Collaborative Research: Orogeny, orography, and unsteady erosion: evolution of the Himalaya

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This research attempts to understand how Himalayan rates of erosion vary as a function of space and time. Some initial data suggest that erosion rates become much steadier at longer time scales. This research hypothesizes that spatial variations in rainfall distributions modulate differences in erosion rates and that specific stream power (the product of discharge and channel gradient) provides a reliable proxy for modern erosion rates. This project will use very high resolution remote-sensing data on rainfall along with digital topography to predict variations in stream power. The effect on erosion rates will be evaluated through cosmogenic nuclide (CRN)analyses of samples from catchments with contrasting stream power. Cooling ages of various minerals from transects in the catchments will be analyzed using thermokinematic models to create reliable reconstructions of temporal changes in erosion rates over longer time scales. The Himalayas are in an active tectonic region, and the proposal presents the hypothesis that topography advects laterally in response to the ongoing collision of tectonic plates. This advection is hypothesized to cause major re-organization of Himalayan drainages and related topography. As advection leads to stream capture and creation of new Transhimalayan rivers with greatly enhanced erosive power, other trunk channels will be beheaded, thereby losing power. Such changes in stream power should be expressed by changes in both erosion rates and topographic relief. The project will test these ideas by reconstructing changes in topographic relief using bedrock cooling ages both from their relief transects and from equal-elevation transects. This project focuses on how mountain belts grow and decay and how climatically modulated erosion may affect where earthquakes occur within them. Recent theory suggests that, in actively deforming mountain belts, spatially varying patterns of rainfall cause contrasts in erosion rates and that these differences in erosion influence where deformation occurs. The Himalayas, with their strong monsoonal rains, large-scale topography, and rapid rates of tectonic deformation, are an ideal setting in which to explore these proposed interactions. Satellite data ground-based observation reveal that the amount of rainfall changes abruptly both along the length and breadth of the Himalaya range. By combining the rainfall data with the topography of the Himalaya, this project's numerical models will predict where erosion should be rapid or slow. Much of the current spatial variation in rainfall along the Himalaya appears to result from the role of large river valleys that slice across the Himalaya: they provide topographic conduits that guide precipitation into the mountains and, thereby, appear to influence where erosion is more or less intense. This proposal contends that these valleys are not permanently locked in place, but they can shift and change drainage patterns. The field and laboratory investigation will document changes in erosion rates through time and changes in the topography of the Himalaya. Over 300 million people live downstream from the Himalaya and utilize the waters that flow from it. This work will provide insight on rainfall, snow, and water balance in remote (and currently poorly known) areas, and it will help facilitate better forecasting of river behavior, a significant benefit for the people of India and Nepal.