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Preprints
https://doi.org/10.5194/esurf-2020-17
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esurf-2020-17
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Mar 2020

Submitted as: research article | 20 Mar 2020

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This preprint is currently under review for the journal ESurf.

Timing of exotic, far-travelled boulder emplacement and paleo-outburst flooding in the central Himalaya

Marius L. Huber1,a, Maarten Lupker1, Sean F. Gallen2, Marcus Christl3, and Ananta P. Gajurel4 Marius L. Huber et al.
  • 1Geological Institute, Department of Earth Sciences, ETH Zurich, Zurich 8092, Switzerland
  • 2Department of Geosciences, Colorado State University, Fort Collins, Colorado 80523, USA
  • 3Laboratory of Ion Beam Physics (LIP), Department of Physics, ETH Zurich, Zurich 8093, Switzerland
  • 4Department of Geology, Tribhuvan University, Ghantaghar, Kathmandu, Nepal
  • anow at: Centre de Recherches Pétrographiques et Géochimiques (CRPG), CNRS, Université de Lorraine, UMR7358, 54500 Vandoeuvre-lès-Nancy, France

Abstract. Large boulders, ca. 10 m in diameter or more, commonly linger Himalayan river channels. In many cases, their lithology is only compatible with source areas located > 10 km upstream suggesting long transport distances. The mechanisms and timing of exotic boulder emplacement are poorly constrained, but their presence hints at processes that are significant for landscape evolution and geohazard assessments in mountainous regions. We surveyed river reaches of the Trishuli and Sunkoshi, two trans-Himalayan rivers in central Nepal to improve understanding of the processes responsible for exotic boulder transport and the timing of emplacement. Boulder size and channel hydraulic geometry were used to constrain paleo-discharges and boulder emplacement ages were determined using cosmogenic nuclide exposure dating. Modelled discharges required for boulder transport, of ca. 103 to 105 m3/s, exceed typical monsoonal floods in these river reaches. Exposure ages range between ca. 1.5 and 13.5 kyrs BP with clustering of ages around 4.5–5 kyrs BP in both studied valleys. This later period is coeval with a broader weakening of the Indian summer monsoon and glacial retreat after the Early Holocene Climatic Optimum (EHCO), suggesting Glacial Lake Outburst Floods (GLOFs) as a possible cause for boulder transport. We, therefore, propose that these exceptional events are climate-driven, but counter-intuitively occur in the wake of transitions to drier and warmer climates leading to glacier retreat rather than during wetter periods. Furthermore, the old ages and prolonged preservation of these large boulders in or near the active channels shows that these infrequent events have long-lasting consequences on valley bottoms and channel morphology. Overall this study sheds light on the possible coupling between large-infrequent events and bedrock incision patterns in Himalayan rivers with broader implications on landscape evolution.

Marius L. Huber et al.

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Marius L. Huber et al.

Marius L. Huber et al.

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Short summary
Large boulders found in two Himalayan valleys show signs of long fluvial transport (> 10 km). Paleo-discharges required to mobilise these boulders, exceed typical monsoon discharges. Exposure dating shows that a cluster of these boulders was emplaced ca. 5 kyrs ago. This period is coeval with a weakening of the Indian monsoon and glacier retreat in the area. We, therefore, suggest that glacier lake outburst floods are likely mechanisms that can explain these exceptional transport processes.
Large boulders found in two Himalayan valleys show signs of long fluvial transport ( 10 km)....
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