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

在整个地质时期，地球的地壳板块经历了两种截然不同的机制。第一种是由于下伏地幔的对流和板块上的力平衡，地球各板块之间的相对运动--我们称之为“板块构造”。第二种机制是所有板块一起运动，这是由于地球内部质量异常的移动和相关的旋转不平衡，这可能导致相对于地球自转轴的重新定向。这种重新定位的运动被称为真正的极地漂移，其进行的速度取决于地球内部的粘性，这在今天和整个地球历史上都是很差的约束。这项研究针对的是大约8亿年前的一段特定时间，当时有人提出，真正的极地漫游比平时快得多--速度足够快，以至于地球表面的某些部分可能在大约100年的时间里移动了足球场那么长的距离。PI试图追踪蒙古和中国的古代板块运动，以使用这个时代的岩石来验证这一假说。他们将利用保存在古老岩石中的磁方向结合U/Pb放射性同位素测年来精确确定它们的年龄。这些数据将被用来约束板块移动的速度和不同板块的相对运动，以确定运动是否与快速的真极移一致。这一结果将向地球科学家提供有关此时板块运动的驱动因素和大陆过去位置的信息。这些数据还有可能限制地球内部随时间推移的粘性。有人提出，大规模的、振荡的真极移(TPW)发生在大约800 Ma，与被称为苦泉阶段的碳同位素间隔有关。最近的进展导致了苦泉阶段的时间量化框架，使这一TPW假说能够在非含碳酸盐岩性，如喷发火山岩序列中得到验证。这个项目试图通过开发蒙古和中国南部的新的古地磁和年代学数据来检验这一假说。这项研究的目标是在地球历史的这一关键时期有力地量化地球的自转稳定性，并允许限制TPW的速率。苦泉TPW假说已成为大陆如何聚集成超大陆的模型的核心组成部分，用于基本推断地球地幔的粘滞性和剩余隆起和过量椭圆度的稳定效应，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Rodinia超大陆构型模型的重要组成部分。该项目试图通过在强有力的地层背景下开发蒙古扎夫汗火山和南中国板溪群的新的地质年代学和古地磁数据来限制这段时间内的极点移动速度。重要的是，这些序列包括：(1)丰富的含有锆石的火山岩，从中可以获得高精度的化学磨损-离子稀释-热质谱(CA-ID-TIMS)年龄和(2)表明原始磁化保持的古地磁可信度测试。经过三年的野外工作、古地磁分析和CA-ID-TIMS U-Pb测年的发展，将集中于为蒙古扎夫汗地体和南中国克拉通提供大约820-750 Ma的高质量磁极路径。这样的路径可以用来进一步测试快速振荡的TPW是否在该间隔期间发生，如果是，以什么速率发生。检验和限制这一假说对于在整个时间段中固体地球和表面的共同演化取得进展以及确定在前寒武纪是否存在能够有力地表明TPW速率高于显生宙中分解的速率的时间段是必要的。拟议研究的更广泛的影响有四个方面：(1)培训和发展加州大学伯克利分校的研究生；(2)让本科生参与基础研究；(3)在实地和两个PIS实验室与蒙古学生进行文化交流；(4)通过与哈佛大学自然历史博物馆(HMNH)一起建造一个教育展示模块，向K-12社区有效地传达一套地球系统历史学习目标。这一教育展览将侧重于亚洲的地质记录，并将包括：(1)可触摸的大岩石样本；(2)实地考察的照片；(3)关于地质年代学和古地磁学的教育展览；(4)解释本研究所使用的地球物理技术的录像带。在HMNH的地球科学展览馆展出后，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，并由国际科学和工程办公室共同资助。

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.