The continental margin of Indochina represents the westernmost margin of the South China Sea, a wide continental rift that in the west is mostly submerged. In the region of south to central Vietnam, the location of this study (Figure 1), the continental margin extends offshore for >100 km before oceanic crust is reached. The extension that led to continental break-up has yet to be fully explained but is known to be related to the subduction of the paleo-Pacific plate, which at the time formed a wide Cretaceous magmatic arc with widespread calc-alkaline magmatism across south to central Vietnam and southern China. Subduction was directed to the northeast beneath southern China and westwards below Vietnam, the eastern margin of the Indochina block (Hall, 2012;Hennig-Breitfeld et al., 2021;Zhu et al., 2004).Extension is imprinted on the landscape and geology of Vietnam but to what extent is not yet clear. It also remains uncertain as to when surface uplift took place. Was uplift staggered over a long time or did it occur within a short
Offshore Southeast (SE) Vietnam is considered a transition zone, with the sedimentary evolution of this area controlled by land-sea interactions, especially by the Holocene sea-level rise. This study presents some new findings regarding Holocene sedimentation and its linkage to the heavy mineral placers within the study area based on high-resolution seismic interpretation and sediment analysis. Our obtained results show that the Holocene sediments directly overlie the Late Pleistocene sedimentary formation, from which they are separated by an erosional/unconformity surface (R1 seismic reflector). Sediments deposited in the Early (~11.7-8.2kyr BP), Middle-Late (8.2kyr BP-present-day) Holocene sub-epochs correspond to the Transgressive and Highstand System tracts, which were closely controlled by the three stages of Holocene sea-level rise. The recent sediments distributed on the seafloor are dominated by sand and gravelly sand, demonstrating high-energy conditions, while the heavy minerals are rich in ilmenite and zircon. Most of these are concentrated along the present shoreline zones, but we do not exclude their accumulation in the paleo-shoreline and incised channels. Two ilmenite dispersion halos (1st and 2nd order) and one zircon dispersion halo (1st order) suggest that ilmenite and zircon are the most dominant heavy minerals while gold is only observed locally. The minerals were potentially derived from the weathering products of the Triassic-Cretaceous ilmenite-, zircon-, and gold-bearing granite and granitoid in central Vietnam.
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