- Controversial end member models for the growth and evolution of the Tibetan Plateau demand quantitative constraints of the lithospheric rheology. Direct determinations of bulk crustal rheology, however, remain relatively sparse. Here we use the flexural rebound of lacustrine shorelines developed during the Lingtong highstand around Siling Co, in central Tibet, to place bounds on the effective elastic thickness (T[subscript e]) and viscosity of Tibetan crust. Shoreline features associated with the Lingtong highstand complex ∼60 meters above present lake level are deflected from horizontal by 2–4 meters over wavelengths of ∼200 kilometers. Optically stimulated luminescence dating of aggradational shoreline deposits indicate that these lake levels were reached at 6–4 ka. Assuming that surface loads were entirely supported by an elastic layer overlying an inviscid fluid, the range and spatial distribution of variations in shoreline elevation are consistent with deflections predicted by a uniform elastic plate with thickness, T[subscript e] of 20–30 km. If viscoelastic relaxation in response to lake withdrawal is complete, our data suggest an average viscosity ≤10¹⁹ Pa s. These results imply that the apparent viscosity of the lower crust inferred over millennial timescales is comparable with that estimated from post-seismic relaxation over decadal timescale.