Tectonics, Topography, And River System Transition In Southeast China: Insights From The Sedimentary Record In Taiwan

The Cenozoic in East Asia is marked by major changes, such as uplift of Tibet Plateau and marginal sea spreading which have had important influence on the tectonic, landform and drainage system in East Tibet, although the timing and nature of such changes remains disputed and the marginal sea spreading in shaping the topography and drainage reorganization in East Asia has not been well studied. The thich Cenozoic sequences in northern margin of South China Sea(SCS) recorded the detailed evolutionary process that topography and river systems had experienced in East Tibet and Southeast China. These sequences were subsequently deformed, uplifted, and exposed in Taiwan as a result of the eastward subduction of SCS and arc-continent collision between northern Luzon arc and continental margin of Southeast Asia. Taiwan represents the “exposed SCS” and it provides one of the best geological windows looking into the details evolution of SCS. In this study, we investigated the breakup unconformity and the geochemistry of mudstones, as well as the U-Pb chronology and Hf isotope of detrital zircon grains in sandstones in Taiwan to understand the initial spreading of the SCS and the evolution of topography and drainage systems in coast area of Southeast China.The basins in northern margin of SCS, such as Pearl River Mouth Basin and Qiongdongnan Basin, develope synrift and postrift sequences, and breakup uncomformity between them. The Cenozoic strata in Taiwan can also be grouped into synrift and postrift sequences. The former is mainly composed of Eocene thickbedded sandstones, siltstones, and coal that deposited under braided river and swamp facies. The postrift sequence is a relatively continuous shallow marine units from Oligocene to Upper Miocene in age, with abundant of marine fossils. The variegated(including brown, offwhite, and celadon) Pinglin Tuff overlying breakup uncomformity is exposed in the axial section of Tsukeng Anticline in Western Foothills. The zircon grains from it show SHRIMP age 39.79 ± 0.39 Ma and 39.53 ± 0.44 Ma, indicating the age of Pinglin Tuff is 39 Ma. Together with biostratigraphy data, the breakup unconformity can be constraint between ~39 and 33 Ma, suggesting that the seafloor spreading in the SCS commenced before ~33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin, and it is consistent with the drilling results in SCS at Site U1435, IODP Expedition 349, 2014. Diachronous southwestward younging of the breakup unconformities of basins suggest seafloor spreading propagating from northeast to southwest.The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values in Cenozoic mudstones in Western Foothills and Hsuehshan Range show an abrupt change between ~31 and 25 Ma。Before ~31 Ma, the ratios of La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th and εNd values are 85.08, 0.49, 2.12, 0.45, 0.22 and-11.8~-9.3(with average value of-10.56) respectively, but it change to 105.4, 0.62, 0.16, 0.31, 0.13 and-14.5 ~-11.9(with average value of-12.96) respectively after 25 Ma. Meanwhile, the U-Pb age spectra of detrital zircon grains from Paleogene sandstones are dominated by Yanshanian(180-67 Ma) age with subordinate or nil Proterozoic(600-1000 Ma and 1600-2000 Ma, 2300-2500 Ma) and Archean(2500-2800) age and the εHf(t) mostly show negative value with narrow range between-24.1 and +10. These results are in contrast to those in Miocene rocks in Taiwan that are dominated by the Indosinian(257-205 Ma) age, and with a lot of Yanshanian, Proterozoic and Archean population, and the εHf(t) demonstrate wide-range between-65 and +15.The same U-Pb age spectra of detrital zircon grains in both Western Foothills and Hsuehshan Range indicate the same sedimentary source for the Paleogene formations. There is a lof of Phanerozoic granites with εNd between-4 and-10 presented in South China. The Yanshanian and Indosinian period zircon grains of them has εHf(t) beween-25 and 0. These characterists are the same with those in Paleogene strata in Taiwan, indicating that these Paleogene sediments sourced from the erosion of Phanerozoic granitic sources exposed in coastal South China before ~31 Ma. After 25 Ma, the lower εNd values of mudstones and a number of Phanerozoic, Proterozoic and Archean zircon grains in Miocene rocks in Taiwan. In comparison with the Yangtze and Cathaysia Blocks, the Lower Yangtze region shared the same feature with the Miocene in Taiwan, suggesting that the main sources had evolved to inland South China after ~25 Ma. The Yangtze Block became the important sedimentary source for Miocene strata in Taiwan. The Paleogene strata in Taiwan have the same U-Pb age spectra of detrital zircon grains with Minjiang River, but the Miocene strata is the same with Yangtze River. Therefore, It can inferred that the Minjiang River may deliver most of sediments to the original region of uplifted Taiwan during Paleogene, but the pathway is less constraint for Miocene sediments.The geochemistry of mudstones and U-Pb age and Hf isotope of detrital zircon grains show abrupt change during ~31-25 Ma, indicating the sedimentary source migrated inland and a major adjustment occured for topography and river systems in East Asia.The diachronism of transition time of provenance in rifted basins in northern and southern margins of SCS is shown. For example, the transition time of provenance in Taiwan is 31-25 Ma, the ODP site 1148 and Pearl River Mouth Basin is 27-23 Ma and the Yinggehai-Qiongdongnan Basin is ~13 Ma. The diachronism of breakup unconformity and transition time of provenance implied that the opening of SCS had great influence on the evolution of sedimentary source of rifted basin, topography and river systems in Southeast China. The rapid uplift of Tibet Plateau and subsidence in coast area of South China contributed to the eastward-tilting topography required to explain the inland incision of rivers. The prototype of modern rivers and topography in East Asia may have established since Early Miocene.