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 reproduc