Topographic disequilibrium, landscape dynamics and activetectonics: an example from the Bhutan Himalayas

Abstract. The quantification of active tectonics from geomorphological and morphometric approaches most often implies that erosion and tectonics have reached a certain balance. Such equilibrium conditions may however be seldom found in nature, as questioned and documented by recent theoretical studies, in particular because drainage basins may be quite dynamic even though tectonic and climatic conditions remain constant. Here, we document this drainage dynamics from the particular case example of the Bhutan Himalayas. Evidence for out-of-equilibrium morphologies have for long been noticed in Bhutan, from major (> 1 km high) river knickpoints and from the existence of high-altitude low-relief regions within the mountain hinterland. These peculiar morphologies were generally interpreted as representing a recent change in climatic and/or tectonic conditions. To further characterize these morphologies and their dynamics, and from there discuss their origin and meaning, we perform field observations and a detailed quantitative morphometric analysis using Chi plots and Gilbert metrics of drainages over various spatial scales, from major Himalayan rivers to local streams draining the low-relief regions. We first find that the river network is highly dynamic and unstable. Our results emphasize that the morphology of Bhutan does not result from a general wave of incision propagating upstream, as expected from most previous interpretations. Also, the specific spatial organization in which all major knickpoints and low-relief regions are located along a longitudinal band in the Bhutan hinterland, whatever their spatial scale and the dimensions of the associated drainage basins, calls for a common local supporting mechanism most probably related to active tectonic uplift. From there, we discuss previous interpretations of the observed landscape in Bhutan. Our results emphasize the need for a precise documentation of landscape dynamics and disequilibrium over various spatial scales as a first-order step in morpho-tectonic studies of active landscapes.