The Neoproterozoic: a transition in the plate tectonic regime?

随着时间的推移，板块构造在地球表面的演变中起着重要作用，因为它不断地改变着大陆和海洋的位置。它始于前寒武纪，在达到今天所见的连续状态之前，经历了一个断断续续的状态。向连续板块构造体制的转变被认为发生在新元古代，并且可能与生物圈、冰冻圈和大气中发生的极端变化有关。古地磁数据在760 Ma之前和615 ~ 565 Ma期间变化大且快，这可能与地壳和地幔相对于自旋轴的相干运动——真极漂移有关。这种机制是由于地幔块体非均质性分布的变化，可能是由于向板块构造过渡。由于新元古代之后的TPW速度要慢得多，古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，这妨碍了充分了解TPW是如何演变的，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：(1)大范围的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极移的板块的纵向运动造成的；（ii）地幔流动不稳定，与间歇性板块构造有关。在这项研究中，我们希望通过多工具的方法来验证这两种情况，该方法结合了新的古地磁数据的获取，760 - 550 Ma的板块模型的阐述，以及利用地幔动力学模拟的TPW模型。在这个项目中，(1)我将从挪威北部的一个完整的Cryogenian （720-635 Ma）沉积岩剖面中获得新的古地磁数据，该剖面代表了Rodinia超大陆的核心Laurentia-Baltica。我还将对该部分确定的富有机质层进行Re-Os地质年代学，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至615 Ma的TPW。然后，(4)我将对760-550 Ma期间进行完整的板块重建，以确定俯冲带和假定的上升流随时间的位置，这将用于(5)模拟地幔流动和TPW。通过对俯冲历史和地幔温度等参数的探索，我将把模拟的TPW与观测的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，这项研究也将为地球的热演化带来强有力的约束。

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.