Analysis of high-resolution digital topography across 21 Plio-Pleistocene grabens in central and southern Tibet shows that high topography occurs along the graben margins. Profiles across these marginal highlands fit curves describing the flexure of anelastic plate under vertical loading. We interpret the features as flexural rift flank uplifts, resulting from regional isostatic compensation of low-density grabens. The short wavelengths of the uplifts (30--40 km) imply a low flexural rigidity for the plateau (D=2--6¿1020 N m), similar to values inferred for the Basin and Range of western North America. In addition, the grabens themselves are extremely narrow compared to typical continental rifts. Taken together, these observations suggest the existence of a low-viscosity lower crust and suggest that isostatic compensation is taking place within this ductile lower crust. Simple channel flow calculations suggest that crustal viscosities below ~1022 Pa s are necessary to accommodate this compensation over reasonable timescales.