NSFGEO-NERC: Collaborative Research: Coupling Erosion, Weathering, and Hydrologic Function in an Active Orogenic System

The collision of continents driven by the movement of the Earth's tectonic plates raises the Earth's greatest mountain ranges. This has fundamental impacts on the habitability of large areas of the continents as well as controlling their geological evolution and moderating global climate. The prime modern example of a continental collision is the northward movement of India colliding with Asia which has raised the Himalayas and Tibet over the last 50 million years. This region sources freshwater for more than one-fifth of the global population and the floodplains of the rivers draining the region provide their food. The Himalayas and Tibet have a major impact on the climate of east and south-east Asia dominated by the summer monsoonal rains. The mountain range is the source of the major natural hazards in the region including earthquakes, landslides and floods. Continental collisions also have a major impact on the geological evolution the continental crust deforming, burying, heating and melting the crust and the associated uplift and erosion provides large amounts of sediment. The continental collisions are also thought to play an important part in moderating global climate on the greater than million-year timescales as the chemical reactions between carbon-dioxide dissolved in rainwater and the eroded sediment enhance the flux of dissolved carbon-dioxide and calcium to the oceans to be deposited as limestone removing carbon-dioxide from the atmosphere and limiting global temperatures.

However the evolution of collisional mountain ranges involves complex interactions between tectonics, erosion and climate. The collision thickens the continents which elevates their surface topography, enhances erosion by landslides and the steeper river profiles. The topographic elevation also alters the local climate, for example by increasing rainfall which enhances erosion by increasing landslide frequency and the ability of rivers to transport sediments. The evolving topography of mountain belts is thought to represent a balance between the thickening of the continental crust by the collision balanced by the tendency of the thickened crust to spread under its own weight and the erosion of the crust forced by the topography and local climatic impacts.

This project will study these complex interactions between the tectonic processes, the erosive processes dominated by landslides and the impact of climate by studying the Melamchi Khola river catchment in Nepal. The catchment spans the dramatic gradient in topography from the foothills to the high mountains, a gradient which is reflected by a marked increase in erosion rates towards the high mountains. The detailed study in one location will enable us to understand the interactions between the controlling processes. For example how does the exhumation rate control landslide formation and the erosion rate. How does the local climate, particularly rainfall, impact this relationship? How does removal of rock by erosion impact the controlling tectonics and propensity for earthquakes and landslides?
The work will involve:
1) Determining how the physical and chemical properties of rock change by fracturing and chemical alteration as groundwater flows through the fractures as they are brought to the surface and the relationship of these changes to both the local tectonics and to climate.
2) Quantify how the topography is shaped by the competing processes of tectonically-driven uplift and erosion by landslides. How are these major processes impacted by the evolution of the rock mass and the groundwater flow through the subsurface.
3) Determine the flow paths of water as rain falls on the mountains and then enters fractured rock before feeding rivers below.
4) Quantify how the chemical alteration of the rock depends on the local climate (temperature and rainfall), groundwater flow paths and erosion rate.