Formation And Evolution Of The Late Triassic Foreland Basin In Western Sichuan

The western Sichuan area,including the western margin of the Yangtze Block and the eastern margin of the Tibetan Plateau,correspond to the western Sichuan Basin and the Songpan-Ganzi Fold Belt.The western Sichuan Basin is separated from the central Sichuan area by the Longquanshan Mountain to the east,bounded by the Beichuan-Yingxiu Fault in the Longmenshan Thrust Belt to the west,by the Qinling Orogenic Belt to the north,and the Kangding ancient land to the south.The Songpan-Ganzi Fold Belt is an inverted triangle,sandwiching between the eastern Kunlun-western Qinling Orogenic Belt,the Longmenshan Thrust Belt and the Qiangtang Block.Prior to the Late Triassic,the western Sichuan Basin,as a part of the passive continental margin of the Yangtze Block,deposited extremely thick marine carbonate strata.Since the Late Triassic,the Indosinian tectonic episode and the collision between the South and North China Blocks ended marine basin in the western Sichuan Basin,and gradually transformed into a continental foreland basin,which was represented by large thickness of fluvial and lacustrine sediments of the Xujiahe Formation.Simultaneous during the Late Triassic,the Songpan-Ganzi terrane continued turbidite deposition.It has long been believed that top-to-the southeast thrusting of the Longmenshan Thrust Belt produced the foreland basin of western Sichuan Basin,and was the main provenance of the the Xujiahe Formation.However,balance and restorationon for geological sections cross-cutting the thrust nappe zone of the Longmenshan Thrust Belt recently reveled top-to-the south with limited uplift,but rather than top-to-the southeast thrusting during the Late Triassic.Moreover,a large scale of uplift was occurred by the formation of the Tibetan Plateau and eastward propagation along the Longmenshan Thrust Belt during the Cenozoic.Therefore,the basin-mountain coupling relationship between the Late Triassic western Sichuan Foreland Basin and the Paleo-Longmenshan Thrust Belt was left unclearBased on previous research,the prototype basin of the western Sichuan Basin in the Late Triassic is the main research object in this thesis.This study focuses on the Xujiahe Formation and makes a preliminary investigation of the Late Triassic flysch deposits in the Songpan-Ganzi Fold Belt.From the outcrops and boreholes data of the Xujiahe Formation,this thesis provides new information for sedimentary facies belt migration and provenance change of the western Sichuan Foreland Basin.Inaddition,detrital zircon dating of the Late Triassic flysch in Songpan-Ganzi Fold Belt were carried out to analyze the relationship between them and the Xujiahe Formation.The results show that the Xujiahe Formation in the Late Triassic can be divided into five segments from the bottom to the top,and the axial main body of the foreland basin presented an east-west trend,and the depositional center had a southward trend from early to late.The maximum deposition ages from the second and fourth segments of the Xujiahe Formation are?221 Ma and-207 Ma.Combined with previous studies,we define the depositional age of the Xujiahe Formation is 228-200 Ma.Paleocurrent data and the age pattern of detrital zircons indicate that the Qinling Orogenic Belt is the main source of the Xujiahe Formation.The results of flysch detrital zircons in Songpan-Ganzi Fold Belt show that the depositional age is 225-222 Ma and the provenance area also includes the Qinling Orogenic Belt.This result indicates that the Songpan-Ganzi flysch basin and the Xujiahe Formation had similar sedimentary time in most of the Late Triassic,therefore,it could not be the main source of the western Sichuan Foreland Basin.During most time of the Late Triassic,the western Sichuan Foreland Basin and Songpan-Ganzi flysch basin were probably separated by the remanents of the early Longmenshan Rift(Late Permian aulacogen).Based on the new results,together with previous researches,the Late Triassic western Sichuan Foreland Basin show a basin-mountain coupling relationship with the Qinling Orogenic Belt.We therefore propose a new tectonic model of foreland basin evolution in western Sichuan Basin that the foreland thrust belt of the Qinling Orogenic Belt gradually progressed southward,providing the main provenance of the Xujiahe Formation and resulting in the southward migration of the sedimentary facies belt and depocenters of the foreland basin.