Quantification of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust. Quartz-strain-rate-metry (QSR) is a convenient tool for performing such measurements once calibrated. We achieve this calibration by identifying the best piezometer-rheological law pairs that yield a strain rate in agreement with that measured on the same outcrop by a more direct method taken as a reference. When applied to two major continental strike-slip shear zones, the Ailao Shan–Red River (ASRR; southwest China) and the Karakorum (northwest India), the calibrated QSR highlights across-strike strain rate variations, from <1 × 10−15 s-1 in zones where strain is weak, to >1 × 10−13 s-1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates on the order of 1.1 cm yr-1 (Karakorum) and 4 cm yr-1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.