We present constraints on the hydration state and rheology of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers record deformational microstructures consistent with shearing in a lithosphere-scale transcurrent fault system. Analysis by Fourier transform infrared spectroscopy indicates that nominally anhydrous phases retain some OH⁻, but bulk rock concentrations are generally restricted to <50 ppm H₂O by weight. From the rock microstructure, we determined differential stress magnitude and active deformation mechanism(s); combined with estimates of hydration state, we constrained the rheology. Recrystallized grain size piezometry shows that the mantle beneath the NAFZ sustained differential stresses of 10–20 MPa, largely independent of depth. The dominant deformation mechanism(s) change with depth; xenoliths extracted from shallower depths record evidence for grain size-sensitive creep possibly in the presence of melt. At intermediate depths, both dislocation creep and grain size-sensitive mechanisms were active, and we did not observe evidence for deformation in the presence of melt. The deepest samples were dominated by dislocation creep. The strong temperature sensitivity of creep mechanisms, combined with the low variability in differential stress, contributes to a stratified viscosity profile ranging from 10¹⁸ Pa s for the deepest samples to >10²² Pa s at shallower depths (assuming a melt-free rheology). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. Vertical stratification in viscosity beneath the NAFZ, the result of melt-present deformation and/or transitions in the dominant deformation mechanism, has important consequences for the seismic cycle of strike-slip fault systems.