Deconvolving Sunda River Systems Using Multi-Proxy Provenance Tools

The rivers draining towards the northeast into the southwestern South China Sea maybe derived from a number of potential sources both in mainland Asia, the Malay Peninsula, as well as the islands of Borneo and Sumatra. Reconstructing the evolution of drainage systems is important if we are to understand how the evolving patterns of rivers that emerged during each sea level low stand period changed through time since these may be important in increasing rates of biological speciation, at least during the Pleistocene. We investigated whether different areas yielded sediments that can be distinguished from one another and from the Mekong and Chao Phraya by analysing modern river sands. Major element compositions are rarely unique for given source areas, being strongly affected by grain size and degrees of chemical weathering. There is no single process that controls the intensity of chemical weathering across SE Asia and this likely reflects the combined influence of temperature, rainfall and tectonics in controlling transport speed and rates of alteration. The different tectonic provinces do however show coherent differences in Sr and Nd isotopes, although it is unclear if Sabah can be distinguished from Sarawak within Borneo. Detrital zircon U-Pb dating is even more effective at resolving these sources. All the samples differ from the relative similar Chao Phraya and Mekong rivers that are dominated by Indosinian, Inthanon Zone (200-300 Ma) grains and a significant “Caledonian" (440-600 Ma) population. In contrast, all samples from the Malay Peninsula are almost entirely Indosinian/Inthanon. All Sumatran rivers contain large proportions of <20 Ma grains, derived from the active arc, as well as 80-140 Ma grains from older Mesozoic arc basement, similar to that seen in the Schwaner Mountains of Borneo. As for the Sr-Nd isotopes the rivers of Borneo has similar populations in Sabah and Sarawak, with the largest being an Indosinian population, but also with large minority populations of Schwaner and Caledonian ages. Borneo is the only source area with a notable 800-1000 Ma “Gondwana” population that is otherwise restricted to the Mekong and Chao Phraya. Our work indicates that these tools can constrain sediment provenance on the Sunda Shelf. They also imply that the continental basement under southern Sumatra has similarity to that of Borneo, especially the Schwaner Mountains, in contrast to some older reconstructions.