deformation of the northeastern plateau in a relatively complete manner.The uplift and climate change process have become an ideal area for studying the uplift of the plateau structure-the coupled process of climate and enviro?ental changes.Based on the high-precision chronostratigraphic framework of the Xunhua Basin,this paper studied the clay mineral content and geochemical element characteristics of the sediments of the Yanggougou section and the Xigou section of the measured stratigraphic section,and studied the study area and the Tibetan Plateau at the same time.The paleoclimate records were compared to explore the relationship between the climatic evolution process in the northeastern margin of the Tibetan Plateau and the uplift of the Cenozoic Qinghai-Tibet Plateau and the formation and evolution of the East Asian monsoon.The study draws the following conclusions:(1)The contents of clay minerals,non-clay minerals,and trace elements in sediments from the Yangquangou section of the Xunhua Basin show that during the late Oligocene-Middle Miocene(27-12.8 Ma),the climate change was mainly distributed in the following nodes: 24.5 Ma,22 Ma,19.4 Ma,and 14 Ma.The high smectite content,high ratio of Rb/Sr and Ba/Sr,and lower gypsum content in the Yanggougou section during 27-24.5 Ma all indicate that the climate of Xunhua Basin was warm and humid during this period.During 24.5-22 Ma,the ratio of clay mineral content in the Yangquangou section(illite + chlorite)/ smectite increased significantly at 24.5 Ma,and the ratios of Rb/Sr and Ba/Sr showed a downward trend,the content of gypsum + halite rised sharply.All these indicate that the climate of Xunhua Basin has obviously become cold and dry during this period.In Xunhua Basin,the climate became relatively warm and humid during 22-19.4 Ma,with higher Rb/Sr and Ba/Sr ratios.The ratio of(illite + chlorite)/smectite decreased slightly,and gypsum + halite content also showed a downward trend.At 19.4 Ma,the content of clay minerals in Xunhua Basin changed significantly.The ratio of illite + chlorite/smectite increased significantly,the ratio of Rb/Sr and Ba/Sr decreased,and the content of gypsum + halite reached a higher value again.This shows that during the period of 19.4-14 Ma,the climate of Xunhua Basin showed obvious aridity characteristics.After 14 Ma,the ratio of(illite + chlorite)/smectite continued to increase,and the calcite and dolomite content increased significantly,indicating that the aridity of the Xunhua Basin during the 14-12.8 Ma period was aggravated by the climatic characteristics.To sum up,during the period of 27-12.8 Ma,the climate evolution process in Xunhua Basin was: warm and humid(27-24.5 Ma)-cold and dry(24.5-22 Ma)-relatively warm and humid(22-19.4 Ma-aridity(19.4-14 Ma)—Intensification of aridity(14-12.8 Ma).(2)The changes of the climate in the Xunhua Basin during the period of 14.6-6 Ma were analyzed through the changes of clay mineral content,non-clay mineral content,major elements and trace elements indicators of sediments in Xigou section of Xunhua Basin.Studies have shown that during the period of 14.6-6 Ma in Xunhua Basin,the climate change was mainly divided into three sections: aridity(14.6-10.8 Ma-relatively warm and humid(10.8-7.3 Ma)-aridity(7.3-6 Ma).During the period of 14.6-10.8 Ma,the content of clay minerals in the sediments of Xunhua Basin was stable overall,and the ratio of(illite + chlorite)/(smectite + kaolinite)was high,and the content of gypsum + halite was high.This shows that the Xunhua Basin has a relatively dry climate.The results are also consistent with the climate features of aggravated aridity after 14 Ma of the Yangquangou section.After 10.8 Ma,the content of clay minerals in Xigou section sediments changed significantly,and the ratio of(illite + chlorite)/(smectite + kaolinite)decreased significantly.At the same time,the major and minor elements indicators also have significant changes,such as the CIA value and Rb / Sr ratio increased significantly.All these indicate that the climate of Xunhua Basin has gradually become relatively warm and humid.After 7.3 Ma,the content of clay minerals in the sediments of Xigou section changed drastically,and the ratio of(illite + chlorite)/(smectite + kaolinite)increased sharply.The results indicated by the change trends of the main elements and trace elements were consistent with those of clay minerals,indicating that the climate of Xunhua Basin was drastically arid after 7.3 Ma.