The Cenozoic Crustal Thinning And Development Of Hyper-extended Rift System In The Northern South China Sea

The South China Sea is the largest marginal sea in the region of Western Pacific.An extension in this region is believed to have started during the Latest Cretaceous-Earlest Eocene and appears to have exploited the location of a pre-existing continental arc along the south coast of China.After multiple episodes of rifting,the breakup of the continental lithosphere during the Early Oligocene(~33 Ma,IODP Expedition 349 drilling core U1435)contributed to the subsequent seafloor spreading of the South China Sea.Site U1148(ODP Leg 184)revealed a significant unconformity at 24 Ma,as evidenced by sharp changes in geochemical parameters,a 2.5–3 Ma hiatus that directly preceded 24Ma.This hiatus is consistent with the interval of a ridge jump to the south between magnetic anomalies 7 and 6b(26–24 Ma).The cessation of seafloor spreading in the South China Sea was synchronous all along the ridge,just after anomaly 5c,at~16 Ma.Classic tectonic models for the evolution of passive margins,involving the process and mechanism of extreme crustal thinning and final continental lithospheric breakup and the development of rift basins,have been developed in the archetypical Atlantic(the mid-Norway-Jan Mayen-East Greenland,the Iberia-Newfoundland,Angola-Brazil),the Alpine Tethys,the Red Sea-Gulf of Aden,the western and eastern India,the western Australia and the Gulf of Mexico rifted margins.There are two end-member extremes of passive rifted margins:(1)volcanic rifted margins and(2)magma-poor rifted margins.The South China Sea is generally regarded as a magma-poor rifted margin because of a lack of identification of Seaward Dipping Reflectors(SDRs),which shares a similar process of continental extension and breakup with the the Iberia-Newfoundland conjugate margins.The northern South China Sea margin is another example of the Iberia-type margin in which the hyper-extended continental crust is separated with oceanic crust by a broad zone of exhumed continental mantle(serpentinized peridotites).In 2016 and 2017,International Ocean Discovery Program(IODP)Expeditions 367and 368 drilled 7 cores in the distal continent-ocean transition(COT)of the mid-northern South China Sea in order to addressed the nature of the basement underlying the syn-rift and post-rift sediments.Howerver,the preliminary results of this discovery suggest that no cores have penetrated the serpentinized peridotites.Instead,the core U1504 penetrated the metamorphic rocks and U1501 penetrated the sediment rocks below the rift-onset unconformity(Tg).It is probably that great difference exists between the tectonic evolution of the South China Sea and the Iberia-Newfoundland conjugate margins.The lithospheric structure of the South China Sea,especially for the northern margin,has been widely invesgated by 3D shear wave velocity model,2D and 3D gravity anomaly inversion,OBH/ESP/OBS/OOS profiles and 2D longcable deep penetration seismic reflection profles,constructing the 3D lithospheric structure in a plate or basin sclae and2D lithospheric structure in an inner-basin scale.High-resolution of 3D lithospheric structure is still unresolved in the northern South China Sea.The distribution and geometry of the border faults in the hyperextended rift basins within the distal margin of the mid-northern South China Sea are still debated.The controls of these border faults on the extensional processes and the final rift structure require further investigation.The Baiyun Sag was interpreted as a graben basin that is controlled by two sets of deep-seated,high-angle normal faults with an opposite dip.One of the oceanward-dipping border faults(B1)has 3000 to 4000 m of vertical offset,while the others have only 390 m of vertical offset on average.Therefore,the oceanward-dipping border fault B1 in the northern slope was regarded as an important structure that controls the extensional process.This observation was so far regarded as indicative of the extensional style of the Baiyun Sag.Similarly,it is proposed that two groups of en echelon fault belts with opposite dips controlled the formation of an overall graben basin.However,other authors have interpreted the Baiyun Sag as a half-graben basin that is controlled by a series of continentward-dipping or oceanward-dipping normal faults.Furthermore,the Liwan Sag was interpreted to be controlled by three continentward-dipping or one oceanward-dipping high-angle normal fault.Our research forcuses on the Cenozoic crustal thinning and its spatial variation,the development of hyper-extended rift system and the mechanisms of extreme crustal thinning and final continental lithospheric breakup in the mid-northern South China Sea.We constructed the crustal structure of the northern South China Sea by 344 2D long-cable deep-penetration seismic reflection profiles(more than 20000 km)in which the Moho unconformity is characterized by a deep-seated,continuous and high-amplitude reflector at a depth beween 8.5 to 10.5 s(TWT).Grids of the depth to the rift-onset unconformity(Tg)and the