Collaborative Research: Did the Pamir Gneiss Domes and Salient form by Northward Underthrusting of India or Southward Subduction and Rollback of Asia?

合作研究：帕米尔片麻岩穹丘和突出部是印度向北俯冲形成还是亚洲向南俯冲和回滚形成的？

• 批准号: 1419751
• 负责人: Bradley Hacker
• 金额: $ 17.38万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-09-01 至 2017-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1419751&HistoricalAwards=false
• 关键词: Collaborative; Research; Did; Pamir; Gneiss

The aim of this project is to better understand how the moving plates that compose the outer rigid shell of the solid Earth behave. These plates may be composed of dense oceanic rock that underlies the deep oceans and/or more buoyant continental rock that makes up landmasses. For decades, it has been recognized that oceanic plates readily sink (subduct) into the Earth along subduction zones, which produce planar zones of earthquakes at depth. More recently, it has been appreciated that continental plates can also subduct deep into the Earth, but how they do so and what drives this process are poorly understood. The best example of active continental subduction is where the Indian plate is moving northward and colliding with the Eurasian plate, resulting in continued development of the Himalaya Mountains, Tibetan Plateau, and Pamir Mountains to the west. East of the Pamir Mountains, there is growing evidence that India is subducting northward beneath Eurasia, but there are only rare earthquakes at depths commonly observed in oceanic subduction zones. The only subduction-like zone of seismicity within a continent on Earth is located beneath the Pamir Mountains, which is the focus of this study. Unlike the case to the east, however, the seismicity is suggestive of southward subduction of the Eurasian plate. In this study, the principal investigators will test two hypotheses for why the Eurasian plate is subducting southward. The first is that the Indian plate has inserted itself into the Eurasian plate, like a wedge splitting a piece of wood, forcing the lower part of the Eurasian plate to subduct. The second hypothesis is that one of several processes could have abruptly increased the density contrast between the the Eurasian plate and deeper Earth beneath it, causing it to subduct rapidly, perhaps initially at rates greater than the rate of the northward motion of India. These hypotheses make contrasting predictions for the geological history of the high Pamir in Tajikistan, which we will test by quantifying the history of faulting, determining the age of the rocks and when and how fast they were brought from depth to the surface, and constraining where the rocks came from. Both of the hypotheses are novel compared to those that have been raised previously for the Pamir or India-Eurasia collision zone to the east. Validating either would thus provide documentation of an underappreciated behavior of continental subduction. In addition to the scientific goals of the project, the proposal contains educational and outreach components that are multifaceted and of societal relevance. They include the training and mentoring of graduate and undergraduate students in a STEM discipline, which will contribute to workforce development in a field (geosciences) that is expanding to address important national needs and challenges. The project is also facilitating scientific exchange between people and institutions in the United States and Tajikistan?the most impoverished country in central Asia whose stability depends heavily on foreign investment. The project is promoting cross-cultural understanding between American and Tajiks and geographic and scientific awareness by giving presentations in multiple venues that encompass culture, geography, and Earth science to local communities. The project will also support outreach activities aimed at advancing scientific understanding and sustainability for Americans for the local Tucson community, including lectures and field trips into the mountains for secondary school students and teachers and constructing an exhibit to be displayed at the University of Arizona's Flandrau Science Center, and for the annual Tucson Gem and Mineral Show--the largest show of its kind in the world.This project is investigating the metamorphic, structural, and magmatic evolution of Cenozoic gneiss domes in the Pamir to test their potential linkages with Miocene to Recent development of the Pamir salient, northward underthrusting of India, and southward subduction of Asian lithosphere. The Pamir orogen is distinguished by a pronounced, northward-convex salient and a spatially extensive, orogen-parallel suite of gneiss domes. Both the salient and gneiss domes are thought to have developed synchronously, largely since Oligo-Miocene time. The thick crust (greater than 65 kilometers) of the Pamir is underlain in the south by a high-velocity mantle interpreted to be northward underthrust Indian lithosphere, and in the north by a southward-dipping zone of intermediate-depth seismicity that has been attributed to intracontinental subduction of Asian lithosphere. This project is testing two end-member 'tectonic drivers' that may genetically link all of these features: (1) A short-lived phase of rapid northward rollback/retreat of a southward-subducting slab of Asian lithosphere, during which the Pamir gneiss domes were exhumed by significant North-South horizontal extension (approximately 140 kilometers) and growth of the Pamir salient. (2) A protracted phase of northward underthrusting/wedging of Indian lithosphere that forced vertical exhumation of Asian mid-crust above it and southward subduction of Asian lithosphere beneath it. These two end-member hypotheses are not mutually exclusive, nevertheless, they make contrasting orogen-scale predictions that can be tested with geologic investigations. We are testing the predictions for the kinematic, metamorphic, and magmatic evolution of the gneiss domes. Our approach integrates geologic mapping and structural analysis to constrain the kinematics of gneiss-dome exhumation; metamorphic petrology, U/Th-Pb geochronology + trace element analyses to quantify the history of prograde and retrograde metamorphism; moderate- and low-temperature thermochronology to quantify the history of exhumation; and U-Pb geochronology and isotope analysis of zircon (hafnium) and titanite (neodymium) to constrain the history and sources of Cenozoic magmatism.

