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

Abstract. Subduction zones represent the only major pathway by which continental material can be returned to the Earth's mantle. Constraining the sediments mass flux through subduction zones is important to the understanding of both petrogenesis of continental crust, and the recycling of volatiles and continental material back into the mantle over long periods of geologic time. When sediments are considered, convergent margins appear to fall into one of 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 where the sedimentary cover is < 1 km. However, as data help define geometry of the global subduction system, the consequences of the two styles of margins on subduction dynamics remain poorly resolved.

In this study, we run systematic 2-D numerical simulations of subduction to investigate how sediment fluxes influence subduction dynamics and 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). Our results show three modes of subduction interface: a) Tectonic erosion margin (high viscosity sediment layer), b) Low angle accretionary wedge margin (low viscosity, thin sediment layer), and c) High angle accretionary wedge margin (low viscosity, thick sediment layer). We find that the properties of the sediment layer modulate the extent of viscous coupling at the interface between the subducting and overriding plates. When the viscous coupling is increased, an erosive style margin will be favored over an accretionary style. On the other hand, when the viscous coupling is reduced, sediments are scrapped-off the subducting slab to form an accretionary wedge. Diagnostic parameters are extracted automatically from numerical simulations to analyze the dynamics and differentiate between these modes of subduction margin. Models of tectonic erosion margins show small radii of curvature, slow convergence rates and thin subduction interfaces, while results of accretionary margins show large radii of curvature, faster convergence rates and dynamic accretionary wedges. These diagnostics parameters are then linked with observations of present-day subduction zones.