Factors Controlling Slab Retreat and the Formation of Back-Arcs: Insights from Numerical Models

Abstract:

Although subduction is a first order plate tectonic process, the factors controlling the dynamics of slab roll-back and back-arc formation are still not very well understood. We present self-consistent thermo-mechanical models to study oceanic subduction, slab retreat, and back arc formation. We focus on two aspects of the subduction process: 1) factors that control retreat of the subduction zone, and 2) those that control the opening of the back arc. The model evolution is calculated using 2D plane strain thermo-mechanical finite element techniques for the finite element solution of incompressible viscous-plastic creeping flows (Fullsack, 1995). The models extend from the surface to 660 km depth. The upper surface of the model is free to move. We investigate interaction of the subducting slab with the overlying plate and focus on factors that may control the opening of a back-arc basin. The down going plate is driven by a kinematic boundary condition, far from the subduction zone. After an initial stage of far-field driven contraction, negative buoyant down welling of the oceanic lithosphere may drive continued subduction zone leading to mature subduction and the formation of an extensional back arc. The models suggest that two primary factors are required for slab retreat and the formation of an extensional back-arc system: 1) Convective destabilization and weakening of the overlying continental back arc region, and 2) sufficient negative buoyancy of the subducting plate.