Study On Thermochronology Of Southeastern Qaidam Basin And Its Tectonic Implications, Qinghai Province, NW China

Qaidam basin is one of largest intramontane basins throughout China which is located to the north of Tibetan Plateau,deformation studies within basin,accurate constraints on magmatic activities and thermal histories of its surrounding mountains,are definitely conductive in better understanding of Tibetan Plateau in terms of its tectonic evolution processes,as well as far-field effects resulting from Indo-Eurasian collisions.Two phases of magmatic activities have recorded in Northern Qaidam Basin by LA-ICP MS zircon U-Pb dating.PhaseⅠoccurred during approximately 493～420 Ma between Ordovician and Silurian.PhaseⅡoccurred during late Permian(257.8±4.0 Ma).Ages of magmatic events in Southeastern Qaidam focused primarily on ages between 250～230 Ma(P-T),regardless of sampling localities,the PhaseⅡhappened in Northern Qaidam can be included in the time interval of Southeastern Qaidam.Based on dating results of this study,in conjunction with previous work in Eastern Kunlun Shan,as well as Southeastern Altyn Shan, we can conclude that entire mountains surrounding the Qaidam basin have undertaken thoroughly magmatic activities during time interval ranging from late Permian to late Triassic time.In eastern Qaidam Basin,the metamorphic and sedimentary Pre-Mesozoic basement had been uplifted due to faulting of the East Qaidam Thrust System and then was separated from the Gonghe Basin due to dextral strike-slip faulting of Wenquan Fault.Apatite fission track analyses and thermochronology from eastern Qaidam Basin show two phases exhumation and thrusting since Mesozoic.PhaseⅠspans from 108 Ma to 61 Ma,showing a reduction of AFT cooling ages from south to north,and could be interpreted as the tilting exhumation of the Qaidam basement and thrusting before and at the beginning of Cenozoic. PhaseⅡoccurs during 26.6～17.8 Ma between Paleogene and Neogene,which is expressed as the out-of-sequence Cenozoic thrusting of East Qaidam Thrust System.According to apatite fission track analyses and thermochronology from Eastern Kunlun shan,three phases exhumation and thrusting events have been indicated since Cenozoic.PhaseⅠoccurred in 52.9 Ma, demonstrating a uplifting event accross Tibetan Plateau subsequent to the Indo-Eurasian collisions, however,it had been eroded shortly after it exhumation and became source areas of sedimentary rocks Kekexili Fm.to the south,Lulehe Fm.to the north.PhaseⅡoccurred between 16.3 Ma and10.0 Ma,the uplifting event mainly centered on the middle Miocene(～15 Ma) accurately witnessed the entire uplifting of Tibetan Plateau for the first time,subsequently,significant climate change and Adakitic rocks eruptions have been recorded.The timing of PhaseⅢ,namely spanning 5.1 Ma to 0.9Ma is consistent with the final uplifting event throughout Tibetan Plateau suggested by some researchers.According to the distribution patterns of granitoids intruded during late Permian to late Triassic times, shortening rate of Eastern Qaidam basement is as much as 50%that of the western Qaidam.Apatitie fission track dating in both Eastern Qaidam and Western Qaidam suggest that tectonically compressional deformation in Miocene initiated from Dulan-wulan in Southeastern Qaidam then propagated westernward.