COLLABORATIVE RESEARCH: Testing proposed rapid true polar wander in the Neoproterozoic Zavkhan Volcanics of Mongolia and the Banxi Group of South China

合作研究：在蒙古新元古代扎夫汗火山和华南板西群中测试提出的快速真实极地漂移

• 批准号: 1547434
• 负责人: Nicholas Swanson-Hysell
• 金额: $ 28.63万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-06-01 至 2021-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1547434&HistoricalAwards=false
• 关键词: COLLABORATIVE; RESEARCH; Testing; proposed; rapid

Through geological time, Earth's crustal plates have moved through two distinct mechanisms. The first is the motion of Earth's plates relative to one another due to convection of the underlying mantle and the force balance on the plate - we call this "plate tectonics". The second mechanism is motion of all of the plates together due to shifting mass anomalies within the Earth and associated rotational imbalance, which can cause reorientation relative to Earth's spin axis. This reorientation motion is referred to as true polar wander, and the rate at it proceeds is dependent on the viscosity of Earth's interior, which is poorly constrained both today and through Earth History. This research targets a particular interval of time around 800 million years ago when it has been proposed that true polar wander was much faster than usual -fast enough that parts of Earth's surface may have moved the distance of the length of football field in around 100 years. The PIs seek to track ancient plate movement in Mongolia and China to test this hypothesis using rocks of this age. They will do so using the magnetic directions preserved in the ancient rocks combined with U/Pb radiogenic isotope dates to precisely determine their age. These data will be used constrain both the rate that plates were moving and the relative movement of different plates in order to determine if the motion is consistent with rapid true polar wander. The results will inform Earth Scientists about the drivers of apparent plate motion at this time and the past position of the continents. The data also have the potential to place constraints on the viscosity of Earth's interior through time.It has been proposed that large-scale, oscillatory true polar wander (TPW) occurred at ca. 800 Ma, associated with a carbon isotope interval known as the Bitter Springs Stage. Recent advances have led to a temporally quantified framework for the Bitter Springs Stage that enables this TPW hypothesis to be tested in non-carbonate-bearing lithologies, such as successions of extrusive volcanics. This project seeks to test the hypothesis through the development of new paleomagnetic and geochronological data from Mongolia and South China. The goal of the research is to robustly quantify Earth's rotational stability through this critical period of Earth history and allow for rates of TPW to be constrained. The Bitter Springs TPW hypothesis has become a central component of models for how continents aggregate into supercontinents, for fundamental inferences into the viscosity of Earth's mantle and the stabilizing effect of the remanent bulge and excess ellipticity, and for the planetary context of Neoproterozoic environmental change. Furthermore, the hypothesis of rapid TPW at ca. 800 Ma has become a significant component of models for the configuration of the supercontinent Rodinia. This project seeks to constrain the rate of pole movement during this interval through the development of new geochronological and paleomagnetic data from the Zavkhan Volcanics of Mongolia and the Banxi Group of South China in robust stratigraphic context. Importantly, these successions contain: (1) abundant volcanic rocks containing zircon from which high-precision U-Pb chemical abrasion-ion dilution-thermal mass spectrometry (CA-ID-TIMS) dates can be obtained and (2) paleomagnetic confidence tests indicating preservation of primary magnetization. Three years of field work, paleomagnetic analysis, and the development of CA-ID-TIMS U-Pb dates will focus on producing high quality pole paths for the Zavkhan Terrane of Mongolia and the South China craton from ca. 820 to 750 Ma. Such paths can be used to further test whether rapid oscillatory TPW occurred through this interval and, if so, at what rates. Testing and constraining this hypothesis is necessary for progress on the co-evolution of the solid earth and the surface through the time period and determining whether there were time periods in the Precambrian where it can be robustly shown that rates of TPW were higher than those resolved in the Phanerozoic. The broader impacts of the proposed research are four-fold: (1) the training and development of a UC Berkeley graduate student; (2) engagement of undergraduate students in basic research; (3) cultural exchange with Mongolian students in the field and in both of the PIs labs and (4) effective communication of a set of Earth systems history learning goals to K-12 communities through the construction of an educational display module in conjunction with the Harvard Museum of Natural History (HMNH). This educational exhibit will be focused on the geological record of Asia and will include: (1) large rock samples that can be touched; (2) photographs from field expeditions; (3) educational displays about geochronology and paleomagnetism; and (4) a video explaining the geophysical techniques used in this study. Following display in the Earth Science exhibit galleries at HMNH, the exhibit module will travel for display in the Earth and Planetary Science Department at UC Berkeley and then to the Mongolia University for Science and Technology in Ulaanbaatar. This work includes extensive international collaboration, and is co-funded by the Office of International Science and Engineering.

