The Neoproterozoic: a transition in the plate tectonic regime?

• Applicant: Dr. Boris Robert
• Subject Area: Geophysics ; Geology
• Term: from 2022 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 499054525

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.

• DFG Programme: Research Grants
• International Connection: Norway, USA
• Cooperation Partners: Dr. Alan Rooney; Professor Harald Walderhaug