The Large-Scale Detachment Fault System And Its Control On The Development Of The Deepwater Basins In The Northern South China Sea

The study of high quality 2D and 3D seismic data and dill data derived from the Northern South China Sea margins enable us to investigate crustal architecture,strcutcutre style,tectonic evolution of deepwater basins and their relationships to lithospheric breakup.In this study,we divide the margin into four domains and build the tectono-sedimentary framework.The study of formation mechanics of deepwater basins is very important because these large sedimentary basins are the most potential and front expolartion areas in the South China Sea.The main results are as follows:The definition of distinct domains represents the strcutral fundaments of the rifted margin.Recently acquired seismic data allow us to further show relationships between the intra-crustal detachment faults,crustal thinning,and development of the distal basins.In detail,basin geometries,sedimentary records,abrupt crystal thinning,the crustal thickness,the border boundary fault and gravity and magnetic anomalies are taken into account,dividing the whole margin into proximal domain,necking zone,distal domain and outer high.The proximal domain is characterized by weak or moderate,rift-related lithosphere thinning.The second domain corresponds to the crustal taper,which is wedgeshaped and crustally defined by the seismic Moho and Top basement.In this paper,we prefer to define distal domain and outer high as the transition from the thinned continental crust to the first oceanic crust.Magmatic intrusion and underplating gabbro emplaced during the syn-rifiting or post-rifting phases make it more difficult to identify the structure and nature of basement with drill hole information.In the necking zone and distal domain,the underlying stretched crust is always accompanied by obvious fault reflections with shallow dips and large-scaled sedimentary basins.One major achievement of our study is the relationships between these detachment faults and rifted basins.The detachment fault systems are interpreted to be the main structural feature of deepwater basin.By integrating well data and published bathymetry data,we built the tectono-stratigraphy framework and mapped the distribution of these detachment faults.Three different tectono-sedimentary units are interpreted to display the different rifting stages of these basins from the onset of high-angle faulting to the onset of the detachment faulting and then to the migration of deformation oceanward.The development of the detachment has been bounded by newly interpreted SD and PD horizons.Thus,the T70(~32 Ma)breakup unconformity is referred to as the first sedimentary deposits overlying the first oceanic crust.In classical models,this horizon is used to separate the syn-rift from the post-rift at the time of the lithosphere breakup and new ocean crust accretion.Obviously,this method is outdated and cannot be used to explain the extensional structure at the scale of the whole rifted margin.The sag sequence passively fills the remaining space in the detachment basin,which could also correspond to the migration of deformation at the margin scale.The zonation of Northern South China Sea and styles of rifting described in the Northern South China Sea are similar to other,well-studied rifted margins.The different phases of extension are identified on seismic data and separated by the important unconformities(Tg,SD,PD,T70).This cycle also reveals the thinning process of the continental crust,leading to the final breakup of lithosphere.During the stretching stage,the associated geometries of faults and basins are interpreted to fit a classic pure shear deformation environment.The border faults are usually high-angle fault with typical wedge-shape syn-teconic sedimentary record.The thinning phase marks the migration and localization of deformation that followed the formation of proximal domain.The image of reflective crustal levels in the middle or lower crust is associated with large detachment faults that border the necking zone.Thus,extensional deformation is transferred to successively lower levels in the crust.Drastic crustal thinning at the level of the evolving necking zone is linked with raised of the underlying mantle.The sag sequence identified in the seismic data also suggests the cessation of detachment faulting in the necking zone locally and successive deformation(detachment faulting)in the distal margin and outer high.The third extensional deformation is exhibited as weak extensional event in the proximal domain.Furthermore,in the distal part of Northern South China Sea,the third extensional deformation could be responsible for the serpentinization of upper mantle due to extreme crustal thinning at which the whole crust becomes brittlement.Outer high is the structure corresponding to the pervasive intrusion and underplating of a wide area of previously hyperextended crustal or/and mantle material.