Timescales of subduction initiation and evolution of subduction thermal regimes

Subduction zones are first-order features of plate tectonics on Earth, yet the mechanisms by which subduction initiates remain enigmatic and controversial. Here, we reappraise the timing of metamorphism of the rock units first detached from the leading edge of the downgoing slab during initiation of the Neotethys subduction, now preserved in the metamorphic sole of the Semail ophiolite (Oman–United Arab Emirates). Using petrochronology and phase equilibrium modeling, we demonstrate that subduction initiated prior to 102–100 Ma at a slow rate (< 1 cm/yr). Subduction stagnated at relatively warm conditions (15–20°C/km) for > 5 Myr before evolving into a faster (≥ 2–5 cm/yr) and colder (∼7°C/km) self-sustained regime. Subduction acceleration (i.e. “unlocking” stage) triggered the onset of slab retreat, large-scale corner flow and fast ocean spreading in the overriding plate at 96–95 Ma, through the progressive change of thermo-mechanical structure of the plate interface. This study reconciles conflicting analogue and numerical subduction initiation models, shedding light on the thermal, mechanical and kinematic complexity of subduction initiation.