| The uplift of the Tibetan Plateau is one of the most important geological events in the Cenozoic.It is a classic case of continental collision,furthermore it had a deep impact on tectonic framework of interior Asia and global climate and environment change.The evolution and uplift history of the Tibetan Plateau has always been one of frontier fields of geosciences.The northeast margin of the Tibetan Plateau is characterized by a series of orogenic belts and sedimentary basins which are controlled by large folds,strike-slip faults and thrust faults.As an area of extremity of collision between Eurasia and Indian plates,the evolution of this tectonic elements provide an important basis to study the dynamical mechanism and space-time evolution of the Tibetan Plateau.Losts of research work has been devoted to this area in different ways,but there still has a dispute about the process of structural evolution.Orogenic belts and sedimentary basins are considered as two basic and essential tectonic units,they constitute the closed contacted tectonic system in space development and formation mechanism.The information of structural evolution usually have been lost or got fuzzy for suffering long time structure activity and denudation in Orogenic belts,by contrast,deposit record is more integrated and continuous in sedimentary basins.Especially the provenance of sediments is the key to understand the history of uplift-denudation of the Orogenic belt in periphery of basins and the space-time deformation of structure.The provenance provides constraints to research the regional tectonic deformation and evolution,and it has become one of the necessary methods to study the Tibetan Plateau.The premise of provenance study is to find the distinction in different source region,then make an accurate judgement about the provenance.The composition of U-Pb age has significant difference from the metamorphose basement,magmatic rock and sedimentary rock of Precambrian,Paleozoic and Mesozoic in the northeast margin of the Tibetan Plateau.However,it has similar features from sedimentary rock of Permian to early Paleozoic in the Kunnan tectonic zone,north of the Western Qinling and the Eastern Qilian block(mainly constitute by Caledonian and Jinning Period zircon)and sedimentary rock of Triassic in the Songpan-Garze massif,east of the Kunzhong tectonic zone and south of the Eastern Qilian block(mainly constitute by later Variscan-Indosinian and early Paleozoic Period zircon).Therefor,the detrital zircon U-Pb ages can be used as a basis to study the sediment provenance but can not accurately distinguish the potential source region only by this method in the northeast margin of the Tibetan Plateau.The composition of orogenic belt's zircon(U-Th)/He ages is connected with tectonic-thermal evolution of basement.And the history of tectonic-thermal evolution has significant difference in the Songpan-Garze massif,the Eastern Kunlun,the Western Qinling and the Eastern Qilian.The distribution of zircon(U-Th)/He ages can be divided to 3 zone from south to north: >260Ma?260-170 Ma and 170-100 Ma in the northeast margin of the Tibetan Plateau.This shows we can divide above orogenic belts to different zone and it is a good supplement for the method of detrital zircon U-Pb age.The purpose of this study is to research the provenance of Cenozoic sediment about Guide basin,Lanzhou basin and Wushan-Tianshui basin in the northeast margin of the Tibetan Plateau.The study is mainly based on detrital zircon U-Pb and(U-Th)/He ages.Here,we present combined investigation of stratigraphy,lithology,paleocurrent,strata deformation,low-temperature thermochronology and provenance for the northeast margin of the Tibetan Plateau,summarizing and rebuilding the history of tectonic evolution during the Cenozoic.The study reveals the change of regional deformation way and orogenic belt range in different period,indicating the important indication significance in the evolution and dynamical mechanism of the Tibetan Plateau evolution.Main productions and results are as follows:1.In the Eocene period(~55-33Ma),the provenance in Guide basin and Lanzhou basin both came from Buqing Mountain,Amne Machin and Xiqing Mountain which belong to the Eastern Kunlun orogenic belt,and there is no signal of bedrock from the Eastern Qilian or Western Qinling orogenic belt.This result shows that east of the Eastern Kunlun orogenic belt occurred intensive uplift and deformation,contrastively,the Eastern Qilian and Western Qinling orogenic belt did not uplift.Guide and Lanzhou basin have the same origin,so it may was a uniform foreland or compressional basin in the area of Guide,Xining and Lanzhou.Riyue Mountain and Laji Mountain were both deep burialism under Paleogene strata in this period.Meanwhile,west of Western Qinling,Daban Mountain and Maxian-Xinglong Mountain may were a depression.2.In the Oligocene period(35-23Ma),the provenance in Guide basin and Lanzhou basin had a trend of extending to north but it was still limited in the Eastern Kunlun orogenic belt.The Paleogene sediment has a two-way provenance in the area of Wushan-Tianshui.The fragment came from the Eastern Qilian in the north and the Western Qinling in the south.And this shows the scale of the Eastern Kunlun orogenic belt becam bigger but limited.The west of Western Qinling and east of Eastern Qilian orogenic belt did not uplift and denudate.And the east of Western Qinling may have a tensional faulted depression because of the eastern extrusion of Songpan-Garze massif contrastive uplift.In the internal and north margin of the Western Qinling orogenic belt had intensive tectonic activity,and they were rapid sinistral strike-slip movement.3.In the early-middle Miocene period,the deposition had a significant change in the northeast margin of the Tibetan Plateau.The sedimentary facies had an abruot change in Guide basin and Lanzhou basin,and a mass of materials were proved that they came from Western Qinling,Riyue Mountain and Laji Mountain.A large scale sedimentary locomotion happened in Wushan-Tianshui basin,the way of sediment supply became only from the Western Qinling.This shows the Western Qinling,Riyue Mountain and Laji Mountain began to uplift in early Miocene,the Western Qinling Fault Zone became thrust faults to north and formed a uniform foreland basin in north of the Qinling.The provenance had a significant change again in the middle Miocene,it should be connected with rapid denudation of orogenic belt and regional planation.4.In the late Miocene period(13-3.6 Ma),large-scale orogenic movement occurred in the northeast margin of the Tibetan Plateau.The provenance research shows tha the Eastern Kunlun orogenic belt,Songpan-Garze massif,Bailongjiang tectonic belt and south of Liupan Mountain began to uplift first in the period of 13-10 Ma.Riyue Mountain and Laji Mountain began to uplift about 8.5Ma,simuitaneously,from Daban Mountain to Maxian-Xinglong Mountain area occurred slightly rise and uplifted Lanzhou Basin basement to end the Neocene deposition.Since 6Ma,all the Western Qinling began to uplift,making the Cenozoic basins collapsed in the northeast margin of the Tibetan Plateau and only some small piedmont and intermountainous basins preserved.5.In The Pliocene period(3.6-1.8 Ma),large-scale orogenic movement occurred again in the northeast margin of the Tibetan Plateau and produced abundant molasse-like deposits in the plateau and surrounding areas.The provenance analysis results shows that the main materials of channeled early Wuquanshan Formation came from the Eastern Kunlun orogenic belt and a handful of them came from the Eastern Qilian or Western Qinling orogenic belt.This proves the intensive uplift occurred again in the Eastern Kunlun orogenic belt in this period.The alluvial fans' materials of later Wuquanshan Formation mainly came from the Maxian-Xinglong Mountains,shows strong uplift and deformation in the middle part of Longzhong Basin.The change of provenance in the Amigang Formation attests the intensive uplift and thrust in noth part of Western Qinling orogenic belt.In The Pleistocene period(1.8-0 Ma),a series of well –preserved fluvial terraces formed with the continually incised of the Yellow River,the change of terraces' provenance shows the Eastern Qilian and Western Qinling are still uplifting.Meanwhile,headward erosion of the Yellow River has occurred internal plateau which in the south of the Eastern Kunlun orogenic belt. |
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