Basin Structure And Its Formation Mechanism In Deep Water Areas Of Qiongdongnan Basin, South China Sea

Recently,more and more worldwide oil and gas reserves are successfully discovered in deepwater areas.Besides focus areas of deepwater hydrocarbon exploration of Gulf of US Mexico, Gulf of Compus in Brazil and West Africa et.al,increasing large oil discovery are appearing in Russia,Norway and Southeast Asia.The geological conditions of deepwater areas at the margin of Northern South China Sea(NSCS) are similar to deepwater areas across world where several good hydrocarbon fileds have been discovered,so it has huge exploration potential.Qiongdongnan Basin(QDNB) is a large rift basin lies to west part of NSCS.Because of few seismic profiles and no exploration wells in deepwater area in QDNB,we know a little about the basin and there is no good news about the oil and gas discovery for a long time.Luckily,a lot of new and good quality seismic and other valuable testing data can be used.And we can comprehensively study the basin structure,evolutional process,formation mechanism and its petroleum significance.This work was supported by a grant from the Major State Basic Research Development Program of China(973 Program),Research Fund for the Doctoral Program of Higher Education of China and a grant from CNOOC.Under the guidance of basin dynamic principle and based on available high-resolution seismic and drilling data,marine magnetic anomaly data and a recently published high-resolution marine gravity data,we will study deepwater area in QDNB in details. The filling sequence during Cenozoic will be studied in the aspects of tectonics,depositional filling,heat,deep background,and as well quantitative basin dynamics modeling will be tried in our work.After that,filling sequence and sequence stratigraphic architecture styles during Cenozoic will be established;Episodes and scales subsidence in study area will be redefined;Also, Integrated of forward models and reversed models,we will discuss the process of lithosphere deformation,especially deformation process in contient-ocean transition(COT).And stress field characteristics of different stages will be discussed;Subsequently profound analysis focused on the relationship between evolution of NSCS and Cenozoic reorganization of plate tectonics;And then typical dynamical evolutional and genesis mechanical model NSCS is to establish through this study. (1)The fundamental tectonic framework in QDNB is determined in my thesis.We describle and analysis the tectonic styles in QDNB.Then the basin can be dividede into three first order tectonic units:northern uplift zone,central depression zone and south uplift zone. Each unit has its unique characteristics.There are several small half-grabens in the northern uplift zone.In the cental depression zone the grabens are very large.And southern uplift zone has half-graben and graben.As the area of central depression zone is very large,it can be further divided into three second order tectonic units:northern depression belt,central uplift belt and south depression belt.Using the data of seismic,geophysical and strata thickness,a very important NW-trending horst belt is distinguished in my thesis.According to that belt,the basin can be divided into western extension zone and eastern extension zone.Conclusively,the structure of QDNB is very complex and a variety of sag structure styles are distinguished in QDNB(2)We systematically analyse the structure styles of main sags and fault zones in deepwarter area.The transfer zones in QDNB are detailed studied and we divided it into three kinds.On the basis of that,the model of relationship between transfer zone and sand distribution are established.Through our study and analysis,three kinds of sag are recongnised,they are half-graben, graben and complex graben.And every kind can be divided into several styles.For example, half-graben can be divided into straight type,listric type,steeping terrace,gentle terrace,terrace zone;As well,granben can be divided into simple graben,bilateral basement fault-controlling sag,bilateral fault-controlling sag;and complex graben can be divided into rotational block and graben-half graben.Meanwhile,on the basis of studying the faults and fault zones,we subdivided the transfer zones in QDNB into three types:convergent,divergent and synthetic. Alternatively,we established the model of transfer zone influencing the distribution of sand.(3)We divided the No.2 fault into three segments and discuss the relationship between segment and geometry of sags.We use two kind of quantitative method(fault activity rate,displace-distance method) to study the activity of No.2 fault,The fault clearly can be divided into three segments:western, central and eastern segment.The former two is terrace type,and the later is right step oblique fault zone.Also,the activity of No.2 fault is mainly activity before 23Ma(seismic reflection is T60),and the whole fault is shown segment movement.The zone between each two segments is an important transfer zone for sand distribution.(4)Using the method of backstripping method,we systematically calculate the subsidence rate in QDNB.During snyrift period,the subsidence rate in the main sags is larger than that of uplift;And postrift period can be divided into two parts.Because the rate from 5.5Ma is much larger than that before 5.5Ma,some areas the value can reach 1000m/Ma.It can be defined as accelerating subsidence period.During post-rift period,the type of shelf-slop is not the same.In the western part of the basin,the fault stop moving during post-rift period and the shelf-slop is shown lateral accuration;and in the eastern part of basin,the minor fault occurred in some times and the subsidence is to some extent controlled by faulting.Above all,the subsidence rate in western parts of basin is faster than that of eastern,especially accelerating period.