Incising Tibet

Abstract:

Convergence between India and Eurasia has produced the Tibetan Plateau, which stands 5 km high over a region of 3 million km²，since collision at ~50 Ma. A key region for understanding the mechanisms and timing of uplift is the SE margin, where major rivers have incised the plateau margin, permitting the use of thermochronometry to time incision. Here we present new (U-Th)/He ages including 17 ages on apatite and 23 on zircon and 12 apatite fission track ages on the southeastern margins of the Plateau to explore rates and timing of incision. All three thermochronometric systems on the Salween suggest a regional change in the last 8 Myr and exhibit progressive younging over ~ 150 km upstream. Apatite (U-Th)/He ages of valley bottom samples from the Mekong become younger upstream whereas apatite fission-track and zircon (U-Th)/He ages show a large scatter from ~ 5 Ma to ~160 Ma, depending on the distance above the thalweg. Similarly, upstream samples north of 29⁰N on the Yangtze yield much younger ages than downstream samples. Inversion of combined thermochronometers reveals a northward propagation of the highest rates of incision along the Salween and Mekong since 8 Ma. These results, together with analysis of river profiles, are best explained by a scenario of plateau incision that occurred in southeastern Tibet before or coeval at 8 Ma. We infer that the southeastern plateau margin was deformed under a complex context with both lateral expansion of the plateau margin and oblique indenting of the India plate localizing shortening and high uplift rates at the corner of the indenter.