The Cenozoic Structural Characteristics And Tectonic Evolution Of The Pearl River Mouth Basin,Northern South China Sea

The Pearl River Mouth Basin(PRMB)is a Cenozoic basin which is located in the northern margin of the South China Sea(SCS).Studying the structural characteristics of the PRMB not only helps to understand the formation and tectonic evolution of the northern margin of the SCS,but also helps to analyze the basic geological feartures and th law of hydrocarbon accumulation.This thesis followed the principle of structural analysis,used the geological-geophysical data to analyze the geometry,kinematics and dynamics of the Cenozoic PRMB.Based on the analysis of representative seismic profiles and structural maps of different structural units in the basin,this thesis analyzes the basin geometry and kinematics(e.g.,fault activity,sedimentary(displaced)area of rifts,depth-to-detachment,extension amout)to reveal the tectonic evolution and temporal and spatial variablitity of different sections of the basin,establish the crustal extension models of different sections,analyze the dynamic mechanism of the crustal stretching and thinning process,and discuss the influence of structural deformation on hydrocarbon accumulation.Five conclusions are as follows.(1)The Cenozoic PRMB displays the three-dimensional structural differences that it can be obviously subdivided both in the east-west and north-south directions,and can also be layered vertically.From the bottom to the top,the basin can be divided into four structural layers: the basement,faulted(including the Wenchang and Enping Formations),faulted-sag transition and continental-slope structural layers.The structural layers are different in the NE-SW direction and asymmetric in the N-S direction.The PRMB is divided into three regions with different structures from west to east along the axial direction.The western part of the PRMB is generally characterized by a small-medium faulted depression controlled by the NE-striking fault developed on the Paleozoic metamorphic rock basement.The middle part is generally a medium-large near-symmetric faulted depression controlled by the NE-and near E-W-striking faults developed on the magmatic rocks and sedimentary rocks formed in the Yanshan period.The eastern part is an asymmetrical medium-sized faulted depression controlled by the NE-,E-W-or NWW-striking curved boundary faults above the Mesozoic sedimentary layers.Different parts are separated by the NNW-trending structure,e.g.,the deep faults,lower uplift or uplift.In the N-S direction,the basin has the structure of three uplifts bounding two depressions.The northern depression zone has the faulted structure controlled by the boundary faults.The boundary faults in the southern depression zone are mostly faulted zones,controlling the sag structure.In the vertical direction,there are significant differences in the basement and faulted structural layer among different structural units.The faulted-sag transition structural layer includes the Zhuhai Formation in the west,the Zhuhai and Zhujiang Formations in the middle part,but does not developes in the east.In the northern depression zone,this structural layer is close to the sag structure,while the southern depression zone displays the faulted-sag or sag-faulted structure.The continental-slope structural layer overtakes the northern uplift zone,while the southern uplift zone shows a slope-like structure.(2)Faults of the PRMB were strongly active in the periods of the Wenchang,Enping and Zhuhai Formations.The fault system has the following characteristics: 1)the dominant fault strikes clockwise rotate from NE to near E-W or NWW;2)the NE-striking faults are strongly active in the period of the Wenchang Formation and 3)the E-W-or NWW-striking faults are strongly active in the period of the Enping Formation.The extension characteristics have the characteristics of temporal and spatial variability.Specifically,the extension rate is strong in the early period but weak in the late period,and strong in the north but weak in the south.The depth-to-detachment controlled by the extension fault system shows the temporal and spatial variability.Specifically,the depth-to-detachment is shallow in the north but deep in the south,and shallow in the early period but deep in the late period.The average extension rate of the basin caused by faults during the Cenozoic is about 12%?23%.During the period of the Wenchang Formation,the average extension rates are about 8%?15% and 4%?5% in the north and south respectively.The average values of depth-to-detachment are about27 km in the Baiyun Sag and about 2?8km outside the Baiyun sag.During the period of the Enping Formation,the average extension rates are about 5% and 3%?4% in the north and south respectively,and the average values of depth-to-detachment are about8?20km and 23?47km in the north and south respectively.(3)The basin experiences three stages of tectonic evolution,including the faulted stage(including the first and second sub-phases of faulted phase),the faulted-sag transition stage and the sag stage with time.The time limit of tectonic evolution stage,tectonic activity characteristics and intensity show spatial difference.The intensity of tectonic activity is strong in the early stage but weak in the late stage,strong in the north but weak in the south.The intensity migrates from northeast to southwest with time.The PRMB experienced the first faulted stage in the period of the Wenchang Formation controlled by the NW-SE direction extension,and second faulted stage in the period of the Enping Formation controlled by the N-S direction extension.The fault activity and extension intensity are strong in the early stage but weak in the late stage.The time limits of different regions entering the faulted-sag transition stage are different.The intensity of the tectonic activity of the basin is strong in the west and south,but weak in the east and north.The western and middle parts experienced the periods of the Zhuhai Formation and the Zhuhai-Zhujiang Formations to enter the faulted-sag transition stage respectively.The eastern part entered the sag stage directly after the second faulted stage.(4)Based on the analysis of basin structure,crust structure and lithosphere structure,the crust extension structure interpretation models of the three parts of the PRMB are established,which are different among different layers,regions and periods.The extensional strength of the basin is stronger in the north that in the south,while the extensional strength of the crust is contrary.The eastern,middle and western parts of the PRMB are controlled by different styles of detachment fault system,and there are spatial differences in the extensional structure deformation in the hanging walls of the detachment faults.The extent of the crustal stretching is strong in the middle(crustal stretching factor is 2.15),second in the east(1.38),and weakest in the west(1.25).The crustal stretching factors of each sags around the Baiyun Sag are distributed in the range of 1.22?1.38.The structural deformation is characterized by transition from brittle deformation to ductile deformation and migration from the upper crust to lower crust or upper mantle.During the period of the Wenchang Formation,the detachment fault systems in the east and west parts were mainly developed in the upper crust,and the Baiyun Sag may be involved in the lower crust.During the period of the Enping Formation,the detachment fault systems were affected by the ductile lower crust and even the upper mantle.The flow of hot materials of mantle from northwest to southeast may be the active force controlling the formation of the PRMB,while the subduction of the Pacific Plate,hard collision between the India Plate and Eurasian Palte and the scissor-like spreading of the SCS may be the passive forces.(5)The tectonic evolution and fault evolution of different parts of the PRMB during different periods affect the evolution of sedimentary facies,and then affect the hydrocarbon accumulation.Generally speaking,the basin experienced the faulted,faulted-sag transition and sag stages,controlling the transformation of sedimentary facies from continental facies to marine facies,from narrow deep lake sedimentary system to broad shallow lake sedimentary system.In the two stages of rifting basins,early faults control the basin structure and development of source rocks,while the second-stage faults connect the deep source rocks and shallow reservoirs.