(3)Based on the characteristics of non-clay minerals and trace elements in Yangquangou section,the provenance of sediments in the late Oligocene-Middle Miocene(27-12.8 Ma)in Xunhua Basin was analyzed.Through the comparison of lithology with the surrounding area and paleocurrent analysis,we discussed the causes of sediment source change in Xunhua Basin.The study shows that the main provenance changes in the Xunhua Basin during the period of 27-12.8 Ma are reflected in two time points of 24.5 Ma and 22 Ma.At 24.5 Ma,the content of plagioclase,K-feldspar,quartz,and calcite in the Yangquangou section sediments dropped sharply.As the main lithology of the northern margin of West Qinling is Triassic flysch,siliceous mudstone and limestone.The Laji Shan exhibits spatial variation in lithologies: the East Laji Shan of upper Devonian intermediate-basic volcanic rock,volcaniclastic rock,and siliceous mudstone;West Laji Shan mainly consists of Mesoproterozoic slate,phyllite,and limestone.Combined with previous research,we consider that the source change of sediments in the 24.5 Ma Xunhua Basin is mainly affected by the uplift of the Laji Shan.The sources of sediments in Xunhua Basin were converted from the source material of West Qinling at 24.5 Ma to the source of the mixture of the West Qinling and the eastern section of Laji Shan,which resulted in a decrease in the content of calcite in the sediment.At 22 Ma,the content of non-clay minerals in the sediments of Xunhua Basin changed remarkably,and this time the content of calcite increased significantly.We consider that the source change of the sediments in the 22 Ma Xunhua Basin was mainly caused by the accelerated uplift of the Laji Shan.The western section of Laji Shan contains more limestone,leading to an increase in the calcite content in the Xunhua Basin sediments.(4)The study of sediments in the Xigou section shows that during the period of 14.6-6 Ma,the provenance changes of sediments in the Xunhua Basin mainly occurred at three nodes: 13 Ma,10.8 Ma,and 7.3 Ma.Based on the evolution of the uplift of the Western Qinling,Laji Shan,and Jishi Shan around the Xunhua Basin,we speculate that the source change of sediments in the 13 Ma Xunhua Basin is mainly affected by the uplift of the Jishi Shan.Since the sedimentation rate in Xunhua Basin has risen at this time,but the magnitude is not large,we speculate that the height of the Jishi Shan uplift is not large at this time.The sedimentation rate in the 10.8 Ma Xunhua Basin increased rapidly,and the non-clay minerals in the sediments also changed significantly.Therefore,we consider that the source change of sediments in the 10.8 Ma Xunhua Basin is mainly caused by accelerating uplift of Jishi Shan.At 7.3 Ma,the non-clay minerals in the Xigou section have undergone dramatic changes,with the sharpest increase in calcite content.Previous studies on the uplifting process of the Laji Shan have shown that the Laji Shan was uplifted again at about 8 Ma,and the limestone in the western section of the Laji Shan caused the content of calcite in the Xunhua Basin to increase during the period.Therefore,the uplift of the Laji Shan resulted in the change of provenance of sediments in Xunhua Basin after 7.3 Ma.(5)By comparing the climatic evolution process of Xunhua Basin with the climate changes of other basins on the northeastern margin of the Tibetan Plateau,we can see that the aridification of the Tibetan Plateau since the late Oligocene can be roughly divided into four stages: 1)The northeastern margin of the Tibetan Plateau is characterized by cold and arid climate at around 25-23 Ma;2)The northeastern margin of the Qinghai-Tibet Plateau shows a trend of increasing aridity in the overall 20-19 Ma;3)Cold and dry climate after 14-13 Ma(aridity);4)Aridification after about 8 Ma.The cold and dry climate around 25-23 Ma is mainly affected by the overall uplift of the Qinghai-Tibet Plateau during this period.The large-scale uplift of the Qinghai-Tibet Plateau destroyed the climate model controlled by the subtropical system of the planetary wind system on the northern margin of the Tibetan Plateau,resulting in formation of Asian monsoon.The aridification characteristics of 20-19 Ma are widespread in Xunhua Basin,Xining Basin and Xinghai Basin.This aridification is mainly related to the adjustment of the plateau from the subtropical arid climate controlled by the planetary wind system to the monsoon climate in the late Oligocene-Early Miocene.The aridification of 14-13 Ma is also widespread in the Qinghai-Tibet Plateau,and its mechanism is generally believed to be affected by the increase of the East Asian winter monsoon and the weakening of the summer monsoon.About 8 Ma of aridification events have been recorded in the Qinghai-Tibet Plateau and even in the globe.The driving mechanism is mainly affected by the rapid uplift of the Qinghai-Tibet Plateau,and the weakening of the East Asian summer monsoon is also considered to be an important factor affecting the aridity during this period. |
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