On Earth, there exist many large orogenic plateaus, where are characterized by a high elevation and low-relief topography. Geologists have extensively studied the growth and development processes of these plateaus. However, why plateaus with high elevation and low-relief topography can still maintain relatively stable terrains over long-time scales is still a puzzling question.
Interaction between river incision and tectonic activity constantly shape the topography of plateaus. It's widely accepted that tectonic uplift increases stream gradient, which conveys more water and sediment via orographic precipitation that facilitates faster incision, driving the headwater erosion of river. However, it is puzzling why rivers have not eroded into active orogenic plateaus further or more deeply to destroy uplifted terrain in some mountains. Some think that the generally dry climate within plateaus makes it harder for rivers to erode and can lead to the formation of internally drained basins. Others believe that the coupling of river erosion and landslides since mountain uplift has resulted in decreasing of landslide frequency and the corresponding declining river erosion capacity as time goes on. Further, others suggest the intense uplift or deformation, glacier and landslide dams, or feedbacks between uplift and erosion at plateau edges have also constrained the dissection of plateau morphology by rivers.
There are usually a series of erosion waves that propagate upstream as river knickpoint migrating upstream. Our Process Geomorphology Team in Nanjing University has been studying the evolution of river geomorphology for over 10 years, and we have found that some large river knickpoints often develop in regions with particularly active tectonic structures. Therefore, we hypothesize that the diverse nature of regional tectonic could prevented upstream migration of river knickpoints and headward fluvial erosion.
To verify the above hypothesis, we take the Yarlung River in the southern Tibetan Plateau as an example, conducted multiple field trips, and collected a large number of bedrock samples for low-temperature thermochronology analysis (Fig. 1). Combined with 3D thermo-kinematic modeling, we constrain the incision history of this river and effect of a tectonic rift on river incision. We found that since the late Miocene, with the accelerated extension of the southern Tibetan Plateau, co-phase tectonic systems inhibited the upstream migration of river knickpoints, thereby maintaining the stability of the topography in the southern Tibetan Plateau (Fig. 2).
Fig. 1 (a-c) Some field Photos of collecting samples in the Yarlung River Valley. (d) Prof. Wang and his students.
Our findings suggest that tectonic activity does not always necessarily enhance river erosion. Diverse activities can also decrease erosion rates in active orogenic belts. This finding also provides a new mechanism for explaining the universal stability of topography and high plateaus in some orogenic belts.
Fig. 2 Schematic diagram illustrating the mechanisms for tectonic activity impeding the river incision. (a) Headwater incision of the river related to the uplift of the southeastern Tibetan Plateau along with intensified monsoon precipitation. (b) Regional coupled tectonic systems with accelerated extension of the plateau driving the formation of river knickpoints, which subsequently adjusts and reduces the regional river gradient, keeping river knickpoints relatively stable and slowing down upstream river incision.