No abstract

The alternating cycle of valley widening through lateral erosion ('strath planation') and valley narrowing through vertical incision into bedrock ('strath terrace abandonment') due to variations in sediment supply (Q¬s) relative to river transport capacity (Qsc) is a common feature in many mountainous environments, yet our understanding of the mechanics of the processes that drive this landscape change remains poorly quantified. Here, we used an experimental and numerical study to identify the geomorphic and hydraulic controls driving the response of mixed bedrock-alluvial rivers to variable sediment supply, water discharge and tectonic tilting. The experimental channels exhibit a multi-stage response of channel narrowing, stripping of the alluvial cover in a downstream migrating incision wave followed by destabilization of the bed and development of a single vertical step in the bed profile ('knickpoint') when the hydraulic conditions are supercritical. In our experiments headward erosion by knickpoints is the most efficient process of strath terrace abandonment, contributing the majority of the total vertical incision in a short period of time. We show experimentally, that knickpoint developing under supercritical flow conditions, driving the rapid response of fluvial systems to upstream perturbations in Qs/Qsc despite no base-level fall. This has implications for the understanding of distributions of strath terrace ages, the inference of baselevel variations from knickpoint propagation, and how landscapes respond to climatic or tectonic perturbations.

This study investigates three‐dimensional flow modes of orogenic plateaus by means of physical modeling. Experiments consist of shortening two contiguous lithospheres of contrasting strength, one being a weak plateau‐type lithosphere and the other a strong craton‐type lithosphere. The lateral boundaries are either totally confined or allow escape toward a lateral foreland on one side. Two synconvergence flow regimes are distinguished, which are governed by the balance between the gravity potential and the strength of the plateau crust and the resistance of its lateral foreland. The first regime implies transversal (orogen‐normal) injection of plateau lower crust into the collision zone as a result of confinement of the plateau by an increasingly stiffer lateral boundary. As a precursor mechanism to channel flow, transversal injection responds to downward thickening of the plateau crust that is forcedly extruded into the orogenic wedge. The second regime is that of collapse‐driven lateral escape of the plateau. This regime is established after a threshold is attained in the interplate coupling in the collision zone, which allows the gravity potential of the plateau to overcome the resistance of its lateral boundary. Under the collapse‐driven escape regime (orogen parallel), such as that governing Tibet during the last 13 Ma, most of the convergence in the plateau and the top and rear of the collisional wedge is transformed into lateral flow and extension.