Lithospheric Dripping in Central Tibet: Underappreciated Factor in Orogenic Plateau Development?

大陆板块的下部比下面的软流圈更致密。如果它由于板块会聚而变厚，例如在印度-亚洲碰撞带内，它可能会变得重力不稳定，并以大片或较小尺寸的水滴沉入更深的地幔。该项目将检验一个假设，即西藏中部4500 - 2000万年前的椭圆形盆地形成于下沉的岩石圈滴状物之上，而幔源火山岩同时沿着盆地边界的地形隆起喷发。如果得到证实，岩石圈滴水理论将提供一个有吸引力的机制来解释在地球所有主要的造山高原中发现的众多神秘的椭圆形盆地。与亚利桑那大学军事和退伍军人事务助理院长合作，该项目将聘请三名本科生退伍军人从事地质领域和实验室研究。退伍军人从现役返回大学的人数越来越多，在科学技术工程数学劳动力中的代表性不足。学生退伍军人参与计划的有效性将进行评估，沿着哪些因素影响退伍军人的职业道路，通过提供入口和出口调查。该项目还将培养两名研究生，并促进美国、中国和西藏人民和机构之间持续的学术、科学和文化交流。除了岩石圈滴水理论外，该项目还将测试新生代缩短、火山作用、和盆地的形成是（i）区域性的向南传播，（600公里长）逆冲系统和岩石圈增厚，随后岩石圈根去除或（ii）长寿命（始新世至中新世早期活动）~200-250公里长的逆冲带和穿越西藏中部的相关挠曲载荷，与产生局部火山高地的不相关地幔过程。这些假说对缩短的时空分布、缩短的盆地充填相对于邻近火山岩年龄的年龄、盆地几何形状及其边缘的性质、沉积物源和扩散模式作出了明确的预测。将采用地质填图、盆地分析、构造分析和解释以及U-Pb火成岩和碎屑锆石年代学相结合的方法来检验这些预测。实地研究将包括主要研究人员，研究生，本科生退伍军人和来自北京青藏高原研究所的合作者，并将重点关注西藏中部三个盆地内和盆地之间的新生代地质。空间覆盖范围将允许评估（i）缩短结构是否局限于盆地或区域性沿着走向延伸，（ii）盆地先前整合为狭长的东西向或更多区域盆地，或在其历史上作为孤立的沉积中心发展，以及（iii）最大盆地沉降和缩短与邻近火山岩的喷发是同时代的。最终，西藏是世界上典型的碰撞高原，项目成果将与世界其他造山高原相关。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The lower part of a continental plate is more dense than underlying asthenosphere. If it gets thickened as a result of plate convergence, such as within the India-Asia collision zone, it may become gravitationally unstable and sink into the deeper mantle as a large sheet or smaller-sized drips. This project will test the hypothesis that 45-20 million year old elliptical-shaped basins in central Tibet formed above sinking lithospheric drips, while mantle-derived volcanic rocks simultaneously erupted along the basin-bounding topographic bulges. If validated, the lithospheric drip theory would provide an appealing mechanism to explain the multitude of enigmatic elliptical basins found in all of Earth’s major orogenic plateaus. In collaboration with the University of Arizona’s Assistant Dean of Military and Veteran Engagement, this project will engage three undergraduate-student veterans in geological field and laboratory research. Military veterans are returning from active duty to college in increasing numbers and are underrepresented in the Science-Technology-Engineering-Math workforce. The effectiveness of the student-veteran engagement program will be assessed, along with which factors influence veteran career pathways, by delivering entrance and exit surveys. The project will also train two graduate students and facilitate continued academic, scientific, and cultural exchange among people and institutions in the United States, China, and Tibet.In addition to the lithospheric drip theory, the project will test the alternative hypotheses that Cenozoic shortening, volcanism, and basin formation in central Tibet are manifestations of (i) southward propagation of regional (600 km long) thrust systems and lithospheric thickening followed by lithospheric root removal or (ii) long-lived (active from Eocene through early Miocene time) ~200-250-km-long thrust belts and associated flexural loading across central Tibet, with unrelated mantle processes generating localized volcanic highlands. The hypotheses make distinct predictions about the temporal-spatial distribution of shortening, age of shortened basin fill relative to the age of adjacent volcanic rocks, basin geometries and the nature of their margins, and sedimentary provenance and dispersal patterns. A combination of geological mapping, basin analysis, structural analysis and restorations, and U-Pb igneous and detrital zircon geochronology will be employed to test these predictions. Field research will include the principal investigators, graduate students, undergraduate student-veterans, and collaborators from the Institute of Tibetan Plateau Research in Beijing, and will focus on Cenozoic geology within and between three basins in central Tibet. The spatial coverage will permit assessment of whether (i) shortening structures are localized to the basins or extend regionally along strike, (ii) the basins were previously integrated as long and narrow E-W or more regional basins, or developed throughout their history as isolated depocenters, and (iii) maximum basin subsidence and shortening were coeval with eruption of adjacent volcanic rocks. Ultimately, insofar as Tibet is the world’s archetypical collisional plateau, project results will have relevance for other orogenic plateaus worldwide.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.