Deciphering contrasts in geomorphic evolution across neighbouring mountain catchments in the High Himalaya of Bhutan using radiocarbon and cosmogenic radionuclide dating

Straddling the Himalayan Arc, Bhutan exhibits an impressive topographic gradient from 170 to 7’600 m a.s.l. Despite these high elevations a defining characteristic of this landscape is the absence of glacial overprint in large portions of the country’s surface. This setting provides a unique opportunity to observe landscape-forming processes such as fluvial erosion and deposition over temporal spans of several glacial cycles.

In this study, we examine two major drainage basins in north-west Bhutan, characterised by three primary geomorphic domains: 1) detachment-limited regimes, characterised by broad alluvial plains followed upstream by 2) transport-limited fluvial systems flanked by high-relief, steep hillslopes which finally transition into 3) low-relief landscapes at high altitudes, which exhibit clear glacial overprinting below the summits of the High Himalayas. Based on the systematic mapping of morphological markers and the collection of soil and rock outcrop records in a series of field campaigns, we have compiled a three-dimensional inventory of sedimentary deposits. To constrain the observed relative age relationships quantitatively, we collected samples for ¹⁴C and cosmogenic radionuclide (CRN) dating in key geomorphic locations. These physical observations were then integrated with longitudinal river profile analyses, providing a conceptual model for the geomorphic evolution of the region.

Our findings highlight significant morphological divergence between the two drainage basins. The Wang Chhu Valley in the west is characterised by broad alluvial plains with negligible fluvial bedrock incision and terrace risers only a few metres high. In contrast, the Punatsangchhu Basin in the east shows narrower valleys with prominent terrace sequences between 10 and 50 metres high. Interestingly, despite being in a similar position in relation to the mountain front, the interior valleys of the eastern basin are approximately 1'000 metres lower than those in the west.

In addition to the valley morphology, also the sedimentological characteristics vary by basin. The western deposits consist of fluvial sequences interbedded with chaotic, sub-angular, blocky facies, which are typical of mass-wasting events such as debris flows. The eastern basin contains similar gravity-flow deposits, as well as lacustrine sediments and massive, fine-grained units containing suspended clasts. These are interpreted as signatures of a previously identified glacial lake outburst flood (GLOF).

By integrating spatial sedimentary data with radiocarbon and CRN ages, we propose a model of landscape evolution defined by prolonged erosional quiescence and sediment aggradation. These stable periods are punctuated with episodic pulses of high sediment supply from hillslopes or high-magnitude catastrophic floods, creating the characteristic present-day morphology of the Bhutanese Himalaya.