Drivers of landscape evolution in eastern Tibet

Abstract

The eastern margin of the Tibetan Plateau is characterized by two end-member morphologies: 2–3-km-deep canyons that are juxtaposed against extensive high elevation, low relief landscapes. Past researchers have highlighted topographic evidence for mobile drainage divides in this region. Active thrusting is thought to drive river gorge incision originating from the east, but within the eastern Tibetan plateau, there is no evidence for focussed thrusting, and no significant post-Cenozoic shortening structures are evident at the surface upstream of the Three Rivers regions. In this region, researchers have attributed the evident landscape transience to different mechanisms, including upward propagation of an incision signal, regional uplift, and local uplift caused by strike-slip motion. Here, we quantify topographic metrics to document the geomorphic response to the proposed tectonic forcing. Specifically, we quantify channel steepness and identify knickpoints to assess evidence for landscape transience and potential accelerated incision triggered in eastern Tibet. We find that ksn and slope systematically increase downstream along the Tongtianhe River (TR) which bisects the study area from NW to SE and is part of the Yangtze River system, but not along adjacent main stem rivers with similar orientation, therefore ruling out the effect of a regional uplift gradient. The spatial distribution of knickpoints does not cluster along or around fault lines, which we interpret to mean that local faulting is not likely a significant factor causing the systematic variations in the topographic metrics. We find that tributaries of the TR exhibit large non-lithological channel convexities, and that the elevation of these knickpoints above the tributaries' mouth increases towards the SE, in the TR's downstream direction. This observation can be best explained by a transient signal propagating upstream along the TR. An additional large non-lithological knickpoint along the TR, as well as a strong local drainage divide disequilibrium between the TR's tributaries and adjacent basins, suggest that waves of incision, potentially set by regional uplift, are propagating up the TR but not up the adjacent major river systems (the Mekong and eastern branch of Yangtze rivers). We propose these waves of incision are the first-order driving force for river arrangements in eastern Tibet. Accordingly, we reconstruct the evolution processes of landscapes in eastern Tibet.