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

大陆板块的下部比下面的软流层密度大。如果它由于板块汇聚而变厚，比如在印度-亚洲碰撞区，它可能会变得重力不稳定，并以大薄片或小水滴的形式沉入更深的地幔。该项目将验证西藏中部4500万年至2000万年历史的椭圆型盆地形成于下沉的岩石圈滴上，而幔源火山岩同时沿着盆地边界的地形凸起喷发。如果得到证实，岩石圈滴流理论将提供一个吸引人的机制来解释在地球上所有主要造山带高原上发现的大量神秘的椭圆盆地。与亚利桑那大学军事与退伍军人事务助理院长合作，该项目将邀请三名本科退伍军人参与地质领域和实验室研究。越来越多的退伍军人从现役中返回大学，而在科学-技术-工程-数学领域的劳动力中，这一比例不足。将通过提供入职和离职调查，评估学生-退伍军人参与项目的有效性，以及影响退伍军人职业道路的因素。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继续进行学术、科学和文化交流。除了岩石圈渗流理论外，该项目还将验证以下假设：西藏中部新生代缩短、火山作用和盆地形成是(i)区域性（600 km长）冲断系统向南扩展和岩石圈增厚随后岩石圈根部移除的表现；（ii）长寿命（始新世至中新世早期活跃）~200-250 km长的冲断带和与之相关的西藏中部弯曲载荷的表现。不相关的地幔作用产生了局部的火山高地。这些假设对缩短的时空分布、缩短的盆地填充物相对于邻近火山岩的年龄、盆地几何形状及其边缘性质、沉积物源和扩散模式做出了不同的预测。将结合地质填图、盆地分析、构造分析和恢复、U-Pb火成岩和碎屑锆石年代学来验证这些预测。实地考察将包括来自北京青藏高原研究所的首席研究员、研究生、本科生退伍军人和合作者，并将重点研究西藏中部三个盆地内部和之间的新生代地质。空间覆盖范围将允许评估(i)缩短构造是否局限于盆地或沿着走向区域延伸，（ii）盆地以前是长而窄的东西向或更多的区域性盆地，或在其历史上作为孤立的沉积中心发展，以及（iii）盆地的最大沉降和缩短与相邻火山岩的喷发同时发生。最终，就西藏是世界上典型的碰撞高原而言，项目结果将与世界上其他造山高原相关。该奖项反映了美国国家科学基金会的法定使命，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.