Far-Field Responses Of The Petroliferous Basins Evolution To The Plate Movement

Located on the eastern Eurasian Plate,China and its adjacent marginal seas are surrounded by the Eurasian Plate,the Pacific Plate and the Indian Plate,and suffer from squeezing,extending and shearing effects by the regional tectonics between continental and oceanic,intraplate and marginal environments,characterizing diverse landform,actively crustal deformation,frequently volcanism and earthquakes.On the background of the Cenozoic plate movement,a series of petroliferous basins generated along the continental margin of China,for example,the Bohai Bay Basin,Qiongdongnan Basin,and the Cenozoic strata of the Qaidam Basin.In this study,we take full advantage of the seismic,logging,drilling core and geochemical data,analyzing the relationship between the intense faulting and downwarping in the Oligocene Nanpu Sag with the westward subduction of the Pacific Plate,and the dextral changes of sedimentation in the Oligocene Lenghu Area with the India-Eurasian Collision,and the oblique accretion on the northern continental margin in the Miocene Qiongdongnan Basin with the dextral strike-slip movement of the Red River Faults from the perspective of basin dynamics.We also discuss the difference of sedimentary fillings,structural evolution,hydrocarbon distribution between the Bohai Bay Basin of stretching environment,the Qaidam Basin of compressional environment and the Qiongdongnan Basin of shearing environment,and elaborate the formation and evolution of petroliferous basins along the continental margin of China in response to the surrounding plate movement.The Nanpu Sag(NPS),located in northwestern Bohai Bay Basin(BBB),East China,has significant oil discoveries in recent years.The effect of intense faulting and downwraping which generated the intense subsidence of NPS in the Dongying Formation(Fm.)(Ed)(28.5-23.8Ma)of the Late Oligocene is presented in this paper.We analyzed the geochemical characteristics of the widespread Cenozoic basalts in NPS from drill core samples.Our results show that the magma experienced negligible crustal contamination and slight fractional crystallization,the features of remarkable positive U,Pb,Sr and Ti anomaly,low Rb/Ba and Rb/Sr values of the NPS basalts were inherited from the source region.The NPS basalts in Ed have distinctive characteristics as sub-alkaline basalts with rare earth elements(REE)patterns parallel to enriched mid-ocean ridge basalts(E-MORB),were generated from a mixture origin of garnet-lherzolite zone with relatively high degree of partial melting(30%-50%)and garnet+spinel-lherzolite transition zone with partial melting degree ranging from 3%to 15%.In contrast,NPS basalts of other Cenozoic periods or Ed basalts of other areas in eastern China are alkaline with light REE enrichment and oceanic island basalts(OIB)-like-pattern,their magmas originated from garnet+spinel-lherzolite transition zone with partial melting degree lower than 5%.Combined with previous studies of the mantle wedge in the mantle transition zone(MTZ)beneath northeastern Asia,forming around 30Ma,we infer that the stagnating of the Pacific slab triggered the asthenosphere upwelling and increased the degree of partial melting in mantle source,relatively weak areas of the North China Craton(NCC)like the Tanlu Fault(TLF)where NPS located on were reactivate and susceptible to be modified.Therefore,the intense faulting and downwarping of NPS in Ed,along with the igneous activities,was a response to the Cenozoic stagnant Pacific slab.In addition,on the background of India-Eurasian Collision,the extrusion forced subsidence center of Huanghua Basin migrating to the Nanpu Sag in the Dongying Formation,made E-W trending faults intensely active under the S-N trending tensile stress.The Cenozoic India-Eurasia Collision is evidently recorded in the Qaidam Basin,which is located in the north margin of the Tibetan Plateau.The present study describes a dextrorotary phenomenon at the end of the Eocene in Lenghu Area in the northern Qaidam Basin,discovered in sedimentary and tectonic records.This phenomenon is interpreted to have been a result of the levorotatory movement along the Altyn Tagh Fault(ATF)based on the following evidence.First,provenance,as analyzed by heavy mineral assemblages,was slightly deflected from southwestward to the westward by about 45°.Second,paleocurrent inferred from dip logging and seismic reflection changed clockwise by approximately 25°.Third,there is evidence of increases in fault activity in the area,especially in northwest-southeast trending branches relative to older west-east trending branches.Increases in faulting coincide with abrupt increases in sediment supply in the Oligocene,after earlier decreases based on total sand content during the Eocene.Our results demonstrate that the northern margin of the Tibetan Plateau synchronously responded to the initiation and termination of the India-Eurasia Collision.The levorotatory strike-slip of the ATF was immediately triggered by the complete collision at the end of Eocene,the strike-slip movement caused the dextrorotary phenomenon in the Lenghu Area.Located on the northwestern margin of the South China Sea,the origin of asymmetrical shelf-slope clinoforms accumulated on the continental margin of the Qiongdongnan Basin since the late Miocene is ambiguous and it likely is the first case of oblique sediment transport and dispersal among the most studied mud-rich shelves of the world during Neogene to Recent sea-level changes.We calculate the paleo-sediment flux required to build the shelf-slope clinoforms based on 2-D seismic data,the results are 3to 17 time larger than the estimated paleo-sediment discharge from the proximal catchment area of Hainan Island.This large mismatch suggests that sediments on the shelf-edge of Qiongdongnan Basin were not only from Hainan Island but more likely from a larger drainage system.The strong(tens km)southward migration of the western portion of the Qiongdongnan shelf-edge break compared to the weak(1-2 km)shelf-edge progradation in the eastern portion,suggests that the Qiongdongnan shelf prism is highly asymmetric and is caused by a large sediment supply centered to the west,with most likely the Red River(Song Hong)as the sediment supply driver.The fine-grained lithology of the studied shelf margin sediments off Hainan Island,especially since the end Miocene,suggests that they were derived mainly from the Red River and were transported southeastward from the Gulf of Tonkin by shelf currents.We propose a model for asymmetric shelf-slope accretion of mud-rich shelves in which a significant part of the sediment-prism volume was obliquely dispersed across the subsiding shelf as fluid mud during highstand sea-levels.The model presented here describes oblique construction of shelf-slope clinoforms in a mud-rich system in response to variations in sediment dispersal and depocenter during multiple sea-level cycles.This model of long-term(?10~7years),lateral asymmetric accretion contrasts with oblique sediment transport in other source-to-sink sedimentary systems around the world.In addition,the intense subsidence of the western Qiongdongnan basement provided larger depositional accommodation for the Red River-derived sediments,which was triggered by the dextral strike-slip movement of the Red River Faults since 10.5Ma.In summary,at the beginning of the Cenozoic,with the subduction of Pacific Plate to the east and the India Plate to the west,the Bohai Bay Basin,the Qaidam Basin,and the Qiongdongnan Basin were generated by the stretching,compressional,and shearing stress in the eastern,western and southern China.The continuous process of the India-Eurasia Plate Collision causes the surrounding terrains of the Tibetan plateau to push out to the northeast,east and southeast respectively,generating lateral extrusion and forcing the continental crust of the western,central and eastern China to move eastward as a cohesive whole.Since then,the crustal motion characteristics of subcontinents within China show a continuity with the Tibetan plateau as the source of power,the tectonic stress field fan-shaped diverts towards the eastern margin.In response to the superposition of the stress and the movement of the Pacific plate,the Qaidam basin,Bohai Bay Basin and Qiongdongnan Basin possess their own tectonic and sedimentary phenomena in the evolution phase.