Early Cenozoic drainage network and paleogeographic evolution within the SE Tibetan Plateau and its surrounding area: Synthetic constraints from onshore-offshore geological dataset

Abstract

The evolution of the major rivers originating from the SE Tibetan Plateau has been a research hotspot due to a close connection between tectonic events, geomorphological shifts and river formation. This study reviews and compiles a large group of provenance analyses including zircon UPb dating and whole-rock geochemistry, in order to provide a systematic interpretation of the drainage evolution of the extensive rivers, mainly represented by the modern Red River, Mekong River, Pearl River, Yangtze River, the coastal South China rivers, etc. There are numerous debates over the potential existence of a paleo-Red River, in other words, whether the Yangtze upper and middle reaches, Mekong and other SE Asian rivers partly or collectively formed a single drainage system during Miocene (or even earlier), and eventually generated thick sedimentary sequences in the South China Sea (SCS). The fragmentation of this continental-scale river is speculated to result from several river captures and reversals during the Cenozoic before evolving into the present-day drainage framework. However, this hypothesis has been increasingly doubted, and our review shows a lack of robust evidence supporting the presence of a unidirectional N-S drainage, since zircon UPb signatures of SE Tibetan River sands, relict Cenozoic terrestrial deposits and contemporaneous offshore sediments fail to match in consistence. Instead, the Mekong River possibly had not achieved its current form until the Middle Miocene, possibly triggered by the Tibetan Uplift to enhance the SE Asian summer monsoon precipitation. Large uncertainties remain over the timing of the Yangtze River formation, with a wide range of age estimates extending from the Earliest Miocene to Holocene. During the early Cenozoic, it was the combination of axial topographic pattern and the prolonged extensional setting of the South China margin that collectively controlled sediment supply and distribution, and induced a dominant eastward transport pathway from eastern Indochina into the southern depression of the northern SCS. It wasn't until the Early Oligocene (or even later) that the topographic inversion of SE Asia by westward to eastward tilting accelerated the headwater erosion and drainage basin enlargement. The Pearl River thereby experienced a significant inland expansion during the Late Oligocene, and reached its near-modern delineation since the Early Miocene. At the same time, some rivers across coastal South China, such as the Min and Jiulong Rivers might also have extended farther to the west. In any case, regional tectonic activity between the Tibetan Plateau and the marginal sea basins shaped the geomorphological and topographic characteristics in a fairly complicated way, and controlled the overall source-to-sink patterns and fluvial system evolution.

The SE Asian continental margin was dominated by a long-lived extension from the Late Mesozoic to Early Cenozoic. The marginal sea basins, including the SCS and the East China Sea (ECS) basins, have been preserved with exquisite sedimentary records of this geological history. Nevertheless, due to the limited drilling penetration, low-resolution seismic profiles, and the lack of a reliable stratigraphic framework, the sedimentary evolution of the offshore basins is still controversial from different perspectives. In this review, we also illustrate the Early Cretaceous–Early Miocene sedimentary evolution of the South and East China Sea regions by synthesizing our newly obtained results and dataset from previous literature, and by displaying the lithofacies patterns in sequential palinspastic restorations. Our preferred paleogeographic scenarios incorporate the conjugate relationship between continental margins and the birth-and-demise of the presumed proto-SCS. The lithofacies patterns, depositional environments as well as the provenance dataset, commonly demonstrate the sedimentary responses to the significant Meso–Cenozoic shift of magmatism and geodynamics along the SE Asian margin, and together have implications for the mechanisms of continental rifting and oceanic crust accretion during the Cenozoic.