Knickpoint evolution on the Yarlung river: Evidence for late Cenozoic uplift of the southeastern Tibetan plateau margin

- Knickpoint and thermokinematic modeling are combined with apatite thermochronology.
- Our results indicate the Yarlung flowed predominantly eastward since at least 9 Ma.
- Uplift of 0.5-2.5 km of southeastern Tibetan margin occurred just prior to 9 Ma.
- Our results are consistent with early drainage reorganization east of Namche Barwa.

A salient geomorphic feature of the Yarlung River is its abundance of large knickpoints, which in many cases coincide with north-south trending rifts. Across one of these rifts, near the town of Jiacha, the Yarlung falls nearly 500 m from an elevation of ∼3500 m over 80 river kilometers, making this the second largest knickpoint on the river. We propose that the Jiacha knickpoint represents a wave of incision migrating upstream through the drainage network in response to a downstream base level fall, not a disturbance in the channel to due rift tectonics.Longitudinal profile slope-area and chi (χ) analysis of Yarlung River tributaries and those of several major rivers in southeastern Tibet indicate several knickpoints are present at ∼3500 m elevation, all resulting from a single regional-scale base level fall. Retreat rates calculated from celerity modeling indicate that the Jiacha knickpoint was located at the upstream edge of the Namche Barwa massif at ∼10 Ma, a history consistent with apatite 4He/3He thermochronometry data and thermokinematic modeling from that region. These data suggest the Yarlung River has flowed in its present course through this area since at least 10 Ma and imply that at least 500 m of incision occurred within this canyon over this time period. The spatial scale of these observations suggests that these knickpoints resulted from surface uplift of southeastern Tibet of 500 to 2500 m just prior to ∼10 Ma. Additionally, our mapped knickpoint locations indicate that reorganization of the drainage network just east of the Namche Barwa massif occurred prior to this time.