Mesozoic Range-basin Coupling And Tectonic Evolution Of The Mid-segment Of The Central China Orogen

Basins and mountains are symbiotic units.No Matter from the perspective of geography or geology,there always is a coupling relationship between orogenesis and basin.Their evolutions are controlled by a kinematics and dynamic mechanism.The Central China Orogen extends in a East-West trending.The Ordos Basin is located to the north and the Sichuan Basin to the south.Since the Mesozoic,here experienced serious of tectonic events,such as the Indosinian movement,the Yanshanian movement,and the Indo-Asia collision in the Cenezoic.Targeting to the key geologic issues,we use structural(Field Mapping,Growth strata,Balance section),geochronologic(Zircon U-Pb,40Ar/39Ar and Apatite fission track dating),and sedimentology(Detrital zircon,Detrital components)methods,try to explore the Mesozoic Basin-range coupling and tectonic evolution of the Mid-segment of Central China Orogen.Regarding to the shortening of the crust,we found that the largest reduction in the crust occurred in our section located around the Huichengcheng Basin:the current distance of ?-?'section,on the north side of the basin,is 65 km in length,and the shortening rate is 57%;The current length of the ?-?' section,across the basin,is 70 km and the shortening rate is 69%;The current length of the ?-?' section on the south is 25 km,and the shortening rate is 62%.Based on the previous studies,the Main structural section of the Central China Orogen is calculated to be 370 km in current length,58%in the average crustal shortening rate,517 km in shortening distance,and 887 km in the original length.Regarding to the initiation age of the Indosinian movement,we identified the growth strata of the Upper Triassic Yanchang Formation in the south margin of the Ordos Basin.Comparing the detrital zircon with the adjacent tuff,the initiation age of the Indosinian movement in the Ordos Basin is 233±3.4 Ma.Two growth strata were identified in the Upper Triassic Xujiahe Formation and the Middle Jurassic Suining Formation on the northern margin of the Sichuan Basin,which means the initiation of the Indosinian and Yanshanian movements respectively and the Indosinian movement started at 216±2.7 Ma.Regarding to the Indosinian tectonic setting,we analized the provenance of detrital zircon and sandstone grain.The results show that the Upper Triassic(Yanchang Formation)in the southern Ordos Basin mainly received deposits from the North China Block;the Lower Triassic(Xipo Formation and Renjiagou Formation)in Huicheng Basin Mainly reveived deposits from the North Qinlling Orogen(Zircons in 534 Ma?400 Ma age),without typical North China basement input(Zircons in 2.5Ga and 1.7Ga?1.9Ga age).The fact indicates that the North Qinling orogenic belt has been uplifted and underwent denudation before the Early Triassic,swperating the North China Block and the South China Block and blocked material exchange between North and South China.According to the detrital zircons from the Hanzhong Basin and the Sichuan Basin,typical detrital materials fom the South Qinling region and the South China block(Zircons in 700 Ma to 900 Ma and 1700 Ma to 1850 Ma age)have been deposited on the north margin of the Sichuan Basin,and the source of the South China South Block has increased gradually to south.Based on the Dickinson and Cawood diagrams,from the perspective of plate tectonics,the south margin of the Ordos basin was considered as the retro-arc foreland area of the orogenic belt before the Indosinian period.The Huicheng basin was a epicontinental foreland basin,and the north margin of the Sichuan Basin was a subduction collision setting.Regarding to basin-range coupling and tectonic evolution since the Indosinian period,this study was focused on the analysis of regional structures and sedimentary basins,combined with the history of regional thermal evolution(40Ar/39Ar dating on different minerals and dating on apatite fission track).The results show that during the Indosinian period,ocean-land transfrom was completed,controlled by the north-south tectonic stress,and formed a series of faults and folds,trending in near east-west direction.The Yanshanian movement was reactivated and the Dabashan,the middle Jurassic growth strata and foreland basins on the north margin of the Sichuan Basin were formed.In the Cretaceous,a series of Cretaceous sediments in the southwest margin of the Ordos Basin and the eastern of the orogenic belt formed.In the Cenozoic,a far field effect of the Indo-Asia collision accelerated forming of the Hanzhong Basin and Fenwei Graben,which iniated from the Cretaceous.