The Neoproterozoic: a transition in the plate tectonic regime?

新元古代：板块构造体系的转变？

• 批准号: 499054525
• 负责人: Dr. Boris Robert
• 金额: --
• 依托单位: Department of Earth and Planetary Sciences
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2022
• 资助国家: 德国
• 起止时间: 2021-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/499054525?language=en
• 关键词: Neoproterozoic; transition; plate; tectonic; regime

Plate tectonics plays a major role in the evolution of Earth’s surface as it changes constantly the position of the continents and oceans through time. It started in the Precambrian and has evolved through an intermittent state before reaching a continuous regime as seen today. The transition toward a continuous plate tectonic regime has been proposed to take place during the Neoproterozoic, and could potentially be linked with extreme changes that occurred in the biosphere, the cryosphere and the atmosphere. The paleomagnetic data display large and fast variations before 760 Ma and from 615 to 565 Ma, which could be related to True Polar Wander (TPW), the coherent motion of the crust and mantle with respect to the spin axis. This mechanism results from changes in the distribution of mantle mass heterogeneities and could be due to the transition toward plate tectonics. As TPW is much slower after the Neoproterozoic, the paleomagnetic signal could have mostly recorded plate motions. Nevertheless, because of the lack of good quality data from 760 to 615 Ma, there is no estimate of TPW, which precludes to fully understand how TPW evolved and therefore to answer whether a transition in plate tectonics occurred during the Neoproterozoic. The present knowledge allows two possible scenarios: (i) the large-scale mantle flow and plate tectonics remained stable, and apparent changes in TPW direction between 780-760 Ma and 615-590 Ma were due to longitudinal motion of the plate where polar wander was recorded; (ii) mantle flow was unstable and associated with intermittent plate tectonics. In this study, we want to test these two scenarios with a multi-tool approach that integrates the acquisition of new paleomagnetic data, the elaboration of a plate model from 760 to 550 Ma, and the modelling of TPW using simulations of mantle dynamics. In this project, (1) I will bring new paleomagnetic data from a complete section of Cryogenian (720-635 Ma) sedimentary rocks from northern Norway that was representative of Laurentia-Baltica, the core of the supercontinent Rodinia. I will also perform Re-Os geochronology on organic-rich levels identified in the section to better constrain the age of the rocks. (2) I will perform laboratory experiments at LMU to identify the directions of magnetization and rock magnetic experiments, which (3) will allow identifying TPW from 760 to 615 Ma by integrating these data with those from the literature. Then, (4) I will perform a full plate reconstruction of the period 760-550 Ma to define the position of the subduction zones and of the presumed upwellings through time, which will be used to (5) model mantle flow and TPW. By exploring several parameters such as the subduction history and mantle temperature, I will adjust the modelled TPW to the observed one to test whether a transition in plate tectonics occurred during the Neoproterozoic. This study will therefore also bring strong constraints on the thermal evolution of the Earth.

板块构造在地球表面的演变中起着重要作用，因为它随着时间的推移不断改变大陆和海洋的位置。它开始于前寒武纪，在达到今天所看到的连续状态之前，已经经历了一个间歇性的状态。向连续板块构造体制的过渡被认为发生在新元古代，可能与生物圈、冰冻圈和大气层发生的极端变化有关。760 Ma前和615 ~ 565 Ma的古地磁资料显示出大而快的变化，这可能与地壳和地幔相对于自旋轴的相干运动--真极移（TPW）有关。这种机制的结果，在地幔质量分布的不均匀性的变化，可能是由于向板块构造的过渡。由于新元古代之后TPW变慢，古地磁信号可能主要记录了板块运动。然而，由于缺乏从760至615 Ma的高质量数据，没有TPW的估计，这排除了充分了解TPW如何演变，从而回答在新元古代板块构造是否发生了转变。目前的认识允许两种可能的情况：（i）大规模地幔流动和板块构造保持稳定，TPW方向在780-760 Ma和615-590 Ma之间的明显变化是由于板块的纵向运动，记录极漂移;（ii）地幔流动是不稳定的，与间歇性板块构造有关。在这项研究中，我们要测试这两种情况下，多工具的方法，集成了新的古地磁数据的采集，从760至550 Ma的板块模型的阐述，并使用模拟地幔动力学的TPW建模。在这个项目中，（1）我将带来来自挪威北方代表劳伦蒂亚-波罗的海（Rodinia超大陆的核心）的一个完整的Cryogenian（720-635 Ma）沉积岩剖面的新的古地磁数据。我还将对该部分中确定的有机物富集水平进行Re-Os地质年代学，以更好地限制岩石的年龄。(2)我将在LMU进行实验室实验，以确定磁化方向和岩石磁性实验，（3）通过将这些数据与文献中的数据相结合，可以确定760至615 Ma的TPW。然后，（4）我将进行760-550 Ma的全板块重建，以确定俯冲带的位置和推测的隆升时间，这将用于（5）模拟地幔流和TPW。通过探索几个参数，如俯冲历史和地幔温度，我将调整模拟TPW观察到的一个，以测试是否在新元古代板块构造的过渡发生。因此，这项研究也将对地球的热演化带来强有力的制约。