Deciphering grain-size reduction as a driver of mid-lithosphere discontinuity formation

The mid-lithosphere discontinuity (MLD) is a fundamental yet enigmatic seismic boundary within Earth's lithosphere, whose origin remains debated despite decades of study. Using thermomechanical models with self-consistent grain-size evolution, we propose a previously unrecognized mechanism for MLD formation. Plate deformation induces pronounced grain-size reduction within the brittle-ductile transition, generating permeability barriers that trap ascending fluids. These fluid-rich layers reproduce the observed seismic velocity drops. Their depth scales with lithospheric thickness and thermal structure and is fine-tuned by rheology evolution through brittle-ductile partitioning. The interplay between grain-size reduction and fluid accumulation imparts dual mechanical behavior to the MLD, stabilizing it within cold, ancient lithosphere or enabling localized failure under thermal perturbation. This framework identifies grain-scale processes as the primary control on MLD formation and links mantle-lithosphere coupling to planetary lithospheric evolution.