| Tectonic evolution of South China sea has always been the important problem of academicresearch. The closed transformation of Proto-South China Sea is one of the key period, and theonshore geological records which can reflect the transformation left in the South ChinaContinental Margin. In this paper, the characteristics and tectonic evolution processes of thethree continental margin basins of the northern South China Sea, Ledong, Nanxiong and Danxia,have been studied by sedimentary and geochemical of the sedimentary rocks. And further toexplore the geological response of the three basins in the evolution of the South China sea.By analyzing clastic composition, heavy minerals, major element, trace elements andtectonic setting and provenance of Ledong, Nanxiong and Danxia basin, the followingconclusions:(1) The tectonic setting of Ledong, Naxiong and Danxia are active tectonic sedimentarybasin, and rock composition maturity is lower, and most are arkose and greywacke. Theprovenance mainly from proximal clastic sedimentary rocks and few contain volcanic detritus.The tectonic environment of Ledong basin is obvious active continental margin retroarcsedimentary characteristics. It provide evidence for the existence suture of the Proto-South ChinaSea in Hainan. Early Cretaceous clastic components and chemical composition of the sandstonein Ledong show its formed in the typical environment of continental margin retroarc; Danxiabasin during the same period, show typical characteristics of foreland basin. Suggests that in thestage of Early Cretaceous, there exists a continental margin arc which can reflect convergentextinction of the Proto-South China Sea in the northern margin of the South China Sea.(2) Sedimentary characteristics of Upper Cretaceous Baowan Formation in Ledong basin ismainly inheritanced from environment of Early Cretaceous continental margin arc, but it is morecomplex in Nanxiong-Danxia basin. In the late Early Cretaceous to early Late Cretaceoustransition phase, Changba Formation shows a stable passive margin sedimentary characteristics,and the Danxia Formation shows the transformation process from foreland basin to rift basin.The process instructs a transformation from compressional foreland environment intoextensional rift environment. And for Nanxiong Basin, it is equivalent to the beginning of the riftbasin development in the Nanxiong basin. The sedimentary features in the Zhutian Formationhave a significant changes, whict obviously reflects the characteristics of the rift environment.Combining with the regional tectonic-magmatic record, we can know that the significant changeoccurred in about90Ma. Due to existing the remaining records of continental margin arc inadjacent areas during the Late Cretaceous, the rift evironment of Nanxiong-Danxia basin can bejudged belong to back arc rift basin. The tectonic transformation changes occurring in Nanxiong-Danxia basin, on behalf of the change of regional stress field in back arc environment.And it is unrelated to northern margin of the South China Sea converging basic reasons unrelatedto the transformation from continent subduction and convergent to the continental rupture.(3) Nanxiong-Danxia basin has obvious inheritance in the development, by compared andintegrated the sedimentary sequence, which clearly reflect the transformation fromcompressional foreland rift to extensional rift in continental margin back arc zone. But the stageof intra-continental rift, which represented by the continental rupture, and the stage of back-arcrift with the system of Proto-South China Sea have not yet a clear mark, pending further study.(4) As the same tectonic activity, it is different to the sandstone formed in the island ormountain arc environment that“predecessor” of intra-continental rift environment may includecratons, orogenic and tectonic subsidence area. Its provenance is very complicated, and it isdifficult to find the markable ingredients or chemical components of detrital indicators about theenvironment of rift. |
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