The Neoproterozoic: a transition in the plate tectonic regime?

板块构造在地球表面的演化中发挥着重要作用，因为它随着时间的推移不断地改变大陆和海洋的位置。它始于前寒武纪，在达到今天所见的连续状态之前，经历了断断续续的状态。向连续板块构造体制的转变被认为发生在新元古代，可能与生物圈、冰冻圈和大气中发生的极端变化有关。古地磁数据在760 Ma以前和615~565 Ma之间表现出大而快的变化，这可能与真极移(TPW)有关，即壳幔相对于自旋轴的相干运动。这一机制是地幔质量不均质性分布变化的结果，可能是向板块构造过渡的结果。由于TPW在新元古代以后要慢得多，古地磁信号可能主要记录了板块运动。然而，由于缺乏760-615 Ma的高质量数据，没有对TPW的估计，这阻碍了对TPW如何演化的充分了解，从而无法回答新元古代是否发生了板块构造的转变。目前的认识允许两种可能的情况：(I)大尺度地幔流动和板块构造保持稳定，在780-760 Ma和615-590 Ma之间，TPW方向的明显变化是由于记录到极移的板块的纵向运动；(Ii)地幔流动是不稳定的，与间歇性的板块构造有关。在这项研究中，我们想用一种多工具方法来测试这两种情况，该方法结合了新的古地磁数据的获取、760-550 Ma的板块模型的精化以及使用地幔动力学模拟的TPW的模拟。在这个项目中，(1)我将带来新的古地磁数据，来自挪威北部的一整段冷生代(720-635 Ma)沉积岩，它代表了罗迪尼亚超大陆的核心劳伦蒂亚-波罗的海。我还将对剖面中确定的富含有机物质的水平进行Re-Os年代学研究，以更好地约束岩石的年龄。(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760-615 Ma的TPW。然后，(4)我将进行760-550 Ma期间的全板块重建，以确定俯冲带的位置和假定的随时间推移的上升流，这将被用来(5)模拟地幔流动和TPW。通过探索俯冲历史和地幔温度等参数，我将把模拟的TPW与观测到的TPW进行调整，以检验新元古代是否发生了板块构造的转变。因此，这项研究也将对地球的热演化带来强烈的制约。

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.