| The continental lithosphere undergoes stretching,thinning,breakup,and subsequent oceanic accretion to form a passive continental margin.The Ocean Drilling Projects(ODP)and International Ocean Drilling Project(IODP)drill holes on the Newfoundland-Iberia conjugate margins and the Northeast Greenland-Norwegian conjugate margins reveal that there are two types of endmembers of structures in the continent-ocean transition zone(COT),which ultimately manifests as the difference in magma volume during the rifting phase.Accordingly,using the above-mentioned two conjugate margins in the North Atlantic Ocean as prototypes,global passive continental margins are divided into"magma-poor"and"magma-rich"models.In the"magma-poor"passive continental margin,the magmatism during the rifting period was very limited,but the lithospheric mantle in the COT was exposed for tens to hundreds of kilometers by crustal-scale faults.In the"magma-rich"passive continental margin,intense crustal extension is accompanied by massive magmatic activity,forming large-scale effusive basalts with seaward-dipping reflections(SDRs)in seismic reflection images and the high-velocity lower crust(HVLC)caused by magma underplating at the bottom of the crust.However,the two end-member models often switch between hundreds to thousands of kilometers along the margin strike,and there are correspondingly obvious lateral variations in the margin structure and the crustal deformation style.Although the"magma-poor"label should only be applied to continental margins where mantle exhumation occurs in the COT,it is a common phenomenon that as long as there is no large effusive basalt(SDRs),even without actual drilling and wide-angle velocity data confirming that there is serpentinized peridotite caused by mantle exhumation in COT,the passive continental margin is classified as"magma-poor"continental margin.As an example of the above trend,the northern margin of the South China Sea(SCS)has been defined as a"magma-poor"type for decades just because no SDRs were found in previous studies.However,the discovery of high-velocity lower crust and igneous syn-rift rocks challenged the label of"magma-poor"in the northern SCS.The term"Intermediate-type"was therefore proposed to distinguish the above two-terminal models.It has been confirmed by the latest IODP expeditions 367/368 because the drilling results denied the existence of serpentinization in the COT of the northern margin of the SCS.Instead,thick basalt has been discovered.However,no consensus has been reached on the margin structure of the northern SCS because the interpretations of the Liwan Sub-basin,the core region defining the northern SCS rifted margin structure,is still debated.In addition,although the"Intermediate-type"model can be applied to some other passive continental margins,e.g.,the Red Sea,the Gulf of California,and Gulf of Lions,etc.,however,this the lateral variation in the crustal structure and deformation patterns of this new type of rifted margins are still poorly understood.There is some limited information indicating the existence of tectonic segmentation of the SCS margin,e.g.,the uneven distribution of the HVLC.However,the specific variations in structural style,rift structures,and magmatism along the margin strike are not fully understood.One of the most important reasons is the limitation of the imaging depth and resolution in previous studies.In this study,a largely unpublished 2D seismic grid(>10 s TWT)along>450 km of the northern SCS imaging the full crustal structure,a large-scale high-resolution 3D seismic data(2.8×10~4 km~2,7.5-9.0 s TWT)covering almost the entire deep-water basins,and～20 wells are used to define the relationship between the basement thickness variations,tectonic style,the correspondence of reflectivity in the images to the seismic velocity structure,and syn-rift magmatism in the northern SCS,aiming to reveal the along-strike structural variations in the northern SCS rifted margin.Finally,a new framework for the classification of passive continental margins is proposed.This study shows that:(1)In terms of crustal structure,the continental crust in the northern SCS has been thinned to varying degrees during the Cenozoic rifting.The regions with a crustal thickness of more than 20 kilometers are mainly concentrated in the Zhu 1 Depression and its adjacent basement highs.The hype-thinned domain with a crustal thickness of<20 km is mainly concentrated in the Baiyun Sub-basin(～5.5 km),the Liwan Sub-basin(～8 km),the Changchang Sub-basin(～6 km)and the Heshan Sub-basin(～7.5 km).In addition,the width of the strong thinning zone varies greatly.The width of the hype-thinned domain in the eastern region represented by the Baiyun and Liwan Sub-basins reaches～250 km,while the width of hype-thinned region in the western region represented by the Changchang and Heshan Sub-basins is just under 100kilometers.In addition,the initial crustal thickness in the Baiyun and Liwan Sub-basins before the Cenozoic rifting phase is 10-25 km,while that in the Changchang and Heshan regions is about 30 km,indicating an apparent difference in the initial crustal structure.The distribution of HVLC in the northern SCS is quite different,mainly concentrated in its eastern region,but rarely distributed in the western region.In addition,there are obvious ductile shear reflections in the lower crust of the Baiyun and Liwan Sub-basins,while the reflections in the lower crust of the Changchang and Heshan sags are homogeneous.In terms of faulting patterns,the main faults in the Baiyun and Liwan Sub-basins are mostly low-angle detachment faults,which eventually sole out near the deep ductile layer of the crust.The main faults in the Changchang and Heshan Sub-basins,however,are steep normal faults,almost cutting through the entire crust.(2)In terms of magmatic activities,there are at least four episodes of syn-rift magmatism in the Baiyun Sub-basin,Liwan Sub-basin,and their surrounding areas,which resulted in the formation of basement uplifting and the corresponding sedimentary unconformity.In contrast,although the magmatic activities in the western region represented by the Changchang and Heshan Sub-basin also exist,almost most of them developed in the post-rift phase.(3)In terms of rift structure,the Changchang Sub-basin is an abandoned narrow rift controlled by continentward-dipping high-angle normal faults.The Heshan Sub-basin is a narrow half-graben controlled by oceanward-dipping high-angle normal faults.The Baiyun Sub-basin is controlled by continentward-dipping low-angle detachment faults.The Liwan Sub-basin is a pull-apart basin jointly controlled by oceanward-dipping low-angle detachment fault and strike-slip faults.The study finally reveals that there are significant structural variations along the strike of the northern SCS rifted margin.Besides,this along-strike variation is mainly manifested in two scales:on the first scale,the northern SCS rifted margin can be divided into two tectonic domains,that is,the Eastern Domain represented by the Baiyun and Liwan Sub-basins and the West Domain represented by the Changchang and Heshan Sub-basins.The Eastern Domain is like the"wide-rift"extension modes with relatively high geothermal gradient,significant syn-rift magmatism,ductile deformation in the lower crust,and widely-distributed HVLC.This Eastern Domain is therefore defined as“hot/ductile lower-crust”mode.Whereas the neighboring Western Domain,defined as a“cold/brittle whole-crust”mode,displays a narrow-rift system with steep crustal-scale faulting and strong rheology akin to magma-poor systems.The two tectonic domains are separated by a large strike-slip fault zone,namely the Yangjiang-Yitongdong Fault Zone.On the secondary scale,margin segmentations can be also observed within a single tectonic domain,which is manifested in the varied rift structures and the boundary faults between them.This research highlights the complex 3D rifting during the development of a passive continental margin.In addition,this study proposes a“two-parameter”classification framework to optimize the traditional“two-endmember”classification framework and the“two-axis”classification framework.Serpentinized peridotite and SDRs are used as a parameter respectively and then are arranged and combined to form four types of rifted margins:“Mantle exhumation”rifted margin with only serpentinized peridotite but no SDRs;“Magma overflowed”rifted margin with only SDRs but no serpentinized peridotite;“Endmember deletion”rifted margin without neither serpentinized peridotite nor SDRs;“Endmember juxtaposition”rifted margin with both serpentinized peridotite and SDRs. |
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