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

喜马拉雅-西藏是地球上大陆-大陆碰撞活跃的典型例子，位于印度和亚洲之间，然而板块尺度的几何形状仍然存在争议。斯坦福大学（Stanford University）和新墨西哥大学（University of New Mexico）的研究人员将对喜马拉雅山和西藏的温泉流体进行取样和分析，这些温泉是更深层结构的示踪剂，以确定印度岩石圈的北边界（以及亚洲岩石圈的南边界），从而确定印度被迫（或下冲）在亚洲之下的北边界。这一努力将提高科学家绘制喜马拉雅和西藏精确剖面的能力，使其达到比以前更大的深度。对地球的新认识？美国最引人注目的山带和高原提高了公众对科学的兴趣，并为将地球科学纳入高中课程提供了一个自然的机会。为此，将制作一个关于该研究项目的新视频和为满足下一代科学标准而设计的相关课程材料。大陆碰撞的岩石圈几何形状仍有争议。关于印度地壳和地幔在西藏下方向北穿透一定距离的单一造山带截面，没有达成一致意见。相反，越来越多的地震学观点支持从西到东的大变化模型，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/西藏多个地震样带的数据。从这些和其他已发表的剖面中对深层反射和转换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，因为它在西藏从西到东变化。在中国西藏（西藏和青海）拉萨地块，温泉中3He/4He比值的初步测量表明，地幔脱气具有显著的空间变化特征。研究小组将在两个野外季节在西藏的60个新地点取样地热和碳流体以及相关的石灰华，沿着几个断面选择它们的可达性、地震数据的可用性和构造/地质背景。该团队将分析3He/4He、22Ne、36Ar、delta13C、delta18O、deltaD、87Sr/86Sr、主要元素和微量元素，以约束流体来源（地幔、地壳、大气、沉积、有机），了解流体混合和路径，并使用多组分化学地温计计算储层温度。地震数据，包括宽带远震和近垂直反射数据，将被重新分析，以更好地了解采样流体的来源区域和输送路径。这些数据将限制印度克拉通地幔岩石圈与西藏初融地幔之间的边界，并有助于确定西藏地下的地幔缝合线。

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.