Temporal-spatial Basin Structure Differences And Their Controlling Factors Of The Cenozoic Basins Around The South China Sea

The South China Sea(SCS)lies at the intersection of the Pacific Plate,Indo-Australian Plate and Eurasian Plate,and experienced a complex tectonic evolution during Cenozoic.Many sedimentary basins developed around the SCS.The study of the temporal-spatial structure differences of these basins is significant to the hydrocarbon exploration in this region.Thus,four representative basins,with different locations and abundant data,are chosen to study the temporal-spatial basin structure differences of the Cenozoic basins around the SCS.The four basins are the Beibu Gulf Basin(BGB)in the northwestern SCS,the Pearl River Mouth Basin(PRMB)in the northeastern SCS,the Yinggehai Basin(YB)in the western SCS and the Reed Bank Basin(RBB)in the southern SCS.Based on the seismic,well-drilling and well-logging data,the geometric characteristics of faults and sedimentary strata are analyzed to distinguish the spatial structure differences of these basins.Then,the temporal structure differences of these basins are reconstructed by using fault active rates calculation and balanced cross-section restoration.Finally,the tectonic setting and analogue modelling are used to discuss the controlling factors of the temporal-spatial structure differences of these basins.Four groups of faults are recognized in these basins,including NE-,ENE-,EW-and NW-striking faults.The BGB and PRMB,in the northern SCS,are controlled by the conjugate fault system,including the NE-(ENE-and EW-)and NW-striking extensional or extensional strike-slip faults.The RBB in the southern SCS is controlled by the conjugate NNE-and NW-striking extensional faults.The YB in the western SCS is controlled by the NW-(SN-)striking faults.Four kinds of structural styles can be identified in these basins,including extensional,strike-slip,extensional strike-slip and inversion(extension first and then compression)structural styles,which indicates that various stresses coexist in space and superimpose in time.In the BGB,the extensional structure is the primary deformation and the extensional strike-slip structure is the secondary deformation.In addition,the inversion structure can be recognized in the local area of the BGB.In the PRMB,the extensional structure is the primary deformation and the extensional strike-slip structure is the secondary deformation.In the RBB,the extensional and inversion structures are the primary deformations and the strike-slip structure is the secondary deformation.In the YB,the strike-slip structure is the primary deformation and the extensional structure is the secondary deformation.The Cenozoic basin structure of these basins is composed of lower syn-rifting structure and upper post-rifting structure.The syn-rifting basin structures are different due to the basin-controlling fault property,arrangement style and formation distribution,and can be divided into three types,including continuous extensional type,transformational extensional type and strike-slip pull-apart type.The continuous extensional type and transformational extensional type are recognized in the northern and southern SCS,and the strike-slip pull-apart type is recognized in the western SCS.These basins experienced the multiple syn-rifting stage and post-rifting stage.The multiple syn-riftings of these basins were jointly controlled by the extensional and strike-slip stresses.Three stages of rifting happened in these basins,whereas,the rifting periods are different.The direction of extension stress of the PRMB,BGB and RBB was clockwise rotated during syn-rifting stage,and the strike-slip stress of BGB and YB became stronger.The transition age from syn-rifting to post-rifting was various.This transition occurred at32 Ma in the PRMB,23 Ma in the BGB and YB,and 16 Ma in the RBB.During the post-rifting stage,different tectonic activities happened in these basins during the thermal subsidence.The Late Miocene strike-slip extensional activity took place in the PRMB,which resulted in the formation of EW-and NW-striking strike-slip echelon faults.The Late Miocene compressional activity happened in the BGB,which led to the formation of NE-to EW-striking folds.The Middle Miocene strike-slip compressional activity occurred in the YB,which caused the inversion of NW-to SN-striking faults from extension to compression.The Middle Miocene compressional activity occurred in the RBB,which was characterized by the flexural deflection and denudation of strata.The temporal-spatial structural differences of these basins are controlled by the movement of the oceanic and continental plates,the associated stress of strike-slip faults activity and the reactivation of pre-existing faults.The transition and superimposition process of these basins are caused by the stress changes derived from the movement of the oceanic and continental plates.For example,the clockwise rotation of extensional stress resulted in the formation of the extensional brush structure in the BGB.And the rotation of this stress is caused by the subduction of the Pacific Plate,the India-Asia collision and the subduction of proto-SCS.Besides,the transition age of the RBB,from syn-rifting to post-rifting,delayed to the end of Early Miocene because of the drag force of the proto-SCS subduction,thus,this basin was the latest one entering into post-rifting.The strike-slip faults result in the different associated structure in the adjacent basins.Different types of local stress are derived from the bend,stepover,and discontinuous parts of the strike-slip faults.For example,large extensional strike-slip duplex formed in the releasing bend of the Red River Fault,controlling the rift structure of the YB.The reactivation of pre-existing faults leads to the structure differences among and inside basins.For example,the structural difference between east and west parts of the PRMB was caused by the reactivation of NW-trending pre-existing faults.