The Himalayan Mountains in Nepal were formed over the last 55 million years by forces associated with the collision of the Indian and Asian continents. Since the onset of the collision, layered rocks draping the upper crust of India have been crumpled and shortened by at least 650 kilometers as the Asian landmass has overridden northern India. This process has produced the highest mountains on Earth. These mountains are important in many respects: they control the climate system of southern Asia, including the all-important Asian monsoon which provides rain in a region that would otherwise be much drier; they store water in the form of glaciers and snow, thus mitigating the effects of drought and providing hydroelectric power to much of southern Asia; they generate the sediments that form the agricultural breadbasket of the region, feeding approximately one quarter of the world?s population; and they embody a dramatic high mountain landscape upon which humans have been inspired to feats of adventure, exploration, and religious devotion. As the mountains have grown, continuous erosion has generated vast quantities of sediment that accumulates along the southern flank of the Himalaya in the Indo-Gangetic foreland basin and in deep sea fans that flank India on the floors of the Arabian Sea and the Bay of Bengal. This sediment provides a record of the history of growth of the Himalaya. Our project aims to reconstruct the history of deformation of the Himalaya in Nepal. The analysis is forensic, as we attempt to retrodeform the rocks that have been folded and thrust faulted in the Himalaya, according to constraints imposed by thermochronology, structural geology, geochronology, and the erosional sedimentary record. Thermochronology measures the time at which a mineral passes through its "blocking temperature" with respect to a particular radiogenic isotopic system en route to the topographic surface as the mountains are growing and erosion is exhuming the rocks. Thermochronological ages can be inverted to assess erosion rates and in some cases the timing of slip on major faults. We will also use Uranium-Lead geochronology to determine the ages of mineral grains in both the eroded sediments and the bedrock source areas of the mountain range. Together these data will allow use to (a) determine the timing of major thrust faulting events within the southern half of the Himalaya; (b) calculate the rate of exhumation of the mountains; and (c) determine precisely the specific sources of the sediments and when they were deposited. Our fieldwork will involve sample collection, and detailed documentation of sections of the sedimentary erosional products where they are uplifted and exposed in the frontal Himalaya. This work promises to elucidate the tectonic and erosional history of the Himalaya at an unprecedented level of detail, and has far-reaching implications for our understanding of changing seawater chemistry, the origin and timing of the South Asian monsoon, and the dynamic interplay between tectonics and climate in orographic systems. Within the context of ongoing research on Himalayan tectonics by numerous international groups, this work fills a gap in understanding of the critical last ~20 million years of deformation of the Himalaya, a timespan during which it is postulated that more than one half of the total Himalayan shortening took place, and during which many of the iconic features of the system developed, including the Main Central thrust, the South Tibetan detachment, the Greater Himalayan leucogranites, and the Lesser Himalayan duplex. Densely populated Nepal and northern India remain seismically active, with potential for a >M8 earthquake in a major seismic gap in the western part of Nepal. Because estimates of earthquake probability are in part based on understanding of the long-term history of shortening in the frontal Himalaya, a small change in the long-term shortening rate would have significant implications for predictions of co-seismic fault slip.
