Variability in flow and temperatures within mantle subduction zones

Abstract

A series of laboratory experiments is used to model three-dimensional aspects of flow in subduction zones, and the consequent temperature variations in the slab and overlying mantle wedge. The effects of longitudinal, rollback and slab-steepening components of motions are considered, along with different thicknesses of the over-riding lithosphere. The results show that the style of plate sinking influences the evolution of subduction zones, both in terms of the speed, orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab. In the simplest case of longitudinal sinking, without rollback motion, velocities in the mantle wedge are 30-40% of the downdip plate speed. Return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds, and there is no mass flux around the edges of a slab segment (of finite width). Rollback subduction leads to flow both around and beneath the sinking slab, with larger velocities in the wedge (up to 150% of the slab speed) and flow focused toward the center of the plate segment. Rollback subduction, in which either the trench migrates or the plate steepens with time, induces shallow and steep wedge return flow trajectories, respectively. The thermal evolution of the plate is strongly influenced by sinking style and rate: highest temperatures are along the edges of the slab for longitudinal sinking, but along the centerline of the slab segment for rollback motion. Slab surface temperatures, when scaled to the mantle, are higher than those given by previous two-dimensional numerical models, and are consistent with recent observational and experimental data on melt compositions.