Do changes in monsoon intensity reposition Himalayan erosion? A combined cyclostratigraphic-detrital thermochronology approach

Lehigh University, Bethlehem PA

Investigators

Abstract

Sediments eroded from active mountains contain a rich record of how those mountains grew and how the combination of processes within the solid Earth combine with the process of erosion to shape landscapes. An important question concerns the significance of the erosional part of this system, and how much the record is dominated by either solid-Earth processes, erosional changes, or both. In Nepal, rocks provide a record of 15 million years of Himalayan topographic change. During this time period the strong South Asian monsoon climate developed, a phenomenon that now focuses intense erosion on the south flanks of the Himalayan mountains in the summer months. There is strong evidence that the strength of this monsoon changes over timespans of 20,000 to 100,000 years. These changes can be used to precisely date sedimentary rocks formed from sediment shed off of growing mountain ranges. By dating minerals found in the sediment, specific locations of erosion can be determined as changes in the strength and location of the monsoon varied. This project will develop a new approach that combines different but complementary dating tools, an approach that will provide a means of reading the sedimentary rock record in far greater detail. This project will also help answer the question: What are the relative roles of climate and rock deformation in shaping mountain landscapes? The project will support training of a Ph.D. student and will also develop public-education videos about the topographic evolution of mountains and the tools geologists use to study sedimentary rock sequences. The project has two technical goals. The first goal involves integrating the methods of rock-magnetic cyclostratigraphy and detrital thermochronology to track changes in the provenance of the Siwalik Group in Nepal through time. Rock magnetics in sediments provide a way of detecting orbital cycles lasting tens of thousands of years, thereby constraining the relative ages of sediments in different locations with 1000-year resolution. Using this high-resolution dating as a framework, single white-mica grains contained in the Siwalik Group sediments will be dated using Ar-Ar analysis to determine when these grains cooled from high temperatures and thus infer where they came from in the sediment source area. The second project goal exploits a well-understood sediment source area in the active Himalayan region. White-mica dating, calibrated at the thousand-year scale by rock magnetic cyclostratigraphy, can be used to examine possible correlations between the intensity of the South Asian monsoon and foci of erosion within the Himalaya. Correlations at these short time scales should be free of any solid-Earth tectonic signal, thus facilitating the examination of climate variations alone, which are attributable to variations in the South Asian monsoon. This project will develop and evaluate combined cyclostratigraphy/detrital thermochronology as an approach to provenance analysis, and then use the results to assess how tectonics, climate, and erosion work together to shape landscapes. Results will also provide more general guidance for interpreting cyclical changes in other mineralogical and geochemical records from synorogenic sedimentary rocks. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.