A Giant Impact Origin for the First Subduction on Earth

Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. Plain Language Summary Plate tectonics remains unique to Earth, but when and how it started is debated. Earth's oldest minerals indicate a clement surface by 4.3 Ga, resembling the modern Earth with its granitic crust and oceans. Granite is most easily explained as originating from subduction. However, the mechanisms for subduction initiation, especially so soon after the Moon‐forming giant impact, remain elusive. Earlier studies indicate that the core‐mantle boundary (CMB) temperature is increased due to accumulation of the impactor's core during the impact. Our recent work further shows that the lower half of Earth's mantle remains mostly solid after this impact and that parts of the impactor's mantle might have survived as the two seismically‐observed large low‐shear velocity provinces (LLSVPs). In this study, we perform whole‐mantle convection models to illustrate that strong mantle plumes can arise, weaken the lithosphere, and eventually initiate subduction ∼200 Myr after the giant impact. Our systematic computations reveal that the hot CMB temperature after the impact is the primary factor determining whether there is early subduction initiation, with enrichment of heat‐producing elements in LLSVPs as another potential contributor. Our model ties the Moon's formation to incipient subduction, providing insights for understanding the diverse tectonic regimes of rocky planets. Key Points The mantle thermochemical structure left by the Moon‐forming impact trigg