Temperature and Dynamic Effects of the Plate Boundary Shear Zone During Subduction

Abstract The plate boundary in a subduction zone is a narrow, weak shear zone, which transitions from brittle to ductile deformation with increasing depth. The exhumed rock record documents the important role of metamorphism, fluid release, and mixing of compositionally distinct materials in controlling the physical properties of the shear zone. The geometry and rheology of the shear zone are also key parameters affecting model predictions of subduction kinematics, including slab sinking and trench retreat. Furthermore, the shear zone modulates mantle wedge flow and thermal structure through the depth of flow field decoupling along the slab interface. However, previous studies have modeled the shear zone either as a weak layer or a slab surface boundary condition with a fixed cutoff depth. These simplifications fail to explicitly link the properties and depth extent of the shear zone to metamorphic and deformational processes occurring in the shear zone. Here, we show that linking the viscosity variation in the shear zone to the basalt‐eclogite transition provides a physical explanation for the inferred globally constant mantle decoupling depth and naturally leads to self‐sustained, one‐sided subduction. In addition, we find that feedback between shallow slab deflection and the shear zone can lead to elevated trench retreat rates. Using an analytic solution, we also show that such feedback controls shallow slab thermal structures through dynamically determined decoupling depths. The model results predict that near the shallow decoupling depth, thermal conditions near the slab surface are favorable for melting of crustal material under moderately wet conditions.