Transtensional Structures And Rapid Subsidence Episode In Huizhou Sag,Pearl River Mouth Basin,Evidences From 2D Seismic Data

The sediment trapped in the continental margin of the South China Sea preserved important information about the Tibetan uplift,East Asian monsoon and Southeast Asian tectonics.The Huizhou sag is one of the important petroleum provinces in the Pearl River Mouth Basin(PRMB)located in the northern continental margin of South China Sea.The basin has several major hydrocarbon accumulations that include petroleum fields such as HZ25-7,HZ21-1,LH11-1 and LF13-2.Based on the structural and sequence interpretation of 2-D resolution seismic data covering the Huizhou sag,this study tried to constrain the basin structure style and build a better tectonic-stratigraphic framework for the basin.Therefore,this work described the geometric features,styles of the faults and quantitatively analyzed their activities.It‘s also proposed a new model for the basin formation mechanism.Integrated with previous researches and the dynamic information in the adjacent areas,this work tries to discuss the transtensional faults evolution and their controlling for the basin development.Lastly,the basin subsidence was investigated quantitatively using the backstripping method to make a better understanding of its origin.The main results are as follows:1.Determing the structure of the Huizhou sag and characterizing the geometry style of their main faults.Based on the structural analysis in the Huizhou sag,the faults characteristics was imaged on the 2-D seismic profiles.The basin presents an asymmetric structural pattern with the depocenters adjacent to the boundary fault,being parallel to the strike of boundary fault.The boundary faults display en echelon arrangement in plane view,which was caused by oblique extension in map view.The faults show a long term activity and stopped their activity ca.5.5 Ma(T30),some boundary faults kept active after 5.5 Ma.2.Reconstructing the evolution of the fault systems in the Huizhou sag and exploring its geodynamical setting.Firstly,from the Eocene to the early Oligocene,mainly NNE-NE and NW trending faults and NE striking graben and half-graben formed in the basin and the set of fault system controlled the deposition in the basin.The fault growth rates are more than 120 m/Ma.Then from the late Oligocene to the early Miocene,the structural activities were relatively small,the fault activity and the fault growth rate decreased sharply(the maximal rate is about 20 m/Ma).Finally,in the late stage from Late Miocene to the present,the structure movement was re-activated,and some faults are also reactivated.The maximum fault growth rate is more than 40 m/Ma.The observed eposided faulting activity in the Huizhou sag is strongly relationship with the regional tectonics.During Paleocene-Middle Eocene,the Pacific and the Indian plates moved NNW and N respectively.The Australia and India began to become a single plate and move together towards the north.During this framework of the SE and E Asia,subduction of the Proto-South China Sea oceanic crust toward the Borneo block start at the period of the Paleocene-Middle Eocene and terminated in the Early Miocene.Therefore,the rollback of the subduction of the Pacific plate and slap pull of the subduction of the Proto-South China Sea.During sea floor spreading of the South China Sea,the basins on the continental margin entered the period of the thermal cooling of the lithosphere.The faulting in the basin was not observed on our data.In addition,the number of the faults in the basin during this stage is much fewer than before,and their activity decreased sharply.From 10.5 Ma,the faults restarted to act again.At that time,to the East of our study area,the Philippine Sea and Pacific plates subducted to the Eurasian plate in the NWW direction and in the south direction,respectively.In addition,the Indian-Australian plate continued its subduction towards the north.This event caused a regional tectonic event in the northeastern South China Sea and affected wide areas compression from east to west progressively,which triggered the neo-tectonics in the northeastern region of the study area.This event is the major tectonical setting for the faulting since Middle-Late Miocene.3.Discerning the anomaly subsidence in the post-rifted stage and discussing its deep earth geodynamics.By detail calculating the subsidence history and stratigraphic development history of the basin,the evolution of the Huizhou sag can be divided into the rifting period(49-32 Ma)with an average of 110 m/Ma for total subsidence and 58 m/Ma for tectonic subsidence and the depression period(32-15.5 Ma)with an average of 37 m/Ma for total subsidence and 19 m/Ma for tectonic subsidence.Since 15.5 Ma after the depression satge,the average subsidence rates are generally >50 m/Ma and 140 m/Ma for tectonic and total subsidence,respectively,which is rapid subsidence than before and is different from the prediction described after the McKenzie model.According to McKenzie‘s model,the subsidence of the post rift basin is governed mainly by the thermal re-equilibration of the lithosphere-asthenosphere system,and its magnitude is related to the rift-induced extension rate(?),which should be an exponential decay function of the rift-induced thermal anomaly.The Huizhou sag is generally in an extensional state during the Cenozoic.It is noteworhty that we calculated the stretching factors for the upper crust and the whole crust in the Huizhou sag.The results show the values for the upper crust and the whole crust in the Huizhou sag are 1.10–1.13 and 1.08–1.31,respectively,which indicated faulting in the Huizhou sag is relatively weak and very small degree of thinning occurred in the crust.This thesis suggests that the anomaly subsidence in the basin is not a simple thermal cooling for the lithosphere described traditionally.We attributed that this anomaly subsidence was controlled by the complex subduction of the plates‘ convergence and subduction surrounded the basin,e.g.the collision between the Nansha and Boneo blocks,the cession of the oceanic spreading of the South China Sea at 16 Ma and the collision between the northern Palawan Island and Luzon arcs.