Provenance Of The Basins In The North Margin Of The South China Sea: Insights From The Well BY7-1-1 In The Pearl River Mouth Basin

The continental margin of South China has a long and complex history that extends back to the Mesozoic when it formed part of an active “Andean type” margin associated with north directed subduction of the Paleo-Pacific Plate. In the Paleocene the margin experienced significant extension leading to a complex passive margin. Whilst considerable research effort has been directed at explaining extension across the South China margin, and the age and mechanisms for opening of the South China Sea, relatively little attention has been paid to the impact of these processes on the evolution of the exposed South China continental margin. Coupling between the sedimentation in a basin and the unroofing-erosion in the surrounding orogens makes it possible to understand the basin-mountain evolution, thermo-tectonic history and surface crustal recycling processes by studying the detritus composition, geochemical characteristics of the basin sediments and the timing and rates of exhumation surrounding orogens.It has been suggested there is a link between re-organisation and uplift of eastern Tibet and/or opening of the South China Sea. Many believe that what connected to this regional uplift event were changes in regional drainage patterns that controlled sediment transfer to basins in the South China Sea. The most recent and widely cited model suggests that the riverdrainage system in eastern Tibet, southwest China and parts of Indochina underwent a series of drainage capture events that progressively removed East Tibet sediment sources from the South China Sea basins leaving sediment inputs from mainly local areas. Our provenance results show that most of the basin sediments could not have come from east Tibet but instead came from a terrain dominated by mid Cretaceous age granitic rocks. Onshore exhumation studies also suggest that the south China margin has only experienced modest levels of erosion since 30-50 Ma hence the most immediate marginal areas could not have been the main source for the sediment in the South China Sea basins. This raises the question wherehas most of the sediment in the South China Sea come from?The purpose of this study is to document the timing and rates of exhumation across the Pearl River Mouth region of the South China margin and the provenance of the basins in the northern margin of the South China Sea.(1) Thermal history modeling of the combined FT and(U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. Initial phase of rapid cooling that youngs to the north; a period of relative(but not perfect) thermal stasis at ~70–60 °C which increases in duration from the south to the north; final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.(2) The Nd isotope data from the ~32 Myr sediments drilled in Well BY7-1-1 show a gradual decrease in(Nd values from-5 to-7.5 at ~25 Myr ago. Starting at ~25 Myr ago, the(Nd values of the sediments decreased significantly to-13.5 and then increased again up to-11.5 in ~11 Myr old sediments. The(Nd isotopic data from Eocene to Oligocene sedimentary rocks from Well BY7-1-1 would like directly reflect a major source contribution from nearby intermediate-silicic plutonic rocks in the Cathaysia Block. The Eocene sandstone obtained from Well BY7-1-1 has yielded zircon dates that cluster in three major age populations: the early Cretaceous(90-120 Myr ago), Jurassic(130-170 Myr ago) and Permian-Triassic(210-260 Myr ago), with few Paleogene(50-70 Myr ago), early Paleozoic(400-450 Myr ago) and Neoproterozoic(800-1000 Myr ago) grains. Zircon age peaks of the Eocene sandstones are consistent with the ages of the igneous rocks exposed around the Pearl River Delta indicating that these local bedrocks contributed the most sediments to the Pearl River Mouth Basin in the South China Sea during the early rifting stages of the basin. Neoproterozoic, Paleoproterozoic and Archean zircon grains become more abundant up section in Miocene sediments and become relatively more common in the middle-upper Miocene samples indicating that the Yangtze Block or northern Cathaysia with abundant Archean and Protozeroic rock units supplied more sediments to this depocenter in the Miocene. Generally the higher(Nd values and a dominated by Yanshanian(200–80 Ma) zircon grains in sediments deposited prior to 25 Ma indicate the detritus in the northern margin of the South China Sea mainly eroded from Phanerozoic granitic sources exposed in coastal South China. Whereas the lower(Nd values and dominated by the Indosinian(257–205 Ma) zircon grains together with Yanshanian, Proterozoic and Archean population in Miocene rock suggests that the main sources had evolved to inland southern China by 25 Ma.(3) The Pearl River and Minjiang River might have been larger in Early Miocene than the present and might have delivered inland material to the northern margin of the South China Sea. This change corresponds to the time of the drainage reorganization in East Tibet, such as Yangtze River, and the regional subsidence resulting from the opening of marginal sea. The combined effects of Tibet uplift and opening of marginal sea might have shaped the topography and river system reorganization in East Tibet. The evolution of topography and drainage systems in southeast China seems to be mainly controlled by the opening of marginal sea.