A major low-angle ductile shear zone containing S-C mylonites involves metamorphosed granitic rocks at the southeastern edge of the Nyainqentanglha mountain range, southern Tibet. Prominent triangular facet geomorphology is developed by valley erosion of the detachment surface, defined by the top of the southeast-dipping mylonitec shear zone. Kinematic criteria consistently indicate a top-to-southeast sense of shear. The ductile shearing deformation is inferred, from isotopic cooling ages, to have occurred during the interval 11-5 Ma (late Miocene). We interpret the shear zone as a regional extensional detachment; its development indicates when the extensional tectonics in this area started, which in turn may mark the time when the maximum sustainable surface elevation, and perhaps crustal thickness, was reached in southern Tibet.