Subduction controls the distribution and fragmentation of Earth’s tectonic plates

International audienceThe theory of plate tectonics describes how the surface of the Earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates, whose areas follow a fractal distribution 2,3. The reconstruction of global tectonics during the past 200 My 4 suggests that this layout is probably a long-term feature of our planet, but the forces governing it are unknown. Previous studies 3,5,6 , primarily based on statistical properties of plate distributions, were unable to resolve how the size of plates is determined by lithosphere properties and/or underlying mantle convection. Here, we demonstrate that the plate layout of the Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. Using 3D spherical models of mantle convection with plate-like behaviour that match the plate size-frequency distribution observed for Earth, we show that subduction geometry drives the tectonic fragmentation that generates plates. The spacing between slabs controls the layout of large plates, and the stresses caused by the bending of trenches, break plates into smaller fragments. Our results explain why the fast evolution in small back-arc plates 7,8 reflects the dramatic changes in plate motions during times of major reorganizations. Our study opens the way to use convection simulations with plate-like behaviour to unravel how global tectonics and mantle convection are dynamically connected

The effect of low-viscosity sediments on the dynamics and accretionary style of subduction margins

Observations of sediments at subduction margins appear to divide them into two classes: Accretionary and erosive. Accretionary margins are dominated by accretion of thick piles of sediments (>1 km) from the subducting plate, while tectonic erosion is favored in regions with little or no sedimentary cover (<1 km). The consequences of the two styles of margins for subduction dynamics remain poorly resolved. In this study, we used 2-D numerical simulations of subduction to investigate how low-viscosity sediments influence subduction dynamics and margin type through plate coupling. We vary the thickness and viscosity of the sediment layer entering subduction, the thickness of the upper plate, and the driving velocity of the subducting plate (i.e., kinematic boundary conditions). Diagnostic parameters are extracted automatically from numerical simulations to analyze the dynamics and differentiate between modes of subduction margin. We identify three margin types based on the extent of viscous coupling in the sediment layer at the subduction interface: (a) tectonic coupling margin, (b) low-angle accretionary wedge margin, and (c) high-angle accretionary wedge margin. In the tectonic coupling case-A nalogous to an erosive margin-high-viscosity or thin-layer sediments increase coupling at the interface. On the other hand, when the viscous coupling is reduced, sediments are scrapped off the subducting slab to form an accretionary wedge. Models that develop tectonic coupling margins show small radii of curvature, slow convergence rates, and thin subduction interfaces, while models with accretionary margins show large radii of curvature, faster convergence rates, and dynamic accretionary wedges. These diagnostic parameters are then linked with observations of present-day subduction zones

Bifurcation of the Yellowstone plume driven by subduction-induced mantle flow

The causes of volcanism in the northwestern United States over the past 20 million years are strongly contested. Three drivers have been proposed: melting associated with plate subduction; tectonic extension and magmatism resulting from rollback of a subducting slab; or the Yellowstone mantle plume. Observations of the opposing age progression of two neighbouring volcanic chains - the Snake River Plain and High Lava Plains - are often used to argue against a plume origin for the volcanism. Plumes are likely to occur near subduction zones, yet the influence of subduction on the surface expression of mantle plumes is poorly understood. Here we use experiments with a laboratory model to show that the patterns of volcanism in the northwestern United States can be explained by a plume upwelling through mantle that circulates in the wedge beneath a subduction zone. We find that the buoyant plume may be stalled, deformed and partially torn apart by mantle flow induced by the subducting plate. Using plausible model parameters, bifurcation of the plume can reproduce the primary volcanic features observed in the northwestern United States, in particular the opposite progression of two volcanic chains. Our results support the presence of the Yellowstone plume in the northwestern United States, and also highlight the power of plume-subduction interactions to modify surface geology at convergent plate margins

Mantle convection

Although the Moon is much smaller than the Earth, dynamic processes took place in its interior and helped shape its surface. It is suggested that compositionally driven convection was dominant in the early evolution after the solidification of the lunar magma ocean -- often also termed as mantle overturn – and that thermally driven convection was mainly active after this overturn phase. Details of these processes are however controversially discussed, but during the last years, improvements in the numerical models and new rheological experiments have led to a better understanding and changed the view about the interior dynamics of the Moon. In this chapter we will discuss various scenarios that have been suggested in the literature, point out their problems and introduce the most likely scenario