Topographic disequilibrium, landscape dynamics and active tectonics: an example from the Bhutan Himalaya

Abstract. The quantification of active tectonics from geomorphological and morphometric approaches commonly implies that erosion have reached a certain balance. Such equilibrium conditions are however rare in nature, as questioned documented by recent theoretical studies indicating drainage basins may be perpetually re-arranging even though tectonic climatic remain constant. Here, we document these dynamics the Bhutan Himalaya, where evidence for out-of-equilibrium morphologies long been noticed, major (> 1 km high) river knickpoints high-altitude low-relief regions mountain hinterland. To further characterize their dynamics, perform field observations detailed quantitative analysis using χ plots Gilbert metrics drainages over various spatial scales, Himalayan rivers to tributaries draining regions. We first find network is highly dynamic unstable, with much divide migration captures. landscape response relatively rapid. Our results do not support idea general wave incision propagating upstream, expected most previous interpretations. Also, specific organization which all located along longitudinal band hinterland, whatever scale dimensions associated basins, calls common local supporting mechanism probably related uplift. From there, discuss possible interpretations observed Bhutan. emphasize need precise documentation disequilibrium scales step morpho-tectonic landscapes.