We show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces, consistent with what is observed in the western Pacific. The dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method. We investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion. The viscosity of the medium is non-Newtonian and temperature-dependent. We also examine the influence of a perfectly plastic yield stress. A new method to model the preexisting weak zone as a graded fault with remeshing is introduced. The first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics. Initially, topography across the plate boundary is characterized by over 1000 m of uplift close to the trench. Subsequently, a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m/Myr for a slab subducting at 2 cm/yr. Tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration. For a plate force of 4¿1012 N/m, subduction initiates even with a fault shear stress of 3 MPa, the resistance most consistent with seismological observations. However, subduction will not initiate when this fault shear stress is >5 MPa. Our results are consistent with plate reconstructions, which predict the initiation of subduction across preexisting weak zones within oceanic basins. ¿ 1998 American Geophysical Union