Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas
<p>Large boulders, ca. 10 m in diameter or more, commonly linger in Himalayan river channels. In many cases, their lithology is consistent with source areas located more than 10 km upstream, suggesting long transport distances. The mechanisms and timing of “exotic” boulde...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-09-01
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| Series: | Earth Surface Dynamics |
| Online Access: | https://esurf.copernicus.org/articles/8/769/2020/esurf-8-769-2020.pdf |
| Summary: | <p>Large boulders, ca. 10 m in diameter or more, commonly
linger in Himalayan river channels. In many cases, their lithology is
consistent with source areas located more than 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 relevant 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 our 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-flood discharge assuming turbulent, Newtonian fluid flow
conditions, and boulder exposure ages were determined using cosmogenic
nuclide exposure dating. Modeled discharges required for boulder transport
of ca. 10<span class="inline-formula"><sup>3</sup></span> to 10<span class="inline-formula"><sup>5</sup></span> m<span class="inline-formula"><sup>3</sup></span> s<span class="inline-formula"><sup>−1</sup></span> exceed typical monsoonal floods in
these river reaches. Exposure ages range between ca. 1.5 and 13.5 ka with a clustering of ages around 4.5 and 5.5 ka 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 exceptional outburst
events in the central Himalayan range could be modulated by climate and
occur in the wake of transitions to drier 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 and infrequent events and bedrock incision
patterns in Himalayan rivers with broader implications for landscape
evolution.</p> |
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| ISSN: | 2196-6311 2196-632X |