Mantle flow induced by back arc spreading

Abstract

I present a simple model for the mantle flow induced by back arc spreading behind a subduction zone. The spreading and the subduction are represented by imposed velocities on the boundaries of a fluid wedge of constant viscosity; the boundary condition on the upper surface is time‐dependent to represent migration of the spreading centre. An analytical solution for the simultaneous flow of the unmelted mantle and an interstitial melt phase is obtained using the Mellin transform. Because the spreading centre is migrating, pathlines of material particles in the wedge differ substantially from the instantaneous streamlines. The solution for the melt flow delineates the mantle source regions for island arc tholeiites (IAT) and back arc basin basalts (BABB), and shows that these source regions are spatially distinct. The source region for BABB is connected to the downgoing slab in the early stages of spreading, but not later. This may explain the observed geochemical evolution of basalts in back arc basins such as the Lau Basin. Copyright © 1989, Wiley Blackwell. All rights reserved