Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding in the central Himalayas

Marius L Huber,Sean F Gallen,Ananta P Gajurel,Marcus Christl,Maarten Lupker

doi:10.5194/esurf-8-769-2020

Abstract

Abstract. 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. 103 to 105 m3 s−1 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.

Highlights

Active tectonics, steep topography, dynamic surface processes and extensive glacier cover expose the Himalayas to a range of catastrophic events that remain relatively rare on observational timescales and are poorly understood
We suggest that the emplacement of large grain sizes in the two studied Himalayan valleys is related to large glacial lake outburst floods (GLOFs) events that followed a time of regional glacial retreat after the Early Holocene Climatic Optimum (EHCO) and at the close of the last glacial maximum
We provide field observations of large boulders ca. 10 m in diameter or more that show lithologic evidence for travel distances of over tens of kilometers in two central Himalayan valleys, the Sunkoshi and Trishuli

Summary

Introduction

Steep topography, dynamic surface processes and extensive glacier cover expose the Himalayas to a range of catastrophic events that remain relatively rare on observational timescales and are poorly understood. Amongst the most striking manifestations of catastrophic events are high magnitude earthquakes (e.g., Avouac, 2003) and resulting widespread landsliding or valley fills (e.g., Schwanghart et al, 2016a; Roback et al, 2018) and lake outburst floods (LOFs), whereby large volumes of impounded water is suddenly released into the fluvial network (Ives et al, 2010; Ruiz-Villanueva et al, 2017). LOF events in the Himalayas received widespread attention as the generated discharges may exceed typical precipitation-induced floods by orders of magnitude (e.g., Costa and Schuster, 1988; Cenderelli, 2000; O’Connor and Beebee, 2009; Korup and Tweed, 2007; Wohl, 2013; Cook et al, 2018). Huber et al.: Timing of exotic, far-traveled boulder emplacement and paleo-outburst flooding sent both a significant hazard (Kattelmann, 2003; Schwanghart et al, 2016b) and an active geomorphic agent of landscape evolution (Wohl, 2013; Cook et al, 2018; Turzewski et al, 2019)

Methods

Results

Discussion

Conclusion