The Influence of Sediment Thickness on Subducting Plate Velocity.

Abstract:

Sediments subducted along convergent margins have been suggested to play an important role in subduction dynamics. Sediments are usually taken to be frictionally or viscously weak and are thought to lubricate the megathrust, thereby promoting fast plate speeds. Yet, global observations show a negative correlation between the amount of sediments and subducting plate velocity. Here, we use 2D visco-elasto-plastic, thermomechanical models to investigate the link between sediment thickness and subduction dynamics. We find that thick sedimentary layers in fact decrease the velocity of the subducting plate. However, it is difficult to understand whether this is due to the mechanical role of sediments as opposed to their effect on the thermal and hence density structure of the subducting lithosphere. Adding sediments on top of the incoming plate in our model setup indeed results in a warmer plate that is also more buoyant. As we seek to isolate the main mechanism decreasing subducting plate velocitiy with increasing sediment thickness, we additionally explore the role of thermal and density structure by varying the plate age. We analyze the integrated shear stress along the interface as a function of sediment thickness and plate age. Increasing the sediment thickness increases the shear stress on the interface mostly at shallow depths, independent of the plate age. Instead young, warm plates with thin to no sediments have high shear stresses related to the deeper portion of the interface. This difference in high shear stress location suggests that the decrease in subducting plate speed observed for increasing sediments is promoted by the development of a large accretionary wedge that increases the load at the shallow interface, hence resistance to subduction. Our results thus indicate that rheological properties of sediments are not the only control on plate kinematics. We are now investigating how the amount of incoming plate sediments and their frictional and rheological properties affect plate speed to better understand subduction dynamics.