3-D Subduction Modeling: The Effects of Back-Arc Boundary Shape on Mantle Wedge Temperature

Analyzing the thermal structure of subduction zones is crucial for understanding the dynamic behaviors of earthquakes, arc volcanism, and material transfer. One key factor that controls subduction zone thermal structure is mantle wedge flow. Because the mantle is brought in from the back-arc by mantle wedge flow, sub-arc mantle temperature depends strongly on the back-arc thermal condition. In this study, we investigate how the choice of back-arc boundary shape effects the calculation of mantle wedge temperature in 3-D thermal modeling by quantifying temperature differences between models with a straight and a curved back-arc boundary. Using the 3-D fine-element code PGCtherm3D, we construct steady-state subduction models with a straight margin (reference model), a concave margin, and a convex margin where in all models the slab dip parallel to the subduction direction is kept at 45 degrees. In the models with a concave or convex margin, we apply either a curved or a straight back-arc vertical boundary and test their effects on mantle wedge temperature. The vertical trench-normal cross-section through the middle of all models have the same geometry to allow equitable comparison. We first apply an isoviscous mantle wedge rheology and a straight back-arc boundary to the two models. The models with a convex margin produces convergent lateral mantle in-flow in the mid-section of the model, discouraging trench-normal mantle in-flow causing lower wedge temperatures (by 90°C) than the reference model. The model with a concave margin shows the opposite effect; divergent flow is produced in the mid-section of the model allowing more hot mantle to flow in, raising the mantle wedge temperature. These results indicate the important role of margin curvature in controlling the mantle wedge flow pattern and subduction zone thermal structure. When we apply a back-arc boundary that has the same curvature as the margin, the mantle wedge temperature changes by -30°C and +5°C for the models with a concave and convex margin, respectively, amplifying the cooling and warming effects of margin curvature. We plan to apply time-dependent mantle wedge rheology to these models, in which the temperature difference between the models with the curved and straight back-arc boundaries is likely to be amplified by the effect of the mantle rheology.