The Tectono-stratigraphic Framework And Geodynamics Of The Zengmu Basin,Southern South China Sea

Zengmu Basin is the largest Cenozoic sedimentary basin in the southern continental margin of the South China Sea.It is under the plate-convergent background and has experienced complex tectonic evolution and regional plate tectonic evolution.The high quality 2D seismic data,drill data,and palacontological data derived from the Zengmu Basin enable us to determine the tectono-stratigraphic framework of this basin.The evolution and the mechanism of formation of the Zengmu Basin can be extracted from the fault activity analysis,subsidence history stripping and numerical simulation.In addition,this hydrocarbon-bearing basin has the highest thermal gradient among the basins in the southwest of the South China Sea.Therefore,this research not only contributes to a comprehensive understanding of the geological evolution of the Zengmu basin and the South China Sea in larger picture,it also benefits industrial exploration for energy resources.The main results of this thesis are as follows:Generally,the Cenozoic strata in Zengmu Basin are divided into 8 cycles by the contact between the most transgressive and the most regressive sediments in offshore and onshore Sarawak.The cycles are determined by high-precision three-dimensional seismic data and are correlated with the global sea level change.The age of these boundaries are accurate and objective since they were ditermined by wells and paleontology beside the seismic data.In this study,the strata are divided on the basis of3D seismic profiles tied by a large nummber of wells in the Nankang Platform.Ten stratigraphic boundaries are determined by well-sesmic correlation and horizon tracking on high-quality seismic profiles which cover the whole basin.The Cenozoic strata divided by ten boundaries are classified into three 1^st order sequences and eight 2ⁿdd order sequences,and the post-Miocene strata are further divided into three 3^rd order sequences.In particular,we mapped a series of key sequence boundaries up from the base boundary of Middle Miocene?T3?in the Kangxi Depression,the deepest part of the Zengmu Basin.The T3 is an importance basin boundary that recorded the tectonic events of the Zengmu Basin.It reaches up to 7 s?TWT?in the Kangxi Depression,providing crucial constraint for the formation of the basin and clues for the degree to which the continental lithosphere was thinned.This research identified three vertically stacked prototype basins–the rift basin,peripheral foreland basin,and passive margin basin–which constitute the tectonic framework of the study area.The prototype basins are separated by the first order sequence boundaries.In the Zengmu Basin,we identified the base Cenozoic?Tg?,Middle Eocene?T8?,and Middle Miocene?T3?boundaries,through seismic interpretation and the analysis of regional tectonics.These unconformities define three episodes of tectonic event,including rifting?Tg?,Sarawak Orogeny?T8?,and Sabah Orogeny?T3?;as such,the rift basin is bounded by the Tg and T8,the peripheral foreland basin,T8 and T3,and the passive margin basin,T3 and seafloor.Regional studies have shown that the rift basin can be only observed locally in the north of the Zengmu Basin,and the peripheral foreland basin is at the present-day part of the Rajang and Crocker Groups,characterised by strong contractional deformation.Madon?2011?defined the location of foreland basin,which indicates that the current Zengmu Basin is situated at all to the forebulge.In the Early Middle Miocene,the forebulge was subjected to strong extension stress and developed the NNE-trending rift system,among which the Kangxi Depression is the largest in size.Within the rift system,the horst blocks controlled the carbonate coral reefs of the Nankang platform,and the grabens hosted the source rocks,thus the structural control on the petroleum system is apparent.Activities of thrust systems associated with Sabah Orogeny brought large amount of clastic sediments to the Zengmu Basin,which resulted in basinward migration of continental slope.The abovementioned prototype basins laid the foundation of structures of the Zengmu Basin.Our research found a typical thin-skin,gravity-driven system that spans a large area?2.56×10⁴ km²?at the continental edge of Kangxi Depression.Detailed structural interpretation of this basin system was undertaken based on the long-distance seismic profiles that cover the continental shelf and the slope.This gravity-driven tectonics can be divided into three structural domains:the extensional,the transitional,and the contractional domains.The gravity-driven structures initiated in the Early Pliocene?ca.4.3 Ma?and lasted to the Late Quaternary.The evolution of gravity-driven system and mud diapir activities were mainly controlled by the progradation of the West Luconia Delta,with little influences of the regional tectonic compression.The process of gravity collapse can be divided into five stages:inital collapse,sealing of overpressure,continued collapse,migration of the gravity-driven deformation,and cessation of the gravitational collapse.We show,through quantitative weighing of gravity spreading vs.gravity gliding,that the gravity-driven deformation in the Kangxi Depression was mainly driven by the gravity spreading.The sediments loading of the West Luconia Delta above the extensional domain are likely to be responsible for the gravitational collapse in the Kangxi Depression,and deltaic loading above the contractional domain is suggested to have led to the cessation of the gravitational collapse.The release of pressure in the detachment layer through the gas chimneys and mud diapir may also play an important role in the deactivation of the gravitational collapse.Integration of the regional tectonics,basin structure and stratigraphic characters,fault activities,subsidence history,and quantitative analysis of Kangxi Depression through stretching modelling has helped the understanding of the Zengmu Basin evolution.The roll-back of the subducted Paleo-Pacific plate contributed to the early stage of continental rifting in the Late Cretaceous and Early Cenozoic.The Luper Line and West Baram Line are the west and the east boundaries of the Zengmu Basin,between which the Sarawak Orogeny occurred in the Eocene.Therefore,these boundaries have important dynamic indication.The Sarawak Orogeny resulted from the subduction of the Paleo-Pacific plate and the ensuing collision of the Zengmu and the Borneo terrains.Accompanied with continental collision,the peripheral foreland basin formed in the subduction plate of the Zengmu Basin.After the Sarawak Orogeny,the subduction of Paleo-pacific plate migrated to the east of the West Baram Line,which led to the closure of Paleo-Pacific ocean and dextral movement of the West Baram Line.As a result of the SE oblique subduction,the partitioned stress generated WNW–ESE extension that developed a series of NNE-trending rift basins in the Zengmu Basin.The NNE-trending extensional basin stopped the activation,when Paleo-Pacific ocean finally closed and when the Sabah Orogeny started to develop.At this moment,the Zengmu Basin entered a stage of passive continental shelf basin.