We investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three-dimensional finite element implementation of the yield stress envelope model. In its initial configuration the three-dimensional model represents a 1000 km by 1000 km area of continental lithosphere. An initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates. The model region beyond the weak trend is uniform. Model conditions are varied by changing the thickness and composition of the crust, the geotherm and extension rate, the strength of the initial weakness and the dimensions of the preweakened and uniform regions. For each model, breakup nucleates at a point on the weak trend in response to an applied constant extension rate. The break propagates rapidly along the weak trend and then slows as it crosses the uniform region. This produces a locked zone in which breakup is delayed, resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus. The total extension required to tear across the 1000-km length of the model is 210 km for 40-km-thick crust, 400 km for 45-km-thick crust, and 710 km for 50-km-thick crust. Smaller increases in total extension occur for elevated geotherms and reduced extension rates. This indicates that prior extension estimates based on subsidence, which exceed 600 km for margin plateaus, can be explained by rift propagation in three dimensions. Differential extension along the model margin is linearly related to the length of the locked zone. The slope of the line, which we call Δ, is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation. Model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in Δ from 0.2 to 1.3. We compare Δ values computed from our model to those estimated from the subsidence of Bonavista Platform and Rockall Plateau conjugate margins in the north Atlantic basin and Georges Bank and Blake Plateau and their conjugate margins in the central Atlantic basin. These results indicate that the differential extension associated with these locked zones, which ranges from 170 km to 550 km, can be predicted in terms of their initial crustal thickness and their length along the margins. ¿ American Geophysical Union 1996