Despite the considerable amount of research conducted in collisional mountain belts over the past few decades, little consensus has been reached regarding the influence that climatic and tectonic processes have on the surface, or how these processes are intertwined. Recent studies in the central Himalaya - a well studied, archetypal collisional mountain belt - have produced conflicting ideas regarding what is the most significant control on surface processes. Farther to the east, the Bhutan Himalaya have not been studied in much detail, but are often labeled as an outlier when compared to the classic Himalayan form and geology. However, the apparently anomalous features of Bhutan make it an excellent environment to test some of the outstanding hypotheses regarding the interplay of climate, tectonics and surface processes. Atypical high-elevation, but low-relief landscapes are perched above and surrounded by deeply incised canyons in the middle latitudes of Bhutan. This study explores the proposition that these landforms represent a pulse of erosion that is sweeping through Bhutan and progressively changing the relief. Such a pulse of erosion signals a change in either tectonics or climate, or both and the characteristics of transient landscapes, including especially patterns of erosion rate, provide a framework to determine what the drivers for the change are. This research project focuses on determining basin-averaged millennial-scale erosion rates from the concentration of cosmogenic radionuclides within river sands. The sampling strategy will cover a wide range of mean annual precipitation, river channel steepness and basin relief. By combining these erosion rates with constraints from low-temperature thermochronology data, the research team will be able to discriminate between climate and tectonic-driven models in Bhutan and test hypotheses regarding the relationships between climate, erosion rate, topography and tectonics that carry implications well beyond the Bhutan field site.
The active nature of the problem addressed in this research has ramifications for many societal concerns including: seismic hazards, landslide hazards, sedimentation, reservoir management, navigability, soil erosion, and response to climate change. These issues are at times painfully clear to those who live in or near active mountain ranges throughout the world. Bhutan, because of its complicated political history and access issues, is one of the most poorly understood regions of the Himalaya. The research team is working closely with the National Environmental Commission of Bhutan to share findings in a meaningful way that better informs decisions in land use management and hazard mitigation. In addition, this research tackles fundamental problems in the understanding of mountain building. Many of the classic models of mountain building in collisional settings make predictions about the relative timing of fault activity; however, these assumptions have been shown to fail in some areas. The broad sample collection scheme will allow examination of uplift patterns across Bhutan to determine if these classic models hold true. Very little is known about the history of fault activity in Bhutan and therefore, any information regarding the pattern of uplift is invaluable to the governing bodies of Bhutan, who determine hazard assessment and control building codes.
The Himalaya are the archetypal example of a continental collision belt, formed by the ongoing convergence between India and Eurasia. Boasting some of the highest and most rugged topography on Earth, there is currently no consensus on how climatic and tectonic processes have combined to shape its topographic evolution – it is has been proposed for instance that climate-driven erosion sufficiently influences the force balance that it can induce an increase in rock uplift and a concentration of seismic hazards but firm, demonstrative data have proven elusive. The Kingdom of Bhutan in the eastern Himalaya provides a unique opportunity to study the interconnections among climate, topography, erosion, and tectonics that inspired this study. The eastern Himalaya are remarkably different from the rest of the range, most strikingly due to the presence of the Shillong Plateau to the south. The faults associated with the Shillong Plateau have accommodated convergence between India and Eurasia, and created a natural experiment to test the possible response of the Himalaya to a reduction in local fault activity. In addition, owing to its position and orientation, the Shillong plateau intercepts moisture otherwise bound for the Himalaya. This has created a natural experiment to test the possible response of the range to a reduction in rainfall. Probably in response to the formation of the Shillong Plateau, the topography of the Bhutan Himalaya is distinct, characterized by low relief landscapes of gently rolling hills perched at high elevations (ca. 3000m) and surrounded by extremely rugged terrain. We focused our attention on the formation of these distinctive landforms with the aim of determining the combination of tectonic and climatic changes that their formation signals and records. The range of plausible formation mechanisms and both the timing and magnitude of proposed effects of tectonic and climatic changes within the Bhutan Himalaya have required us to employ a range techniques to unravel and illuminate the possible influences of climatic and tectonic changes. The project was designed as complementary to an earlier, related study directed by Dr. Kip Hodges using a different set of tools (thermochronology as opposed to the cosmogenic isotope emphasis of this study). The PhD student on funded on this project (Byron Adams) worked on both projects to ensure full and optimal integration of results. In conjunction we have used many techniques within this study ranging from: analysis of modern topography, modeling of erosion rates on million-year timescales using thermochronometric data; measurement of millennial-scale erosion rates using cosmogenic radionuclides; and testing the plausibility of hypotheses using landscape evolution models. This combination of techniques yielded robust constraints the evolution of the Bhutan Himalaya at a reasonably high temporal and spatial resolution. From this study we have drawn three primary conclusions. 1) We have constrained the cooling histories of seventeen bedrock samples using three different thermochronometers (e.g. we know how long ago each rock cooled to ~70, 190 and 400°C) from the metamorphic core of the Bhutan Himalaya. Using a thermal model to convert bedrock cooling histories into depth histories, we have demonstrated a decrease in erosion rate from ~11-4 million years ago. We have attributed this change to a reduction in fault slip rates across the Himalayan mountain belt, due to increased accommodation of plate convergence across the Shillong Plateau. 2) Analyses of our field observations and a new suite of basin-averaged, millennial-scale erosion rates elucidate high rock uplift rates in the interior of the range – farther back from frontal faults than previously predicted. We have used a landscape evolution model to show that a similar pattern in rock uplift would produce low-relief landscapes like those observed in Bhutan. We hypothesize that this pattern in rock uplift rate could be produced by an active, blind, hinterland duplex. It follows that these landscapes were formed during surface uplift, and not transported from lower elevations. Using our extensive erosion rate dataset we show that these landscapes began forming ~1.5 Ma and have been uplifted ~800 m since that time. Thus we can conclusively demonstrate clear tectonic drivers that explain much of the unusual character of the Bhutan Himalaya. 3) Analysis of our ~70 new cosmogenic radionuclide measurements also shows that millennial-scale erosion rates are coupled with modern rainfall rates. Relationships between topographic metrics and erosion rates show a fundamental difference between the drier interior of Bhutan and the wetter foothills. These data allow that a climate change may indeed have contributed to the formation and uplift of the enigmatic low-relief, high-elevation surfaces in Bhutan. A final set of analyses and model runs currently being completed as part of the preparation of an additional research paper will determine quantitatively how important this climatic contribution has been.