在整个地质时代，地球的地壳板块通过两种不同的机制移动。第一种是由于地幔的对流和板块上的力平衡而导致的地球板块之间的相对运动——我们称之为“板块构造”。第二种机制是由于地球内部移动的质量异常和相关的旋转不平衡，所有板块一起运动，这可能导致相对于地球自转轴的重新定向。这种重新定向的运动被称为真正的极移，其进行的速度取决于地球内部的粘度，这在今天和地球历史上都很难受到限制。这项研究的目标是大约8亿年前的一段特定时间，当时有人提出，真正的极地漂移比平时快得多——快到地球表面的部分地区可能在大约100年内移动了一个足球场的距离。pi试图追踪蒙古和中国的古代板块运动，用这个时代的岩石来验证这一假设。他们将利用保存在古代岩石中的磁方向，结合U/Pb放射性同位素日期来精确确定它们的年龄。这些数据将用于约束板块运动的速率和不同板块的相对运动，以确定运动是否与快速的真极移相一致。这些结果将使地球科学家了解这个时期明显板块运动的驱动因素和大陆过去的位置。这些数据也有可能限制地球内部随时间的粘度。有人提出，大尺度振荡性真极漂移（TPW）发生在约800 Ma，与一个被称为苦泉期的碳同位素间隔有关。最近的进展已经为苦泉阶段提供了一个暂时的量化框架，使TPW假说能够在非含碳酸盐岩性中进行测试，例如挤入式火山序列。本项目旨在通过开发来自蒙古和华南的新的古地磁和地质年代学数据来检验这一假设。该研究的目标是在地球历史的这一关键时期对地球的旋转稳定性进行可靠的量化，并允许TPW的速率受到限制。苦泉TPW假说已经成为大陆如何聚集成超级大陆的模型的核心组成部分，对地幔粘度和残余隆起和过度椭圆的稳定效应的基本推断，以及新元古代环境变化的行星背景。此外，约800 Ma的快速TPW假说已成为罗丁尼亚超大陆构造模型的重要组成部分。本项目旨在通过发展蒙古扎夫汗火山和华南板溪群在坚固的地层背景下的新的地质年代学和古地磁数据，来限制这段时间内的磁极运动速率。重要的是，这些序列包含：(1)丰富的含锆石的火山岩，从中可以获得高精度的U-Pb化学磨损离子稀释-热质谱（CA-ID-TIMS）日期；(2)古地磁置信度测试表明原始磁化的保存。通过三年的野外考察、古地磁分析和ca - id - tims U-Pb测年，将重点为蒙古Zavkhan地体和华南克拉通约820 ~ 750 Ma的极径构造提供高质量的极径。这样的路径可以用来进一步测试快速振荡TPW是否在这段时间内发生，如果发生，以什么速率发生。检验和限制这一假设对于研究固体地球和地表在整个时期的共同演化，以及确定在前寒武纪是否存在可以强有力地表明TPW速率高于显生宙分辨率的时期是必要的。拟议研究的更广泛影响有四个方面：(1)加州大学伯克利分校研究生的培训和发展；(2)本科生参与基础研究；(3)与蒙古学生在实地和pi实验室进行文化交流；(4)通过与哈佛自然历史博物馆（HMNH）合作建设教育展示模块，向K-12社区有效传播一套地球系统历史学习目标。这个教育展览将集中于亚洲的地质记录，包括：(1)可以触摸的大型岩石样本；（二）野外考察照片；(3)地质年代学、古地磁教育展示；(4)一段解释本研究中使用的地球物理技术的视频。在HMNH的地球科学展览馆展出后，该展览模块将前往加州大学伯克利分校的地球和行星科学系展出，然后前往乌兰巴托的蒙古科技大学。这项工作包括广泛的国际合作，并由国际科学与工程办公室共同资助。