该项目的目的是更好地了解构成固体地球刚性外壳的移动板块的行为方式。这些板块可能由深海下方的致密海洋岩石和/或构成陆地的浮力较大的大陆岩石组成。几十年来，人们已经认识到海洋板块很容易沿着俯冲带沉入（俯冲）到地球中，从而在深处产生平面地震带。最近，人们认识到大陆板块也可以俯冲到地球深处，但人们对它们如何俯冲以及驱动这一过程的因素知之甚少。大陆俯冲活动的最好例子是印度板块向北移动并与欧亚板块碰撞，导致喜马拉雅山脉、青藏高原和帕米尔高原继续向西发展。在帕米尔山脉以东，越来越多的证据表明印度正在向北俯冲到欧亚大陆之下，但只有在大洋俯冲带常见的深度才会发生罕见的地震。地球上大陆上唯一的类似俯冲的地震活动带位于帕米尔山脉下方，这是本研究的重点。然而，与东部的情况不同，地震活动表明欧亚板块向南俯冲。在这项研究中，主要研究人员将检验两种假设，以解释欧亚板块向南俯冲的原因。第一个是印度板块插入欧亚板块，就像楔子劈开木头一样，迫使欧亚板块下部俯冲。第二个假设是，几个过程中的一个可能突然增加了欧亚板块与其下方更深的地球之间的密度对比，导致其快速俯冲，最初的俯冲速度可能大于印度向北运动的速度。这些假设对塔吉克斯坦帕米尔高原的地质历史做出了对比预测，我们将通过量化断层历史、确定岩石的年龄以及它们从深处带到地表的时间和速度，并限制岩石的来源来测试这些预测。与之前针对帕米尔高原或印度-欧亚大陆东部碰撞带提出的假设相比，这两个假设都是新颖的。因此，验证其中任何一个都将提供大陆俯冲未被充分认识的行为的记录。除了该项目的科学目标外，该提案还包含多方面且具有社会相关性的教育和推广内容。其中包括对研究生和本科生进行 STEM 学科的培训和指导，这将有助于该领域（地球科学）的劳动力发展，该领域正在不断扩展，以满足重要的国家需求和挑战。该项目还促进了美国和塔吉克斯坦之间的人员和机构之间的科学交流。塔吉克斯坦是中亚最贫困的国家，其稳定在很大程度上依赖于外国投资。该项目通过在多个场所向当地社区提供涵盖文化、地理和地球科学的演讲，促进美国和塔吉克人之间的跨文化理解以及地理和科学意识。该项目还将支持旨在促进美国人对当地图森社区的科学理解和可持续发展的外展活动，包括为中学生和教师举办讲座和到山区进行实地考察，并建造一个在亚利桑那大学弗兰德劳科学中心展出的展览，以及一年一度的图森宝石和矿物展——世界上同类展览中规模最大的展览。该项目正在调查变质、结构和矿物的变化。 帕米尔新生代片麻岩穹丘的岩浆演化，以测试其与中新世的潜在联系，以及帕米尔高原的近期发展、印度向北俯冲和亚洲岩石圈向南俯冲。帕米尔造山带的特点是有明显的北凸凸起和空间广泛、与造山带平行的片麻岩穹顶。人们认为，突出穹顶和片麻岩穹顶是同步发育的，主要是从渐新世时期开始的。帕米尔高原的厚地壳（超过 65 公里）的南部是高速地幔，解释为向北逆冲的印度岩石圈，北部是向南倾斜的中深度地震活动区，归因于亚洲岩石圈的陆内俯冲。该项目正在测试两个端元“构造驱动因素”，它们可能在基因上将所有这些特征联系起来：（1）亚洲岩石圈向南俯冲板片快速向北回滚/后退的短暂阶段，在此期间，帕米尔片麻岩圆顶因显着的南北水平延伸（约140公里）和帕米尔凸起的生长而被挖掘出来。 （2）印度岩石圈向北的下冲/楔入的长期阶段，迫使其上方的亚洲中地壳垂直折返，以及其下方的亚洲岩石圈向南俯冲。这两个端元假设并不相互排斥，然而，它们做出了对比的造山带规模预测，可以通过地质调查进行检验。我们正在测试对片麻岩穹丘的运动学、变质作用和岩浆演化的预测。我们的方法整合了地质测绘和结构分析来约束片麻岩穹顶折返的运动学；变质岩石学、U/Th-Pb年代学+微量元素分析，以量化顺行和逆行变质作用的历史；中低温热年代学以量化挖掘历史； U-Pb年代学和锆石（铪）和钛矿（钕）的同位素分析，以确定新生代岩浆作用的历史和来源。