Metamorphic and mechanical feedbacks facilitate plate boundary weakening during subduction initiation

The generation of lithosphere-scale subduction plate boundaries requires mechanical weakening of rocks in order to localize immense strain. Geologic constraints on the mechanical properties and evolving thermal structure of the proto-plate interface during subduction initiation are needed in order to (in)validate proposed micro-physical mechanisms, or hypothesize new mechanisms, that can facilitate such weakening. Recent petrologic and geo-/thermo-chronologic data demonstrate that the thermal structure of the plate interface cools through time following initial underthrusting. Therefore, cooling must lead to rheological weakening in order for a plate boundary to develop, which runs counter to conventional models of temperature-dependent rock rheology. Here, we compare microstructures and metamorphic mineral assemblages from rocks that formed during various stages of subduction zone development in order to quantify changes in interface strength as a function of temperature. We document first-order differences in metamorphic mineralogy, microstructures, and inferred deformation mechanisms that translate to punctuated decreases in interface viscosity from hot initial convergence, to warm self-sustaining subduction, to a cold, mature plate boundary. We suggest that these cooling-induced changes may facilitate feedbacks required for a subduction plate boundary to initiate and sustain weakness. Our interpreted rheological changes successfully explain observed changes in upper plate stress state during subduction initiation, timing and chemistry of proto-forearc ophiolite lavas, and kinematically-constrained delay times between plate convergence and the onset of arc volcanism.