Break-up Unconformities In The Surrounding Margins Of The South China Sea:Spatial-Temporal Distribution,Evolution Model And Formative Mechanism

The South China Sea experienced an entire process comprising continental margin rifting,breakup,seafloor spreading,and the final closure,which is one of the best natural laboratories to investigate the Breakup Unconformity of the margins surrounding South China Sea and their response to the seafloor spreading.In this paper,we use regional 2D seismic profiles on the northern,western,and southern continental margins of the South China Sea,combined with IODP drilling data,drag data,and gravitational and magnetic data from,to explore the properties of various tectonic domains of surrounding margins as well as the corresponding breakup unconformities.Firstly,major regional unconformities have been recognized along the margins surrounding South China Sea.The characteristics and ages of these unconformities were determined according to the results the IODP drillings,chronostratigraphic data and drag samples.Breakup unconformities were then identified regarding the relationship with the start of seafloor spreading.The results suggested that the Breakup Unconformity have obvious discrepancies between the Eastern and Southwestern Sub-basins,together with clear conjugation pattern between the northern and southern margins.In the Eastern Sub-basin,two episodes of unconformities formed at 32 Ma and 23 Ma,corresponding to surfaces T7 and T6,respectively.T7 is represented by a rifting-sag bounding surface to a rifting-rifted sag surface and then to a inter rifting boundary.In the meantime,for the T6,it is manifested as a surface from conformity in the sag to a surface between rifted sag and sag,and then to a surface between rift and sag.Two breakup unconformities occur in adjacent areas of the Eastern Sub-basin.In contrast,continental margins surrounding the Southwestern Sub-basin only develops one singe breakup unconformity,which mimicked the T6 on the margins around the East Sub-basin.Meanwhile,the proximal,necking,distal and outer domains and oceanic crusts are subdivided according to the definition and identification criteria,together with the structural pattern from the seismic sections,depth of Moho,seafloor topography and magnetic anomalies.Our results indicate that the tectonic domain is centered at the relic ridge and displayed an obvious asymmetric shape which is wider in the north and narrower to the south.The lateral variation of the nature of the breakup unconformity closely corresponds to the division of the tectonic domain.Obviously,the geodynamic background of different tectonic domains determines the lateral variation of the geological structure of the breakup unconformity to some extent.Then,we analyze the evolutionary pattern of the breakup unconformity around South China Sea and focuses on the problem of the era of breakup unconformity.The occurrence of two breakup unconformities strongly suggest that breakup unconformities on both flanks of the Eastern Sub-basin are not diachronous across the whole continental margin,whereas the rifting-sag surface are surely diachronous.Actually,the variation of the nature of breakup unconformity can easily cause the illusion of diachronous,and the two breakup unconformity in East Sub-basin add the complexity of it.Accordingly,the delay of sag stage in some areas that indicated by theappearance of carbonate platform don't represent the delay of breakup unconformity formation,since the breakup unconformity is an inner rifting surface when seafloor spreading starts.In contrast,the breakup unconformity that parallel to the continental margin is diachronous(i.e.,younger in northeast versus older in southwest),which are commensurate with the gradually opening of South China Sea.Finally,the comprehensive analysis reveals the internal relations between the evolutionary evolution of South China Sea and the corresponding features of breakup unconformity,and the similarities and differences between South China Sea and the Atlantic poor-magmatic continental margin.The results show that both of them have the similar tectonic style zoning,which is all influenced by the stress background in continental margin,such as stretching and thinning.In the period of seafloor spreading,breakup unconformity will vary their nature with the preexisting tectonic style in each region.In addition,the South China Sea and the Atlantic Ocean both open in a certain direction,that is northeast to southwest in South China Sea and south to north in Atlantic.All these observations systematically suggest that,although some similarities in tectonic subdivision between South China Sea and Atlantic,as influenced by regional location and tectonic scales,South China Sea is characterized by a more complex regional stress.The complex regional stress of South China Sea was also increased,due to the strike-slip system in the west and the collision-compression during a later period in south.That makes South China Sea more prone to the tectonic responses to passive continental margin.As compared with some localized response of continental margins at 32 Ma,the widespread occurrence of the breakup unconformity at 23 Ma in the South China Sea strongly suggests a prominent plate event beyond the scale of South China Sea's opening.