RIFT FLANK UPLIFT IN TIBET - EVIDENCE FOR A VISCOUS LOWER CRUST

Analysis of high-resolution digital topography across 21 Plio-Pleistoc ene grabens in central and southern Tibet shows that high topography o ccurs along the graben margins. Profiles across these marginal highlan ds fit curves describing the flexure of an elastic plate under vertica l loading. We interpret the features as flexural rift flank uplifts, r esulting from regional isostatic compensation of low-density grabens. The short wavelengths of the uplifts (30-40 km) imply a low flexural r igidity for the plateau (D = 2-6 x 10(20) N m), similar to values infe rred for the Basin and Range of western North America. In addition, th e grabens themselves are extremely narrow compared to typical continen tal rifts. Taken together, these observations suggest the existence of a low-viscosity lower crust and suggest that isostatic compensation i s taking place within this ductile lower crust. Simple channel flow ca lculations suggest that crustal viscosities below approximately 10(22) Pa s are necessary to accommodate this compensation over reasonable t imescales.