A source-to-sink multiproxy analysis of late Eocene regional drainage evolution in the Beibuwan Basin of the South China Sea

Provenance studies involving regional-scale drainages that traverse lithostructurally complex terranes are challenging, especially if similar-aged zircon populations are distributed across source areas. In this paper, we study the late Eocene succession of the Beibuwan Basin in the northwestern part of the South China Sea, which is near the junction of the Yangtze, Cathaysia, and Indochina Blocks. We apply a zircon-based multiproxy approach that integrates UPb dating, Hf isotopes, and U–Pb/fission track double dating, and sedimentological evidence to understand source-to-sink pathways. Detrital zircon geochronology analysis reveals significant spatial provenance variation between western and northern samples, with unimodal and multimodal age peaks, respectively. Although both samples contain a large amount of Caledonian ages, provenance determination is uncertain based on the statistical analysis of age distributions alone. Compared with northern samples, combined zircon Hf isotope and fission track analysis suggest that western samples may have been sourced from the Song Chay Massif in the Yangtze Block, rather than from the nearby Yunkai Massif in the Cathaysia Block, as indicated by previous studies. Seismic stratigraphic studies indicate the existence of a large-scale axial deltaic clinoform in western areas, which may be linked to the development of an extensive palaeodrainage originating from the Song Chay Massif in the northwest. This southeast-flowing palaeodrainage was potentially a precursor to the palaeo-Red River and Pearl River tributaries. These findings validate that zircon-based multiproxy analyses effectively resolve provenance complexities in diverse regions, but emphasise that integration of sedimentological evidence is important to further refine the source-to-sink model.