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

喜马拉雅山-西藏地区是印度和亚洲之间活跃的大陆与大陆碰撞的典型例子，但板块规模的几何形状仍然存在争议。斯坦福大学和新墨西哥大学的研究人员将对喜马拉雅山和西藏的温泉流体进行采样和分析，这些温泉是深层结构的示踪剂，以确定印度岩石圈的北限（以及亚洲岩石圈的南限），即印度被迫（或被掩埋）到亚洲之下的北限。这项工作将提高科学家们绘制喜马拉雅山和西藏的精确横截面的能力，其深度比以前更大。对地球上最引人注目的山脉和高原的这种新认识增强了公众对科学的兴趣，并提供了将地球科学纳入高中课程的自然机会。为此，将制作一个关于该研究项目和旨在满足下一代科学标准的相关课程材料的新视频。大陆碰撞的岩石圈几何仍然存在争议。对于印度地壳和地幔在西藏下方向北渗透一定距离的单一造山剖面，目前尚未达成一致。相反，越来越多的地震学观点支持从西到东发生巨大变化的模型，从地壳逆冲到岩石圈分层（俯冲回滚）再到去斑点。斯坦福大学帮助获取和解释了喜马拉雅山/西藏多个地震横断面的数据。这些和其他已发表的剖面图对深层反射体和转换器的解释为印度和西藏岩石圈的几何形状提供了可测试的模型，因为它在西藏自西向东变化。对温泉中 3He/4He 比率的初步测量表明，中国西藏（西藏和青海）拉萨地块存在显着且空间变化的地幔脱气现象。研究小组将在两个野外季节期间，沿着根据其可及性、地震数据可用性和构造/地质环境而选择的几条横断面，在西藏多达 60 个新地点对地热、含碳流体和相关石灰华进行采样。该团队将分析 3He/4He、22Ne、36Ar、delta13C、delta18O、deltaD、87Sr/86Sr、主量元素和微量元素，以限制流体源（地幔、地壳、大气、沉积物、有机物），了解流体混合和路径，并使用多组分化学地温测量法计算储层温度。 将重新分析地震数据，包括宽带远震数据和近垂直反射数据，以更好地了解采样流体的源区和传输路径。这些数据将限制印度克拉通地幔岩石圈和西藏早期融化地幔之间的边界，并有助于确定西藏下方的地幔缝合线。

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.