Collaborative Research: Exhumation History of the Indian Lesser Himalaya: Discriminating Tectonic Models with Implications for the Neogene Isotopic Composition of Seawater

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The uplift of the Himalayan Mountains due to collision of the Indian and Eurasian plates is the textbook example of continental collision in plate tectonics. The effect that uplift of this orogenic belt has had on the Earth surface system processes and lithospheric dynamics has been dramatic and far-reaching. This project will use an integrative multidisciplinary approach to better understand timing and sequence of thrust fault deformation and exhumation of crustal rocks in the 'outer' Himalayan zone of the Lesser Himalaya of India. The research will greatly improve our understanding of compression-driven crustal deformation processes associated with mountain building. These results will provide a means to critically evaluate the potential relationship between monsoon intensity and thrust fault propagation, as well as document the importance of Himalayan uplift and exhumation and chemical weathering on ocean chemistry. The isotopic seawater record of the ratios of osmium (187Os/188Os) as recorded in sedimentary rocks has been used as a proxy for global chemical weathering, and the results of this study will provide data with profound influences on future global oceanographic and climate models. In addition to the scientific objectives of this study, potential benefits to society and national interests include providing support for an early career researcher at a major research institution; graduate and undergraduate student training in a science, technology, engineering, and mathematics (STEM) discipline; with international collaboration of a U.S. graduate student and post-doctoral researcher with Indian scientists; education and outreach activities aimed at U.S. and Indian K-12 equivalent students; and development of web-based informational contributions for the general public. The project will promote broadening of participation of underrepresented groups in science, and the results of the work will dissemination of research results through professional science presentations, the peer-reviewed scientific literature, and incorporation of research results in into publicly accessible community databases. The project involves a high-resolution geo/thermochronometric investigation of bedrock and foreland basin deposits of the Himalayan frontal system in north India to constrain exhumation timing for distinct zones in the Lesser Himalaya. This work will focus on a series of strike-perpendicular transects across major thrust sheets for (Uranium-Thorium)/Helium dating of zircon and apatite, coupled with detailed facies analysis and provenance studies via zircon Uranium-Lead-Helium (U-Pb-He) double dating of foreland basin deposits. Contrasting models have been proposed for the sequence of thrust propagation of Lesser Himalayan blocks, notably the time of exhumation and structural affinities of the 'outer' Lesser Himalayan zone. These models postulate structural emplacement ages for the 'outer' Lesser Himalayan zone that range from the late Eocene to less than about 14 million years ago--a potential 14 to 30 million-year discrepancy. However, recent studies of foreland basin provenance data suggested the 'outer' Lesser Himalayan zone was likely exhumed at approximately 16 Ma, which corresponds with a noted increase in monsoon intensity. The weathering of 'outer' Lesser Himalayan zone rocks enriched in radiogenic 187Os has been tied to a pronounced increase in Neogene seawater 187Os/188Os at 16 million years ago. Our comprehensive study will constrain the sequence of thrust propagation and exhumation timing of the 'outer' Lesser Himalayan zone, allowing us to: 1) differentiate between contrasting kinematic models for Himalayan thrust belt evolution, and 2) test the hypothesis that exhumation and weathering of distinct Himalayan tectonic zones was the principal driver of secular changes in Neogene seawater chemistry. Preliminary (U-Th)/He geochronologic data offers intriguing support for the hypotheses of a direct link between Himalayan weathering and seawater chemistry.This award is co-funded by NSF's International Science and Engineering Section.