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Abstract As the largest terrestrial planet in the solar system, Earth experienced a prolonged major accretion, ending with the Moon-forming giant impact (MFGI), whereas the direct evidence and origin of the impactor Theia remain elusive. Recent computational studies indicate that parts of the impactor Theia mantle may persist above Earth’s core–mantle boundary as the large low-velocity provinces (LLVPs), yet it remains unclear how these results were affected by the initial size of Theia fragments after the MFGI. Here I explore such influence in whole-mantle convection simulations, assuming that the Theia debris size follows the size distribution of the main-belt asteroids, which provides a natural estimation of collision debris for the ill-constrained parameter during extreme impacts. The results demonstrate that the asteroid-sized Theia debris can survive Earth’s 4.5-billion-year convective history as large-scale thermochemical structures resembling the seismically observed LLVPs. The results also demonstrate that rheologically strong Theia fragments are more capable of long-term preservation compared to those with weaker compositions. The inferred viscosity of Theia fragments aligns with that proposed for LLVPs from noble gas isotope evidence for a dry plume mantle source and agrees with global mantle attenuation constraints from seismic normal modes. These findings provide insight into the physical mechanism of preserving ancient geochemical signatures in Earth’s mantle, support an inner solar system provenance for the impactor Theia, and further help explain the isotopic homogeneity between Earth and the Moon. 

Abstract 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.