The essential features of observed topography, free air gravity, and the geoid over subduction zones have been reproduced with a dynamically self consistent viscous flow model of a subduction zone with a faulted lithosphere. A fault between the overriding and subducting plates is crucial to understanding lithospheric deformation and gravity in a subduction zone. The topography for models with a fault is characterized by a deep and narrow trench and a broad and shallow back-arc depression, quite different from viscous models without faults which only have a broad and large depression over subducting slabs. For a model in which a slab is 600 km long and 80 Myr old, the resulting trench is approximately 3 km deep and 100 km wide and the back-arc basin is about 1.5 km deep and 600 km wide; the model is characterized by a long wavelength geoid high of about 30 m over the slab, a local geoid low of 10 m over the trench and a 150 mgal gravity low over the trench. The fact that trench depth and width are reproduced in models with a small resisting shear stress, 50 bars, suggests that convergent plate boundaries are probably weak. ¿ American Geophysical Union 1992