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外层刚性壳的移动板块的行为。这些板块可能是由深海下面的致密海洋岩石和/或构成大陆块的更有浮力的大陆岩石组成的。几十年来，人们已经认识到，海洋板块容易下沉（潜没）到地球沿着俯冲带，这产生了平面地震带的深度。最近，人们认识到大陆板块也可以深深地俯冲到地球中，但他们如何做到这一点以及是什么推动了这一过程，人们对此知之甚少。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继续发展。在帕米尔山脉以东，有越来越多的证据表明印度正在向北俯冲到欧亚大陆之下，但在大洋俯冲带通常观察到的深度只有罕见的地震。地球上大陆内唯一的类似俯冲的地震活动带位于帕米尔山脉之下，这是本研究的重点。然而，与东部的情况不同，地震活动暗示着欧亚板块向南俯冲。在这项研究中，主要研究人员将测试两个假设，为什么欧亚板块向南俯冲。第一个是印度板块已经插入欧亚板块，就像一个楔子劈开一块木头，迫使欧亚板块的下部俯冲。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和它下面的地球深部之间的密度对比，导致它迅速俯冲，最初的速度可能比印度向北运动的速度更快。这些假说对塔吉克斯坦帕米尔高原的地质历史做出了对比性的预测，我们将通过量化断层的历史，确定岩石的年龄以及它们何时以及以多快的速度从深处带到地表，并限制岩石来自何处来进行测试。这两种假说都是新的相比，那些已经提出了帕米尔或印度-欧亚大陆碰撞带以东。因此，验证任何一个都将为大陆俯冲的未被充分认识的行为提供证据。除了该项目的科学目标外，该提案还包含多方面的、具有社会意义的教育和外联内容。其中包括培训和指导STEM学科的研究生和本科生，这将有助于在一个正在扩大以应对重要的国家需求和挑战的领域（地球科学）的劳动力发展。该项目还促进了美国和塔吉克斯坦的人员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。该项目通过在多个地点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性的外联活动，包括为中学生和教师举办讲座和山区实地考察，并在亚利桑那大学弗兰德劳科学中心举办展览，以及每年一度的图森宝石和矿物展--世界上同类展览中最大的一次。这个项目正在调查变质，构造，帕米尔新生代片麻岩穹隆的构造演化和岩浆演化，以检验它们与帕米尔凸起的中新世至近代发展、印度向北的俯冲和亚洲岩石圈向南的俯冲之间的潜在联系。帕米尔造山带的特点是明显的，北凸凸起和空间广泛，造山带平行套片麻岩圆顶。突出和片麻岩圆顶被认为是同步发展，主要是因为渐新世-中新世时间。帕米尔的厚地壳（大于65公里）的下方是在南部的高速地幔解释为向北俯冲印度岩石圈，并在北部的中间深度地震活动，已被归因于亚洲岩石圈的陆内俯冲南倾带。该项目正在测试两个端元“构造驱动因素”，它们可能在遗传上联系所有这些特征：（1）亚洲岩石圈向南俯冲板块快速向北回滚/后退的短暂阶段，在此期间，帕米尔片麻岩圆顶因显著的南北水平延伸（约140公里）和帕米尔凸起的增长而被挖出。(2)印度岩石圈向北俯冲/楔入的一个长期阶段，迫使其上方的亚洲中地壳垂直折返，而其下方的亚洲岩石圈向南俯冲。这两个端元假说并不相互排斥，但它们作出了可通过地质调查检验的造山带规模预测。我们正在测试对片麻岩穹丘的运动学、变质和岩浆演化的预测。我们的方法结合地质填图和构造分析来约束片麻岩穹隆折返的运动学，结合变质岩石学、U/Th-Pb地质年代学+微量元素分析来量化逆冲和退变质作用的历史，结合中低温热年代学来量化折返的历史，结合岩石学和岩石学的方法来量化折返的历史。锆石（铪）和钛铁矿（钕）的U-Pb年代学和同位素分析，以确定新生代岩浆活动的历史和来源。