Preprints
https://doi.org/10.5194/esurf-2020-105
https://doi.org/10.5194/esurf-2020-105

  18 Dec 2020

18 Dec 2020

Review status: a revised version of this preprint is currently under review for the journal ESurf.

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

Martine Simoes1, Timothée Sassolas-Serrayet2, Rodolphe Cattin2, Romain Le Roux-Mallouf2,a, Matthieu Ferry2, and Dowchu Drukpa3 Martine Simoes et al.
  • 1Université de Paris, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France
  • 2Géosciences Montpellier, Université de Montpellier, CNRS, Université des Antilles, Montpellier, France
  • 3Department of Geology and Mines, Thimphu, Bhutan
  • anow at: Géolithe, 38920, Crolles, France

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.

Martine Simoes et al.

 
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Status: final response (author comments only)
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Martine Simoes et al.

Martine Simoes et al.

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Short summary
Elevated low-relief regions and major river knickpoints have for long been noticed and questioned in the emblematic Bhutan Himalayas. We document the morphology of this region using morphometric analyses and field observations, at a variety of spatial scales. Our findings reveal a highly unstable river network, with numerous non-coeval river captures, most probably related to a dynamic response to local tectonic uplift in the mountain hinterland.