Collaborative Research: Helium-isotope Systematics Along Seismic Profiles in Tibet to Study Geometry of Indian and Tibetan Lithosphere

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Himalaya-Tibet is Earth's type example of active continent-continent collision, between India and Asia, yet the plate-scale geometry remains controversial. Researchers from Stanford University and the University of New Mexico will sample and analyze fluids from hot-springs, which are tracers of deeper structure, in the Himalaya and Tibet to locate the northern limit of the Indian lithosphere (and southern limit of the Asian lithosphere), hence the northern limit to which India has been forced (or underthrust) beneath Asia. This effort will improve scientists' ability to draw accurate cross-sections through the Himalaya and Tibet to greater depths than previously possible. This new understanding of Earth?s most dramatic mountain belt and plateau enhances public interest in science, and offers a natural opportunity to integrate earth science into high-school curricula. To this end, a new video about the research project and associated curricular materials designed to meet the Next Generation science Standards will be produced.The lithospheric geometry of continental collision remains controversial. There is no agreement on a single orogenic cross-section in which Indian crust and mantle penetrate a certain distance northwards beneath Tibet. Instead, growing seismological opinion supports models with large changes from west-to-east, from crustal underthrusting to lithospheric delamination (subduction roll-back) to de-blobbing. Stanford has helped acquire and interpret data from multiple seismic transects across Himalaya/Tibet. The interpretations of deep reflectors and converters from these and other published profiles provide testable models for the geometry of Indian and Tibetan lithospheres as it varies from west to east across Tibet. Pilot measurements of 3He/4He ratios in hot springs show significant, and spatially variable, mantle degassing across the Lhasa block in Chinese Tibet (Xizang and Qinghai). The research team will sample geothermal and carbonic fluids and associated travertines at up to 60 new locations in Tibet during two field seasons along several transects chosen for their accessibility, availability of seismic data, and tectonic/geologic setting. The team will analyze 3He/4He, 22Ne, 36Ar, delta13C, delta18O, deltaD, 87Sr/86Sr, major and trace elements to constrain fluid sources (mantle, crustal, atmospheric, sedimentary, organic), to understand fluid mixing and pathways, and to calculate reservoir temperatures using multi-component chemical geothermometry. Seismic data, both broadband teleseismic and near-vertical reflection data, will be re-analyzed to better understand the source regions and transport pathways of the sampled fluids. The data will constrain the boundary between Indian cratonic mantle lithosphere and Tibetan incipiently melting mantle and help define the mantle suture beneath Tibet.