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

这个项目的目的是为了更好地理解构成固体地球外壳的运动板块的行为。这些板块可能由位于深海下面的致密海洋岩石和/或构成大陆块的浮力更强的大陆岩石组成。几十年来，人们已经认识到，海洋板块很容易沿着俯冲带下沉（俯冲）到地球上，这在深处产生了平面地震带。最近，人们认识到大陆板块也可以俯冲到地球深处，但人们对它们是如何俯冲的以及是什么驱动了这一过程却知之甚少。活跃大陆俯冲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、青藏高原和帕米尔山脉向西持续发展。在帕米尔山脉以东，有越来越多的证据表明印度正在欧亚大陆下向北俯冲，但只有在海洋俯冲带常见的深度才会发生罕见的地震。地球上大陆内唯一类似俯冲的地震活动性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，地震活动性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个关于欧亚板块为何向南俯冲的假设。第一种是印度板块插入欧亚板块，就像楔子劈开一块木头，迫使欧亚板块的下部俯冲。第二种假设是，几个过程中的一个可能突然增加了欧亚板块和它下面更深的地球之间的密度对比，导致它迅速俯冲，最初的速度可能比印度向北运动的速度还要快。这些假设对塔吉克斯坦高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的历史、确定岩石的年龄、它们从深处被带到地表的时间和速度、以及限制岩石的来源来检验这些预测。与之前提出的帕米尔或印度-欧亚碰撞区以东的假设相比，这两种假设都是新颖的。因此，验证这两种说法都将为未被充分认识的大陆俯冲行为提供文件。除了项目的科学目标外，该提案还包含多方面的和与社会相关的教育和推广内容。其中包括对STEM学科的研究生和本科生进行培训和指导，这将有助于该领域（地球科学）的劳动力发展，该领域正在扩大，以应对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两国人民和机构之间的科学交流。中亚最贫穷的国家，其稳定严重依赖外国投资。该项目通过在多个场所向当地社区介绍文化、地理和地球科学，促进美国人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图森当地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，并在亚利桑那大学弗兰德罗科学中心举办展览，以及为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）举办展览。本项目研究帕米尔新生代片麻岩穹窿的变质、构造和岩浆演化，以测试它们与中新世到帕米尔隆起的新近发展、印度向北逆冲和亚洲岩石圈向南俯冲的潜在联系。帕米尔造山带的特点是一个明显的北凸凸起和一个空间上广泛的、与造山带平行的片麻岩穹窿套件。突出圆顶和片麻岩圆顶被认为是同步发展的，主要是在渐新世至中新世。帕米尔的厚地壳（大于65公里）在南部被高速地幔所覆盖，这被解释为北冲的印度岩石圈，在北部被一个中深度地震活动的南倾带所覆盖，这被归因于亚洲岩石圈的大陆内俯冲。该项目正在测试两个端元“构造驱动因素”，这两个端元“构造驱动因素”可能在遗传上将所有这些特征联系起来：(1)亚洲岩石圈向南俯冲的板块向北快速回滚/后退的短暂阶段，在此期间，帕米尔片麻岩丘被显著的南北水平延伸（约140公里）和帕米尔凸起的生长挖掘出来。(2)印度岩石圈向北逆冲/楔入期的延长，迫使其上的亚洲中地壳垂直掘出，其下的亚洲岩石圈向南俯冲。这两种端元假说并不是相互排斥的，然而，它们做出了对比鲜明的造山带规模预测，可以通过地质调查来检验。我们正在测试对片麻岩圆顶的运动、变质和岩浆演化的预测。我们的方法将地质填图和构造分析相结合，以约束片麻岩-穹顶发掘的运动学；变质岩石学、U/Th-Pb年代学+微量元素分析量化进、退变质史；中低温热年代学量化挖掘历史；锆石（铪）和钛矿（钕）的U-Pb年代学和同位素分析，以约束新生代岩浆活动的历史和来源。

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.