Late Paleozoic Paleogeographic Reconstruction And Tectonic Implication Of The Beishan Region, NW China

The Beishan Region,located in the southernmost of the Central Asian Orogenic Belt(CAOB),is the critical region to reconstruction of the accretionary history of the CAOB.The Late Paleozoic amalgamation between southern Beishan blocks and Dunhuang Block(DH)plays an important role in the accretionary orogenic process between the CAOB and North China–Tarim Block.The Upper Paleozoic sedimentary sequence provides a complete record of the tectonic evolution.The topographic reconstruction based on stratigraphy,sedimentology,and provenance can be used to distinguish archetypes of sedimentary basins in the different tectonic settings and to discuss phases of an accretionary orogenic system.Integrating newly obtained paleontologic and radiometric data,this study established a high-resolution stratigraphic correlation of the Upper Paleozoic formations(groups).Sediment routing systems from source to sink were reconstructed through sedimentology,petrology,and provenance methods in the Shuangyingshan and Shibanshan blocks(SYS and SBS),southern Beishan Region.These data were interpreted in a topographic and filling model of a marine intracontinental rift basin in the Beishan Region.The coupling system between sedimentary basins and orogenic belts was discussed based on the correlation among megasequences of 15 regions in the Tianshan–Xingmeng Belts.The Upper Paleozoic strata here are divided into three megasequences(MS)separated b two major unconformities each with a huge sedimentary hiatus.They are,in ascending order: pre-rift Devonian MS1(Queershan Group,Sangejing,and Dundunshan formations),syn-rift Mississippian–Guadalupian MS2(Baishan,Shibanshan,Ganquan,Shuangbutang,Jushitan,and Jinta formations),and inversion Lopingian MS3(Fanshankou and Hongyanjing formations).During the Middle Ordovician–Late Devonian,the northward subduction of the Liuyuan Ocean resulted in the Huaniushan Arc and a back-arc basin filled with Sangejing and Dundunshan formations of the MS1 on the southern margin of the SYS.The southward subduction caused the Lucaotan Arc and Lower Devonian Queershan group sourced from arc volcanic rocks(ca.415 Ma).The major unconformity between the MS1 and MS2 indicates the collision between SYS and DH–SBS.Up to the Visean–Moscovian,the intracontinental rift system in the southern Beishan Region was composed of several semi-isolated half-grabens including Shibanshan,Hongliuyuan,Yinaoxia,and Jijitaizi basins.These small hydrologically limited basins were mainly filled by carbonate platform,reefs,and tidal flat sediments.Braid stream facies on the top of this sequence indicate the continental environment in the Hongliuyuan and Yinaoxia basins.Detritus of these formations derived from Precambrian basement,Lower Paleozoic intrusive,volcanic and clastic rocks of the DH,SYS and SBS(ca.2456 Ma,1807 Ma,906 Ma,450 Ma,and 408 Ma)exhumated by the previous collisional uplift.During the Sakmarian–Capitanian,a large marine rift basin was formed with incentive felsic explosive and basaltic flood volcanism.This sequence shows an upward-deepening trend from an alluvial fan,delta to shallow marine facies,and a successive erosion of the Lower Permian volcanic strata/Lower Paleozoic intrusive pluton(ca.300 Ma and 448 Ma).The topographic pattern is made up of a depocenter and two grossly symmetry basin boundaries.Sequences on the northern margin are composed of carbonate platform,alluvial fan and delta facies with detritus mainly from the Lower Paleozoic intrusive pluton,while those on the southern margin of fan-delta and delta facies with detritus mainly from the Lower Permian volcanic strata.An intercontinental environment in the rift climax stage is supported by the offshore sedimentary facies and pillow basalts in the depocenter.The Lopingian MS3 is made up of continental fluvio-lacustrine sequences with felsic volcanic beds.The disconformity between MS2 and MS3 and shrinking sedimentary distribution is consistent with features of the closure of aulacogen basins.The Mississippian–Guadalupian marine rift basins developed in the intracontinental environment without magmatism after the post-collisional stage.The archetype of these basins can be distinguished from back-arc rift basins by their sedimentary sequences,erosional record,and geometric pattern.The syn-rift MS2 represented by transgressive marine and volcanic strata in the Beishan Region can be well-correlated with Pennsylvanian–Guadalupian megasequences in Inner Mongolia.The Devonian/Carboniferous nonmarine-marine facies transition are respectively exposed on the southern margin of the Central Tianshan–Hanshan–Shalazashan–Xilinhaote Block proposes the closure of the southern brach of the Paleo-Asian Ocean.