Moho unconformity were used to draw contour maps.The thickness of continental crust could be calculated by subtracting the depth to the rift-onset unconformity(Tg)from the depth to the Moho unconformity.The results suggest that there is obvious spatial varation of the Cenozoic crustal thinning in the northern South China,which is characterized by the deferences in the width of slight/moderate/strong crustal thinning zone from west to east and the development of aborted hyper-extended rift systems(Xisha Trough,Baiyun Sag,Liwan Sag and northern depression of the Southwest Taiwan Basin)from the continent to the ocean.From 2007 to 2015,the CNOOC conducted several 3D seismic surveys in the Baiyun Sag.Now,the area of the 3D seismic data is approximately 21000 km~2.The records to 8 s or 9 s(TWTT)provide high-quality imaging of the Cenozoic sediments and normal faults,allowing industrial interpretation in a large 3D space.Moreover,the breakup unconformity was penetrated by 19 wells,providing a strong constraint for the seismic interface T70(~33 Ma)and the seismic interfaces above T70.The Wenchang Formation was only revealed by Well 2 which provide a strong constraint for the seismic interface T80.Ninety long-cable deep-penetration seismic reflection profiles was acquired in 2015 by the CNOOC and has a total length of~5300 km.The maximum record is 12 s(TWT).A large amount of 2D and 3D seismic reflection data were used to interprete the major border fault and key seismic interfaces in the Baiyun Sag,Liwan Sag and Changchang Sag in order to construct the extensional fault system and rift structure.The Baiyun Sag shows an asymmetric rift structure that is controlled by a continentward-dipping detachment system,which involves low-angle or even flat-lying detachment surfaces that are overlain by an array of continentward-dipping normal faults and oceanward-tilted fault blocks.Large vertical and horizontal offsets were observed along these normal faults rather than the previous suggestion of short faults with small offsets.The decollement level is between the brittle upper crust and ductile lower crust.The Liwan Sag shows an asymmetric rift structure that is controlled by a single large-scale oceanward-dipping low-angle or even flat-lying detachment fault with a horizontal disaplcement of~60-70 km.There are no splay extensional normal faults underlying the detachment surface which was greated deformed by magmatic activities at~33 Ma and24 Ma,corresponding to continental breakup and a ridge jump to the south,respectively.The decollement level is between the brittle upper crust and ductile lower crust.The Changchang Sag show an asymmetric rift structure that is controlled by a single continentward-dipping high-angle listric detachment fault which is still active at present.The decollement level is between the crust and mantle.The transition from pure shear to simple shear extensional deformation occoured at~33 Ma.We analysised the mechanism of extreme crustal thinning and final continental lithospheric breakup on the basis of the the crustal and rift structures of the hyper-extended rift systems and the drilling cores of the International Ocean Discovery Program Expeditions 349,367 and 368 in the distal margin of the mid-northern South China Sea.The tectonic characteristics of the distal margins between the mid-northern South China Sea and archetypal magma-poor and magma-rich rifted margins were comparative analysised.The results suggest that the mechanism of extreme crustal thinning involves the successive detachment faulting towards the continent(Baiyun Sag),the depth-dependent lithospheric stretching and upwelling of lithospheric mantle(Liwan Sag)and the brittle of the entire continental crust after strong extension.The final continental lithospheric breakup in the mid-northern South China Sea was contributed by a tectonic-magmatic event which occurred at~33 Ma and exploited a zone of hyper-thinned continental crust.Howerver,the final continental lithospheric breakup in archetypal magma-poor rifted margins was contributed by a tectonic-magmatic event which exploited a zone of exhumed serpentinized peridotites and in archetypal magma-rich rifted margins was contributed by a tectonic-magmatic event which exploited a zone of incruded or underplated thin crust.Compared with the distal margin of the archetypal magma-poor and magma-rich rifted margins,the mid-northern South China Sea shows great difference in detachment fault system(the array of detachment faults,dip angle,horizontal displacement and decollement level),differential crustal thinning in terms of time and space,vertical and lateral lithospheric structure,nature of the continent-ocean boundary,magmatic activity,mechanism of continental lithospheric breakup and topography of top-basement surface.The analysis of these differences provides a better understand of the tectonic evolution of the South China Sea,especially for the processes of continental stretching,extreme thinning and final breakup and the development of rift basins with differential extensional fault system rift structure and tectonic evolution.