Amphibole enrichment weakens the mid- to lower crust of southern Tibet

A weak mid- to lower crust is commonly invoked to explain uplift, lateral flow, and lithospheric removal beneath southern Tibet, yet its origin remains unresolved. Because present-day geotherms are generally too cool for widespread melt weakening, we tested whether precollisional arc magmatism produced a hydrous mineralogical inheritance that weakened the mid- to lower crust. Coupled phase-equilibrium modeling and experimentally calibrated flow laws show that geologically plausible bulk H2O contents of 1.0−1.5 wt% stabilize amphibole-rich assemblages containing 50−65 vol% amphibole at depths of 30−60 km. This hydration-driven transformation from dry granulitic assemblages to amphibole-rich amphibolite lowers viscosity by up to two orders of magnitude and produces slightly buoyant crust, with predicted viscosities overlapping geodetically inferred values for ductile flow beneath southern Tibet. Below ∼60 km, amphibole breakdown produces garnet- and clinopyroxene-rich assemblages, increasing density and promoting eclogitization and possible delamination. We propose that inherited amphibole enrichment created a weak mid- to lower crust, whereas subsequent amphibole breakdown localized the transition to dense, gravitationally unstable lowermost crust.