| The age,extent and nature of the uplift of the Tibetan Plateau are currently hotspots and difficulties in domestic and foreign research.The Lhasa terrane is located in the central and southern hinterland of the plateau,adjacent to the Himalaya terrane in the south,and the Qiangtang terrane in the north.It is the most important part of the plateau.Research on its uplift process can provide key evidence for the overall uplift status of the plateau.Meanwhile,the planation of the Lhasa terrane is widely developed and well preserved,which completely recorded the history of regional planation and uplift.Because different chronological methods have diverse theoretical foundations and assumptions,it is difficult to make a macro comparison of relevant information obtained through multiple methods.The use of a single thermochronological method to conduct a large-scale comparative study of the geological and geomorphological bodies in this area can not only obtain systematic and reliable evidence,but also fully evaluate,distinguish and connect to the previous work,which makes it so far the most promising area for breakthroughs.The apatite fission track has a low sealing temperature(about 60?-120?).It is sensitive to shallow crustal processes and can capture the details of evolution that are not effective in other mid-to-high temperature thermochronology.Compared with other low-temperature thermochronological methods,the apatite fission track has the advantages of perfect basic theory,mature track annealing behavior research,wide measurable range,and low test cost.It is the best choice for research on large-scale geological and geomorphological objects.A large number of igneous rocks are exposed in the Lhasa terrane,which is an ideal rock body for the study of apatite fission tracks.Therefore,this study chose this area to conduct a systematic thermal chronology study of apatite fission tracks,and a total of 48 age data and related annealing parameter data were obtained.Based on the data,we carried out the comparison and analysis of the age macroscopic information and the information of a single rock body,made full use of the existing chronological evidence in this area combining with the thermal history simulation results,and we excavated their geological significance and obtained the preliminary understanding as follows:(1)Except the samples XZ85(48.8±3.9Ma)and XZ92(39.3±3.7Ma)in the Anduo-Bango area in the northern part of the Lhasa terrane are younger,the ages of the other samples fall in the Mesozoic era(74.4±3.3Ma-154.1± 9.0Ma),with the track length range of 13.42±1.28?m-14.55±0.86?m,and the track length is relatively short;the geological background of the Changdu-Nangqian area in the east is the most complicated.The sample age(15.7±2.8Ma-100.8±22.2Ma)and the track length(13.07±1.01?m-14.81±1.10?m)are quite various,which means the rock mass may have evolved independently;in the southern part of the Lhasa terrane,most samples in the Damxung-Xigaze-Lhasa and Linzhi-Bomi areas are not older than 35 Ma.The age of the samples can roughly reflect the rapid stripping events that occurred during this period.The track length ranges from 14.06±1.37?m to 14.91±0.97?m.The track length is relatively long,and the rock mass where the surface sample is located quickly passed through the partial annealing zone.(2)The results of thermal history simulation of samples that can obtain a sufficient number of track lengths show that the study area has experienced three major rapid stripping events on a macroscopic scale: the one occurred in the northwest(Amdo-Bango area)in the middle of the Epoch(about 120-100 Ma),the one occurred in the eastern part(Qamdo-Nangqian area)during the Paleocene-Eocene(about 60-45 Ma),and the one occurred in the southern part(Dangxiong-Xigaze-Lhasa area and Linzhi-Bomi area)during the Miocene(approximately 20-15 Ma and 7 Ma).(3)The Lhasa terrane and its surrounding have typical structural geomorphology,spreading in layers.Through field investigation,quantitative analysis of geographic information system technology,and the comparison of three-dimensional images from Google Earth,this paper confirmed that the planation of this area is divided into two levels.Among them,the Amdo-Bango area is the most well-preserved,and the other areas are relatively fragmented and have small area.The main planation is about4600-4900 meters above sea level,and the top surface of the mountain is about5000-5400 meters above sea level.According to the chronological results obtained in this study,combined with previous research,it is preliminarily obtained that the early quasi-plainization stage of the top surface of the mountain could be 33-25 Ma,and the early quasi-plainization stage of the main planation plane could be 15-7 Ma.(4)Based on the evidence obtained in this study and previous work of other research,this paper found that the central plateau represented by the study area was first uplifted in the late Mesozoic,and then gradually expanded eastward under the continuous compression of the Indian plate.It extended to the current eastern part of the present plateau in the end of the Paleocene-the beginning of the Eocene.And in the Oligocene-early Miocene,it experienced a period of relatively quiet tectonic structure,and the flattening effect was strong,which formed the top surface of the mountain.The 15-7Ma plateau is mainly deformed by plastic extrusion or rigid rotation,which provides dynamic conditions for the formation of the main planation surface. |
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