A Simple Picture of Slab-Driven Mantle Wedge Flow Patterns

In this study, mantle wedge flow patterns that are predicted by 3-D numerical models with varying subduction parameters are evaluated based on the directions of the mantle inflow from the back-arc region and the down-dip outflow. The outflow directions generally parallel the subduction direction regardless of subduction parameters as it is coupled with the subducting slab. Then, what characterizes the mantle wedge flow pattern is the inflow direction, and the key parameter that controls the inflow direction is subduction obliquity, which is defined here as the angle between the subduction direction and the strike-normal axis of the subducting slab beneath the flowing mantle. The inner angle of the inflow direction with the outflow direction increases with subduction obliquity. As defined, a change in the strike of the slab leads to a change in the inflow direction. Such change is common along curved margins as the strike of the slab tends to follow that of the margin, or vice versa. Along convex-arc-ward margins, the mantle inflow is deflected towards the slab curvature but with reduced vigor, resulting in a cooler mantle wedge. Along concave-arc-ward margins, the mantle inflow is deflected away from the curvature but with increased vigor, resulting in a hotter mantle wedge. These effects increase with decreasing radius of curvature. The effect of change in the slab dip along the margin also affects the inflow direction through its impact on the strike of the slab, but its effect is relatively small. We express the inner angle between the inflow and outflow directions as a function of obliquity based on the modeling results and apply the function to predict the sub-arc mantle inflow directions in the circum-Pacific and neighboring regions away from slab edges. In most regions, the inner angles are less than 45° with some exceptions in Caribbean, eastern Aleutians, Marianas, northern Sumatra, and Hikurangi. At a scale of 1000s of kilometers, the most of the margin segments are either straight or curved concave-arc-ward with large radii of curvature, where only small along-margin variation in the mantle wedge temperature is expected. A large drop in the mantle wedge temperature by up to a couple of hundred degrees is expected at short segments with a sharp convex-arc-ward curvature, such as the Kuril-Japan and Bonin-Mariana junctions.