Numerical studies of back-arc convection and the formation of marginal basins

Abstract

Marginal basins that occur behind island arcs display geological and geophysical features that imply an extensional origin and a spreading mechanism, possibly driven by the heating of the basin floor from below. The origin and evolution of these basins appear to be associated with the subduction process of the oceanic lithosphere. The mechanism we propose for the formation and spreading of marginal basins is the convective flow induced in the mantle by the subducting slab. This convective current brings hot mantle material to the base of the lithosphere behind island arcs. The combination of material upwelling, heating of the lithosphere and flow induced tension initiates spreading and the formation of marginal basins. Quantitative investigation of this mechanism has been carried out utilizing numerical calculations using both constant and variable viscosity models. Numerical results indicate that the induced flow can generate a 1 km topographic rise and a tensile stress of about 100 bars. Together with the thinning of the lithosphere due to heating, this induced current is probably sufficient to generate inter-arc spreading at about 5–10 million years after the initiation of subduction. The horizontal scale of the spreading center obtained from the numerical models is also consistent with the observations over many marginal basins.