New gravity and topographic elevation data crossing the Altyn Tagh and West Kunlun faults in northern Tibet are used to test key features of a recently proposed tectonic model for the northern boundary of the Tibetan Plateau. The observations, collected in 1997 and 1998 at 468 stations along three profiles, were converted to Bouguer gravity anomalies to constrain flexural models for isostatic compensation of the large relief associated with these faults. Initially, we failed to reproduce the main features of the gravity anomalies with any isostatic model that assumes that the main contribution to the observed gravity anomalies is from the flexure of the Moho. By integrating sedimentary information derived from exploration seismology in the Tarim and Qaidam basins, we obtained satisfactory fits to the data using simple models of isostasy. No elastic strength across the Altyn Tagh is required to fit the gravity data, suggesting that the fault breaks the entire lithosphere beneath it and has negligible thrust component currently or in the past. This result is consistent with tectonic models that create the thickened crust of northern Tibet through thick-skinned thrust sheets progressively overriding Asia to the northeast as the Altyn Tagh, behaving as a classic transform fault, propagates in that direction. Farther west, the gravity observations across the West Kunlun are best explained by significant underthrusting of the high topography by an elastic plate with effective thickness between 30 and 40 km, in agreement with geologic interpretation and modeling of earlier, but very sparse, gravity observations.