Cenozoic Fault System Of Zhu ? Depression And Its Control On The Basin Strcuture

Zhu ? Depression is a Cenozoic extension basin and located in the north of South China Sea along the passive continental margin.The basin underwent three evolution stages: rifting stage,depression stage and tectonic reactive stage,consequently,the related sedimentary pattern were formed,including the the faulted continental sedimentary face in the lower sequence,the depression marine sedimentary face in the upper sequence.The space-time different of the basin have featured different characteristics,which resulted from the combination effect of multi-direction,multi-stage and multi-type basin structure.And the development and evolution characteristics of fault system and basin structure are important to the oil and gas exploration in this area.Therefore,this paper mainly focused on of space-time difference of fault system and basin structure by using 3-D seismic surveys such as Enping sag,Panyu 4 sub-sag of Xijiang sag and western Huizhou suverys.Combined analysis of 3-D seismic data and logging data,we depicted the characterization of fault system,divided the types of fault system,compared the characteristics of fault system in different layer and built the structure mode of basin.The tectonic evolution was reconstructed by using the fault activity rate,equilibrium recovery and extension rate,which describe the basin structure and its vertical evolution and superimposition process.The geo-dynamics of the basin was discussed by combined analysis of the regional geological background,the genetic mechanism of fault development and basin evolution.Concrete results embody the following aspects.Four groups of faults were distinguished in this basin according to their strike,including NE-,ENE-,WNW-and EW-striking which could be decided into three types,pull-apart,strike-slip and strike-slip extension.The present fault system featured different characteristic in different layers.In deep layer,the faults mainly strike NE-NEE,nearly EW and NW,all of them are extension faults.In the middle layer,NE-NEE faults gradually stop active,and the NWW-,nearly EW-striking faults become active and control the evolution of the basin.In the shallow layer,the NWW-and EW-striking faults active intensively,and the NE-striking gradually stop active,all of them behave as strike-slip faults.All in all,the Zhu ? depression was a fault-depression basin.And this basin featured different characteristics from west to east,including the Enping sag is a single side detachment half graben,the Xijiang sag is a both sides rollover half graben and the Huizhou sag is a double fault with half graben and rap sag.Meso-Cenozoic evolution of Zhu ? depression mainly divided into three stages,the rifting stage,the depression stage,and the tectonic reactive stage.During rifting stage,the basin controlling fault changed its strike from NE-NEE to NWW-nearly EW.During depression stage,the fault activity became weak,the basin was in an inactive tectonic stage overall.During tectonic reactive stage,pre-existing NWW and nearly EW-striking faults reactivate and have strike-slip characteristics.According to fault development characteristics,faults can be divided into terminal activity fault,newly generated fault,successional-reconstructive fault and successional fault.In different sag of various different characteristics during each evolution stage,the diversity of fracture system development due to the complexity of basin superposition relationship.Enping sag developed under the superposition of strike which changed from NE to nearly EW to NWW,Xijiang sag developed under the superposition of strike which changed from NE(NEE)to nearly NWW,Huizhou sag presents nearly EW sustainable development fault of successional fault of superposition.Differential evolution and superimposition in different dynamic background of Cenozoic is the basic reason that result in the spatio-temporal differences of basin structure.The study area was in NW-SE-trending extensional environment.The decrease of subduction rate and the withdrawing of the Pacific Plate result in the chasmic stage of Wenchang with NE-trending or NEE-trending basin controlling faults.Controlled by the rotating extrusion of Indochina block and subduction of proto-south China Sea,the regional stress field changed to NS-trending tensional stress and it resulted in chasmic stage of Enping with EW-trending of NWW-trending basin controlling faults.Due to the extensional stress centre of lithosphere migrated to extended center of the South China Sea,the clock of northern margin of South China Sea including Pearl River Mouth basin was in the stage of post-rifting subsidence.The double side dislocation between Indian plate and Philippine plate lead to the formation of dextral shear stress field in neotectonic stage(after Hanjiang stage).