The Hanjiang River is the second-largest river in Guangdong Province, China. The modern sedimentation pattern of the Hanjiang subaqueous delta has been significantly affected by changes in the sediment delivered by the Hanjiang River. Based on multiperiod charts and columnar samples from the Hanjiang subaqueous delta, the influence of a sharp decrease in the sediment flux from the Hanjiang River on the deposition of the Hanjiang subaqueous delta in the past 60 years was studied through a combination of chart-based analysis of the evolution of erosion and deposition and the analysis of sediment samples. The results showed that the significant reductions in sediment fluxes from the Hanjiang River and Rongjiang River had obviously spatially differentiated effects on the deposition pattern of the Hanjiang subaqueous delta. The significant decreases in sediment fluxes from the Hanjiang River and the Rongjiang River caused the sediment grain size of the columnar samples in the near-estuary area of the Hanjiang River subaqueous delta to coarsen upward, the deposition rate to decrease, and the depositional state to change to an erosional state. Since the sediments discharged into the ocean after reservoir impoundment are mainly fine-grained suspended sediments, the supply of coarse-grained sediments in the Hanjiang subaqueous delta decreased, resulting in the upward fining of the columnar samples. The spatial response pattern of the Hanjiang subaqueous delta to the sharp decrease in sediment flux into the sea differed significantly from those of the Yangtze and Yellow subaqueous deltas.

This study utilized single-channel seismic, multi-channel seismic, and multibeam bathymetric data to examine the distribution and geomorphological background of geological hazards in the Zhongsha Islands region of the South China Sea. We elucidate the regional geological structure and its evolution while focusing on the types and characteristics of submarine hazards since the Quaternary Period. By integrating geomorphological, tectonic, and sedimentary factors, we analyzed the primary drivers of shallow geological hazards in the region. Our findings reveal that seabed topography, tectonic activity, and sedimentary processes critically influence hazard formation, particularly in geomorphic units prone to disasters, such as submarine slopes and canyons. Igneous rocks in the region display medium-acid to medium-basic compositions, with notable developmental stages during the Himalayan and Yanshan periods. From the Paleogene to the Middle Miocene, tectonic activity intensified, significantly thinning the lithosphere. By the Middle Miocene, the crust stabilized into its present configuration, marking the formation of key tectonic units in the region. Multiple phases of sedimentary evolution, influenced by the Cenozoic tectonic movements, further contribute to the region’s susceptibility to geological hazards.

The intrusive bodies in the study area are categorized into two intrusive units, γπK₂ and ηoπK₂, based on rock structure characteristics and intrusive contact relationships. The primary lithologies are granite porphyry and quartz monzonite porphyry. Through LA-ICP-MS zircon U-Pb dating, the weighted mean ages of zircon ^206Pb/^238U for these two units were determined to be 84.09 ± 0.86 Ma and 86.10 ± 0.76 Ma, respectively. According to geochemical characteristics and Sr-Yb granite classification, the quartz monzonite porphyry is classified as Zhejiang-Fujian type granite, and the granite porphyry as Nanling type granite. The study explores the geodynamic significance of granites in the area based on Sr-Yb granite classification, identifying the lower crust metamorphic facies as amphibolite facies. The source rocks are primarily basic granulites and felsic granulites, reflecting a thermally active lower crust with poorly developed fluids. Additionally, Sr-Yb granite classification aids in determining the crustal thickness of the lower crust, which is crucial for understanding continental geological evolution. This paper discusses the optimal locations and conditions for mineralizing fluid precipitation, noting that tungsten-tin mineralization is associated with Nanling type granites and that the mineralization process generally occurs later than the diagenetic process under conditions of low pressure and low oxygen fugacity. Tungsten-tin prospecting should focus on areas where the surrounding rock temperature ranges from 300 to 500 °C, with particular attention to the contact zones of sedimentary rocks or near structural fault zones.