(5)The migration of subsidence center and pattern change of uplift-depression are detailed analysis in my thesis,we divided the tectonic evolutions stage.Using the method of migration of subsidence centers,subsidence analysis,ancient topography and balance sections,and also considering the important tectonic events,we divided the evolution of basin into three parts: snyrift,post-rift and accelerating subsidence period.The snyrift can be divided into three periods (T70 and T80 is their interface).In the first period of snyrift,the sag is NE-trending and the scale is very small.At in the second and third period of snyrift,the subsidence center of basin is mainly in central depression and the direction is NE—WE—NWW trending.And they are superimposed.And during post-rift period,the center of subsidence is migrated from NE-SW trending.(6)We systematically analyse dynamical background of basin development.On the basis of discussing basin structural geometry and kinematics,we analyse dynamical background of basin development and compare it with the circum regional tectonic event.After that,several key factor controlling basin evolution are get by us:indiochina block rotating,South China Sea spreading,middle-down ductile flow.(7)We have systematically concluded the geophysics and geology characters about continent-ocean transfer zone(COT) across the world,and it is first time to describe the COT characteristics of NSCS.Firstly we systematically conclude the academic fruit about COT in the world,we discuss the COT characteristics in NSCS in the aspect of geology,geophysics and marine topography. And we define the COT in NSCS as the place of slop.Because in the place of Slop in NSCS, mantle accelerating uplift,gravity gradient change fast and the value is increasing from big negative to positive.And also we conclude the mechanism drawn by other scientist,and believe QDNB is the best lab studying basin analyzing mechanism in COT(8)Using basin modeling,It is the first time to find depth-dependent strenching in deepwater area of QDNB.Based on theory of lithosphere stretching flexural cantilever model,we calculated and analyzed lithosphere stretching process in QDNB.Integrated of results of forward models and reversed models of upper crust,whole crust and whole lithosphere,the ratio of upper crust is small(β=1.1～1.4);As well,the stretching value of whole crust is changed much more than that of upper crust,theβprofile shows that from shelf to deepwater basin the valve ofβchange from 3.6 to 1.5;also,the big change occur in the valve ofβin lithosphere.The amount from shelf to deepwater changed from 3 to 4.2.Integrated with characteristics of lithosphere structure and rheology in NSCS,we believe that the deepwater area experience the process of depth-dependent strenching.(9)We proposed the evolutional model of depth-dependent strenching in deepwarter in NSCS,and distinguish development models of Qiongdongnan Basin type and Pearl Mouth River Bain type in COT.Through detailed studying plate movement of SCS and circum plate activity, and also considering the development of deep dynamics in study area,the development of basins in NSCS can be divided into three stages:introplate rifting,post-rifting subsidence and accelerating subsidence stage.In introplate rifting stage,NE or NEE-trending rifted basins occur because the crust in NSCS experienced strenching and thinning.This process is due to pro-NSC subduct toward south area.And the subsidence in this stage likes Mckenzie model.Then the spreeding jumped from northwestern sub-ocean basin to southwestern sub-ocean basin.And the very important break unconformity boundary is formed.From then on the fault stop moving or fewer minor fault are formed.The basin mainly experience subsidence causing by lithosphere cooling.5.5Ma,because of stretching and thinning of down crust,large scale of accelerating subsidence occurred in basin.In addition,according the location of basin lies to ridge,we are first time to define development models of Qiongdongnan Basin type and Pearl Mouth River Bain type in COT.The main innovations of this thesis may be as follows:①the geology and geophysics characteristics of basins in COT are studied specifically and systemically.And we detailed study the geology,geophysics and topography data of NSCS, we determined the lithosphere attribute of NSCS.Then we conclude that deepwater area of QDNB is in COT.Hence,the lithosphere attribute of QDNB is determined and we can do some basin model on the evolution of QDNB.②Under the guidance of basin dynamic principle and based on recent available high-resolution seismic and drilling data,marine magnetic anomaly data and a recently published high-resolution marine gravity data,integrated with skills of subsidence center migration, subsidence history characteristics,ancient topography and balance sections,we analyse the basin structure in detail.The fundamental basin structure and tectonic styles are described and analysis respectively.A new classification system for tectonic styles is proposed in our paper.We also study the main sags and fault belts in QDNB,and discuss the relationship between transfer zones and distribution of sand.④It is first time to discovery lithosphere depth-dependent strenching in deepwater area in QDNB,and we believe it may be the reason for accelerating subsidence.Integrated with analysis of structure geology and kinematics,and tectonic events around our study area,we discuss main sedimentary basin forming dynamics.Then we distinguish two types of basin in COT in NSCS, "Qiongdongnan Basin" type and "Pearl Mouth River Basin" type basin in COT.And we proposed a new model of lithosphere depth-dependent strenching for forming process for NSCS.