Reinterpretation of the northern South China Sea pre-Cenozoic basement and geodynamic implications of the South China continent: constraints from combined geological and geophysical records

The pre-Cenozoic northern South China Sea(SCS)Basin basement was supposed to exist as a complex of heterogeneous segments,divided by dozens of N-S faulting.Unfortunately,only the Hainan Island and the northeastern SCS region were modestly dated while the extensive basement remains roughly postulated by limited geophysical data.This study presents a systematic analysis including U-Pb geochronology,elemental geochemistry and petrographic identification on granite and meta-clastic borehole samples from several key areas.Constrained from gravity-magnetic joint inversion,this interpretation will be of great significance revealing the tectono-magmatic evolution along the southeastern margin of the Eurasian Plate.Beneath the thick Cenozoic sediments,the northern SCS is composed of a uniform Mesozoic basement while the Precambrian rocks are only constricted along the Red River Fault Zone.Further eastern part of the northern SCS below the Cenozoic succession was widely intruded by granites with Jurassic-to-early Cretaceous ages.Further western part,on the other hand,is represented by meta-sedimentary rocks with relatively sporadic granite complexes.To be noted,the western areas derived higher-degree and wider metamorphic zones,which is in contrast with the lowerdegree and narrower metamorphic belt developed in the eastern region.Drastic collisions between the Indochina Block and South China continent took place since at least late Triassic,resulting in large-scale suturing and deformation zones.At the westernmost part of the northern SCS,the intracontinental amalgamation with closure of the Meso-Tethys has caused fairly stronger and broader metamorphism.One metamorphic biotite granite is located on the suturing belt and yields a Precambrian U-Pb age.It likely represents the relict from the ancient Gondwana supercontinent or its fringes.Arc-continental collision between the Paleo-Pacific and the southeast China Block,on the other hand,results in a relatively narrow NE–SW trending metamorphic belt during the late Mesozoic.Within the overall geological setting,the Cenozoic SCS oceanic basin was subsequently generated from a series of rifting and faulting processes along the collisional-accretionary continental margin.

Tectonics of the offshore Red River Fault recorded in the junction of the Yinggehai and Qiongdongnan Basins

The Red River Fault,which originated from the southeastern margin of the Tibetan Plateau,has a great significance for obtaining a further understanding of the regional tectonics,topography and river catchment evolution,as well as the petroliferous sedimentary basin formation.The junction of the Yinggehai and Qiongdongnan Basins(YQB Junction)is the key to understanding when and how the strike-slip deformation on the South China Sea resulted from the collision between the Indian and Eurasian plates.In this study,we show regional seismic profiles,3D seismic and drilling core data to analyze the tectonostratigraphy in the YQB Junction,aiming to identify its tectonic framework and the associated faults system.A transitional domain from the strike-slip zone to the extensional deformation zone was mapped,which consisted of the No.1 Fault and the Zhongjian Uplift.The strike-slip faulting in the YQB Junction was active during the Oligocene-Early Miocene,with a period of strong faulting in the Early Oligocene.Integrated with the regional tectonic evolution,a coevolution model of strike-slip and extensional deformation in the YQB Junction and the adjacent area was built.In the Eocene,the YQB Junction was controlled by the NW-SE extension and formed a series of distributed rifts bounded by the NE-striking faults and filled up with proximal sediment.In the earliest Oligocene,a NW-trending strike-slip fault began to develop in the YQB Junction and crosscut the NEstriking normal faults.Since the occurrence of the strike-slip faults,the NE-striking faults,to the west of the No.1 Fault and the Zhongjian Uplift,failed to grow.However,to the east of the No.1 Fault and the Zhongjian Uplift,the faulting continued to develop until the latest Late Oligocene.The faulting of the NW-trending faults was observed to be active until the earliest Middle Miocene.Since then,with the exception of some diapiric structures and associated small-scale faulting in the Yinggehai Basin,we did not observe any basement-involved faulting.Our results will improve our understanding of the tectonics in the southeastern margin of the Tibetan Plateau and the South China Sea.