RECONSTRUCTING THE CENOZOIC EROSION OF EASTERN TIBET FROM THE MARINE STRATIGRAPHY OF THE SOUTH CHINA SEA

Uplift of the Tibetan Plateau following India-Asia collision might be expected to have generated an erosional record in the marginal seas of East Asia because the rivers that feed the deltas of the region are largely sourced in eastern Tibet. As the plateau surface was elevated gorges have been incised into the edge of the plateau. Furthermore, intensification of the monsoon linked to uplift would have increased summer precipitation and the erosive power of the rivers, resulting in increased mass delivery to the margins. The Red River in particular flows directly from Tibet into the South China Sea with minimal onshore storage of sediment, such as seen in the Yangtze system. Only ca. 5% of the sediment lies under the Hanoi Basin, with the bulk lying in the Song Hong-Yinggehai Basin between Vietnam and Hainan Island. This basin is ca. 15 km deep and was generated as a pull-apart basin starting after 45 Ma. Drainage evolution models and simple mass balance calculation show that the Red River must have lost significant drainage since the Oligocene due to capture into adjacent systems. Nd isotope data from sedimentary rocks in the Hanoi Basin indicate that the basin fill closely resembled the modern river by the Early Miocene, suggesting that major drainage capture occurred in the Oligocene-Early Miocene. Nonetheless, the flux of eroded material into the Red River delta is reconstructed as being as fast during the Pleistocene as at any time in the geological past. The Middle Miocene is identified as a period of strong erosion, followed by much lower rates in the Late Miocene. We suggest that this reflect changes in monsoon intensity. New data from ODP Site 1148 offshore the Pearl River estuary now indicate monsoon intensification by ca. 24 Ma, but with more arid conditions during the Late Miocene slowing erosion. Together our data point to eastern Tibetan uplift to significant altitudes by the Early Miocene but with monsoon intensification and continental erosion also controlled by global climate change, especially Antarctic glaciation after 14 Ma.