The Delamination and Underplating of the Subducting Crust, and Slow Subduction

Abstract:

Subducted, compositionally buoyant, crustal materials may form instabilities at the slab-mantle wedge interface which have the potential to feed crustal plumes that rise through the mantle wedge. The end result of this would be underplating of slab crustal material, below the over-riding plate, beneath arcs. This behaviour has been invoked to explain many features of arc magmatism, from variable sedimentary geochemical signatures to its discrete nature in space and time. It has also been proposed as a key mechanism that has led to the formation of Earth’s continental crust, at least in part, over time.

We set up a 2D, dynamic, numerical model of subduction with a compositionally layered subducting crust in order to investigate this process of crustal delamination and underplating. Upon undertaking a parameter sensitivity study of this model we recognise that subduction convergence rate has a particularly strong control on this behaviour with slow subduction zones favouring the formation of crustal instabilities. Therefore, we further quantify the effect of subduction velocity on the rate of crustal material flux into the mantle wedge and the composition of this delaminated material. A detailed literature study of observed arc magmatism throughout the Cenozoic, exhibits a correlation between convergence rate and certain geochemical signatures. Such signatures are often suggested to be linked with the melting of crustal materials in subduction zones. Here, we propose and illustrate that this correlation requires the operation of crustal delamination in slow subduction zones.