OLIGOCENE-EARLY MIOCENE REORGANIZATION OF PALEODRAINAGE ALONG THE SOUTHEASTERN MARGIN OF THE TIBETAN PLATEAU

Drainage network evolution along the southeastern margin of the Tibetan Plateau is coupled with tectonic and topographic changes associated with the collision of Eurasia and India. The potentially integrated paleodrainage of the Red, Yangtze, and Mekong Rivers are the subject of intense scrutiny as their response to tectonic forcing may shed light on the geophysical and geomorphologic evolution of this region of the plateau. Previous studies, which focused on zircon populations of the trunk streams or at proposed capture points, suggest early Miocene, middle Miocene or Quaternary drainage reorganization. This study uses detrital zircon double dating, with broad spatial and temporal coverage to examine the potential nature and timing of reorganization.

Zircons were extracted from sandstones collected from Paleogene and Neogene deposits that span an 800 km NW-SE transect along the SE margin. Analysis of the probability density functions was performed using Kolmogorov–Smirnov tests, PDF crossplot analysis, and percent area similarity. Samples were compared to PDFs of zircons from modern river sands of the Red, Mekong, and Yangtze Rivers as well as to populations from potential sources throughout the region.

In Midu Basin sediments of Yunnan, China, there is clear evidence of southward Oligocene paleodrainage, which may be a link between the paleo-Red and paleo-Yangtze river. Within the same basin, Paleogene deposits (~35-23 Ma) show a dramatic change in sediment source and ZHe ages and may correspond with early Oligocene initiation of the Ailao Shan Shear Zone and larger scale paleodrainage changes throughout the region. Analysis also indicates there is Neogene western-sourced sediment deposited in the Yanyuan Basin just north of the current Yangtze River, suggesting any reversal of the Yangtze likely occurred prior to deposition. In addition, it seems reasonable that a paleo-Mekong River was established at Lanping Basin; ~50 km to the east of its current position during the Miocene and has subsequently migrated west. Combined, the data leads to the conclusion that the reorganization likely occurred during the Oligocene-early Miocene and is most likely the result of lateral shearing along the series of large strike-slip faults in and around the SE Plateau Margin and not surface uplift related to lower crustal flow.