Did a single large river system once drain the margin of SE Tibet and flow into the South China Sea?

Largely on the basis of geomorphological features it has been proposed that in the Cenozoic a large single river system once drained the SE margin of the Tibetan plateau and flowed southwards into the South China Sea. This palaeodrainage model proposes that river capture and drainage reversal accompanied Neogene uplift of the SE margin of Tibet leading to a progressive reduction in drainage area to eventually form a drainage similar in catchment area to that of the modern Red River. To test this model we examined detrital zircon U-Pb ages from Cenozoic sediments deposited in the South China Sea Yinggehai Basin, offshore Red River delta, and presumed palaeo-Red River deposits deposited in the Jianchuan Basin near the modern day river headwaters in China. If a large palaeo river system once drained the southeast margin of Tibet we would expect to find a wide age range diagnostic of a large drainage area encompassing at least four major tectonic terranes that welded together in the Mesozoic. We would also expect samples from both basins to show a similarity in zircon age distributions through time. Samples from the Yinggehai Basin that span the Oligocene and mid Miocene (circa 25-13 Ma), the time at which a large drainage system may have existed, contain a consistently identical provenance with the bulk of zircon sources recording circa 250 Ma and 100-110 Ma ages. Zircon typology indicates a tholeitic/alkaline I- type granite terrain for the Cretaceous ages. Zircon U-Pb data from Oligocene sediments collected from the onshore Jianchuan Basin also record a narrow range of ages and are also dominated by 250 Ma sources although by contrast Cretaceous grains are virtually absent. Importantly, both basins exhibit a very narrow range of detrital zircon ages suggestive of a restricted drainage. According to the large single drainage model a progressive reduction in drainage area initiated in the middle to late Miocene as east Tibet experienced surface uplift. We do find evidence for a major change in the marine basin sediment provenance between circa 13 and 3 Ma when the range of zircon ages dramatically expands to include Palaeozoic and Proterozoic sources accompanied by loss of the Cretaceous ages. These changes coincide with shifts in sediment accumulation in the South China Sea Basins. So what might have caused this? The young and restricted age range of zircons from Oligocene and early Miocene samples are hard to reconcile with the existence of a large regional scale drainage system. Based on the provenance data we envisage a relatively small palaeodrainage system with restricted and localized erosion linked to strike-slip deformation. The Late Miocene to Pliocene change in South China Sea basin provenance can be linked regional uplift and a phase of incision by the Red River in the Pliocene rather than any major drainage re-organization connected to uplift along the eastern margin of Tibet.