Evolution of the paleo-Mekong River in the Early Cretaceous: Insights from the provenance of sandstones in the Vientiane Basin, central Laos

摘要

The Cenozoic was a time of rapid change in the large river systems of the southeastern Tibetan Plateau. Many previous studies proposed a connection between the upper reaches of the Salween, Mekong, Jinsha (Upper Yangtze) Rivers and paleo-Red River which then flowed to the South China Sea. However, little is known about the pre-Cenozoic source-to-sink river pathways of the Tibetan Plateau. Using 848 detrital zircon U-Pb ages derived from the Lower Cretaceous samples located in the Vientiane Basin, combined with petrographic, statistical, and Multidimensional Scaling (MDS) analyses, we present a detailed history of the paleo-Mekong River. All samples can be divided into three groups based on their age distribution. Samples in Group 1 from the lower part of section show major peaks in the Wutai (2400-2600 Ma), Luliangian (1700-1900 Ma), Jinningian (750-1000 Ma), Indosinian (200-290 Ma), and Yanshanian (150-200 Ma) periods, and are mainly sourced from the Southern Qiangtang terrane. These samples bear no resemblance to modern river sands in the MDS diagram. As such, we infer that these samples represent a unique paleo-river course from the Southern Qiangtang terrane. Samples in Groups 2-3 at the middle and upper part of section yield age peaks at the Luliangian, Jinningian, Caledonian (400-490 Ma), Indosinian, and Yanshanian periods, and are sourced from the Sichuan Basin, Songpan-Garze and the Northern Qiangtang terranes, which have characteristics similar to those of the modern Mekong River and Jinsha River. In our paleo-drainage model, the upper reach of the paleo-Mekong River did not connect to the Upper Salween River and flow into the Red River during the early Early Cretaceous period, as has been proposed previously. In the late Early Cretaceous, the upper segment of the paleo-Mekong River, which drained from the Sichuan Basin and Songpan-Garze terrane, and the upper reach of the Jinsha River were tributaries of a single Mekong River before collecting in the Vientiane and Khorat Plateau basins. These provenance and drainage changes (or lack thereof) were driven by the collision between the Lhasa and Qiangtang terranes similar to 120 Ma and the subsequent uplift and denudation of the Northern Qiangtang and Songpan-Garze terranes.