I investigate a number of lithospheric deformation models based upon the concept that the motions of points near the North America-Pacific plate boundary are a linear combination of North America and Pacific velocities. This concept entails a shear strain transition zone in the vicinity of the boundary. The best of these models fit 95% of the variance in 139 Very Long Baseline Interferometry (VLBI) length and transverse velocity observations taken over 5.5 years and require only 90% of the relative plate motion predicted by RM2. Residual misfits in the megashear models are largely explained by a 6.1-mm/yr component of Basin and Range spreading perpendicular to the plate boundary. Instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the San Andreas Fault. Some of this deformation will be recovered through elastic rebound; the rest will be permanently set through plastic processes. Thus depending on the elastic-plastic composition of the borderlands, the width and pattern of strain accumulation in the megashear over geological time may or may not be significantly different than what is currently observed. Because VLBI data have been assembled for only a small fraction of the earthquake cycle, plastic and elastic strains are indistinguishable. Other information will have to be brought to bear to discriminate elastic and plastic behaviors and reconcile instantaneous and geologic deformation near the North America-Pacific margin. ¿ American Geophysical Union 1988