INVESTIGATING DRIVERS OF EROSION IN A TECTONICALLY-QUIESCENT MONSOONAL RIVER BASIN, SONG GIANH, CENTRAL VIETNAM

Interpretation of the geologic record in terms of tectonic- or climate-driven erosion requires an understanding of modern sediment production and transport processes. However, unravelling drivers for erosion in continental systems is difficult because these processes are often closely linked. Here, we addressed this complexity by targeting a fluvial system in a largely tectonically quiescent setting to constrain climate-erosion linkages. We examined the development of the Song Gianh Basin (~3500 km²) in central Vietnam over the Holocene to assess how monsoon strength influenced sedimentation. The small size and simplicity of its tectonic setting permits a basin-wide analysis of sediment production in contrast to larger fluvial systems. Using several geochemical and detrital geochronological provenance methods on siliciclastic bulk sediments, we constrain sediment provenance in the Song Gianh and quantify downstream sediment mixing to the offshore Song Hong-Yinggehai Basin (SH-YB).

Geochemical data indicate greater erosion in the high, wetter reaches of the Song Gianh. In contrast, detrital zircon U-Pb ages argue for greater flux from the drier northern tributaries. We propose this mismatch represents disequilibrium in erosion patterns driven by changing monsoon strength and human agriculture across the region. Calculated erosion rates from AFT and in-situ ¹⁰Be indicate significant volumes of sediment (132,000–158,000 km³; 17–21% of total) are delivered from coastal Vietnam to the SH-YB, the primary depocenter of the Red River. This flux does not negate drainage capture in the Red River but can partly account for the discrepancies between preserved and eroded sediment volumes in earlier models.

We further applied OSL dating to constrain timing of aggradation along the Song Gianh. Strong monsoonal rainfall controls focused sediment generation in the steep, upper reaches of the Song Gianh and drove Early Holocene (7.4–8.5 ka) terracing. Late Holocene (~550 and 150–300 yr B.P.) aggradation instead reflects high sediment flux related to farming intensification, neither climatic nor tectonic changes. We argue that human-induced erosion dilutes the modern Song Gianh with weathered soils, indicating that no natural erosion signal representing climatic conditions is transmitted offshore.